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Improving organic
crop cultivation
Edited by Professor Ulrich Köpke
University of Bonn, Germany
BURLEIGH DODDS SERIES IN AGRICULTURAL SCIENCE
E-CHAPTER FROM THIS BOOK
http://dx.doi.org/10.19103/AS.2017.0029.09
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic
crop cultivation
Michael J. Goss, University of Guelph, Canada; Adrian Unc, Memorial University of
Newfoundland, Canada; and Wilfried Ehlers, Georg-August University, Germany
1 Introduction
2 Key aspects of organic farming affecting availability and use of water
3 Developments in water management in organic agriculture
4 Conclusion
5 Where to look for further information
6 References
1 Introduction
1.1 Water on earth and in the atmosphere
The global water cycle gives a simple overview of how water associated with the soil,
geological substrata, plants, animals and the atmosphere link to each other. As plants
largely, but not uniquely, exploit water in the soil we need to understand how they are
able to access this resource and know how large a volume is available. However, soil and
plants also lose water to the atmosphere and hence the balance between these three
parts of the cycle is critical for identifying whether and how the losses can be replenished,
either in the short or long term. Water may seep through soil and into underlying rock
strata, which can represent an important mechanism by which nutrients may be lost by
leaching from the rooting zone of a crop. In some circumstances, this can be beneficial
because it prevents the build-up of salts and the development of salinization. There are
times when water needs to be removed from soils through constructed drainage schemes,
for example, when there are barriers to natural drainage that cause waterlogging. This
can prevent roots from exploring a sufficiently large volume of soil to anchor plants and
capture enough mineral nutrients. If soils are not sufficiently permeable, incident rainfall
may be so intense that water cannot infiltrate but flows from the field over the soil surface,
which can lead to erosion and loss of mineral and organic matter fractions of the soil. We
therefore have to consider both the static and dynamic aspects of water availability in the
soil.
Improving water management in organic crop
cultivation
Improving water management in organic crop
cultivation
Chapter taken from: Köpke, U. (ed.), Improving organic crop cultivation, Burleigh Dodds Science Publishing,
Cambridge, UK, 2019, (ISBN: 978 1 78676 184 2; www.bdspublishing.com)
Improving water management in organic crop cultivation
2
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
1.1.1 Water, weathering and soil formation
The first step in the process of soil formation is the weathering of parent rocks. Water
contributes to this process through both physical and chemical actions. The process of
physical weathering splits both rocks and minerals but does not change their chemical
composition. Water plays a critical role in the splitting of rock by getting into cracks, where
it expands on freezing. In arid regions, differential expansion of minerals under the heat
of the sun can also split rocks. The fragments formed are transported down from higher
elevations into valleys by surface water. Here the fine material can be deposited as the
flow of water slows, covering the valley bottoms and effectively levelling the topographical
features. After small rocks, large fragments and gravels are deposited, sand particles,
which range in size from 201 to 2000 µm, settle out. Finer particles, ranging in size from 2
to 20 µm, silt, can also precipitate out. Silt particles tend to have a relatively large surface
area-to-volume ratio and hence are easily subject to weathering.
Chemical weathering involves the breakdown of minerals by hydration, hydrolysis and
dissolution. The direct effects of water are greatly enhanced by the presence of protons or
hydronium ions (H3O+) that are derived from organic and inorganic acids. Thus, eventually,
even the sparingly soluble silicates are finally broken down. The rate of mineral degradation
increases with temperature. Consequently, many tropical soils are strongly developed at
greater depths than those in cooler climates but at the same time they may only provide
an inadequate supply of essential plant nutrients. In contrast, weathering and soil genesis
in colder regions only penetrate to shallow depths of the mineral stratum.
During soil genesis, soil-specific minerals are formed, but at later stages of development
these may themselves degrade. Disintegration of solid particles liberates solutes, such as
cations, silicic acid, and iron and aluminium compounds. Water transports these solutes
together with colloidal solids deeper into the soil or even into the rock strata below.
Disintegration, displacement, precipitation and leaching are essential parts of the soil-
forming process that depend on water. In temperate-humid regions, for example with
limestone parent material, the first step towards soil formation is the decalcification of
the rock fragments. Calcium ions leach out and the pH decreases, resulting in the decay
of ‘primary’ minerals and the formation of ‘secondary’ silicate minerals that belong to the
‘clay’ fraction. These fine particles are on average smaller than 2 µm. During clay formation
the soil colour turns brown, with the colour being dependent on the amount of organic
matter and the level of oxidation of iron and manganese oxides. With time, the clay may
start to migrate within the soil from near the surface to a lower layer, a soil-forming process
called ‘lessivage’. Another process, ‘podzolization’, involves the breakdown of clay by
chemical weathering, and the release of sesquioxides of iron and aluminium, which can
leach along with soluble humic acids to deeper layers.
In these ways, characteristic soil horizons are formed that are diagnostic of distinct soil
types. In arid regions, where soil water is more likely to be lost by evaporation, water flows
up towards the soil surface during the more prevalent rain-free periods. As a result, any
solutes transported from the subsoil are deposited in the surface layers.
Soil genesis is accompanied by the formation of soil structure, which also is essentially
dependent on soil water. Water causes some clay minerals to swell and shrink; furthermore,
the soil matrix can become further divided by planes of weakness or fissures. Ice lenses,
1. In many countries, the lower limit for the size of sand is taken as 50 µm.
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 3
formed by freezing temperatures, can separate the soil matrix into aggregates of
characteristic size and form. Aided by the action of plant roots and mycorrhizal fungi,
aggregates made up of sand, silt and clay-sized particles can be formed, which are
cemented together by inorganic oxides, amorphous aluminosilicates and organic matter
(Goss and Kay, 2005). Both roots and fungal hyphae release organic mucilage that under
dry conditions can enhance the stability of soil aggregates (Reid and Goss, 1981, 1982).
1.1.2 Water storage capacity and the availability of water in soil
The space between the solid particles of the soil, both within and between soil aggregates
is made up of pores of different effective sizes. It is within this pore space that the storage
and movement of water takes place. Just as a sponge, a clod of dry soil will absorb water.
Initially, the rate that the water is soaked up may be relatively rapid but it declines with
time and will eventually stop. At that point the forces in the soil sample causing the water
to enter the pore space within the clod are neutralized. These forces originate on the
surfaces of clay particles as ‘short range’ London–van der Waals forces, the negative
electrostatic forces associated with clay minerals and from the adsorbed positively charged
counter ions. In addition to these forces, there are capillary forces that are the result of the
adhesion of water molecules to the surface of solid soil particles and the cohesive forces
between water molecules. In most soils, water wets the surface of particles, which causes
the surface of the water within a pore to form a concave meniscus at its boundary with the
air. In some dry soils, the nature of the organic matter covering soil particles prevents the
surfaces from wetting and the water may form a convex surface, effectively preventing it
from entering the pores.
Because of the complexity of soil pores in terms of their structure, particularly the
pore necks created where, for example, three touching particles form part of the pore,
some air is entrapped as a soil profile wets up during a rainfall event. Prolonged rain will
eventually result in the soil becoming saturated with water. When the rain stops, water
filling the coarser pores will start to drain from the soil profile and this will continue for
some time. The volume of water retained by the soil ‘against gravity’ after two days of
drainage without further rain is commonly referred to as the field capacity. In reality, the
soil commonly continues to drain slowly for longer than two days, so the water content
at field capacity is really just a convenient concept for calculating water storage capacity.
The other important value is the limit of water extraction by plants, the permanent wilting
point. At that point, the adhesive forces are too strong for plants to absorb any of the
residual water remaining in the soil. The difference between these two water contents,
field capacity at the wet end of the scale and permanent wilting point at the dry end, is
identified as the soil water content potentially ‘available’ to plants. Available water content
varies with proportions of sand, silt and clay that constitute the soil. These proportions
define the textural classes to which soils belong.
The total porosity of the soil is simply the proportion of the volume of voids in the
total volume of soil. The sizes, shapes and volumes of individual pores are defined by
the spatial distribution of the solid component of the soils and their propensity to form
compact or loose aggregates. Small mineral particles, with greater surface electrostatic
potentials per unit mass, associate more closely and this causes a greater proportion of
the total soil porosity to occur as smaller pores. Thus, the average equivalent diameter
of individual pores varies among soils. Smaller pores will retain water more strongly than
Improving water management in organic crop cultivation
4
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
larger pores; this phenomenon is known as capillarity. The energy by which a capillary can
retain water can be simply expressed in terms of hydraulic head as (Or and Wraith, 2000):
hm 14.84
rm
()
=
()
µ
(1)
where h is the height that water rises in a capillary with a radius r. Thus, for two soils
with equal total porosity, the one with a larger proportion of pores in the small diameter
range will retain more water. On the other hand, coarse-textured soils, such as sandy soils,
which have most of their pores in the larger size domain (e.g. Unc and Goss, 2006) hold
water less strongly than the smaller, capillary, pores. Thus, in general, sands and clay soils
have the least available water, with silt and silty-clay loams commonly having the greatest
amount of available water (Fig. 1).
1.1.3 Drivers of water movement
In the previous section we described how water moves into dry soil and now we need to
consider how the forces at the surface of soil solids combine with others to cause water to
move through the soil. We have also discussed the movement of water from upland areas
down into the valley. In many parts of the world, where there is a lot of water cascading
from one level to another, a part of the energy in the moving water is captured to generate
hydroelectric power. The potential of water to do work is greater in the uplands than
when it has fallen to the valley and part of that energy released can be used for turning
the blades of turbines. In an electrical circuit, the rate at which the charge moves (the
unit is coulomb per second) is the current I (unit is ampere) that is related to the potential
difference V (unit is volts) between the battery terminals. We can write an equation to
describe the movement of the charge:
Figure 1 Changes in water content at field capacity and at the permanent wilting point as soil texture
becomes finer from sand to clay. The maximum value of available water content occurs at the boundary
between silt loam and clay loam texture.
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 5
IVR=
(2)
where R is the resistance (unit is ohm = volt/ampere) in the wiring.
We can write a similar equation for water moving through the soil:
qK z=∆∆
()
φ (3)
where q is the volume of water moving through an area per unit of time (cm3 H2O per cm2
cross-sectional area per unit of time), K is the reciprocal of the hydraulic resistance to flow,
the hydraulic conductivity. The difference in water potential between two points in the soil
is given by ∆ϕ, and this is calculated over a distance of ∆z. If we make our calculations
on the basis of the weight of water, this results in the unit for ϕ being centimetre, a unit
of length. This can greatly simplify the calculations required to determine the flow rate of
water, as the unit for K will be cm per unit of time.
As described above, ϕ is the total potential of the water. It is the summation of the
matric potential, gravitational potential, the osmotic potential and the pressure potential.
The matric potential results from the adsorption of water on the surfaces of mineral
and organic materials constituting the solid matrix of the soil, as described above. As
the soil dries, matric potential becomes the major component of the total potential.
The gravitational potential results from the height of water above a datum position.
Osmotic potential results from the presence of solutes dissolved in the water. For osmotic
potential to play an important role the presence of a semipermeable membrane to
separate two bodies of water is required. In the soil, knowledge of osmotic potential will
therefore be important for water to be absorbed by plant roots, as the water has to cross
the outer semipermeable membrane enclosing the cytoplasm of cells of the epidermis
or the endodermis of a root (for more detail see Section 1.2). An air–water interface also
acts as a semipermeable membrane, so under some circumstances the osmotic potential
of the soil solution can be important for the loss of water by evaporation. However, even
in soils subject to regular applications of soluble fertilizer, the concentration of ions in soil
solutions is unlikely to influence the transport of water except in localized regions around
fertilizer pellets applied when soils are relatively dry. Pressure potential develops if a water
body is being compressed. In soils, the pressure potential is important for movement of
water below the surface of the water table.
Equation 3 is known as the Darcy equation and establishes that a flow of water only occurs
when there is a difference in its potential (∆ϕ) between locations and the movement is from
the point with the higher potential to that with the lower potential. The potential gradient
(∆ϕ/∆z) is the difference in potential divided by the distance (∆z) between locations, which
could be, for example, the distance between two soil layers or soil horizons. If the topsoil is
at equilibrium, no water movement takes place. If we measure the gravitational potential,
Z, from the soil surface, the value of Z will decrease from Z = 0 cm to Z = –10 cm at a point
10 cm below the surface (Fig. 2). If we assume that ϕ depends only on Z and the matric
potential (ψ), if there is no flow, there is no difference in total potential: ∆ϕ = 0 cm. Then
ψ at the soil surface (ψ0) must differ from the value at 10 cm below the surface (ψ−10) by
−10 cm, that is, ψ−10 = ψ0 + 10 cm (Fig. 2a). If rainfall then occurs, the potential of water
at the soil surface will increase. The rainwater will be attracted to the particle surfaces
but that also reduces the strength of the attraction forces, which become less negative
and hence the matric potential approaches 0. In consequence, the potential gradient
will increase from the soil surface to the point 10 cm below. Water will then start to flow
downwards from the soil surface. Depending on how dry the soil was at the start of the
Improving water management in organic crop cultivation
6
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
rainstorm, the duration and intensity of the rainfall will determine the depth to which the
soil wets. Some time after the end of the storm, the soil above a depth of 10 cm may
reach a new equilibrium where no flow is taking place although the matric potential near
the surface will again be 10 cm lower than that at the 10-cm depth. If water starts to be
lost to the atmosphere by evaporation, the attractive forces at the surface of soil particles
start to increase and the matric potential starts to decrease. In consequence, water will
move upwards as the total potential gradient increases (becomes more negative) from the
10-cm depth (Fig. 2b).
We can now look at the field capacity and permanent wilting point in terms of matric
potential. In soils with a deep unsaturated profile, and thus a thick layer above the
watertable, the water held in pores at a matric potential of −330 cm is often taken as the
amount at field capacity. This means that the largest water-filled pores are approximately
9 µm in diameter. At the permanent wilting point, the diameter of the largest water-filled
pores is about 0.2 µm, so the matric potential is about −15 000 cm. In shallower soils, the
water potential at field capacity may be closer to −100 cm, but the potential at the wilting
point may be less affected. Although in reality these matric potentials for the maximum
and minimum ends of the plant available water in a field soil are only approximate, they
are convenient parameters to use for comparing soils in the laboratory.
The growth and productivity of crops can be affected by both the excess and lack of
available water.
1.1.4 Subsoil drainage schemes
When climatic conditions are such that rainfall dominates the hydrologic cycle, the soil
profile can become saturated and water drains below the depth that plant roots are able
to exploit the resource. This allows the water to contribute to groundwater resources.
Some soils, particularly those dominated by clay content, drain so slowly that the profile
can become waterlogged, and this greatly impairs crop growth and production. Field
drainage schemes, which consist of perforated pipe drains that collect the water from the
top metre of soil, transport this water off the field to natural watercourses or constructed
drainage ditches. The required spacing of the perforated drains depends on the hydraulic
conductivity of the subsoil, and in deep clay soils this can require close spacing, which can
be prohibitively expensive. If the clay can be moulded, it is possible to create temporary
round channels in the soil, ‘mole drains’, by use of a mole plough. In poorly permeable
soils, effective drainage schemes with closely spaced drains can be created by back filling
Figure 2 Components of the soil water potential under moist conditions. (a) Equilibrium conditions
after rain. (b) Evaporation taking place at the soil surface.
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 7
the trench formed when inserting the pipe drains with gravel and then drawing the mole
plough through the trenches and perpendicular to them.
Mole drains can function for 3–4 years and allow a spacing of about 2 m. The evidence
from fall-sown field crops suggests that the aim of the drainage scheme should be to
prevent the top 50 cm of the soil from remaining waterlogged (Belford, 1981) for extended
periods. Critical durations for the waterlogging taking place at depths shallower than
50 cm depend on the prevailing temperatures, which control the rate at which the oxygen
levels in the soil decline (Cannell et al., 1985). For spring-sown crops, such as pea (Pisum
sativum L.), even two days of waterlogging can result in significant reduction in shoot
growth, pod formation and seed development (Cannell et al., 1979), in part because of the
warmer temperatures that prevail after the crop is sown in early spring.
1.1.5 Irrigation systems
To augment the water available through rainfall and snow, it may be necessary to irrigate
crops, choosing from a wide variety of schemes.
In flood irrigation, water flows into the soil from a supply channel at a sufficient rate to
adequately cover the soil. This approach relies on the formation of a flat, even surface
with a very small slope to ensure the whole field receives water. The main drawback with
such schemes is the large loss of water by evaporation from supply channels and from the
flooded soil.
Border strip irrigation involves forming dykes parallel to the slope that enclose strips
into which the crop is seeded in rows aligned across the slope. The rows of the crop help
to spread the water across the strip.
Furrow irrigation requires the formation of furrows or dykes between individual or groups
of crop rows that are sown on ridges or raised beds. The water infiltrates laterally into the
ridges or raised beds, which have a greater hydraulic conductivity than the flooded soil in
the furrows.
In basin irrigation an area is surrounded by a dyke that allows water to pond but prevents
it from running off the field. Ponding helps to ensure relatively uniform infiltration into soils
that have poor permeability.
Sprinkler irrigation requires more technical equipment and the water is supplied under
pressure so that fine droplets can be formed to simulate natural rainfall. Lines of sprinkler
jets can move automatically up and down a field to apply a uniform distribution of water.
Alternatively, the line of jets can be arranged to form the arm of a centre pivot, uniformly
irrigating circular or semicircular areas over which the jets pass.
For crops of greater market value, drip irrigation can be used with water being supplied
to individual plants through a drip nozzle. By limiting the area of wet soil at the surface, the
system reduces the volume of water lost by evaporation before it can be used by the crop
and can focus the irrigation on the soil volume containing the plant roots.
Another method for reducing unnecessary losses is to supply the water via subsurface
pipes, sub-irrigation. Some combined systems have been developed that allow the water
intercepted by a drainage scheme to be collected in a holding pond and then to be used
to sub-irrigate the crop when the available water declines (Tan et al., 1999; Bonaiti and
Borin, 2010).
All of these techniques for irrigation need a clear understanding of the local hydrological
cycle as it pertains to the specific crop being produced. That in turn requires a knowledge
of the processes involved in water use by each crop.
Improving water management in organic crop cultivation
8
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
1.2 The soil–plant–atmosphere continuum
As identified in Section 1.1, there are three interactive components important to the
hydrologic cycle: the soil, the plant and the atmosphere. The previous sections have
provided an overview of the soil component and now we consider the plant component.
The water potential within plant cells is dependent on the osmotic potential, which is
related to the presence of mineral ions and organic compounds, such as sucrose in the
vacuole. These solutes are osmotically active and lower the water potential within the
plant cell. The reference level for the osmotic potential is pure water with no solutes.
Osmotic activity of solutes assumes the presence of cell membranes: the plasmalemma,
which separates the protoplasm from the external medium, and the tonoplast between the
protoplasm and the vacuole. These cell membranes seem to behave as if impermeable to
solutes but permeable to water; however, they are capable of the facilitated transport of
ions and other solutes that allow the build-up of a higher solute concentration inside than
outside the cell. This difference in concentration facilitates the movement of water across
the membrane through special structural units, aquaporins, within it and in consequence
lowers the solute concentration inside the cell.
In a fully turgid plant cell, the protoplasm (the cellular content) is replete with water. A
turgid cell, together with those in close proximity, gives the tissue some rigidity, which
in turn determines the form of herbaceous plants. When the cell cannot absorb more
water it is unable to induce water movement towards itself. Under those conditions the
total water potential (ϕ) of the cell is zero. The osmotic potential, Ω, that has drawn water
into the cell is balanced by the reaction to deformation exerted by the cell walls and
surrounding turgid tissue, resisting further expansion that is required to accommodate
more uncompressible water. The result is that the cell contents become pressurized. This
positive pressure, the turgor pressure, increases the energy level of the water. Expressing
turgor pressure in terms of the unit volume of water establishes another component of
water potential, the pressure potential, P. It has as a reference level the value for water
at atmospheric pressure. In a fully turgid cell at equilibrium, the pressure potential is at a
maximum:
φ=Ω+P
(4)
When plant cells are subject to extreme drying, some water remains under tension within
the microfibrillar structure of the cell walls. This means that we should write Equation 4 as:
φψ=++Ω P
(5)
where ψ is the potential resulting from the water attraction to a surface (matric potential),
this time the surfaces are within the cell wall. However, when a cell is fully turgid, ψ will be
zero. Hence:
P=−
Ω=
when
φ
0 (6)
The uptake of water by roots and its movement within the plant do not rely on the
expenditure of metabolic energy. The water simply flows from sites of higher potential to
sites of lower potential. Water flow within the plant, from the epidermis of the root through
the cortex to the xylem in the central stele, and from the stele to various organs of the
plant, and finally from the leaves to the atmosphere, is caused by differences in total water
potential, just as in the case of water flow through the soil.
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 9
Evaporation is important in the transfer of water from the soil and from the plant to the
atmosphere. Hence, before continuing with the overview of water moving through the
plant, we need to consider the process of evaporation a little further. For evaporation from
the soil, water changes its state from a liquid to vapour at or near the soil surface. This
change requires the input of energy, most of which comes from the radiant energy of the
sun. The water molecules diffuse into the air and away from the soil surface, depending
on differences in the vapour pressure. The key values here are the saturated vapour
pressure of the air (es), which is the maximum value the air can accommodate and which
depends on air temperature, and the actual vapour pressure (e). The difference between
es and e (es – e) is known as the saturation deficit. The larger the saturation deficit the
more water vapour can be held in the air mass. In addition to diffusion, the transfer of
water vapour away from the soil will be much quicker if it takes place as a mass flow, also
referred to as convection. Convective vapour movement can result from local heating
of the air near the soil surface or can be caused by wind. All of these determinants of
evaporation, energy supply, saturation deficit and convection, depend on the climatic
factors that create evaporative demand or the potential evaporation. Evaporation from
moist soil is influenced to a much greater degree by atmospheric conditions than by the
characteristics of the soil surface.
Solar energy reaching the earth’s surface, the global or total radiation RT, arrives as short
wave radiation, ranging between 300 and 3000 nm, and may be a little less than half that
reaching the outer part of the atmosphere in more humid regions but can reach up to 70%
of that value in arid zones with little cloud. The instantaneous value of RT varies according
to the time of day, cloudiness, atmospheric turbidity, season, latitude, slope aspect and
altitude. The fraction of RT reflected from the surface, the fraction r, (the albedo) depends
on the colour of the surface and its roughness. Of the radiation that is absorbed, a fraction
is re-radiated to the atmosphere in the form of long wave radiation (RL). The net radiation,
RN, available to keep the air and soil warm and for use by plants in photoassimilation
(photosynthesis) is given by:
RR1r R
NT L
=−
()
− (7)
RGHL
EA
N=+
++
(8)
where G is the energy flux used to heat the soil and H is the fraction used to heat the air.
Only a small part of the energy flux is used for photosynthesis, A, and the remainder
of the net radiation goes into evaporating water, LE. The term LE, the latent heat flux, is
made up of the latent heat of vaporization, L, with a value of 2.45 kJ g−1 at 20°C, and the
evaporation rate, E g cm−2 day−1. The heat of vaporization is required to change liquid
water into a gas. The energy flux used for evaporation is only identified if some of it is
taken from the evaporating body itself, which will cool down.
Potential evaporation can be calculated from the energy balance equation (Eq. 8) but
when taking into account the ease of measurement it is better to combine it with the
aerodynamic aspect. Penman (1948) developed such a combination method. The Penman
equation for the daily evaporation from a water surface is given by:
LE RGLE
P
Na
=∆
()
−
()
+
∆+
γ
γ1 (9)
for the potential evaporation rate, Ep (g cm−2 day−1), we can write:
Improving water management in organic crop cultivation
10
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
ERGL E
Pa
=∆
()
−
()
+
{}
∆+
()
γγ
N1 (10)
where ∆ is the slope of the function of saturated vapour pressure versus temperature
(mbar °C−1); γ is the psychrometric constant (mbar °C−1); Ea (mm day−1) is a ventilation–
humidity term, which takes into consideration the influence of wind speed and the vapour
pressure deficit of the air on evaporation; and L, RN and G are as defined for Eq. 8.
As with the process of soil evaporation, three conditions have to be met for evaporation
from a plant leaf. Within the leaf, water has to change from the liquid to the vapour phase,
an energy-consuming process. Again, the energy comes from the radiant energy of the
sun. Secondly, a drop in vapour pressure is necessary to start the diffusion of the water
molecules in the vapour phase from the intercellular spaces of a leaf through stomatal
pores and out from the confines of the plant. Finally, the water vapour must be removed
from the leaf surface to the atmosphere. The latter occurs by diffusion across a thin
boundary layer, but that transport is greatly enlarged by the mass flow driven by the wind.
Liquid water is supplied to the leaf through the vascular bundles, which are branched
subdivisions of the conducting tissue of the stems and end in the mesophyll as a fine
network. Vascular bundles contain the two conducting systems of a plant, the phloem that
transports organic and inorganic solutes and the xylem that mainly conducts water and
mineral ions. The main conducting units of the xylem in most crop plants are the vessels.
As water comes to the end of a vascular bundle in a leaf it moves into the mesophyll cells.
In some plants there is a really distinct layer of closely packed columnar cells with many
chloroplasts, the palisade layer, and a layer of more loosely packed cells, the spongy
mesophyll. In both cases, some water moves through the cells of the mesophyll to the
epidermal cells. This outer layer of cells is typically covered with a waxy layer, the cuticle,
that is not impervious to water and is responsible for some 3–5% of the total water lost
from the leaf. However, the larger proportion of the water in the mesophyll evaporates
from the cell walls into the intercellular spaces. The largest of these spaces are associated
with structural pores in the epidermal layer, the stomates, and form the substomatal cavity.
Stomates provide the main conduits for water vapour to escape from the leaf. Importantly,
stomates are the essential openings for carbon dioxide (CO2) to enter the leaves to be
available for photosynthesis. Two specialized cells of the epidermis, the guard cells, form
stomatal pores. Because of differential thickening of the cell wall, the guard cells subtend
a large pore between them when fully turgid, but as the cells lose turgor pressure the pore
size gets smaller. By closing their stomates, plants can greatly reduce their water loss as
the supply from the soil declines. However, the closing of stomates also cuts off the supply
of CO2 for photosynthesis.
We can now track the pathway of water through the soil to the root surface, into the
root and from there through the stem into the leaves and finally into the atmosphere, the
transpiration stream. The component potentials in plant cells vary over a much greater
range of values than do those in moist soil. Depending on how turgid the cells are, ϕ for
plant cells is much lower (approximately –10 000 cm H2O) than ϕ for moist soil (ranges
between −100 and −1000 cm H2O). As a consequence of the potential drop between soil
and plant, the water will tend to move automatically from the soil towards the root. Once
at the root surface it can be absorbed at any point along the root, although the specific
water uptake rate, UR, will be greater in the region of the root hair zone, where on the one
hand cells are not suberized as a means of impeding radial flow, and on the other hand the
surface area of the root hair cell is enhanced by the presence of the hair and by the fact
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 11
that the root hair can penetrate some distance into the soil away from the drier interface
with the root.
Water may be absorbed into root hairs of the epidermal cells or, if the epidermis has been
lost, by entering into passage cells of the exodermis as the root ages. In either case, water
crosses the semipermeable plasmalemma of the cells and enters the cytoplasm. Once it is
within the cytoplasm it can pass from cell to cell by diffusion through the plasmodesmata,
which are cytoplasmic connecting strands that link cell through adjoining cell walls.
This is the symplastic pathway. Alternatively, water can move through the cell walls and
intercellular spaces of the root but not cross a plasmalemma without immediately entering
the cells. As a bulk flow, water can be conducted between cells, along neighbouring
cell walls or through intercellular air spaces. This route is called the apoplastic pathway.
Some water may move across the root via a mixture of the two routes but always crosses
the plasmalemma or even the membrane surrounding the internal vacuole of a cell, the
tonoplast. This is known as the transmembrane pathway.
Regardless of which of these three pathways carries the water across the root it will
eventually arrive at the innermost layer of the cortex, the endodermis, where it has to
enter the cell cytoplasm because the apoplastic pathway is impeded by suberization of
the cell wall. The flow of water into the xylem of the root from the endodermis is via
the transmembrane pathway, where water moves because of the osmotic potential. The
vessels of the xylem have no protoplasm and the water enters the apoplastic pathway.
In this part of the apoplastic pathway it is the cohesion between water molecules that
maintains a constant column of water between the root and the leaves. The evaporation
of water driven by the vapour pressure deficit lowers the water potential in the leaves that
then drives the water movement through the plant.
It is appropriate at this point to identify how much water is involved in evapotranspiration
(ET) from a field over the main growing season. A potential ET of 700 mm per season is
fairly typical in Europe and North America. So, from a 1-ha field that ET is equivalent to
7 × 106 L or 7000 t of water. In herbaceous plants, about 80% of the fresh weight is due
to water, which is used to provide rigidity to the structure and a relatively small amount,
about 1%, is used in photosynthesis. The main role of transpiration and evaporation from
the leaf is to keep the leaf temperature from exceeding that which causes the efficiency
of the CO2 fixation process to decline. Evaporation from the soil will cool the soil surface
and atmospheric boundary layer above it but will have less benefit for processes in the leaf
compared with transpiration.
1.3 The soil water balance
All the water that is lost by evaporation from the soil (E) and is transpired by the plants
(T) originates from the precipitation (W) falling on a field. Some precipitation falls onto
leaf surfaces and evaporates directly rather than entering the soil; this is referred to as
interception (I). Of the water that enters the soil some may drain to below the rooting
zone of the crop and be lost (D), in quantities governed by soil hydraulic parameters as
described in Sections 1.1.2 and 1.1.3. Some (R) may simply run off over the surface of the
soil and enter a watercourse. Depending on the soil water storage capacity, there may be
changes in the amount of water (∆S) in the soil at any time. All these components can be
expressed in terms of the equivalent depth (mm) of water
WETIDR S
=+++ ++∆
(11)
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Determining E and I separately from T is difficult when the crop covers the soil and it is
convenient to combine the values into the ET of the plants. From the field water balance,
if run-off does not occur, then:
ET WSD=−∆−
(12)
In Table 1 the components of the water balance, identified in Eq. 12, are shown for a
clay soil under winter wheat. The values were obtained from measurements in the field
comparing crops grown following removal of the harvest residues or their incorporation by
ploughing. The main difference was the greater water loss to field drains when the residues
were incorporated, possibly because of greater conductivity of the topsoil resulting from
the associated tillage. The Penman equation used to calculate potential ET assumes loss
from a short grass sward and underestimates losses from a relatively tall crop, such as
wheat by approximately 15% (Ehlers and Goss, 2016). In the experiment described, the
values were between 0 and 21% underestimated.
1.4 Efficiency of water use by crops
The fact that stomates are the point of control for water leaving the plant, and consequently
for CO2 entering, draws attention to the efficiency with which the plant makes use of these
two resources. The ratio of the dry matter produced by a crop (DM) to the amount of
water used (WU) provides a measure of that efficiency. If we simply consider the amount of
water that moves through the plant in transpiration (T), the ratio DM/T is the transpiration
efficiency (TE), which can be expressed in units of grams of plant dry matter per litre
of water transpired or kg dry matter per m3 H2O. However, from a crop management
perspective it is easier to determine the ratio of amount of shoot dry matter produced to
the total water lost from the field in ET. That is the water use efficiency (WUE). We could
also take into account the other ways that the water, which landed on the soil, has been
redistributed and left the field without directly involving the plants of the crop.
Table 1 Components of the field water balance of a clay soil under winter wheat. The harvest residues
were removed or ploughed under. Measurements of soil water content and soil water potential were
made from the end of tiller formation in spring until harvest. The potential evapotranspiration was
calculated from an automatic weather station on the site and changes in water storage determined
by neutron scattering. Drainage was calculated from downward flows based on soil water potential
gradients. The water used by the crop was equated to the evapotranspiration term in the water
balance equation
Harvest residues
removed
Harvest residues
incorporated
Precipitation (mm) 148 148
Change in soil storage (mm) −107 ± 10.5 −105 ± 2.2
Drainage 30 ± 1.3 44 ± 2.4
Water used by crop from water balance (mm) 225 ± 11.8 209 ± 4.6
Potential evapotranspiration using the
Penman equation (mm)
204 204
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Improving water management in organic crop cultivation 13
Starting with the water use efficiency defined above we can write:
WUE
DM
ET
= (13)
Remembering that the transpiration efficiency (TE) is DM/T and therefore DM = TET, we
can rewrite Eq. 13 as:
WUETET
ET
TET
E+T
TE
1+
ET
=== (14)
In this equation E/T is the ratio of evaporation from the soil to the transpiration through
the plant. We can therefore add the water lost to run-off and drainage into the water
balance equation and can describe the water use efficiency for plant production WUEp:
WUETE
ER
DT
P=+++
()
1 (15)
and finally, if we consider water that has been used by competing weeds Tweed we can write:
WUETE
1ERD
TT
P
weed
=++++
()
(16)
Equation 16 allows different management options to be considered to ensure that the
water available to a crop can be used effectively. Ensuring that the maximum amount
of water possible can be stored in the soil and not lost in run-off is clearly an important
objective as is reducing evaporation when water is scarce.
2 Key aspects of organic farming affecting availability
and use of water
The understanding of how water is stored in soil and used by crops allows us to consider
how management practices can affect these key aspects.
2.1 Soil management
Clearly, any changes made to the soil can have considerable impact on both the potential
for water to enter the soil and to be held available to crops. As indicated in Section 1.1.2 a
key aspect in the storage of water is the distribution of pores of different sizes within the soil.
These pores can be located within and between the structural units of the soil.
2.1.1 Soil structure
Organic matter is essential for the formation and stabilization of soil aggregates because
of its binding and cementing properties. Increasing the organic matter content of soil
is generally associated with improved soil structural stability because the aggregates
are more resistant to tension-free water, so that pores between and within them are
Improving water management in organic crop cultivation
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© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
more stable. Fungal hyphae and roots can enhance the creation of soil aggregates by
enmeshing soil mineral particles (Miller and Jastrow, 2000). Arbuscular mycorrhizal fungi
(AMF) form symbiotic relationships with about 90% of land plants, including most but
not all crop plants. The dead hyphae of these fungi, along with decaying root material,
can form the core of soil aggregates (Goss and Kay, 2005), but living hyphae produce
glycoproteins, glomalin and glomalin-related soil proteins (Wright and Upadhyaya,
1996; Rillig, 2004b), which enhance stability through their cementing properties. ‘Sticky’
by-products, such as polysaccharides, of the activity of both bacteria and fungi as
well as the release of root exudates cement aggregates of soil mineral and organic
particles and thus lead to greater aggregate stability. Any management practices that
enhance microbial activity and thus the production of polysaccharides tend to give soil
greater structural resilience. Importantly, when plants dry the soil during transpiration,
the stabilization of soil can increase. However, just adding organic matter to soil is not
sufficient to increase stability (Fig. 3a), so there is also an organic matter quality aspect.
The increase in aggregate stability reduces their disruption (Fig. 3b) and enhances the
infiltration rate (Fig. 3c). After rainfall, the infiltration was found to decline with bigger
changes occurring in soil with greater stability (Fig. 3c). A small amount of particle
detachment from aggregates, possibly in combination with organic matter, had a large
impact on the ability of soil to allow water to infiltrate (Fig. 3d). The greater reduction
in infiltration likely resulted from blocked pores, whereas the disruption of the more
stable aggregates still permitted good infiltration (Fig. 3d). However, Reinhart et al.
(2015) found that the relationship between stability of aggregates and infiltration was
not constant and for larger aggregates it could even be negative. The effects of the
variation in aggregate stability resulting from the different quality organic material on
infiltration are evident from Fig. 4; the results shown in Figs. 3 and 4 are from the same
study by Ekwue (1992).
Many comparative assessments of organic farms with conventional operations suggest
that the soil organic matter (SOM) content is generally greater under organic agricultural
systems, although that does not always have to be the case (Huntley et al., 1997; Trewavas,
2004; Crittenden and de Goede, 2016; see also Watson, Chapter 3). The general benefits
to water management of maintaining a large SOM in soil are also evident in terms of
water availability. SOM has the net effect of reducing soil bulk density (Hudson, 1994) and
attracts water molecules to its surfaces. Even though some of the water associated with
organic matter may not be available to plants, Hudson (1994) established that for soils
ranging from sands to silty clay loams, the water content at field capacity was increased
more by an increase in SOM than it was at the permanent wilting point, thus increasing
the available water content. An increase in SOM not only enhances water storage in soil
micropores, but the greater development of stable aggregates will also lead to a larger
proportion of inter-aggregate spaces, soil mesopores. These pores hold water at relatively
large potentials between −2.5 and −300 cm (Regelink et al., 2015) and hence are able
to drain water more efficiently (Boyle et al., 1989). The presence of free-draining inter-
aggregate spaces and channels formed by soil fauna, such as earthworm, reduces the risk
of ponding with all the negative effects on infiltration, soil aeration and related chemical
and biological functions.
A decline in aggregate stability can lead to a number of changes in soil structure, including
the formation of surface crusts and a dense plough layer which may then be more easily
compacted further by heavy machinery. Surface crusts are commonly produced by the
energy of raindrops impacting soil surface aggregates and by the slaking and dispersion
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 15
(Folorunso et al., 1992; Carrizo et al., 2015) of soil particles in the presence of tension-free
water on the soil surface. Importantly, surface crusts can greatly impair the infiltration of
water from rainfall, irrigation or snowmelt, resulting in more loss in run-off from the soil.
Surface sealing can have the same effect and can result from the surface application of
liquid manure rather than solid manure (Unc and Goss, 2006). Increases in soil density in
the plough layer horizon can result as the soil naturally consolidates with changes in soil
water content, but it can also result from compaction due to wheeled traffic associated with
sowing, pest and weed control, nutrient application and harvesting (Hamza and Anderson,
2005). The treading of soil by grazing animals, as well as when they are herded between
paddocks, can result in soil compaction of grassland (Hamza and Anderson, 2005).
Figure 3 Importance of organic matter for soil structural stability and infiltration. (a) Relationship
between soil organic matter and the stability of soil aggregates (closed circles). Although the stability
tends to be greater at larger soil organic matter content, the values were not correlated. Nevertheless,
the particle detachment (open circles) was inversely related to organic matter content (R2 = 0.89,
p < 0.001). (b) Particle detachment is strongly related to aggregate stability. (c) Relationship between
infiltration into soil and aggregate stability (closed circles) and after rainfall (open circles), the infiltration
declined with bigger changes occurring in soil with greater stability. (d) The change in infiltration was
inversely related to the amount of particle detachment. Based on Ekwue (1992).
Improving water management in organic crop cultivation
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2.1.2 Tillage
Tillage systems have been designed to produce a suitable seedbed for crops as well as
control weeds and incorporate organic amendments. What has been clear for more than
a century (Sturtevant, 1887) is that the use of inversion tillage, for example, mouldboard
ploughing, for weed control and seedbed preparation is detrimental to the soil. Much of
the negative impact comes from the increased breakdown of SOM and the reduction in
beneficial soil organisms, including AMF and earthworms.
In North and South America and the Mediterranean basin, no-till techniques have
been developed and refined to plant crops without prior tillage and restricting any soil
disturbance to the layer above the depth of seed placement. As no-till soils are cooler and
wetter in the early spring season, this can lead to retarded and reduced mineralization and
nitrification. Consequently, in Northern Europe emphasis has been more on shallow or
reduced tillage practices (e.g. Crittenden et al., 2015). In both cases, the lack of inversion
allows retention of harvest residues on the soil surface to protect it from raindrop impact,
slaking and dispersion, thereby enhancing infiltration and reducing the risk of both wind
and water erosion of the soil.
Although tillage may lead to an immediate increase in total soil porosity in the tilled
layer, mainly in form of larger inter-aggregate pores (Lipiec et al., 2006), this is often
counterbalanced by more compaction in the subsurface soil layer in comparison with a
no-till management (Tebrügge and Düring, 1999). Moreover, tillage-induced soil porosity
is not resilient as the plough layer compacts easily during the growth season especially
when using heavy machinery. On the other hand, repeated mechanical disturbance
and enhanced aeration due to repeated tillage increase the breakdown of SOM and
thus breakdown of soil aggregates with the concurrent loss of the biological functions
Figure 4 The variation in organic matter and associated stability of soil aggregates resulting from
contrasting organic and inorganic amendments applied in agronomic treatments. Based on Ekwue
(1992).
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 17
associated with the lost aggregates (Jiang et al., 2011). This can cause soils to consolidate
and become compact more readily after tillage (Tebrügge and Düring, 1999), eventually
causing them to have a smaller water storage capacity. Moreover, consistent tillage at a
given depth can lead to formation of a compact layer, a plough pan, that delays or stops
water infiltration into the deeper soil layers, reducing the water storage in the entire soil
profile, often leading to water saturation and erosion of the top layer. Nevertheless, by
omitting intensive tillage, efficient weed control, especially of perennials, is limited under
conditions of organic farming in temperate-humid climates (Zikeli and Gruber, 2017).
Future strategies involving the occasional no-till seeding of grain legumes (Massucati and
Köpke, 2011) and the use of ‘bioherbicides’ (Giepen et al., 2014) applied with precision
farming measures (Ammann, 2009) may help to overcome these limitations.
Earthworms are commonly classified according to their lifestyle. Epigeic earthworms
live close to the surface, where they are important for incorporating surface litter into
the soil. In contrast, anecic species live within the soil and characteristically form vertical
burrows that are permanent and can be important in the rapid infiltration of water to
depth (Ehlers, 1975). The anecic group will collect material from the soil surface, and
pull it deep into their burrows before eating it, but they will explore the different layers
for nutrients. Endogeic earthworms also tend to live below the surface but their burrows
are mainly horizontal within a soil layer and are not permanent. Differences in earthworm
numbers and in the predominance of different groups can exist within fields under the
same management treatments, largely because of the impacts of tillage (Crittenden and
de Goede, 2016). Differences in earthworm populations between management systems
also tend to reflect the intensity of tillage (Ehlers, 1975; Barnes and Ellis, 1979; Berry and
Karlen, 1993; Crittenden and de Goede, 2016), the length of leys (Scullion et al., 2002; Han
et al., 2015) and extent of natural and improved grassland (Fraser et al., 1994). Scullion et
al. (2002) found in an extended survey that the most consistent difference between organic
and conventional farmland was the biomass of the earthworm population, which was more
frequently greater under organic soil management. However, the smaller biomass under
conventional management comprised fewer but more mature individuals. No consistent
differences between these management systems were found in the deep burrowing anecic
worms, which contrasts with the consistently smaller number in ploughed soil compared
with no-till land (Ehlers, 1975; Barnes and Ellis, 1979).
One other important tillage practice is the use of equipment to minimize the slope and
to guide water to areas where it can be used, thus managing water storage and preventing
run-off. Repeated contour tillage enables a semblance of terracing to be developed.
Forming laneways that can be put under permanent cover (grassed waterways) can
prevent uncontrolled and potentially erosive water loss. On a smaller scale run-off zones
can be guided into pits prepared for individual or groups of plants, called the Zaï farming
approach.
2.1.3 Nutrient supply
A key aspect of the field water balance is the proportion of water passing to the atmosphere
in transpiration relative to that evaporated from the soil surface. In the leaf, there is a
balance between the water that passes out through the stomates and the inwards diffusion
of CO2. The factor that influences the relative water loss from the soil through evaporation
and transpiration is the proportion of the soil that is shaded by leaves, strictly the area of
green leaf covering unit area of soil, the leaf area index (LAI). As a plant canopy develops,
Improving water management in organic crop cultivation
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© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
the area of ground covered increases with each new leaf but quite quickly the newer
leaves shade a part of the older leaves so the LAI increases more slowly. In field crops,
once the ground is fully shaded by the leaves, the radiant energy will largely be used
in transpiration of water by the plant rather than evaporation from the soil. The rate of
development of LAI at the start of the main growing season is therefore important for the
effective use of water in the soil. When the LAI has a value of 1, the transpiration rate of
the plant is about 40% of its maximum potential.
Depending upon the crop, as LAI reaches a value between 3.5 and 4.5 its transpiration
is about 90% of its maximum potential rate. Over this same range of LAI, the net radiation
component of soil evaporation may decline by 3–5% of its maximum potential rate, with
a further reduction associated with the aerodynamic term (Ritchie, 1972). Both the LAI
and the net assimilation rate of the leaves depend on the nutritional status of the plant.
Although nutrients can be supplied to conventionally farmed crops as very soluble mineral
fertilizers, organic operations depend on materials that need to be mineralized or are only
sparingly soluble. In consequence, this could be an important constraint to optimizing
production if the start of the growing season is cool and wet. As organic matter releases
its nutrients more slowly over the growth season this can limit the development rate of LAI.
Improved yields have also been the result of extending the period for which leaves remain
green, which may also require enhancing the nutrient supply towards the end of season or
preventing early senescence from pest and disease attack.
The selection of crops and the means to protect them against competition, pests and
diseases is the purpose of cropping systems.
2.1.4 Crop rotations
Crop rotation is the ordered and planned cycling of crops over time in an arable field;
the crops tending to be grown in pure stand with the plant species involved being
annuals, biennials or perennials (Ehlers and Goss, 2016). Ideally, the crops selected
will make optimum use of the local environmental conditions, taking account of solar
radiation, temperature, water supply and soil fertility. Consideration needs to be given to
having species that contrast in rooting habit, such as depth and form (tap or fibrous root
system). In this way, a crop can take up nutrients that leached below the rooting system
of a predecessor. Taproots can generate continuous pores in the soil that can provide
pathways for roots of subsequent crops (Cresswell and Kirkegaard, 1995; Han et al., 2016)
to access water held in subsoil horizons by avoiding the penetration resistance of the
bulk soil (Gaiser et al., 2012). The biopores produced may also allow water to infiltrate
rapidly to depth (Cresswell and Kirkegaard, 1995; Kautz, 2014). Fibrous root systems
are commonly considered to support soil aggregation and the stabilization of structure,
thereby increasing water-holding capacity.
The crops selected may be sequences of plants, all providing saleable components,
such as seed, edible leaf, stem fibres and storage roots, or include crops that are grown
to enhance or protect the soil. The latter may not be grown ‘in season’, so that they will
not survive over a cold winter, but their shoots will still protect the soil from raindrop
impact and the nutrients taken up during their growth period will be recycled. Staple
crops, such as wheat, barley, maize, oilseed and grain legumes, may form the main focus
of the rotation, but mixtures, such as alfalfa-grass or grass-clover, may also be grown for
fodder and support sustainability goals.
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 19
In North America and Europe, winter cover crops have been extensively investigated
as a means of protecting the soil from the erosive forces of wind and water as well as
to increase SOM, thereby enhancing water infiltration and storage. Some of the species
used include those that are grown out of season, such as spring oat, but others, such as
rye, have to be killed before the next crop can be sown. There is evidence that, with the
appropriate selection and prolonged use of winter cover crops, there can be significant
increases in SOM, particularly if associated with no-till (Blanco-Canqui et al., 2011, 2015).
The greater SOM content can also be associated with improved stability of soil aggregates
(Hermawan and Bomke, 1997). Folorunso et al. (1992), working with soils that tended
to form a surface crust, observed considerable reductions in the surface strength in the
presence of cover crops compared with their absence. Abdollahi and Munkholm (2014)
found that the resistance to penetrometer was significantly (p < 0.05) smaller between 32
and 38 cm below the soil surface where the brassica cover crop fodder radish (Raphanus
sativus L. var. oleiformis) had grown than where it was not. In their experiment, the
compacted zone was associated with a plough pan, considered to have predated the
reported experiment (Abdollahi and Munkholm, 2014). In a related experiment, total
porosity and air-filled porosity for pores >30 µm diameter at 12 to 16-cm depth in the
plough layer horizon were significantly reduced under reduced tillage systems (no-till and
harrowing). Irrespective of the tillage treatment, Abdollahi et al. (2014) found that growing
a winter cover crop created continuous macropores, which enhanced permeability to air
and water and reduced the impedance to root growth. Chen and Weil (2010) investigated
root growth of cover crops through compacted soil and found that the best penetration
was observed for fodder radish followed by rapeseed, whereas rye was the least effective.
In their experiments, Folorunso et al. (1992) found that either the steady-state rate of
infiltration or the cumulative total of water infiltrated over 6 h was greater with cover crops
than without. The residues from cover crop shoots can provide a mulch to reduce soil
evaporation, aid infiltration as well as reducing run-off (Blanco-Canqui et al., 2012) and
provide protection from erosive forces (Unger and Vigil, 1998). The plants can also help
dry the soil surface layers in spring to aid early planting (Unger and Vigil, 1998).
Cover crops can also help in controlling weeds by competing with their seedlings for
light, water and nutrients (Teasdale and Mohler, 2000; Kolodziejek and Patykowski, 2015) or
by the release of allelopathic chemicals (Reberg-Horton, 2005; Blanco-Canqui et al., 2015).
The physical presence of mulch comprised of cover crop material can adversely affect weed
emergence, depending on the proportion of the ground being covered (comparable with
LAI) and the solids component of the material (Teasdale and Mohler, 2000).
Pathogen and pest control are important factors in selecting crops and the order in which
they are to be grown. Selections can be made to prevent pests and diseases building up in
the soil and locality by separating two susceptible crops with one that is not susceptible.
3 Developments in water management in organic
agriculture
Many aspects of agricultural production require change if there is to be sufficient food to
meet the demands of another almost two billion people by 2050. The land area required
for production is declining because of urbanization and land degradation. Salinization,
Improving water management in organic crop cultivation
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© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
compaction and desertification of soils are increasing and we recognize the damage to
the earth that is associated with the clearing of forest. Furthermore, the land that could be
brought under production is often marginal for arable agriculture. The conclusion is that
the productivity per unit area of land has to increase. At the same time, farms have to grow
more produce but without further impacting the wider environment or creating food that
is itself of poor or even harmful quality. The build-up of toxic metals in soil and activities
that can lead to increased levels of antibiotic resistance genes in soil microbes have to
be prevented. Solutions have to be found to prevent invasive species taking over as new
weeds and halting the threats to crops or stored produce from migrating insects. But this
cannot be at the risk of harmful effects to wildlife or humans. Contamination of water
resources with plant nutrients, pharmaceuticals and hormonally active compounds has
to stop. More importantly, the concerns over water resources for agricultural production
continue to mount as the competition with the supply of potable water for cities becomes
more acute.
The required intensification of production must extend to those areas, where at present
it is resource limited and levels must be maintained where the impacts of agriculture have
contributed to global warming and environmental degradation. Plant breeding is vitally
important so that improved water use efficiency can come from enhanced transpiration
efficiency as well as from more efficient irrigation, crop protection and weed control.
Additionally there is a need to enable plants to be more effective in growing in soils with
increasing levels of salinity.
3.1 Managing soil limitations through enhanced soil biodiversity
One of the most exciting developments in the last twenty years has been the ability to
identify the different groups of organisms that interact within the rooting zone of plants.
The new generation of molecular techniques allows the components of local food webs,
previously hidden in the soil, to be identified and quantified. The understanding of the
roles that different groups have in the soil is beginning to make possible the identification
of practices, which can be adopted in arable agriculture (Goss et al., 2017). Local soil
and environmental conditions and plant species will likely influence the range, diversity
and functioning of microorganisms across the soil–plant continuum (Tahtamouni et al.,
2016). Soil management practices that favour stable microbial ecosystems may be critical
for long-term sustainability of agricultural production (Lucero et al., 2014). Landmark
papers have identified the critical activities of AMF in natural ecosystems that determine
the community dynamics of higher plants (van der Heijden et al., 1998; Helgason et al.,
1998; van der Heijden and Horton, 2009). Meanwhile, the contribution that AMF make to
enable their host plants respond positively to threats from both biotic and abiotic stresses
is becoming clearer, as is the range of abiotic stresses that plants can gain protection
against through their symbiotic community. These contributions include resistance to
drought (Augé et al., 2001; Augé, 2004), salinity (Liu et al., 2016), toxic metals (Ahmed et
al., 2006) and metalloids (Alho et al., 2015), enhanced supplies of nutrients (Al-Karaki and
Clark, 1998) and defence against soil-borne pathogens (Akhtar and Siddiqui, 2008). There
is also evidence that weed competition can be affected (Qiao et al., 2016). But we also
know that there are interactions in the soil between AMF and other organisms, particularly
the so-called helper-bacteria, which enhance the potential for AMF to colonize new plants
and improve their ability to support the host. A key development has been the recognition
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Improving water management in organic crop cultivation 21
that, via the fungal component of a mycorrhiza present in the soil, many host plants can be
linked together and communicate with each other. It appears possible that knowledge of
one host being attacked is communicated to all linked hosts, which then start the process
of initiating production of compounds that can counter the perceived threat.
The practices that provide most support for AMF can be divided into those that make
the soil more amenable to their survival and those that provide more mycotrophic hosts
(Goss et al., 2017). Reduced non-inversion tillage and no-till cultivation dominate the first
category together with the retention of harvest residues. Critical in the second category is
the development of rotations that sequence crops, which are members of the grass family
(Poacea), with dicots, particularly legumes (Fabaceae), together with using cover crops to
prevent soil being devoid of growing plants for extended periods. Non-mycorrhizal break
crops in the rotation have to be followed by strongly mycorrhizal plants to maintain the
efficiency of the whole system.
4 Conclusion
Improvement in water management in organic production systems requires the
adoption of practices that would be considered appropriate for building soil quality
with respect to nutrients: build the organic matter levels in the soil and protect it from
the effects of erosion and excess wetness. The move away from inversion tillage, always
using the mouldboard plough, to shallower soil disturbance, which keeps residues on
the surface, balances the adoption of selected rotations that can include cover crops.
The evidence points to these practices building SOM; supporting stable, functional,
microbial communities and preventing and alleviating compaction and erosion.
These approaches help to reduce run-off and support better infiltration and storage
of rainwater and irrigation water. The choice of tillage system, crops and cropping
sequences can also help in the integration of AMF into the farming system, which can
help protect the crops not only from water shortage but also from many other biotic
and abiotic stresses.
5 Where to look for further information
The following organizations are involved in research related to soil conditions and plant
growth applicable to organic agricultural practices:
ETH Zurich University (https://www.ethz.ch/en/the-eth-zurich/sustainability/research-for-
sustainable-development/natural-resources.html)
Rodale Institute (https://rodaleinstitute.org)
The Organic Agriculture Centre of Canada - Dalhousie University (https://www.dal.ca/faculty/
agriculture/oacc/en-home.html)
USDA Agricultural Research Service National Laboratory for Agriculture and The
Environment (https://www.ars.usda.gov/midwest-area/ames/nlae/)
USDA Natural Resources Conservation Service (https://www.nrcs.usda.gov/wps/portal/nrcs/
main/national/water/)
Improving water management in organic crop cultivation
22
© Burleigh Dodds Science Publishing Limited, 2019. All rights reserved.
Specific information on water and plant production is contained in Ehlers, W. and
Goss, M. (2016). Water Dynamics in Plant Production. 2nd Edition. CAB International,
Wallingford, UK. This book contains an extensive reference list on water in the soil–plant–
atmosphere continuum and also gives an account of the processes of water requirements
together with water uptake and use by crop plants.
The potential for supporting the role that mycorrhizal fungi play under modern
agricultural practices is evaluated in the Functional Diversity of Mycorrhiza and
Sustainable Agriculture. Management to Overcome Biotic and Abiotic Stresses, written
by Goss, M. J., Carvalho, M. and Brito, I. (2017). Academic Press, Elsevier, San Diego,
CA, USA. Further developments will be available through the Institute of Mediterranean
Agriculture and Environmental Sciences, University of Évora, Portugal (http://www.
icaam.uevora.pt.).
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