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AGRONOMIC PRACTICES FOR WATER
MANAGEMENT UNDER SMALLHOLDER
RAINFED AGRICULTURE
TRAINING MANUAL No.4
Nile Basin Initiative (NBI)
Regional Agricultural Trade and Productivity Project (RATP)
By:
Bancy M. Mati
2012
Citation
Mati, B.M. 2012. Agronomic Practices for Water Management under Smallholder Rainfed Agriculture.
Training Manual 4. NBI/NELSAP - Regional Agricultural and Trade Programme (RATP),
Bujumbura, Burundi.
Illustrations and diagrams drawn by: Munene M. Muverethi
Contacts:
NELSAP/Regional Agricultural Trade and Productivity Project
Quartier Kigobe Sud, Kigobe Main Road, Plot No: 7532/C
P.O Box: 4949
Bujumbura- BURUNDI
Nile Basin Initiative – NELSAP/RATP
ii AGRONOMIC PRACTICES FOR WATER MANAGEMENT
UNDER SMALLHOLDER RAINFED AGRICULTURE
About this Training Manual
The Nile Basin Initiative (NBI) is a partnership of the riparian states (Burundi, Democratic Re-
public of Congo, Egypt, Ethiopia, Kenya, Rwanda, Sudan, Tanzania and Uganda, Eritrea is partic-
ipating actively in the NBI as an observer) that seeks to develop the river in a cooperative manner,
share substantial socioeconomic benets, and promote regional peace and security through its
shared vision of “sustainable socioeconomic development through the equitable utilization of, and
benet from, the common Nile Basin water resources”. NBI’s Strategic Action Program is made up
of the Shared Vision Program (SVP) and Subsidiary Action Programs (SAPs). The SAPs are mandated
to initiate concrete investments and action on the ground in the Eastern Nile (ENSAP) and Nile
Equatorial Lakes sub-basins (NELSAP).
NELSAP through its sub basin programs implements pre-investment programs in the areas of
power, trade and development and natural resources management. As part of its pre-investment
framework, the Regional Agricultural Trade and productivity Project (RATP), in concert with the
NELSAP, intends to promote and disseminate best practices on water harvesting and small scale
irrigation development as a contribution towards agricultural development in the NEL Countries.
NELSAP has previously implemented completed a project called Efcient Water Use for Agricul-
ture Project (EWUAP). One of the recommendations of EWUAP was the need to develop Train-
ing/Dissemination materials on “adoption of low cost technologies for water storage, conveyance, distribution,
treatment and use for agriculture that can be adapted by communities and households of the rural and peri-urban
poor”. This Training Manual is the initiative of NELSAP, for that purpose.
This Training Manual summarizes the major components of water conservation techniques prac-
ticed in rainfed smallholder agriculture. It focuses more on soil moisture retention and soil fertility
management. It covers four specic technologies adaptable by smallholder farmers in the Nile
Basin countries. These are; crop husbandry, vegetative barriers, cover crops, mulches, soil nutrient
management, conservation tillage and agroforestry. For each intervention, the salient characteris-
tics of the technology are described, as well as the design, management and maintenance.
This manual is meant to improve the skills of engineers, technicians, extension workers, managers
and practitioners engaged in soil and water management, especially those working in smallholder
agriculture in Africa. It is meant to inform, educate, enhance knowledge and practice targeting
smallholder agricultural livelihoods in the NEL region. The information contained here may not
be exhaustive and thus, readers are encouraged to seek further information from references cited
in this publication and elsewhere.
Acknowledgements
The publication of this booklet was supported by the Nile Basin Initiative’s NELSAP-RATP.
RATP is a technical assistance project nanced by the Canadian International Development Agen-
cy (CIDA). The author wishes to thank all the institutions and individuals who provided data/
information for the publication of this manual. Special thanks to Innocent Ntabana, Gabriel Ndi-
kumanaAdamu Zeleke, Habtu Bezabhe, Espoir Bagula, Faith Livingstone, Samuel Mungai, Francis
Koome, Jean Jacques Muhinda, Maibo Malesu, Jimmy Musiime and Mary Kakinda among others.
The views expressed here are not necessarily those of CIDA or NBI, as the content is solely the
responsibility of the author.
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Table of Contents
About this Training Manual ........................................................................................................... ii
Acknowledgements ....................................................................................................................... ii
Denion of Key Terms ................................................................................................................ vi
1. INTRODUCTION ......................................................................................................................... x
1.1 Water management in rainfed agriculture .................................................................................. x
1.2 Agronomic conservaon pracces ..............................................................................................x
1.3 The case for water conservaon .................................................................................................1
1.4 Advantages of water conservaon ..............................................................................................2
1.5 Limitaons faced ........................................................................................................................2
1.6 Basic principles of improving water management .....................................................................2
2. AGRONOMIC CROP MANAGEMENT PRACTICES ..........................................................................4
2.1 Crop husbandry ...........................................................................................................................4
2.1.1 Crop selecon ....................................................................................................................4
2.1.2 Early planng ......................................................................................................................4
2.1.3 Contour farming .................................................................................................................5
2.1.4 Seed priming ......................................................................................................................5
2.1.5 Re-seeding grasslands ........................................................................................................6
2.2 Crop Rotaon .............................................................................................................................6
2.3 Intercropping ...............................................................................................................................6
2.3.1 Relay cropping .................................................................................................................... 7
2.3.2 Nurse cropping ................................................................................................................... 7
2.3.3 Strip cropping ..................................................................................................................... 8
2.4 Soil culvaon .............................................................................................................................8
2.4.1 Tillage pracces ..................................................................................................................8
2.5.2 Timing of llage. .............................................. ..................................................................9
2.5.3 Tillage depth ......................................................................................................................9
2.5.4 Seed bed preparaon ............ ............................................................................................9
2.5.5 Trench Farming ................................................................................................................... 9
2.5.6 Double digging ................................................................................................................ 10
3. VEGETATIVE BUFFERS ...............................................................................................................12
3.1 What are vegetave buers? ................................................................................................... 12
3.2. Grass Strips .............................................................................................................................. 12
3.3 Hedgerow Intercropping .......................................................................................................... 13
3.4 Trash Lines .............................................. .................................................................................. 13
4. COVER CROPS .......................................................................................................................... 15
4.1 What are cover crops?.............................................................................................................. 15
4.1.1 Ulity of cover crops ....................................................................................................... 15
4.1.2 Suitable cover crop types ................................................................................................ 15
4.1.3 Selecng suitable species ................................................................................................ 15
4.1.4 Advantages of cover crops .............................................................................................. 16
4.1.5 Major limitaons of cover crops ..................................................................................... 16
4.2 Permanent soil cover ............................................................................................................... 16
4.2.1 Benets of permanent soil cover .................................................................................... 17
4.2.2 Limitaons of permanent soil cover ................................................................................ 17
5 MULCHING ...............................................................................................................................18
5.1 What is mulching? .................................................................................................................... 18
5.2 The case for mulching .............................................................................................................. 18
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5.3 Types of mulches ...................................................................................................................... 18
5.3.1 Crop residues .................................................................................................................. 19
5.3.2 Green mulches ................................................................................................................ 19
5.3.3 Gravel-sand mulches ...................................................................................................... 19
5.3.4 Plasc mulching (plasc lms) ........................................................................................ 19
5.3.5 Alternate row-mulching .................................................................................................. 20
5.4 Mulching techniques: ............................................................................................................... 20
5.4.1 Planted mulch ................................................................................................................ 20
5.4.2 Live mulch ....................................................................................................................... 20
5.5 Benets of mulching ................................................................................................................. 20
5.6 Limitaons of mulches ............................................................................................................. 21
6. SOIL NUTRIENT MANAGEMENT ...............................................................................................22
6.1 What is soil ferlity? ................................................................................................................. 22
6.2 What is soil nutrient management? ......................................................................................... 22
6.3 Chemical ferlizers ................................................................................................................... 23
6.3.1 Inorganic ferlizers .......................................................................................................... 23
6.3.2 Rock phosphate ............................................................................................................... 24
6.3.3 Superphosphate .............................................................................................................. 24
6.3.4 Potassium ferlizer types ................................................................................................ 24
6.3.5 Commercially available ferlizers .................................................................................... 25
6.4 Organic ferlizers ...................................................................................................................... 25
6.4.1 Organic manures ............................................................................................................. 26
6.4.2 Farmyard manure ............................................................................................................ 26
6.4.3 Compost manure ............................................................................................................. 26
6.4.4 Fored compost ............................................................................................................ 27
6.4.5 Liquid manure ................................................................................................................. 28
6.4.6 Plant tea .......................................................................................................................... 28
6.4.7 Compost baskets ............................................................................................................. 29
6.4.8 Green manure ................................................................................................................. 29
6.5 Integrated soil ferlity management ........................................................................................ 30
6.5.1 Improved fallow .............................................................................................................. 30
6.5.2 Biomass transfer ............................................................................................................. 31
6.6. LEISA technologies .................................................................................................................. 31
6.6.1 Organic farming ............................................................................................................... 31
6.6.2 The case for LEISA ........................................................................................................... 31
7. CONSERVATION TILLAGE .......................................................................................................... 32
7.1 What is conservaon agriculture? ............................................... ............................................. 32
7.2 What is conservaon llage ..................................................................................................... 32
7.3 Types of conservaon llage .................................................................................................... 33
7.3.1 Minimum llage .............................................................................................................. 33
7.3.2 Zero llage ....................................................................................................................... 33
7.3.3 Spot llage ...................................................................................................................... 34
7.3.4 Deep llage ..................................................................................................................... 34
7.3.5 Strip culvaon ............................................................................................................... 35
7.3.6 Stubble mulch llage ....................................................................................................... 35
7.3.7 Ridging ............................................................................................................................ 36
7.3.8 Tied Ridging ..................................................................................................................... 36
7.4 Tools and equipment for conservaon llage .......................................................................... 37
7.4.1 Special features of CA equipment .................................................................................. 37
7.4.2 Hand-jab planters .............................................. ............................................................. 37
7.4.3 Hoe openers .................................................................................................................... 38
7.4.4 Animal-drawn planters ................................................................................................... 39
7.4.5 Magoye ripper ................................................................................................................. 39
7.4.6 Tractor-drawn ned implements ..................................................................................... 39
7.4.7 Two wheeled tractors ..................................................................................................... 40
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7.5 Advantages of conservaon llage ........................................................................................... 40
7.6 Limitaons of conservaon llage ........................................................................................... 41
8. AGROFORESTRY .......................................................................................................................42
8.1 What is agroforestry? ............................................................................................................... 42
8.2 Benets of agroforestry ............................................................................................................ 42
8.3 Limitaons ................................................................................................................................ 43
8.4 Types of agroforestry systems .................................................................................................. 43
8.5 Characteriscs of suitable agroforestry tree species ................................................................ 44
8.6 Tree establishment and care .................................................................................................... 44
9. SELECTED REFERENCES.............................................................................................................46
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Denition of Key Terms
Te r m Denition/Brief description
ASAL Arid and semi-arid lands
Available water The amount of water held in a soil that plants can use.
Available water holding
capacity
The total amount of water a soil prole can hold for plant uptake. It
depends on soil depth, texture, structure and organic matter content.
Bulk density The apparent density of a soil, measured by determining the ov-
en-dry mass of soil per unit volume
Compost
The decomposed organic matter that includes one or more of the
ingredients like farmyard manure, slurry, compost manure, crop resi-
dues, kitchen wastes, hedge cuttings, grain husks and other materials.
Conservation farming
The holistic application of conservation tillage alongside other agro-
nomic practices (e.g. manuring, crop rotations, mulching) to reduce
labour and preserve the natural state of the soil. Also called conser-
vation agriculture.
Conservation tillage Tillage that tries to preserve the soil, water, crop residues and biologi-
cal status of the soil with as little disturbance as necessary.
Contour (line) An imaginary line joining all points of the same elevation on a land
surface.
Conventional tillage
Land preparation that involves several soil turning and operations
such as digging, ploughing, discing, harrowing, rotavating, or a com-
bination of several soil turning operations depending on the crops to
be grown
Cover crops Crops grown to cover the soil during the cropping season, fallow
periods or between harvest and planting of commercial crops.
Crop rotation Planting different crops on the same piece of land every successive
season
Cultivation Tillage operations before or after planting to keep the crop free of
weeds.
Deep percolation Downward movement of water below the root zone under the force
of gravity, eventually arriving at the water table.
Depression storage Temporary holding of rainfall in crevices, hollows and surface de-
pressions
Diversion ditch
A channel made across the slope to protect cultivated land from
external runoff, normally with a gradient of 0.25-0.5%, also called
cut-off drain
Evaporation The amount of water that leaves a water surface or land as vapor.
Evaporation can be benecial or non-benecial.
Farmyard manure
Animal droppings from domestic livestock such as cattle, goats,
sheep, pigs, chicken including the sweepings, urine, remnants of fod-
der and animal beddings
Field capacity (FC) The maximum amount of water held in a soil, measured a few days
after it has been thoroughly soaked and allowed to drain freely.
Green manure
Leguminous green vegetation grown for the sole purpose of im-
proving soil fertility, and which is incorporated into the soil during
cultivation
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Horizontal interval The horizontal distance between two conservation structures
Hydraulic conductivity
The rate at which water can pass through a soil material, usually mea-
sured under saturated conditions to ensure water is moving through
the soil via gravity and positive head pressure.
Inltration Entry, absorption and downward movement of water into the soil
Inltration capacity Limiting rate at which falling rain can be absorbed by a soil surface in
the process of inltration.
Inltration rate
The rate at which water enters the soil prole. Inltration rate can be
relatively fast, especially as water enters into pores and cracks of dry
soil. As the soil wets up and becomes saturated, the inltration rate
slows to the point where surface runoff occurs.
Interception Catching and holding of rainfall above the ground surface by leaves,
stems and residues of plants
Interow Movement of soil water through a permeable layer in a downslope
direction parallel with the ground surface, also called throughow
LEISA Low External-Input Sustainable Agriculture technologies
Liquid Limit The moisture content at which a soil begins to ow and behave like a
liquid.
Liquid manure A mixture of farmyard manure, urine, green manure and other soil
nutrient additives which is prepared in liqueed form
Minimum tillage Reduced tillage operations on a farm to the bare minimum required
for crop production
Mulch A layer of crop residue or other material, placed on the soil surface.
Mulching
The practice of covering cropped land with a layer of loose materi-
al such as dry grass, straw, crop residues, leaves, compost inorganic
covers.
Nitrogen-xing
The ability of certain small organisms (bacteria, algae) to convert
atmospheric nitrogen into a form which can be used by plants. These
organisms live on or near the roots of legumes
Organic farming Growing crops without the use of articial inorganic fertilizers,
chemical pesticides and other “external” additives
Overland ow Water owing over a sloping ground surface to join a channel or
stream
Overtopping Water owing over the top of a bund or ridge, and is usually undesir-
able
Perennial (crop) A plant that lives for three or more years and which normally owers
and fruits at least in its second and subsequent years.
Permaculture
A system of farming in which farmland is maintained under some
crop cover throughout the year. It is also known as permanent soil
cover.
Permanent wilting
point (PWP)
The soil water content at which water is no longer available to plants,
which causes them to wilt because they cannot extract enough water
to meet their requirements.
Plastic limit The moisture content at which a soil changes from a semi-solid to a
plastic state
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Rainfed agriculture
Agricultural systems whereby natural rainfall is the predominant
source of water for growing crops, trees or pasture on that eld. It
also includes crops grown with ood ows harvested from excess
rainfall runoff.
Salinity Soils having high concentration of soluble salts
Saturation The moisture content at which all soil pores are completely wa-
ter-lled.
Semi-arid Fairly dry climate with average annual rainfall of about 300-700 mm,
with high variability in rainfall.
Slope gradient The angle of inclination of a slope, which may be expressed in de-
grees or as a percentage.
Slurry A mixture of animal dung, urine and water.
Soil and water conser-
vation (SWC)
Activities that maintain or enhance the productive capacity of land in
areas affected by or prone to soil erosion.
Soil erosion The movement of soil from one part of the land to another through
the action of wind or water.
Soil fertility
The capacity of a soil to produce crops by supplying nutrients (mac-
ro and micro) in correct proportion and in adequate amounts over a
long time.
Soil moisture Water held in the soil and available to plants through their root sys-
tem, also called soil water.
Soil moisture prole The depth to which water inltrates into the soil, also called inltra-
tion boundary
Soil porosity
The percentage of a given volume of soil that is made up of pore
spaces. Soils are oven-dried to measure bulk density, so porosity is a
measure of air-lled pore space
Sub-humid A humid climate with average annual rainfall of roughly 700-1000
mm.
Surface runoff Excess rainfall which runs off the surface of the land, it includes both
overland ow and stream-ow
Surface sealing
When soil forms a sort of clay cement after rain, because the nest
grains clog the soil pores, preventing water inltration. Also called
clogging up
Sustainable land man-
agement (SLM)
The use of land resources, including soils, water, animals and plants,
for the production of goods to meet changing human needs, while
simultaneously ensuring the long-term productive potential of these
resources and the maintenance of their environmental functions.
Terrace
A piece of land whose slope steepness and/or length has been re-
duced by either construction works, or by creating barriers across the
slope, so as to absorb and/or reduce surface runoff
Tillage Preparation of the land for planting, or all the operations undertaken
to prepare a seed bed in agriculture
Transpiration Water that is taken up by plants from the soil and then lost to the air
through small openings in the leaves of plants.
Vertical interval
Spacing between two conservation structures determined on the
basis of the difference in ground elevation, also referred to as vertical
distance.
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Vertisol
A type of montimorillonitic clay (also called black cotton soil) with
high clay content, which cracks when dry and is difcult to till when
wet
Water conservation The control, protection, storage, management and utilization of wa-
ter resources in such a way as to optimize productivity
Water harvesting Activities where water from rainfall and/or surface runoff is collect-
ed, diverted, stored and utilized.
Water logging State of land where the water table is located at or near the surface
resulting in poorly drained soils, adversely affecting crop production
Water storage capacity Maximum capacity of soil to hold water against the pull of gravity,
also called eld capacity
Water table Upper limit of the ground water
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1. INTRODUCTION
1.1 Water management in rainfed agriculture
Water management in rainfed agriculture includes the conservation, control, protection, storage
and utilization of water resources in such a way as to optimize crop and livestock production using
natural rainfall. It also includes managing water for ecosystem services and natural habitats. Water
management involves:
(i) Making optimal use of rainfall,
(ii) Reducing water losses (through runoff, evaporation, unnecessary transpiration,)
(iii) Increasing efciency of irrigation water use,
(iv) Selecting best suited crops and cropping methods,
(v) Reducing losses of stored water and in conveyance
(vi) Improving water availability (e.g. through soil moisture storage, aquifer recharge).
Water management therefore improves the availability of rainwater, surface runoff or irrigation
water for agricultural purposes, reduces the present size of water demand and protects water re-
sources from being polluted or wasted. Water management also involves selecting best suited crops
and cropping methods, using crops of high water use efciency, making use of structures, practic-
es and technologies that are efcient in water capture and soil moisture retention. Sometimes, wa-
ter conservation on its own is enough to ensure crop production improvements, especially where
water harvesting is either not feasible or desired. Rainfed water management is closely related to
soil and water conservation.
1.2 Agronomic conservation practices
Agronomic measures involve land husbandry, cropping, use of equipment and vegetative measures
and all the crop husbandry activities that are implemented on a farm, from land preparation to har-
vesting (gure 1.1). There is little earth moving or re-shaping of land. Instead terraces may form
naturally through erosion and deposition. These methods used encompass a range of technologies
and practices that facilitate optimal moisture retention in the soil and its use by desirable plants
such as crops and pastures. Agronomic measures on their own are best suited to high rainfall areas
and areas on gentle slopes, but they are even more benecial in semiarid zones and areas with steep
slopes when combined with structural methods.
Direct conservation of rainwater in the soil is sometimes referred to as in-situ water harvesting, or
in-situ conservation. These are techniques which optimise the retention of direct rain falling on a
piece of land, through improved inltration and reduction of other losses such as runoff and evap-
oration (Figure 1.1). It is distinguished from water harvesting in that there is no deliberate effort to
channel water from some other area (or catchment) onto the target cropped land.
Sustainable Land Management (SLM), on the other hand, is a broader term to encompass the
use of land resources, including soils, water, animals and plants, for the production of goods to
meet changing human needs, while simultaneously ensuring the long-term productive potential of
these resources and the maintenance of their environmental functions. This training manual is part
of a series of ten manuals, which put together, relate to SLM. This particular manual, however,
focuses on water conservation in rainfed systems under smallholder agriculture.
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Figure 1.1 Example of Agronomic conservation measures (photos by A. Rakotondralambo)
1.3 e case for water conservation
Water conservation is necessary in all agricultural systems, be they large scale or small-scale, mecha-
nized or manual, commercial or subsistence, in wet areas or dry areas, fertile or infertile soils, culti-
vated food crops or in pasture lands, poor or rich farmers. However, whereas water conservation in
very wet areas or wetlands involves some amount of land drainage (see Training Manual 9 in these
series), this particular training manual is more concerned with water conservation under smallhold-
er agriculture in the low to medium rainfall areas, which cover vast areas of Africa.
Typically, the arid, semi-arid and dry sub-humid areas (ASALs) receive low rainfall amounts spread
within a few months, or weeks of the year. The rainfall events are erratic and unreliable even during
the rainy season, falling in a few, heavy storms. As a result, soil water retention is poor as much of
the rain water runs off the surface, causing ooding and soil erosion.
Another feature of African soils is that they are highly weathered with low organic matter contents,
thus poor fertility and in low water retention properties. In some cases, the soils e.g. Alsols and
Luvisols, have soil sealing properties which inhibit inltration, further increasing the incidence of
surface runoff even after small storms.
High evaporation losses are another problem hampering crop production in smallholder farms.
The high diurnal temperatures coupled with poor land cover result in much of the soil moisture
getting lost as unproductive evaporation.
Another factor is rudimentary tillage practices such as use of the animal-drawn plough which cre-
ates a hard pan within the topsoil prole, further inhibiting inltration. Poor agronomic practices
such as failure to use manure or fertilizers, poor cover on the soil, and all activities that increase
runoff are to blame for declining soil fertility. Therefore, water conservation practices are needed
that prevent water from damaging the land but rather improve the absorptive capacity of the land
to hold more water, just where it falls (in-situ conservation).
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1.4 Advantages of water conservation
• Conserving water makes it available for crops, livestock and domestic use over a longer peri-
od.
• Controlling soil erosion improves crop or pasture yields.
• Conservation measures improve the supply of fuel and forest products.
• They increase the value of the land.
• Terraces make cultivating steep slopes easier.
• More and better livestock fodder is available, for example from grass strips, hedge barriers
and terrace embankments.
• Employment opportunities in soil- and water-conservation work increase.
1.5 Limitations faced
• Fragmented land ownership makes it difcult for farmers to invest optimally in soil and water
management systems.
• Agronomic water and soil conservation requires lot of labour
• The farmer still has to rely on natural rainfall (unlike irrigated systems), thus there are weath-
er-related risks including very low rainfall or oods.
• extremes can cause damage to crop production
1.6 Basic principles of improving water management
There are many ways of improving soil-moisture storage and conservation, some of which are de-
scribed elsewhere in the Training Manuals 1, 5 and 7 of these series). They can be divided broadly
into physical conservation structures and agronomic measures. Physical measures (described in
Training Manual 5) involve building permanent structures, usually of soil or stone, to control the
ow of water. Agronomic or vegetative measures are covered by this manual include the use of
vegetation, soil fertility enhancement and various tillage practices. In general the basic principles
are the same as the ones described below:
(a) Improving soil inltration
The basic principle for improving inltration is by loosening soil and creating roughness on the
surface. Depending on the characteristics of both the soil and rainfall, the methods includes rip-
ping, deep chiseling, surface covers, contour plowing, ridging and pitting techniques.
(b) Reducing evaporation losses om the root zone
The main source of water loss from the root zone is through evaporation from bare soil and tran-
spiration by weeds. The aim should be to reduce these losses and allow most of the water in the
root zone to be transpired by useful crops. There is a strong relationship between crop transpira-
tion and dry matter production. Based on the nature of evaporation process, surface covers using
mulches and crop canopy will reduce evaporation. However, most mulches will increase evapora-
tion during dry spells by prolonging the rst stage of evaporation. Tillage is also an effective way of
controlling evaporation by disrupting capillary continuity. Therefore, weed control is an important
soil and water conservation measure. The weeds should be controlled when still very young, so as
to effectively control the loss of water.
(c) Improving soil fertility
The effective utilization of water is important. Thus, soil water management goes with improve-
ments in soil fertility so as to get due benets of water conservation. Water conservation on soils
which are infertile results in wastage of resources. The reverse is also true, that is, applying fertilizers
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where the soil-moisture is limited does not yield much. Thus, although shortage of soil-moisture is
often the bigger constraint, improved soil fertility has high returns on investment in the long run.
(d) Improving crop water-use and productivity
Several agronomic practices are used for ensuring that crops use soil-water effectively and produc-
tively. These include:
• Selection of crops and varieties having growth patterns which match the soil-water availabil-
ity patterns of the given locality.
• The adjustment of sowing times so as to ensure that the periods of critical water require-
ment by plants coincide with the periods of adequate available soil-water.
• A judicious fertilizer use commensurate with the status of soil, nutrient needs of crops, plant
population and an available soil-moisture.
• Crop rotation including fallowing for purpose of using the difference in crop characteristics
to restore soil structure and fertility.
• Limited supplementary irrigation to carry the crop through a particularly damaging dry spell.
(e) Technologies and practices
These technological practices for implementing water conservation without structural measures
can be categorized as follows:
(i) Agronomic crop management practices,
(ii) Use of vegetative barriers,
(iii) Soil nutrient management,
(iv) Mulching,
(v) Conservation tillage, and
(vi) Agroforestry
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2. AGRONOMIC CROP MANAGEMENT PRACTICES
Agronomic practices are all the preparatory and crop husbandry activities that are implemented
on a farm during the crop production, starting from land preparation till harvesting. Ensuring a
healthy crop stand is in itself a water conservation measure. This is because a good crop stand
covers the soil well, sustainably makes use of nutrients and protects the land from excessive evap-
oration as most of the water is transpired as productive water consumption. The main reason for
water conservation is to achieve increased productivity, which is affected by other factors such
as seed type, variety, crop spacing, date of planting, weed control, fertility, pest and disease man-
agement. The agronomic practices relating to water management that support crop production
include the following:
2.1 Crop husbandry
Crop husbandry or how well the crop is managed in the eld is also a water management interven-
tion, because it affects the overall value of water to crop productivity. There are many operations
depending on crop type, climate, available technology and labour. However, good crop husbandry
includes; early or optimum planting schedules, e.g. dry seeding, improved tillage and eld prepara-
tion, use of best crop variety available, soil fertility, weed control, pest and disease management as
well as appropriate timing of all operations. Good crop management reduces soil erosion by water
and wind to tolerable levels and can improve soil fertility. Selection of appropriate crops for the soil
and slope, use of suitable cropping systems and rotations to keep the soil covered.
2.1.1 Crop selection
Crop selection, in itself is a water management intervention because the crop should match the
amount of water available for optimum production. Crops vary considerably in their water demand,
drought resistance and drought avoidance (gure 2.1). It is recommended to grow fast-growing
plants (but without greatly increased water requirements) that shorten the time in which water is
lost by transpiration and evaporation. High-yielding crops that require no appreciable increase in
water supply, and give high yields without increasing water demand and have high water-use ef-
ciency should be selected. The idea is to give priority to crops with high water productivity.
Figure 2.1 (a) Sorghum is drought resistant (Photos by B. Mati)
(b) High yielding drought tolerant cowpeas
2.1.2 Early planting
Planting annual crops early at the beginning of the rains or dry-planting just before the onset of the
rains has several advantages: It increases the chances of a crop reaching maturity before the rains
end, and as a result of early growth shading the soil surface, evaporation is reduced enabling more
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water to become available for transpiration. This increases the efciency of water use by the crop
and so increases yields. These effects are also favoured by the ush of inorganic nitrogen and other
nutrients liberated at the beginning of the rains from the decomposition of dead soil micro-organ-
isms. Interaction between the additional nutrients and soil water enhances crop growth and yield.
Crops planted early usually also benet from less pest problems.
2.1.3 Contour farming
Contour farming is when tillage, planting and other farm operations are done along the contour
(gure 2.2). It is one of the simplest methods to reduce surface runoff and control soil erosion.
Contour farming is done on its own or on terraced farms. On slopes of 4 to 6 percent, contour cul-
tivation can sufce to control soil erosion. It can reduce water loss (runoff) by 50 percent and soil
loss by about 50 percent compared to up and down hill cultivation. It is advisable to compliment
this technique by other interventions.
Figure 2.2 Contour cultivation combined with deep tillage (photo by Bancy Mati)
The effectiveness of contour farming decreases with increase in slope gradient and slope length,
and increasing intensity of rains. If the rainfall exceeds the surface detention capacity of the con-
touring system, concentrated runoff owing downstream unchecked can lead to accelerated ero-
sion and even severe gulling. Therefore, contour farming alone is not sufcient to control erosion
on steep, long slopes, erodible soils, and during erosive rains. The major drawbacks of contour
farming are frequent turning involving extra labour and machinery time, and loss of some area that
may have to be put out of production. Planting of crops should be in rows to permit inter-tillage
as described later.
2.1.4 Seed priming
Seed priming is pre-germination of seeds before planting by soaking them in water to hasten
germination and emergence. This ensures faster establishment thus providing ground cover to
protect the soil from erosion. It also ensures that crop growth is more advanced in areas with
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erratic rainfall, so that a crop yield can be assured. Crops such as melons andpumpkins can be
pre-germinated in paper tubings one month prior to the rains.
Maize seeds can also be primed before planting where rainfall is assured. Soaking the seed for as
little as 5-10 hours can reduce the time to emergence by 10 hours which may be crucial in enabling
seedling roots to grow down to below a rapidly drying or crusting soil surface. For most crops,
soaking the seed for 12 hours is usually sufcient, but up to 24 hours are needed for rice and maize.
Seed priming apparently does not work for nger millet.
2.1.5 Re-seeding grasslands
The re-seeding of grasslands or rangelands used for grazing livestock is a water management activ-
ity because when the land is covered by a good stand of grass, it absorbs rainfall, reducing runoff
erosion and improving productivity. Rangelands that have been denuded by overgrazing can be
re-seeded with selected grass varieties. Rehabilitation can be improved if re-seeding is accompa-
nied by other conservation activities such as scratch-plowing the surface with a tined implement,
or building stone scour checks to reduce runoff losses. The grass can be cut and baled for sale or
use in times of drought. Natural grass should also be encouraged to revegetate rangelands as it is
likely to be more sustainable. Some common natural grass varieties include common nger grass
(Digitaria eriantha), Rhodes grass (Chloris gayana), common thatching grass (Hyparrhenia hirta), spear
grass (Heteropogon contortus), and goose grass.
2.2 Crop Rotation
When one annual crop is grown year after year on the same piece of land, the yields gradually
decrease. The soil becomes less fertile, which reduces its water-holding capacity, thus becoming
loose and more easily carried away by running water and wind. More weeds appear and there is an
increase in pests and diseases. If maize, for example, is grown continuously on the same eld, the
stalk borer becomes a problem. This situation can be avoided by crop rotation because, when a
crop is not grown over a period of two or three seasons, its pests and disease-causing organisms
tend to disappear.
Crop rotation involves planting different crops on the same piece of land every successive season.
Depending on the farmer’s needs for food, income, a three to four or ve-year rotation pattern
is possible. Thus, crop rotation requires careful planning of both the season and the plots where
each crop will be planted. Rotation of crops is recommended to control diseases, control insects
and pests, and enable plants to extract nutrients from different soil horizons. Plants that take more
nutrients from the soil should be followed by those that need less and legumes that replace soil
nutrients.
Crop types suitable for rotations
Choice of varieties is important. Varieties which have proven excellence in irrigated or high rainfall
areas are generally unsuited for dry land conditions. Variety requirements for dry farming include:
• Short-stemmed varieties with limited leaf surface minimize transpiration.
• Deep, prolic root systems enhance moisture utilization.
• Quick-maturing varieties are important in order that the crop may develop prior to the hottest
and driest part of the year and mature before moisture supplies are completely exhausted.
• Leguminous plants which x nitrogen in the soil .These include beans, garden peas, Lucerne,
and cowpeas.
2.3 Intercropping
Intercropping is the growing of different types of crops on the same piece of land at the same
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time (gure 2.3). This reduces the chance of total crop failure in case of unexpected events such
as drought, weeds, pests and plant diseases since most of these attack selected types of crops. It
also increases the total production from a piece of land.
Figure 2.3 (a) Intercrop of maize and cow-
pea (Source: CFU-Zambia, 2007)
(b) Intercrop of maize and beans
(photo by Espoir Bagula)
Intercropping protects the soil from erosion, makes better use of water, sunlight and nutrients
and provides your family with a better variety of food crops. For example, planting a row of
beans between rows of maize and cassava or peas, beans and soya beans between strips of millet.
Intercropping also enables a farmer to plant green manure and food crops together.
Important considerations for intercropping
• Crops belonging to the same plant family should not be planted together as they could be
attacked by the same pests and diseases, e.g. tomatoes and potatoes, maize, sorghum and
millet.
• The taller crops should not have a dense canopy that cuts out light from those below. The
maize plant canopy does allow a reasonable amount of light for shorter plants like beans and
groundnuts.
• The roots should not explore the same soil layer, i.e., plants with a shallow root system should
be intercropped with plants with a deep root system, e.g., bananas (shallow rooter) and avo-
cado (deep rooter). This is aimed at avoiding competition for nutrients and moisture. At the
same time, nutrients from different soil layers are exploited.
2.3.1 Relay cropping
This is a farming practice whereby an annual crop is grown under an already established crop or
one that is nearing harvest, e.g. Maize planted with beans. After about three months, the beans
are ready for harvest while the maize is still growing. For instance, cassava can be planted under
the maize after the beans are harvested. In relay cropping, two or more annual crops are grown
together although not planted at the same time. This method reduces the need for re-ploughing
the garden and ensures continuous soil cover by the crop from direct sun heat or being eroded by
rainwater.
2.3.2 Nurse cropping
This involves growing an annual crop together with a perennial tree crop to provide ground cov-
er during the establishment of the sparsely spaced tree crop. This provides soil cover while also
improving the overall productivity of land. A runner crop e.g. mukuna beans, melons, pumpkins
planted under fruit trees e.g. papaya (gure 2.4). Mango provides a green mulch as well as nitrogen
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xation in the case of a legume annual crop.
Figure 2.4 (a) Bean nurse crop under pa-
paya
(photos by Bancy Mati)
(b) Sweet potato nurse crop with banana
2.3.3 Strip cropping
Strip cropping is a technique that serves to control erosion and increase water absorption thereby
maintaining soil fertility and plant response. In effect, it employs several farming practices such
as crop rotation, contour cultivation and stubble mulching. By growing in alternating strips crops
that permit erosion and exposure of soil and crops that inhibit these actions, several functions are
performed, such as; slope length is maintained, movement of runoff water is checked, silt in run-
off water is ltered by the different strips, dense foliage of the erosion resisting crop prevents rain
from beating directly on the soil surface and inltration of rainwater by soil is increased.
The crop strips are normally planted perpendicular to either the slope of the land or the prevailing
wind direction, according to whether water or wind presents the more serious erosion potential.
Additionally, crops which do not resist erosion should be rotated with crops which do. Examples
include growing groundnuts in strips with maize which is very effective in controlling runoff. The
normal seed rate of leguminous crops other than groundnut does not provide sufciently dense
canopy to prevent raindrops from beating the soil surface; it should be raised to three times the
normal seed rate.
2.4 Soil cultivation
Soil cultivation is done to break up the soil crust so as to improve water inltration and to create a
better environment for plants. What ever method is used for cultivation, it helps to loosen the soil
and open it for plant roots. It also helps to reduce weeds further improving the crop cover and its
productivity. Although conservation agriculture is advocated for, not all agricultural enterprises
and soil conditions are suited to conservation tillage. There is therefore need, wherever possible
to till the land for various types of crops and local circumstances. It is important to take care not
to cause soil compaction, as this could result in short rooting depths of many plants. Some of the
agents of soil compaction are animals, machinery, people walking, and continuous cultivation at
the same depth.
2.4.1 Tillage practices
Tillage involves seedbed preparation as well as weed control. Tillage should be done using appro-
priate equipment for each type of soil and at the right moisture content. Ploughing when the soil is
in the proper condition, wears the soil into thin layers, and forces the layers past each other. If the
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soil is too wet when ploughed (especially if it is heavy), the soil crumbs or granules are destroyed,
thus puddling or compacting the soil. When the soil is too dry, the soil tends to pulverize and form
dust. Tillage should therefore aim to:
• Result in a rough, cloddy surface that increases the moisture absorption and reduces runoff, as
well as erosion from wind and water.
• Controls and destroys weeds that compete with crop for sunlight, nutrients, and water.
• Destroys or prevent the formation of a hard pan which can develop after repeated shallow
ploughing or harrowing. This hard pan can stunt root growth, reduce water storage, and check
the capillary rise of water from the subsoil.
• Promotes bacterial activity by aerating soil, encouraging the decay of residues and the release
of nutrients.
2.5.2 Timing of tillage.
Ploughing, like planting, is sensitive to moisture and neither should be done when soil is either too
wet or too dry. In the arid and semiarid zones, proper moisture conditions are likely to occur only
at the beginning of the rainy season and should be done on the same day. If possible, planting
should immediately follow ploughing, with seed rows centered on the furrow slices.
Testing soil for correct moisture content
The proper soil moisture condition for ploughing can be easily determined in the eld. The usual
test is to squeeze a handful of soil. If it sticks together in a ball and does not readily crumble under
slight pressure by the thumb and nger, it is too wet for ploughing or working. If it does not stick
in a ball, it is too dry. When examining soils, samples should be taken both at, and a few inches
below the surface. Soil that sticks to the plod or to other tools is usually too wet. A shiny, unbroken
surface of the turned furrow is another indication of excessive soil moisture. In general, sandy soils
and those containing high proportions of organic matter bear ploughing and working at higher
moisture contents than do heavy clay soils.
2.5.3 Tillage depth
Generally, heavy clay soils should be ploughed deeper than light, sandy soils, in order to promote
circulation of the air and bacterial activity. Deep ploughing on sandy soils, which are naturally po-
rous and open, tends to disconnect the seed bed from the subsoil and increases evaporative loss of
moisture through capillary action. Deep ploughing need not necessarily be done annually. Depend-
ing on soil and rainfall, a deep ploughing can be done every 2 to 5 years.
Deep ploughing in some clay and loam soils can reduce yields for one or two seasons afterward, as
a result of an acidic subsoil. This may be dealt with by liming the soil (neutralizing the acidity) or
by varying the depth of the ploughing slowly so that the acidic subsoil is exposed a little at a time.
This practice also eliminates the plough pan.
2.5.4 Seed bed preparation
In general, smaller seeds require a ner, seed bed than larger seeds. Seeds germinate and plants
grow more readily on a reasonably ne, well prepared soil than on a coarse, lumpy one, and thor-
ough preparation reduces the work of planting and caring for the crops. Even then, it is recom-
mended not to overdo the tillage operations. The seedbed should be brought to a granular rather
than a powder-ne condition for planting.
2.5.5 Trench Farming
Trench farming involves digging a small furrow measuring about 0.6 m deep and 0.6 m wide (g-
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ure 2.5). The furrow is packed with crop residues (manure is added if possible), then backlled,
resulting in a bed. The trenches are normally meant for incorporating large amounts of organic
matter in the soil, and thus may end up being higher than the ground.. The purpose is three-fold:
to improve soil fertility, inltration and moisture storage capacity. Trench farming maximizes soil
moisture storage in the crop root zone by soaking and storing most of the rain water. It is used to
grow eld crops or vegetables. The trench stores enough moisture to guarantee a crop yield even
when there are only 2-weeks of rainfall. The trench can be re-used with good results for up to four
crop seasons.
Figure 2.5. Illustration of trench farming with crop inside the trench
Another trench farming variation is also used for banana crops, in a system where a small trench is
dug slightly ahead but uphill of the banana plant, and then lled with trash. During the rains, water
accumulates within the trench and is held as soil moisture by the trash. During the dry season, the
banana roots seek this moisture through hydrotropism and help the plant survive droughts while
increasing yields.
2.5.6 Double digging
Double digging is a soil tillage practice in which a farmer can use small plots of land intensively
over a long period. Double digging is commonly applicable in situations where the farmer has little
cultivable land area or where the soils are too poor to support healthy crop growth. In very poor
soils, the incorporation of compost manure during double digging greatly improves the fertility of
the soil and enables the farmer to get good yields. Double-dug beds are meant to improve soil po-
rosity and break hard pans by creating a deep layer of loose organic soil which is used for intensive
cultivation to produce higher yields. The practice aerates the soil, improves water absorption and
retention, allows plants to use available nutrients more efciently and increases rooting depth. They
are mainly used for cultivating high-value cash crops such as vegetables (gure 2.6).
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Figure 2.6 (a) Double dug sunken beds
with vegetables (photos by Bancy Mati)
(b) Double dug raised bed with vegetables
The construction technique involves preparing the garden beds by digging out the topsoil and
subsoil separately. The bottom of the trench is further tilled to improve inltration. The topsoil
is then mixed with organic manure and returned to the bed. Care is taken not to step on the bed
in order to avoid compaction. The commonly recommended dimensions of a double-dug bed are
approximately 1.5 x 7 m wide and 60 cm deep. The bed is lled with about six wheelbarrows of
compost, which can be used for four consecutive cropping seasons before the process needs re-
peating. High-value crops are then grown on the beds with very good results since the bed absorbs
more water than in conventional tillage. Farmers can adapt this method in various ways, digging
less deeply when the soil is rocky or when labour is scarce, changing the length of the beds and
adding a variety of organic materials. With double digging therefore, the farmer can grow crops
closely in small plots of land and get high yields repeatedly over a long period.
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3. VEGETATIVE BUFFERS
3.1 What are vegetative buers?
Vegetative buffers comprise natural vegetative strips either planted for the purpose of soil and
water conservation, or left un-ploughed during land preparation. They are normally planted across
the slope and have a small width to allow runoff to slow down and deposit sediments, while also
improving water inltration. Vegetative buffers can be living hedges such as tree and shrub strips,
grass strips or dead plant residues as in the case of trash lines. Runoff velocity can be reduced
drastically by planting vegetative buffers. They are sited to high rainfall areas, where the survival
of the vegetation is assured, and on gentle slopes, they can sufce as a soil and water conservation
barrier. In dry areas, special varieties of trees, shrubs and grasses are used, such as euphorbia, si-
sal, croton and makarkari grass. Vegetative buffer strips are suitable on gently rolling topography
with complex slopes, where contour strips are difcult to establish. The main types of vegetative
barriers include:
3.2. Grass Strips
A grass strip is a vegetative buffer, in which grass is planted in dense strips, about 0.5 to 1 m wide,
along the contour, and the space between the strips used for crop production (gure 3.1). These
lines create barriers that minimize soil erosion and runoff, through a ltering process. Silt builds
up in front of the strip, and with time, bench terraces are formed. For this reason, grass strips are
sometimes grouped together with structural soil and water conservation measures.
Figure 3.1 (a) Grass strip on terrace bank for
fodder (photo by Janvier Gassasira)
(b) Grass strip with bean crop
(photo by Bancy Mati)
The spacing of the strips depends on the slope of the land and is calculated to be equivalent to
intervals of regular terrace spacing. They are effective soil and water conservation on soils that
absorb water quickly, and on slopes as steep as 30%. On gentle sloping land, the strips are made
with a wide spacing (20-30 m), while on steep land the spacing is about 10 to 15m. The grass needs
to be trimmed regularly, to prevent spreading to the cropped area. Grass strips are suited to wetter
areas where their survival is more assured.
Grass strips are not costly and require little labour to install. They combine characteristics of both
biological and structural measures. Grass strips are a popular and easy way to terrace land, espe-
cially in areas with relatively good rainfall, where grass is used also as fodder. The grass is cut and
normally used as livestock fodder or as mulch. Many grass varieties are used, such as napier, guin-
ea and guatemala grass, Axonopus micay, Brachiaria brizantha, Brachiaria decumbens, Brachiaria mutica,
Cenchrus ciliaris, Eragrotis curvula, Molasses grass, Panicum antidotala, Panicum coloratum, Panicum maximum,
Paspalum c conjugatum, Paspalum decumbens, Paspalum notatum, Pennisetum purpureum, Setaria vetiveria spp.
and Vetiveria zizanioide.
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Sometimes, natural vegetative strips are left unploughed during land preparation leaving a living
buffer strip, especially in dry areas where grass strips have a slim chance of survival. The main
drawback with grass strips is that they harbour rodents and in dry areas, they may not survive the
dry spells.
3.3 Hedgerow Intercropping
Hedgerow intercropping or alley cropping involves growing leguminous tree shrubs in narrow
strips across the slope (gure 3.2), then the shrubs are lopped and the material used as a green
mulch. Crops are planted within the space between the hedges. The system has the benets of
protecting the soil from runoff and erosion, since the lopped tree branches and leaves are used
as mulch, and the tree stems act as a barrier to surface runoff. Moreover, nitrogen xation by the
hedge roots and its incorporation through pruning is supposed to replace the need for nitrogen
fertilizers thus saving costs. Competition for moisture between crop and hedges is a major limita-
tion in dry areas. Despite this limitation, hedgerow intercropping can be quite effective in soil and
water conservation.
Figure 3.2 (a) Hedgerow intercropping of
Calliandra with beans (photos by Bancy Mati)
(b) Hedgerow intercropping of Callian-
dra with maize and kales
Characteristics of a good tree species for hedge-row intercropping
• The tree should have a light, open crown that lets sunlight through.
• Ability to resprout quickly after pruning, coppicing or pollarding.
• Productive capacity that includes poles, wood, food, fodder, medicinal or other products.
• Good leaf litter making nutrients available at appropriate times in the crop cycle.
• Few and shallow lateral roots (or `prunable`).
• Ability to assist in nitrogen xation.
• Resistance to droughts, ooding, soil variability and other climatic hazards.
• Deep thrusting tap-root system
Some commonly grown leguminous trees suitable for hedge-row intercropping include; Centrosema
pubescens, Desmodium buergeri, Medicago sativa, Mucuna puriens, Phaseolus acontifolius, Psohocarpus palustris,
Pueraria phaseocoides, Stizolobium deeringianum, Stylosanthes guianensis, Trifolium alexrium, Vigna catjang,
sesbania sesban, caliandra calothyrsus and leucena sp.
3.4 Trash Lines
Trash lines are buffer strips created by arranging the previous season’s crop residues or any other
dead vegetative materials (“trash”), in lines across the slope (gure 3.3). Trash lines act more or
less like grass strips, by providing a buffer which slows down runoff and traps eroded soil, while
dissipating the energy of runoff beyond the strip at non-erosive velocities. Thus, trash lines can be
used to develop bench terraces over time (progressive terracing) through the natural phenomena
of erosion and deposition. The spacing between trash lines is calculated as terrace spacing for con-
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vention. They are preferred in areas where the slope is gentle, crop residue are available and there
is no termite problem.
Figure3.3 Illustration of trash lines made with maize crop residues
Making trash lines is a traditional technique in many parts of Africa, although it is not widely prac-
ticed these days. This is due to the competition for crop residues for other uses such as livestock
feed or fuel. In the wetter areas where farm sizes are restricted, farmers prefer to feed crop residues
to livestock, while in the dry areas trash lines are associated with termite infestation. Trash lines
have the advantage of low labour requirement, and the material is locally available on the farm.
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4. COVER CROPS
4.1 What are cover crops?
Coer crops are those crops grown for the purpose of covering the soil during the cropping sea-
son, fallow periods or between harvest and planting of commercial crops. They utilize the residual
moisture in the soil. They are also used for protecting the soil, even when it is not cultivated.
4.1.1 Utility of cover crops
Cover crops are grown to protect the soil from leaching, erosion and to improve soil fertility (gure
4.1). They build up organic matter in the soil, improve soil structure, suppress weed growth and
increase soil fertility through nitrogen xation. They also, shade the soil to reduce uctuations in
temperature and improve soil moisture conservation. Legumes, such as beans and peas, or grasses
are often used. They cover the ground surface between a widely spaced perennial crop, such as
young fruit trees such as mango, papaya, citrus, coffee, or between rows of grain crops such as
maize. Cover crops can be a source of food, fodder and mulch and may provide some cash income.
However, they may also provide a refuge for rodents and pests
Figure 4.1 (a) Pumpkins used as a cover
crop (photos by Bancy Mati)
(b)Water melons cover crop under papaya
4.1.2 Suitable cover crop types
A cover crop should be of a slow growing variety to minimize competition for water and nutrients
with the main crop. It should be planted as soon as possible after tillage to be fully benecial. This
can be done at the same time as sowing the main crop, or after the main crop has established to
avoid competition. Growing grass or leguminous cover crops once every two to three years may be
necessary for the sustainable management of soil and water resources.
The most commonly used cover crops are sweet potatoes, melons, crotolaria, mucuna bean and
various types of grasses. Cover crops are grown mostly in the wetter zones for their other utilities
such as food and fodder.
4.1.3 Selecting suitable species
A wide range of cover crops can be used for soil and water conservation. In addition, cover crops
are also used for food crop production and this is an important consideration in choosing an ap-
propriate cover crop. The choice of an appropriate cover crop for different soils and ecological
regions depends on many considerations, including:
• The ease and economics of establishment, including the availability of seed;
• Quick ground cover and growth rate during the off-season;
• Low possibility as alternative hosts for pests and cover for wildlife;
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• Low canopy height;
• Deep-root system and low consumptive water use;
• Ability to suppress weeds;
• N-xing rather than N-consuming;
• Feed value for livestock;
• Growth duration (i.e., permanent versus annual);
• Shade tolerance; and
• Ease of management for growing a food crop with conservation tillage.
4.1.4 Advantages of cover crops
Cover crops have many advantages for the sustained use of natural resources (e.g., restore fertility,
control weeds, avoid repeated seeding and cultivation trafc, conserve rain, and reduce energy
costs). In addition to controlling pests, cover crops improve soil physical properties and reduce soil
erosion.
4.1.5 Major limitations of cover crops
Cover crops are not suitable for dry areas with annual rainfall of less than 500mm, as they might
compete for water with the main crop. Under such conditions it might be better to keep the weeds
and natural vegetation as cover. They may not do well on soils with low phosphorous.
4.2 Permanent soil cover
Permanent soil cover (or permaculture), is a special system of farming with cover crops, in which
farmland is maintained under some crop cover throughout the year (gure 4.2). It is practiced like
re-lay intercropping, such that a crop is planted before harvesting of anannual crop. But the meth-
od differs in that a permanent cover can also be grown alongside annual crops.
Permanent soil cover facilitates soil protection from surface runoff, crusting or evaporation, while
also optimizing the productivity of land. A permanent soil cover helps to protect the soil against
the impacts of rain and sun, provide the micro and macro organisms in the soil with a constant
supply of “food”, and alters the microclimate in the soil for optimal growth and development of
soil organisms, including plant roots. The types of cover crops applied include different legume
species like beans, forage and agroforestry species such as Stylosanthes guianensis, Mucuna spp, Dolichos
lablab, Pueraria phaseoloides, Cajanus cajan, Vigna umbellat. Grass leys are sometimes also used in per-
manent soil cover, especially where the main crop are trees.
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Figure 4.2 Permanent soil cover of groundnuts
after maize (photos by A. Rakotondralambo)
(b) Permanent soil cover using crop
residue with zero tillage
4.2.1 Benets of permanent soil cover
• Improved inltration and retention of soil moisture resulting in less severe, less prolonged
crop water stress and increased availability of plant nutrients
• Higher diversity in plant production and thus source of food and fodder,
• Habitat for diverse soil life: creation of channels for air and water, and biological activity
• Soil regeneration is higher than soil degradation
• Increased organic matter recycling
• Reduced rain drop impacts on soil surface resulting in reduced crusting and surface sealing
• Improved inltration and consequential reduction of runoff and erosion
• Mitigation of temperature variations on and in the soil
• Better conditions for the development of roots and seedling growth,
• integrated management and reduced competition with livestock or other uses e.g. through
increased forage and fodder crops in the rotation
4.2.2 Limitations of permanent soil cover
Requires higher levels of soil moisture availability to reduce competetion withthe main crop.
Thus, the method is suitable for irrigated elds or high rainfall areas,
There is danger of pest infestation since the land is not rested between growing seasons,
Can be quite labour intensive, and
targeted use of herbicides for controlling cover crop and weed development may be costly.
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5 MULCHING
5.1 What is mulching?
Mulching is the practice of covering the soil between crop rows or around trees with a layer of
loose material such as dry grass, straw, crop residues, leaves, compost or inorganic membranes
(gure 5.1). Mulching is normally done to conserve soil moisture, reduce runoff ows, evaporative
losses and wind erosion, prevent weed growth, enhance soil structure and control soil temperature.
Most smallholders do mulching only for special crops such as tomato, cabbage and potatoes due
to the shortage of crop residues. It is practiced by farmers in the wetter areas due to the availability
of vegetative materials. The importance of mulches in reducing surface runoff, soil erosion and
evaporation losses cannot be overstated.
Figure 5.1 Mulched coffee eld
(photo courtesy of Mary Kakinda)
(b) Mulched banana plantation
(photo by Bancy Mati)
5.2 e case for mulching
Mulching has positive effects by reducing raindrop impact, improving rainfall inltration through
enhancement of soil structure and reduce runoff and erosion. Mulching also increases the biolog-
ical activities in the soil, increasing the activity and species diversity of soil ora and fauna, such as
earthworms. Mulching results in increased biomass carbon, and improved crop growth; nutrient
enhancement and inuence on crop growth. Mulching also reduces loss of water by evaporation.
It also protects the soil from wind damage.
Mulching needs to be monitored carefully to avoid related problems. Too much mulch prevents ad-
equate air ow and encourages pest and fungal disease and , in dry areas termites. These problems
can be avoided by using mulch during the dry season and applying mulch two weeks after planting
to allow the seedlings to develop.
5.3 Types of mulches
There are different types of mulches, depending on the source and method of mulch procurement
and application (gure 5.2). Although a wide range of materials is used as mulch, the most practical
and feasible material is the residue from a previously grown crop. The technological methods of
mulch farming differ on the basis of whether mulch is brought in or produced in situ. Mulch can be
spread on a seedbed or around planting holes and it can also be applied in strips
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Figure 5.2 Types of mulch materials and associated agricultural practices
5.3.1 Crop residues
Use of the previous season’s crop residues is the most prevalent method of mulching. The residues
are carefully arranged on the soil surface of cultivated elds. It is difcult to get adequate residue
material from the previous year’s harvest. However, mulch densities range between 30 percent
and 70 percent can be obtained from the previous season’s crop residues. In certain cases, the res-
idues or grasses to be used for mulch are out-sourced from outside the farm. Grass mulches are
particularly useful where there are natural grasses which are cut and taken to the farm as mulches.
The grass should be dried before applying as this reduces the chance of it rooting. Sometimes, the
mulch can be covered with a layer of soil to protect it against wind.
5.3.2 Green mulches
Green mulches are usually leguminous plants that cover the ground as runners, grown together
with other crops. They are sometimes also termed as green manure because of the ability of the
companion legume to x nitrogen in the soil. The legume could be cut and incorporated into the
soil while green as manure. Alternatively the legume is used as a cover crop. Other types of crops
such as pumpkins and water melons have proved useful green mulches.
5.3.3 Gravel-sand mulches
Gravel and sand mulching involves applying gravels or rock fragments onto the soil surface, pref-
erably in rows. Crops are then grown in the rows between the gravel/sand cover. This has several
functions, such as: (i) as a water harvesting system in which the gravel-sand mulch acts as a catch-
ment, from which runoff can be channeled onto the cropped rows, (ii) as a mulch cover to reduce
surface runoff and conserve the soil, (iii), by covering the ground to reduce evaporation losses,
the stones provide a capillary fridge since they are porous, thus reducing water losses by capillarity
from the more compact soil layers beneath, and (iv) reducing weed infestation in the rows covered
by the gravel. Once the gravel mulch has been applied, the cropped rows are tilled using minimum
tillage techniques e.g. strip tillage, spot tillage or direct seeding. The system is suited to areas with
large volumes of stones and/or gravel.
5.3.4 Plastic mulching (plastic lms)
Plastic material is also sued for mulch crops such as vegetables and fruits, particularly pineapple.
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The plastic sheets are arranged across the slop in strips in between crop rows. Some plastic mate-
rials are bio-degradable while others are less degradable and must be removed from the eld and
disposed of at the end of the season. Plastic mulching can be expensive, and is used for high value
crops. It serves several functions, e.g. the spaces covered by plastic smother out weeds, hence it acts
as a weed control. The plastic-covered areas act as a catchment which increases runoff generation
between the crop rows to improve the efciency of micro-catchment water harvesting. Plastic
mulches also reduce disease and pest incidences, especially soil-borne pests. In cold areas, black
plastic mulches increase soil temperature thus improving crop performance. For water harvesting,
a simple way of achieving effective plastic mulching is to buy a large plastic sheet, then puncture
holes with spacing equivalent to crop spacing. Then plant directly through the holes. Care should
be taken to ensure that plant roots get adequate air and the soil is not overheated. After crop har-
vesting, the plastic lm is collected and re-used.
5.3.5 Alternate row-mulching
Alternative row mulching is sometimes preferred to full mulching, because it reduces the re risk.
It is most effective if applied at the start of the rains, as it intercepts and increases water take-up,
but it is frequently more practical to mulch towards the end of the rains when grass is available.
When crop remains are used for mulching, nutrients are released more slowly, so that more manure
or fertilizer has to be applied.
5.4 Mulching techniques:
There are different ways of applying a mulch. It can be put on the soil surface, or incorporated
during cultivation of the land. Two methods predominate.
5.4.1 Planted mulch
In this system, legumes, grasses and other suitable plants such as Grevillea and Tithonia can be plant-
ed along boundaries, on soil band or in spaces between crops or in fallow land. These can be cut,
left to dry and used as mulch. Legumes, however, do not need to be dried rst.
5.4.2 Live mulch
For perennial crops like bananas, creeping legumes such as lablab and mucuna bean can be planted
between the rows and periodically uprooted to provide organic materials. The principle of live
mulch is based on mixed cropping. The live mulch must be a ground legume which grows below
the main crop. Live mulches are not recommended in situations where soil moisture stress occurs,
as the live mulches will compete with crops for moisture.
5.5 Benets of mulching
A principal benet of mulch farming is reduction in runoff and soil erosion.
• Mulching helps to retain soil moisture by limiting evaporation, suppressing weed growth and
enhancing soil structure, reducing runoff, protecting the soil from splash erosion and limiting
the formation of crust.
• Mulching reduces uctuations in soil temperature which improves conditions for micro-organ-
isms. It is commonly used in areas affected by drought and weed infestation.
• Mulching protects the soil from the direct impact of rain drops, thus maximizing water perco-
lation and protects the soil against water and wind erosion. It also reduces water loss from the
soil by evaporation and maintains conducive soil temperatures for germination and growth of
crops and for the life of soil organisms.
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• Mulch improves soil conditions by improving soil plant and animal activity and adding organic
matter to the soil. Good mulch also controls the rapid growth of weeds.
• Mulch can be produced on the farm. A mixture of deliberately grown grasses and legumes
forming a cover can be cut down to provide the mulch. This mixture can be grown together
with crops on bands or in separate elds.
5.6 Limitations of mulches
• The major limitations lie in the large quantities of residues required (usually 4 t/ha/yr)
• However, mulch materials are sometimes not available as farmers use them for other uses
e.g. as fodder, fuels or construction material.
• There is some additional labour involved in mulch collection or procurement and appli-
cation. Consequently, mulch farming is likely to be feasible on a small scale for high-value
commercial crops.
• Weeds can be a problem if some grass species are used and mulches can provide a possible
habitat for pests and diseases. Use a mixture of fast and slow decomposing material and
break large pieces of crop residue before application.
• The mulch layer should not be too thick; otherwise this can lock-up nutrients in the soil.
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6. SOIL NUTRIENT MANAGEMENT
6.1 What is soil fertility?
Soil fertility refers to the capacity of a soil to produce crops by providing adequate supply of
nutrients in correct proportions, resulting in sustained high crop yields. In addition, a fertile soil
has good rooting depth, good aeration and good water holding capacity. It is also necessary that
there is a strong presence of soil organisms e.g. earthworms, adequate amounts of organic matter,
the right pH balance and no adverse soil-borne pests and diseases. Efcient farm management
practices should result in greater stimulation of activities of soil organisms, nutrient additions to
the soil, minimal nutrient exports from the soil and optimal nutrient recycling within the farming
system (gure 6.1). However, the concept of “poor” and “fertile” soil may mean different things
to different communities and conditions. But it should be possible to say accurately whether a soil
is fertile or not.
Figure 6.1 Effects of soil nutrient management comparing adjacent farms
Left- without nutrients, and right-with proper soil fertility (photo by Bancy Mati)
Soil fertility management goes beyond just improving the nutrient levels in the soil. It is the holistic
improvements made to a soil and its ability to produce crops, including water management and
weed control. Thus, the term also includes soil and water conservation practices like terracing as
well as water harvesting and drainage of waterlogged soils. Agronomic measures such as conser-
vation tillage, deep tillage, residue mulching, contour tillage, crop rotations, intercropping, cover
cropping and agroforestry are all fertility enhancing. Soil fertility management also includes off-
farm activities such as biomass transfer, improved fallows and rotational grazing and proper care
of rangelands. However, this Chapter is concerned with soil nutrient management.
6.2 What is soil nutrient management?
Soil nutrient management, are the activities and additives used to enhance the chemical and bio-
logical constituents of the soil so as to improve its fertility. Nutrient management involves adding
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manure and fertilizers to the soil in the right amounts to provide the required plant nutrients for
vigorous crop growth. Good crop cover is an indicator of soil nutrient availability and is linked to
protection of the land from excessive runoff losses and soil erosion (see Chapter 1 in this manual).
Thus availability of nutrients (macro and micro) in the soil is one way of achieving water conser-
vation resulting in higher yields and improving water productivity. There are many methods of soil
nutrient management, such as addition of fertilizers, manures, and low input technologies.
6.3 Chemical fertilizers
Fertilizer is any organic or inorganic material added to a soil to supply one or more plant nutrients
essential for improving the productivity of crops. Fertilizers are divided into two groups; inorganic
and organic. In both types, fertilizers typically provide both macro nutrients and micro-nutrients.
Macronutrients are the elements consumed in larger quantities and are present in plant tissue. They
include: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur
(S). Micronutrients, on the other hand are consumed in smaller quantities and are present in plant
tissue on the order of parts per million (ppm). They include: boron (B), chlorine (Cl), copper (Cu),
iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).
6.3.1 Inorganic fertilizers
Inorganic fertilizers sometimes referred to as commercial or chemical fertilizers contain the major
crop nutrients needed by plants, such as nitrogen, phosphorus, and potassium. These three essen-
tial nutrients should naturally occur in healthy soil, but some soils are decient of them or plants
require more nutrients than are available. Most inorganic fertilizers are rated based on the percent-
age of nitrogen, phosphorous, and potassium, with a rubric called NPK (Table 6.1).
Table 6.1. Common basic fertilizer types and their nutrient contents
Fertilizer name Components N (%) P2O5 (%) K2O (%)
Ammonium nitrate 33-0-0 33 0 0
Ammonium sulphate (ASP)* 21-0-0-24S 21 0 0
Diammonium phosphate (DAP) 18-46-0 18 46 0
Potassium chloride (muriate of potash) 0-0-60 0 0 60
Potassium sulfate** 0-0-50-16S 0 0 50
Potassium magnesium sulfate*** 0-0-22-23S-11Mg 0 0 22
Triple super phosphate (TSP) 0-46-0 046 0
Urea 46-0-0 46 0 0
Note: *Ammonium sulfate contains 24% sulfur (S)
**Potassium sulfate contains 16% S
***Potassium magnesium sulfate contains 23% S and 11% magnesium (Mg)
Source: Maguire et al, 2009.
The addition of nitrogen to the soil encourages growth of stems and leaves by promoting protein
and chlorophyll. More owers, larger fruits, and healthier roots and tubers result from added phos-
phorus. It also helps plants resist certain diseases. In addition, potassium from potash, thickens
stems and leaves by fostering protein development. Different plants require different pH levels. A
soil’s pH can be lowered or made acidic by applying inorganic fertilizer such as aluminum sulfate
or ammonium sulfate. Lime increases soil pH making it more alkaline. Sometimes blood meal or
other organic matter can also affect acid levels in the soil.
The rst step in applying the correct rate of fertilizer is calculating crop nutrient requirements. A
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soil test is the only way to measure how much phosphates and potash are available in soils, and soil
tests are available through several private and public laboratories as nitrogen is much more mobile
in soils and must be applied every year to cereal crops. Nitrogen requirements are based on the
crop to be grown and the soil type that inuences yield goals.
Blended fertilizers are mixes of these fertilizer materials that are made to vary the N-P2O5-K2O
ratio to meet crop requirements. The phosphate fertilizers are categorized as natural phosphates,
either treated or processed, and also by products of phosphates and chemical phosphates.
6.3.2 Rock phosphate
Rock phosphate is a naturally occurring sedimentary rock, which contains over 10% phosphorous
and 38% calcium oxide as the main ingredients. It is mined in some countries including at Minjingu
near Lake Manyara, in Tanzania, from where it derives its name as “Minjingu rock phosphate”.
This fertilizer type has its advantages and disadvantages. The advantage is that with adequate rain-
fall this fertilizer results in a long growing period which can enhance crops. Powdered phosphate
fertilizer is an excellent remedy for soils that are acidic and has a phosphorous deciency and re-
quires soil amendments. However, the disadvantage is that although phosphate fertilizer such as
rock phosphate contains 25 to 35% phosphoric acid, the phosphorous is insoluble in water. It has
to be pulverized to be used as a type of fertilizer before rendering satisfactory results in garden
soil. Thus, rock phosphate is used to manufacture superphosphate which makes the phosphoric
acid water soluble. Rock phosphate is relatively affordable and can be used directly to correct phos-
phorous deciencies in the soil especially targeting decient parches in the farm. It has a positive
residual effect in soils over several seasons and does not cause soil acidity.
6.3.3 Superphosphate
Superphosphate fertilizers are available in three different grades, depending on the manufacturing
process. These are:
• Single superphosphate containing 16 to 20% phosphoric acid;
• Dicalcium phosphate containing 35 to 38% phosphoric acid; and
• Triple superphosphate containing 44 to 49% phosphoric acid.
Triple superphosphate is used mostly in the manufacture of concentrated mixed fertilizer types.
The greatest advantage of using superphosphate fertilizer is that the phosphoric acid is fully water
soluble. Thus, when superphosphate is applied to the soil, it is converted into soluble phosphate.
This is due to precipitation as calcium, iron or aluminium phosphate, which is dependent on the
soil type to which the fertilizer is added, be it alkaline or acidic garden soil. Phosphorus deciency
is a common problem in most farms, hence most soil types can benet from the application of
superphosphate fertilizers. Used in conjunction with an organic fertilizer, it should be applied at
sowing or transplant time.
6.3.4 Potassium fertilizer types
Chemical potassium fertilizers should only be added when there is certainty that there is a Potassi-
um deciency in the soil. Potassium fertilizers also work well in sandy soils as they respond to the
application. Crops such as chillies, potato and fruit trees all benet from this type of fertilizer since
it improves the quality and appearance of the produce. There are basically two different types of
potassium fertilizers:
• Muriate of potash (Potassium chloride) and
• Sulphate of potash (Potassium sulphate).
Both muriate of potash and sulphate of potash are salts that make up part of the waters of the
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oceans and inland seas as well as inland saline deposits.
Muriate of potash
Muriate of potash is a gray crystal type of fertilizer that consists of 50 to 60% potash. All the
potash in this fertilizer type is readily available to plants because it is highly soluble in water. Even
so, it does not leach away deep into the soil since the potash is absorbed on the colloidal surfaces.
Muriate of potash is applied at sowing time or prior to sowing.
Sulphate of potash
Sulphate of potash is a fertilizer type manufactured when potassium chloride is treated with mag-
nesium sulphate. It dissolves readily in water and can be applied to the garden soil at any time up
to sowing. Some gardeners prefer using sulphate of potash over muriate of potash.
6.3.5 Commercially available fertilizers
The different types of chemical and organic fertilizers that are usually commercially available can
be categorized further into:
• Complete inorganic fertilizers: – these types of inorganic fertilizers contain all three major
macronutrients, Nitrogen (N), Phosphorous (P) and Potassium (K). On the containers, macro-
nutrients are depicted as a ratio, e.g. 2:3:2 (22). Complete inorganic fertilizers are usually applied
at a rate of 60g/m2 or roughly 4 tablespoons per square meter.
• Special purpose fertilizer: – these types of fertilizer are formulated especially to target certain
plants’ requirements or certain soil deciencies.
• Liquid fertilizers: – these types of fertilizer come in a variety of formulations and even include
organic fertilizer, complete fertilizer as well as special purpose fertilizer.
• Slow-release fertilizer: – these types of fertilizer are formulated to release their nitrogen at a
steady pace. On the packs of these fertilizer that are available commercially usually are depicted
as 3:1:5 (SR) where the SR indicates slow-release.
• Fertilizer with insecticide: – these are types of fertilizer that are prepared and combined with
an insecticide.
There are many other different types of chemical fertilizers in different formulations because dif-
ferent plants require different nutrients and different pH levels in the soil. However, organic fertil-
izers have more diversity, and do not burn plant roots, get into ground water, or affect surrounding
growth as is the case when using the different types of chemical fertilizer and NPK amendments.
Generally, farmers are discouraged to use fertilizers because it is not a traditional practice among
most communities. High fertilizer costs and poor prices from staple foods grown in the region
mean that economic gains have not been quite evident. Even though expensive, the use of inorgan-
ic fertilizers needs to be promoted, as many types of soil lack adequate levels of phosphorus and
nitrogen. The major type of fertilizer used in the region is DAP. The quantity of fertilizer used de-
pends on the socioeconomic level of the farmers, with richer farmers using more fertilizers. Field
trials on maize and sorghum with and without fertilizer application in semi-arid areas have shown
that a substantial yield increase occurred when fertilizers are used along with water conservation
practices.
6.4 Organic fertilizers
Organic fertilizers are those obtained from biological materials such as farmyard manure, crop
residues, bat guano, compost, peat moss, wood ash and other soil amendments. They have many
benets over chemical fertilizers. For instance, organic fertilizers do not harm plants if applied in
excess and have long-term positive effects on the soil without damaging ground water. The vegeta-
tive materials also help to aerate the soil, insulate the ground against temperature change, and add
needed nutrients. However, they may have lower concentrations of nutrients and are thus required
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in larger quantities. Organic fertilizers are rich in organic nutrients and minerals and are gradually
released to the soil, hence have a long lasting effect. There are various ways of preparing organic
fertilizers as described below.
6.4.1 Organic manures
These are soil nutrient amendments made from organic materials, particularly those obtained from
within the farm, such as crop residues, farmyard manure, weeds and tree prunings. There are sev-
eral types of organic manures as well as the methods to prepare them. These include all types of
farmyard manures, composts, green manures, liquid manures, slurry and fortied composts. Crop
residues left on the soil also act as manures.
6.4.2 Farmyard manure
Farmyard manure is the animal droppings obtained from the sheds of various domestic livestock
such as cattle, goats, sheep, pigs and chicken. It also includes the sweepings, urine, remnants of
fodder and animal beddings and other biological materials found in animal dwellings (gure 6.2).
Farmyard manure is easier to convert into humus since it is an advanced stage of vegetative ma-
terial that has been digested and deposited by animals. It contains useful nutrients especially Ni-
trogen and carbon, which are derived from biological materials. Farmyard manure should rst be
composted before application to crops in the farm.
Figure 6.2 Farmyard manure (photos by Bancy Mati) (b) Compost manure
6.4.3 Compost manure
Compost manure is the decomposed organic matter that includes one or more of the ingredi-
ents like farmyard manure, slurry, compost manure, crop residues, kitchen wastes, hedge cuttings,
grain husks and other materials. Compost is made by converting the large amounts of vegetation,
such as crop remains, garden weeds, kitchen and household waste, hedge cuttings, and garbage,
into valuable plant food called humus. When properly made and applied, compost is immediately
available as plant food without the need to be rst broken down by soil micro organisms. The use
of compost allows a farmer to obtain a good crop without the need to apply expensive and toxic
chemical fertilizers.
Composting is the natural process of turning organic materials, such as crop residues and farmyard
manure, into valuable plant food or humus. The ingredients that produce good quality compost,
such as leguminous residues and manure, are just as important as the methodology of compost-
ing. The normal procedure is to rst make a foundation onto which ashes are spread to prevent
termite infestation. Then layer after layer of dry crop residues (chopped), green vegetation e.g.,
Lantana camara or Tithonia diversifolia and topsoil are placed over each other, wetting with fresh water
(non-chlorinated). The heap is then covered with soil and a stick driven into the middle to act as a
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thermometer (gure 6.3). The compost is turned (and wetted) after around 22 days and the com-
post is ready for use within 45 days, depending on temperatures.
Figure 6.3 (a) Illustration of the arrangement
of a composting system
(b) Composting manure by a group of
farmers (photo by Bancy Mati)
6.4.4 Fortied compost
The making of fortied compost involves addition of low levels of fertilizer or nely-ground
phosphate rock and urea to improve the quality of compost prepared from crop residues, particu-
larly cereal-based residues such as maize stover, rice husks or wheat straw (gure 6.4). Low quality
organic materials such as maize stover or wheat straw with a wide C:N are fortied with small
amounts of fertilizer and manure for preparing fortied compost.
Figure6.4 Arrangement of a fortied compost under preparation
The process involves making the compost in the normal way but between the layers of residues, a
nitrogenous fertilizer, e.g. DAP or rock phosphate is sprinkled. A layer of organic soil and farm-
yard manure are then spread over to act as “starter inoculants” and water sprinkled over. This
forms one layer, ve layers are packed in this way till the compost heap is done. It is then covered
with soil and left to compost in the normal way (see compost manures). For most poor soils, the
method recommends the application of 2 t/ha of fortied compost applied as a pre-plant applica-
tion at the start of the rainy season. Fortied composts carry the advantages of mineral fertilizers
as well as those of organic matter addition to the soil.
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6.4.5 Liquid manure
Liquid manure is a mixture of farmyard manure, urine, vegetative materials and other soil nutrient
additives which is prepared in liqueed form. The liquid manure is made in drums where all the
ingredients are mixed with water, forming slurry. In preparation, some goat or cattle manure is
soaked in a net/bag within a drum as well. The bag is then suspended using a stick in a drum of
water (Figure 6.5). The drum is covered and left to stand. The bag can contain 30-50 kg of manure
in 200 liters of water. The contents are stirred with a stick at least every 3 days to quicken the re-
lease of nutrients into the water. The process takes 10-15 days for the liquid manure to mature. The
mixture is ready when the color of the water changes to dark brown.
The mixture is a good liquid fertilizer for top dressings growing crops. It can be applied to a variety
of crops including vegetables. It is applied around the roots of the plants. Before use, the mixture
is diluted as in the case of plant teas ratio 1:2. Slurry from an animal shed can be directed into a
shallow pit, where it turns into liquid manure after several days. But the pit is kept well covered.
Liquid manure can also be obtained from the sludge efuents of biogas digesters.
Figure 6.5 Illustration of the preparation of liquid manure
6.4.6 Plant tea
Occasionally, liquid manure is prepared in the form of “plant tea.” Plant teas are especially nec-
essary to quickly provide the crop with adequate natural plant food during the growing season,
and as a top dressing. Plant teas are derived from plants and contain many minerals in addition to
hormones that promote growth. Plants used should be broad-leaved young shoots that can easily
decompose in water. Commonly used tree species include; stinging nettle, comfrey, amaranthus,
mukuna and Tithonia deversifolia.
Preparation of plant tea
In preparation, plant teas the green sappy leaves and young branches of leguminous trees are
chopped into small bits and put in the drum up to three-fourths full. Then the drum is lled with
water. The mixture is left to stand. The material rots in the water, releasing nutrients. When the
process is activated, the solution begins to smell like cow urine. To quicken the process, the solution
is stirred every three days. Plant teas are ready for use when the solution turns a rich green color
and has a pleasant smell. Depending on type of plant material used and the temperature, the plant
tea is ready for use within two to three weeks.
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Application
Plant teas should be used as soon as they are ready. The solution may remain effective for fourteen
days but will gradually lose its useful nitrogen. The solution should be applied directly to plants. It
may be too concentrated and can scorch plants. Thus, it should be diluted to reduce concentration
by adding water, by a ratio of 1:2 (one part solution to two parts water). Plant teas are applied to
the soil around the root zone of an actively growing young plant. It should not be applied on the
leaves. It is advisable to cover the treated area with dry soil or mulch in order to minimize loss of
nutrients, especially nitrogen, through solar radiation. Plant teas can be applied for three consec-
utive days, then wait for the plant to show the effects of accelerated growth of new leaves and
branches. Plant teas may be applied to leafy crops and fruit crops .It is not necessary to use them
on root crops.
6.4.7 Compost baskets
A compost basket refers to a method of composting in-situ, i.e. composting in which the crop
utilizes the compost as it decomposes, and thus is expected to last longer. Compost baskets are
woven from twigs and driven into the prepared beds at a spacing of 1 m as follows: holes of at
least 15 cm deep and 30 cm wide are dug along the centre line of the prepared bed at a spacing of
about 1 m. Sticks of about 60 cm long are then driven into the ground around each hole, and long
exible twigs woven around to form above ground baskets. The baskets are lled with manure and
well-decomposed household wastes. The manure is translated from the soil below the basket into
the root zone through natural processes. Due to hydrotropism, the roots also tend to grow towards
the basket (gure 6.6). The compost basket also absorbs and retains a lot of water which the plants
can withdraw during the dry season. It holds the composting materials in place and therefore
minimizes nutrient depletion by run-off and stray animals. Birds, too, cannot scatter the compost
materials. It is a popular method of water conservation for banana plantations.
Figure 6.6 (a) Illustration of a compost bas-
ket with banana crop
(b) Making a compost basket for banana
(photo by Bancy Mati)
6.4.8 Green manure
Green manuring is the use of crops, weeds, and other leguminous plants to maintain or improve
soil fertility. Green manures help loosen the soil and let in needed air, and hold soil and water in
place. Weeds should not be burnt, but instead, they are dug into the soil as green manure. Some
crop remains like those of beans and peas that add nitrogen (an important plant nutrient) to the
soil is used. Other plants suitable are: Leucaena, sunn hemp, Tithonia, Sesbania, lablab, Pannicum, Calli-
andra, elephant grass, Lantana camara, which are lopped and placed on the soil surface to act both as
a green manure and as a mulch (gure 6.7). Crop elds left to fallow should be planted with green
manure plants to increase the rate of soil fertility renewal. Also, intercrop green manure plants with
food crops: plant green manure plants in-between rows of food crops, but make sure you minimize
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competition for water, sunlight and nutrients from other crops.
Figure 6.7 Trimming markhamia hedge
for green manure (photos by Bancy Mati)
(b) Lopped markhamia prunnings for use as
green manure
Green manure is made by digging into the soil three of four weeks before planting food crops.
This allows green manure to rot completely and to provide plant nutrients. It is advisable to dig
the green manure into the soil before any owering. If the green manure has tough stems, they are
chopped into small pieces before being incorporated into the soil.
6.5 Integrated soil fertility management
Integrated soil fertility management (ISFM) is achieved when all the components necessary to
promote healthy and sustainable crop growth are applied to farmlands. ISFM encompasses both
LEISA and conventional methods of implementing sustainable agriculture. It includes the use of
mineral fertilizers, organic matter, manures, tillage operations, water management and even control
of pests and diseases. Mineral fertilizers such as NPK, CAN, TSP, Urea and rock phosphate as well
as biological measures, involving agroforestry, green manures, improved fallows that utilize Sesbania
sesban, Crotalaria grahamiana, lablab or Tephrosia vogelii, and fortied composts can be used at various
combinations. Combinations of two or more interventions is usually encouraged under ISFM in
order to address the diversities in basic soil nutrient decits, management systems and water man-
agement improvements brought about by biological measures, especially for long-term impacts on
crop productivity. The focus of any soil fertility replenishment should be integrated nutrient man-
agement involving the application of inorganic and organic soil nutrients, as well as technologies
that reduce the risks of acidication and salinisation.
6.5.1 Improved fallow
An improved fallow is the enrichment of a natural fallow with leguminous trees or shrubs to im-
prove soil fertility. It is achieved by deliberately planting leguminous tree species or broadcasting
seeds of leguminous shrubs onto land which has been left fallow. This ensures that benecial
plants colonize the fallow to improve nutrient replenishment and to x nitrogen. Some suitable
shrub species include Sesbania sesban, Crotalaria grahamiana, Tephrosia vogelii and Tithonia diversifolia
(gure 6.8).
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Figure 6.8 Improved fallow with Tephro-
sia vogelii (photos by Bancy Mati)
(b) Improved fallow with Tithonia diversifo-
lia
6.5.2 Biomass transfer
This is the incorporation into the soil of leafy shrubs, which accumulate high concentrations of
nutrients in their leafy biomass and mineralize rapidly. It can also be dened as a form of cut and
carry mulching and the shrubs are widely distributed along farm boundaries in the humid and
sub-humid tropics of Africa. The shrubs include Lantana camara and Tithonia diversifolia. In small-
holder agriculture, Tithonia diversifolia is commonly used as biomass material because it is readily
available, easy to propagate and relatively richer in nutrients. One ton of dry weight Tithonia di-
versifolia leaves contains an average of 33 kg of nitrogen, 3.1 kg of phosphorous and 30.8 kg of
potassium.
6.6. LEISA technologies
LEISA stands for Low External-Input Sustainable Agriculture technologies. It is a broad term used
to describe low-input farming systems which include; Alternative Agriculture, Low-Cost External
Input Agriculture, Bio-Intensive Agriculture, Sustainable Agriculture and “Permaculture”. In its
most extreme form, low-input agriculture is known as organic farming.
6.6.1 Organic farming
Organic farming (or organic agriculture), is a farming system which utilizes natural products such as
locally available biological material and natural methods to maintain soil fertility and to keep crops
and livestock healthy. The approach keeps the land productive using materials found on the farm.
Organic farming meant to promote environmentally sound means of production. Compared to
conventional farming systems, where much effort goes into bringing chemical inputs and animal
feeds from outside the farm, organic farming instead makes full use of what is found on the farm.
In contrast, organic agriculture does not allow use of inorganic fertilizers, pesticides, vaccines and
medicines. Instead, manures are used and organic pesticides made from plants and local materials.
The organic farmer puts effort into improving soil fertility through composting, proper cultivation,
rotation of crops, mixed planting, growing trees, proper care of crops and animals and the natural
control of pests and diseases. Because of the better natural balance, agricultural products fetch a
much higher price in niche markets than conventionally grown crops and, in general, ensure good
health and environmental safety all around.
6.6.2 e case for LEISA
The main limitation with organic farming is that sometimes, locally made pesticides fail to work.
Also, some farmers do not have manures or crop residues (the latter are fed to livestock) and thus
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soil fertility could be hampered unless mineral fertilizers are used. LEISA technologies offer a
middle ground between puritanical organic farming and harmful effects of conventional agricul-
ture. These technologies are more realistic and practicable by smallholder farmers. Nearly all the
technologies and management practices described in this manual have some component of LEISA
technologies.
7. CONSERVATION TILLAGE
7.1 What is conservation agriculture?
Conservation agriculture systems are systems that utilize soils and crops with the aim to reduce the
excessive mixing-up of the soil and maintain the crop residues on the soil surface in order to mini-
mize damage to the environment. CA is based on enhancing the natural biological processes above
and below the ground. Interventions such as mechanical soil tillage are reduced to an absolute
minimum, and the use of external inputs such as agrochemicals and mineral fertilizers are reduced.
Also, organic nutrients are applied in requisite quantities and in such a way as not to interfere with,
or disrupt the biological processes.
CA is intended to provide and maintain optimum conditions for crop root-zone development by
retaining maximum possible depth for crop roots to function effectively and in capturing high
amounts of desired plant nutrients and water. CA also promotes benecial biological activity in
the soil in order to maintain and rebuild soil structure, contribute to soil organic matter and avoid
physical or chemical damage to plant roots. CA is characterized by three principles which are linked
to each other, namely:
(i) Permanent organic soil cover.
(ii) Diversied crop rotations in the case of annual crops or plant associations in case of
perennial crops
(iii) Continuous minimum mechanical soil disturbance or conservation tillage.
Since permanent crop cover and crop rotations have been covered in Chapter 2 of this
manual, this Chapter focuses on conservation tillage.
7.2 What is conservation tillage
Conservation tillage, are land cultivation techniques which try to reduce labour, promote soil fer-
tility and improve soil water conservation. It aims at reducing the negative effects of conventional
tillage such as soil compaction, formation of pans, disturbance of soil fauna and moisture loss. The
method tries to reduce labour in land preparation through tillage systems that promote soil fertility
and soil and water conservation.
Conventionally, tillage is conducted to prepare a seed bed and also to control weeds. However,
conventional tillage overworks the soil and can destroy the structure and cause compaction. This
has negative effects on soil aeration, root development and water inltration among other factors.
More important, but less noticeable, is the destruction of soil microbiology by disturbance and
turning over of soil, which is then exposed to drastic atmospheric and climatic conditions.
Conservation tillage, therefore, takes care of this by applying four main principles: 1) zero or
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minimum soil turning, 2) permanent soil cover, 3) stubble mulch tillage, and 4) crop selection and
rotations. An important aspect of conservation tillage practice involves ripping the land with tined
implements or sub-soiling the land immediately after crops are harvested, to break the plough pans
using suitable equipment as described below.
7.3 Types of conservation tillage
Conservation tillage is a broad term encompassing a range of technologies and practices. Some of
the more common methods used in smallholder agriculture are described bellow:.
7.3.1 Minimum tillage
Minimum tillage involves land cultivation in which soil turning operations are reduced from what
is conventionally normal for a given crop or area. Thus, minimum tillage assumes many forms,
from reduced tillage operations, strip cropping, spot tillage and even zero-tillage. Normally, special
tined equipment are used to open the soil just enough to allow a seed to be planted. Weed control,
a major function of minimum tillage, and is achieved through one of several options such as:
(i) applications of herbicides and growth regulators;
(ii) (ii) inter-cultivation based on secondary tillage;
(iii) (iii) manual hoeing/slashing; (
(iv) iv) smothering by cover crops and planted tallow, and
(v) (v) suppression by mulches.
The minimal cultivations reduce water losses because of a reduction in soil disturbance from till-
age. In the long term the soil structure is improved. Less surface compaction and smearing at depth
from the shares of the plough should increase rooting depth and therefore the drought tolerance
of crops. There are many variations to minimum tillage as described below:
7.3.2 Zero tillage
Zero tillage, or no-till system, is minimum tillage at its most absolute. It involves growing a crop
in a eld which has had no tillage operations preceding the planting. The land is planted by direct
seed drilling without opening any furrows or pits (gure 7.1). Old crop residues act as a mulch
and weeds are controlled using herbicides. In the dry areas of Africa, zero tillage is hampered by
poor inltration, since most ASAL soils have surface-sealing problems. Moreover, the costs of
herbicides can be prohibitive to small holder farmers whose agriculture is more labour based than
capital based.
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Figure 7.1 (a) Zero tillage planting with
digging stick (photos by A. Rakotondralambo)
(b) The bean crop after establishment
7.3.3 Spot tillage
Spot tillage sometimes called pitting or auger hole cultivation, involves using special digging tools
or augers, to excavate small holes just enough for one or two seeds of grain. The land is not tilled
and the holes are dug over the old crop residues, while weeds are controlled with herbicides. The
digging of small planting pits with hand-hoes can be quite efcient in concentrating surface water
and plant nutrients as well as breaking hard plough pans. The technique is labour intensive, but sim-
ple and is an efcient way of assuring a crop survival even when rainfall is inadequate and resources
such as fertilizers and manure are unavailable (gure 7.2).
Figure 7.2 Spot tillage (planting basins) with
residue mulch covering (Source: CFU Zambia,
2007)
(b) Measuring depth of planting hole
7.3.4 Deep tillage
Deep tillage or sub-soiling is practiced on soils having surface crusting properties or those prone to
developing hard pans. Special equipment known as sub-soilers, capable of digging down below the
plow depth are used. Most sub-soilers require tractor-drawn power. However, manual sub-soilers
have also been developed by innovative smallholder farmers The equipment comprises a long hoe
that can cut into about 30 cm of soil. It is made from old car-springs cannibalized from junk cars
and therefore, is quite durable and low-cost. Deep tillage may be required once every 3 years, to
break soil crusts developed from prolonged use of the mold-board plow.
Manual sub-soilers have also been developed by innovative farmers. The equipment comprises a
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long hoe that can cut into about 30 cm of soil, which is made from old car-springs cannibalized
from junk cars and, therefore, is quite durable and cost-effective. The sub-soiler is used once in 3
years, to break soil crusts developed from prolonged use of the mold-board plow.
7.3.5 Strip cultivation
Strip cultivation or No-till strip cropping, is a method of minimum tillage. It involves tilling the
land in strips at the position of the crop rows, leaving the rest of the land untilled, to generate run-
off and reduce labour (gure 7.3). Tined implements are usually used. The animal-drawn tined im-
plements e.g. the “magoye ripper,” can be used. It digs 25-30 cm into the soil breaking the plough
hard pan. It can also be used to make furrows about 80 cm apart. Where access to equipment is
possible, the operation can be advanced to simultaneously insert seeds (and even fertilizer) into the
soil while breaking the hard pan in the same single pass.
Figure 7.3 (a) Strip cultivation using oxen
(Source: CFU Zambia, 2007)
(b) Strip cultivation with stubble mulch
7.3.6 Stubble mulch tillage
Stubble mulch tillage involves retaining the past season’s crop residue by incorporating it into the
soil during primary tillage. The residues are chopped and spread on the surface or incorporated
during tillage with tined implements such as the chisel plough (gure 7.4). Stubble mulch tillage
reduces labour and farm-power requirements, and as such, it is cost-effective. The system results
in improved soil structure, thereby reducing direct impact of raindrops on bare soil, and thus min-
imizing soil erosion. Moisture retention capacity of the soil is also enhanced by the residues; hence
crop survival is better during dry spells or drought.
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Figure 7.4 (a) Stubble mulch tillage with trac-
tor (Source: CFU Zambia, 2007)
(b) Stubble mulch tillage done with
oxen (Photo by Bancy Mati)
7.3.7 Ridging
Ridging, which is the construction of contour ridges (or contour furrows), involves making narrow
earthen bunds along the contour at a spacing of about 1-2 m. The ridges may be on the contour
with graded furrows draining into a grassed waterway or the ridges may have short cross-ties to
create a series of basins to store water. In most smallholder farms, ridging is normally done for
crops such as potatoes, tobacco, groundnuts and even for maize. Ridging for maize sometimes
involves “earthing” up the maize rows during the weeding process, albeit the maize is rst planted
on the at. Ridging systems are mostly suited for areas with an annual rainfall ranging from 350 to
750 mm. Ridging redirects surface runoff across the slope, improving inltration and thus prevent-
ing soil erosion. In dry areas, the crop is planted at the side of the ridge so as to be closer to the
water conserved, without the danger of water-logging (gure 7.5). Ridging is suited to deep soils
and gentle topography. The ridges may be made every season. Alternatively, in a semi-permanent
ridge-furrow system only the necessary repair is done at the onset of a new cropping cycle.
Figure 7.5 (a) Ridging with sweet potato crop
(Photos by Bancy Mati)
(b) Ridging with maize crop
7.3.8 Tied Ridging
Tied ridges are a modication of the normal contour ridges. They are made in semi-arid areas
especially for water conservation. The technique involves digging major ridges that run across the
predominant slope, and then creating smaller sub-ridges (or cross-ties) within the main furrows
(gure 7.6). The nal effect is a series of small micro-basins that store rainwater in-situ, enhancing
inltration. Depending on the system, the crop is planted at the side of the main ridge, to be as
close as possible to the harvested water while also avoiding water logging in case of prolonged
rains. Tied ridges have been found to be very efcient in storing the rain water, which has resulted
in substantial grain yield increase in some of the major dryland crops such as sorghum, maize,
wheat, and beans.
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Figure 7.6 Sketch Illustration of tied ridging
7.4 Tools and equipment for conservation tillage
There is a wide range of equipment and tools used in smallholder agriculture in Africa, whether
for conventional or conservation agriculture. The hand hoe, digging stick, pick axe, ox-drawn im-
plements and other traditional tools can be used for conservation tillage, including planting and
cultivation. However, African smallholder agriculture will not be upgraded with these rudimentary
tools. They are slow, labour intensive, tiresome, inefcient and cause drudgery, which is a major
reason youth shun agriculture. Even then, manual and animal drawn equipment are still necessary,
and in some cases, quite effective and could be all that a farmer has. Therefore, this section will
present tools and equipment used for conservation agriculture, including manual, animal-drawn
and tractor drawn implements.
7.4.1 Special features of CA equipment
Equipments used in conservation tillage sometime differ to some extent from those used for con-
ventional agriculture. The basic features are that these equipments should be able to dig deeply
into the soil, while causing as little disturbance to the soil surface as possible. Tillage equipments
such as the moldboard plough are not used in CA because they turn the soil. If possible the equip-
ments should be capable of multiple functions. This could include ability to handle residues, open
an appropriate slot, meter seed and perhaps fertilizer, distribute this to the openers, place it in the
soil in an acceptable pattern, and cover and pack the seed and the fertilizer. There are many types
of small-scale minimum tillage equipments available, each suited to different sources of power
and eld conditions. For instance, tined implements such as furrow openers used in minimum till-
age which allow cutting into the soil to create a relatively deep and quite narrow furrows, without
turning the soil. They are especially suitable for strip cultivation and stubble mulch tillage. Others
include; tined cultivators, augers, hand jabbers, animal-drawn planters, rippers, sub-soilers, power
tillers and powered tractors.
7.4.2 Hand-jab planters
Hand-jab planters (sometimes called dibblers) are normally used for sowing seeds under no-tillage
systems. Hand-jabbers may have either separate hoppers for seed and fertilizer or one hopper for
seed only. Figure 7.7 illustrates a typical double-hopper jab-planter. In design, a seed metering de-
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vice is used on hand-jabbers in the form of a rectangular plate placed inside the hopper. When the
handles are pulled apart, the seeds drop into the holes, which are delivered to the outlet and the
discharge tube. Plates with different hole sizes are available according to the seed size. Seeding rates
can be adjusted according to the number of holes in the seed plate that are exposed in the outlet.
Fertilizer amounts can also be similarly adjusted.
Figure 7.7 (a). Sketch of a hand-jab
planter with seed and fertilizer hoppers
(b) A hand-jab planter with seed and fertilizer
hoppers (photo by Bancy Mati)
One of the advantages of hand-jab planters is that they do not require access to animal or tractor
power, are low cost, light and easy to operate, although some skill is required. Thus, they are easy to
use by a woman, which increases the available labour pool for small farmers, although no-tillage it-
self reduces labour demand. By planting seeds in pockets, there is minimal soil disturbance so weed
seed germination is minimized, resulting in easy hand hoeing between plants. The small size of the
devices makes them suitable for operation on hilly, stony and stumpy areas and for intercropping
(e.g. sowing mucuna between maize rows) and for planting in fallow areas.
Hand-jab planters are suited to light soils since penetration can be difcult in harder soils in the
absence of some form of tillage. Some clay soils may also stick to the blades when working in wet
conditions and seed coverage may be affected by the V-shaped pockets and minimal disturbance.
Hand-jab planters can also be adapted in conventional tilled agriculture as they reduce labour and
introduce precision in planting of seeds.
7.4.3 Hoe openers
Openers are seed drills designed for manual planting. They can be attached with seed and fertilizer
hoppers. Small-scale planters with tined openers have independent adjustment for the fertilizer
opener so that fertilizer can be placed deeper than the seed. Although placing fertilizer beneath the
seed in no-tillage does not always result in the best crop yield, with small-scale drills and planters
it is a more realistic option than placing fertilizer alongside the seed because the latter option re-
quires the fertilizer opener to be operating in new ground, which requires more energy than when
both openers (seed and fertilizer) operate at different depths in a common slot. In any case, placed
fertilizer within the seed zone is better than surface broadcasting causing slow crop access and
increased weed growth.
Generally, tines require less force than double disc openers, which contributes to maintaining a
uniform seeding depth if a suitable depth control mechanism is included. Tines are preferred in
hard soils, although their drag force may become excessive for the power available. Tines are also
more susceptible to blockage with residues and are unsuitable in stony areas.
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7.4.4 Animal-drawn planters
Animal drawn planters for CA have the same basic design that allows seed placement in the soil
with minimum disturbance (Figure 7.8). They are generally lighter, less expensive and more adapt-
able to hilly and stony areas. They also have a greater range of adjustments to allow for the diversity
of small farms, seed types and animal pulling capacity. Residue handling is often easier with small
scale machines as a result of fewer rows and openers.
Figure 7.8 (a) Tined openers for ani-
mal-drawn no-till planters (source: Ribeiro et al,
2006)
(b) Animal-drawn planter with fertilizer
hopper (photo by Bancy Mati)
7.4.5 Magoye ripper
The Magoye ripper is a tined implement designed in Zambia especially for conservation tillage. The
implement is basically a narrow tine (in this case a hexagonal-section 25 mm diameter carbon steel
rod – probably from discarded rock drilling bits) with a low angle of attack (<15°). This congu-
ration gives good penetration at low draft. The tine is reversible and moveable to compensate for
wear. The tine and adjustment assembly are mounted on a steel plough frame.
The Magoye ripper has two variations, the sub-soiler and the ripper. The ripper is a tool for rip-
ping and producing a weed-free seedbed along the line of planting. This obviates the need for
ploughing, but does increase the level of management required to control the inter-row weeds
(either mechanically or chemically). There is provision for attaching wings for ridging and the wing
mounting plates alone produce a narrow weed-free furrow, suitable for hand planting with a stick
or jab planter. The Magoye ripper is normally pulled by oxen and is quite popular with smallholder
farmers engaged in conservation tillage
7.4.6 Tractor-drawn tined implements
There is a wide range of tractor pulled implements for CA adaptable to smallholder farming. These
include small tractor tined cultivators and planters requiring up to 50 hp. The machines have the
same straw-cutting (smooth disc) and slot forming (tine or double disc) openers as the single-row
machines and most are capable of applying fertilizer at seeding time. Some models provide bulk
seed and/or fertilizer hoppers in a similar manner to larger machines, while other models are set
up as multi-row precision seeders (gure 7.9).
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Figure 7.9 Small 4-wheeled tractor with
planter (photo by Bancy Mati)
(b) 4-wheel tractor with chisel plow
(photo by Omar el Seed)
7.4.7 Two wheeled tractors
Two-wheeled tractors, sometimes known as power tillers, provide an option to mechanise agricul-
ture on small farms at less cost than conventional four wheeled tractors (gure 7.10). They can be
used on land as small as 0.2 ha and utilize less energy. Two wheeled tractors can be used for both
conventional and conservation agriculture, depending on the type of tools and attachments used.
They offer versatile use and can be adapted as cultivators, planters, weeders, rotavators or attached
to a trailer and sued to transport agricultural produce (gure 7.11).
Figure 7.10 Two-wheeled power tiller
with tined cultivators (photos by Bancy Mati)
(b) Two-wheeled tractor mounted with sub-
soiler
(a) Weeding tyne (b) Ridger (c) Bund former (d) Trailer
Figure 7.11 Accessories used with power tillers (Source: Ribeiro et al, 2006)
7.5 Advantages of conservation tillage
Labour saving: The substitution of conventional tillage by conservation agriculture allows a more
even distribution of labour over the year, because of the elimination of ploughing and harrowing
activities and the use of cover crops and herbicides.
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Agronomic benets: Adopting conservation agriculture leads to improvement of soil productivity:
Organic matter increases, there is in-situ soil water conservation and improvement of soil struc-
ture, and thus rooting zone. Also, the reduced trafc by agricultural machinery reduces soil com-
paction, improving inltration of water into the soil.
Improved soil fertility: The added crop residues increase of the organic matter content of the soil.
In the beginning this is limited to the top layer of the soil, but with the years this will extend to
deeper soil layers. Organic matter plays an important role in the soil: fertilizer use efciency, wa-
ter holding capacity, soil tilth, rooting environment and nutrient retention, all depend on organic
matter.
Enironmental benets: There are improvements in water quality as there are less sediments erod-
ed from land. Also, increased micro-biodiversity in the soil due to incorporation of crop residues
on the soil surface and also reduced splash-effect of the raindrops. This results in higher inltration
and reduced runoff, leading to less erosion. The residues also form a physical barrier that reduces
the speed of water and wind over the surface, of which the latter reduces evaporation.
Carbon sequestration: Conservationagriculturealsohassignicantbenetsatthegloballevel,
including: carbon sequestration in the organic matter accumulated in the soil from the crop resi-
dues and cover crops. There is also less leaching of soil nutrients or chemicals into groundwater,
recharge of aquifers through better inltration, less pollution of the water and less energy/fuel
used during tillage.
7.6 Limitations of conservation tillage
Conservation tillage may carry high initial costs of purchasing the specialized equipment and the
new management skills. In addition, conservation tillage may not work well in areas prone to ood-
ing or soil with surface sealing properties. The need to use herbicides to control weeds is another
limitation for smallholder farmers, where the cost of capital is higher than that of labour. In addi-
tion, smallholder farmer contain a multiplicity of crop enterprises for which herbicide use could
pose a limitation. Sometimes, it takes several years before the advantages are realised so it should
be considered a long-term project.
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8. AGROFORESTRY
8.1 What is agroforestry?
Agroforestry refers to “a dynamic, ecologically based natural resources management system of
trees in farms and in the landscape, diversies and sustains far increased social, economic and
environmental benets for land use at all levels.” It involves planting trees or shrubs in the farm,
or keeping those that are already there. This can be in the form of interspaced trees, borders or
shelterbelts. An agroforestry system should hold a diversity of plants with different root systems,
drawing water from different soil layers with different growing periods (gure 8.1). Tree-roots are
deeper than most agricultural crops and they can reach water & nutrients from deeper layers than
crops. The idea is combine species with different properties in such a way that the resources are
optimally used. Any negative interference or competition with crop should be minimal, such that
the net productivity of land is increased compared with sole crops.
Figure 8.1 Agroforestry is growing trees, shrubs and crops (photo by Bancy Mati)
8.2 Benets of agroforestry
• Agroforestry has the potential to increase the organic matter and nitrogen content of soils.
The growing of multipurpose tree species compatible with existing farming systems is
important in soil management.
• Agroforestry trees cushion the impact of raindrops on the soil, hence reducing the amount
of rain-splash erosion. Their roots bind the soil together.
• Planted along contours, agroforestry trees interrupt the ow of water running off the
surface. They prevent soil erosion, conserve soil water, and improve soil fertility and micro
climate. The environmental benets of trees include soil conservation, bio-diversity con-
servation, and conservation of terrestrial carbon.
• The trees shade the soil, reducing the soil temperature and cutting the amount of water
that evaporates into the air. They also serve as a wind break, reducing incidence of wind
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erosion.
• Agroforestry trees recycle nutrients from deep in the soil, and leguminous trees x nitrogen
that can benet plants. They provide nutrient inputs to crops by capturing nutrients from
atmospheric deposition, biological nitrogen xation, availability of nutrients from deep in
the subsoil (deep nitrate capture) and storing them in the bio-mass.
• In biomass transfer systems, the transfer of biomass from one site to another provides
nutrient inputs which become available when the biomass is decomposed in the soil.
• Trees can also enhance nutrient cycling through conversion of soil organic matter into
available nutrients (especially N and P). It is, therefore, possible to recycle nutrients through
litter-fall, root decay, and green manure.
• Agroforestry has other environmental benets such as improving water retention in a
watershed, increasing biodiversity, greening the environment and supporting wildlife, and
livestock.
• Apart from helping conserve water and soil, agroforestry can provide many other ecolog-
ical, economic or social benets. These include the provision of poles for building, fruits
for sale and consumption, fuel wood, and fodder for livestock.
8.3 Limitations
The main limitation with agroforestry is that for some crops, the shading is undesirable and may
lower yields. Also, there is some competition for nutrients and water especially in dry areas. Trees
take up space on a farm and this may discourage farmers with very small plots of land to adopt
agroforestry. The biggest limitation, however, is the lack of knowledge by farmers, on which tree
species best suit their cropping systems, where to get them from, and how to manage them to op-
timize agricultural productivity on relatively small farms.
8.4 Types of agroforestry systems
Agroforestry systems are often complex systems, which allow the implementation of a number of
water & soil conservation measures. Thus, there are many ways to practice agroforestry, and some
of them e.g. hedge-row intercropping, grass-strips, vegetative buffers, improved fallows and strip
cropping have been discussed in previous sections of this manual.
Other methods include the following
• Planting trees in woodlots, or farm forestry systems in which a part of the farm is set aside
for pure stand of forest trees.
• Orchards are a variation of woodlots except that they carry fruit trees. This is an agrofor-
estry system as much as it is a horticultural enterprise
• Interspersed planted with crops within the farm.
• Contour planting of hedges forms biological terraces naturally, stops the soil and water
from washing downhill.
• Fodder trees and grass leys used for grazing add plant biomass to the farm and provide
agroforestry benets
• Multipurpose tree systems whereby trees have benets e.g. grevillea robusta provides timber,
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rewood from trimming its branches, shade and good leaf litter
• Indigenous trees could be retained on the farm, when new land is opened for cultivation,
• Natural bush hedges running across the slope which act as inltration buffers
• Trees planted on farm boundaries
• Agroforestry trees planted to soil conservation structures e.g. terraces to stabilize them,
• Trees planted across the wind direction as wind breakers, and
• Sylvopastoral systems combine pasture and trees.
8.5 Characteristics of suitable agroforestry tree species
The important qualities of a good agroforestry tree species is one that has a light, open crown that
lets sunlight through. The tree should have ability to x nitrogen and good leaf litter which would
easily convert into nutrients available at appropriate times in the crop cycle. It should have few and
shallow lateral roots and a deep thrusting tap root system so that it draws most of its water from
subsurface soil layers. The trees should have the ability to resprout quickly after pruning, coppicing
or pollarding. A good agroforestry tree should be resistant to droughts, ooding, soil variability,
and other climatic hazards. It should have a productive capacity that includes poles, wood, food,
and fodder, medicinal and other products. It should not compete with crops for water or nutrients.
Tree species with all these characteristics are few to nd. But a number of trees and shrubs have
varying benets and are commonly used for agroforestry in tropical zones. Typical species include
trees such as Grevillea Robusta, and fruit trees such as papaya, mango, orange, avocado, guava, and
other indigenous trees. Also, shrub species used in various soil and water conservation practices
and for livestock feed are agroforestry trees. They include; sesbania sesban, caliandra calothyrsus, Cassia
siamea, Lantana camara, Tithonia diversifolia, Crotalaria grahamiana, Tephrosia vogelii, caliandra calothyrsus
and leucena sp. There are numerous other indigenous tree and shrubs which local people know are
good for growing with crops.
8.6 Tree establishment and care
The establishment of agroforestry trees is an important step which requires much preparation
and care. First suitable tree species are selected, their seeds collected and planted in a nursery. The
seedlings are carefully tended in nursery beds during the dry season. The young seedlings should
be kept under shade to protect them from the harsh conditions from the sun’s heat. Later, when
the seedlings are close to transplanting, they are removed from the shade and given enough time
to grow and harden in the open (gure 8.2). Depending on tree species, the nursery seedlings are
transplanted at the earliest opportunity, so that they take root in the eld.
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Figure 8.2 Young tree seedlings in a shaded
nursery (photo by Janvier Gassasira)
(b) Hardening tree seedlings in the sun
before transplanting (photo by Bancy Mati)
When ready, the seedlings are transplanted into large planting holes at the end of the dry season
or at the onset of the rains. They are watered liberally at rst in order to encourage the growth of
deep roots in the planting hole and vigorous branch and leaf growth. Rapid leaf growth means
that the roots will be well nourished; these will then increase in length and penetrate deep into the
ground in search of the capillary fringe. When rainfall is inadequate, water application should be
very limited, just enough to keep the tree alive. This avoids the formation of shallow rooting which
is detrimental to of the growth of the deep thrusting tap roots. If the tap roots reach the capillary
fringe by the end of the rainy season, the seedling will survive the dry season. Otherwise, it will
need watering until the rains begin again.
As with all other agricultural enterprises, agroforestry trees should be tended with care. This could
involve activities like pruning, watering, addition of fertilizers and manures where necessary, re-
moval of unwanted vegetation and weeds, pest and disease control. Finally, trees are planted with
a purpose. Therefore, harvesting of the tree products or the tree itself is part of agroforestry
management.
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