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Int. J. Agron. Agri. R.
Mbindah et al. Page
52
RESEARCH PAPER OPEN ACCESS
Evaluation of the combined use of organic and inorganic
fertilizers on bulb onion (
Allium cepa
L.) yields in West Ugenya
Sub-County, Kenya
Benedict Mbindah*1, George Karuku1, Charles Gachene1
Department of Land Resources Management and Agricultural Technology Faculty of Agriculture,
University of Nairobi, Nairobi, Kenya
Article published on March 30, 2018
Key words: Cattle manure, Inorganic fertilizer, Allium cepa, Yield.
Abstract
Low yields due to declining soil fertility continue to be a major constraint to onion production in Kenya,
necessitating imports to meet market demand. A field experiment was carried out for two seasons in West
Ugenya Sub-County during the 2015 and 2016 short and long rains seasons, respectively to evaluate the effect of
combining organic and inorganic fertilizers on the soil nutrient status, growing period and yield of bulb onion
(Allium cepa L.). The experiment was a Randomized Complete Block Design (RCBD). Treatments, each
replicated three times were: T1 (5Mega grams ha-1 cattle manure), T2 (46kg P ha-1 x 26kg N ha-1 inorganic
fertilizers), T3 (unfertilized control), and T4 (half T1 x half T2) were evaluated. Data from T1 and T4 at the end of the
two growing seasons showed significantly (P≤0.05) higher mean yields compared to the control with highest bulb
yield (25.2Mg ha-1) recorded in T4. Also, significantly (P≤0.05) higher soil available P and total organic carbon
was recorded in T4 at the end of season II compared to the other treatments in the same season. Seasonal
variation in rainfall amount led to considerably lower yields in the short rains (season II), compared to long rains
(season I). Observed data concluded that onion yields could be significantly increased by combining organic and
inorganic fertilizers at the rates of 2.5Mgha1 cattle manure containing 2% N, 0.6% P and 2.3% K, with 23kg P ha-1
x 13kg N ha1 inorganic fertilizers.
* Corresponding Author: Benedict Mbindah benedictmbi@yahoo.com
International Journal of Agronomy and Agricultural Research (IJAAR)
ISSN: 2223-7054 (Print) 2225-3610 (Online)
http://www.innspub.net
Vol. 12, No. 3, p. 52-64, 2018
Int. J. Agron. Agri. R.
Mbindah et al. Page
53
Introduction
The bulb onion (Allium cepa L.) is adapted to growing
in different agro-ecological zones (Nguthi et al., 1994)
and prefer medium textured soils with a pH range of
6-7 (Muendo and Tschirley, 2004), optimal
germination soil temperature of 15 - 25°C (Jaetzold et
al., 2009), and can be established either by direct
seeding or seedling transplanting (Muvea et al.,
2014). Although onion has been grown in Kenya over
a long time, production per land area is still low at a
national average of 15Mgha-1 compared to Korea
(67Mgha-1), Spain (54 Mgha-1), Egypt (36Mgha-1),
Ghana (17Mgha-1) and Ethiopia (10Mgha-1)
(FAOSTAT, 2012). In most parts of Kenya just like in
the study area of West Ugenya sub-county, onion is
mainly grown at small scale level, and soil fertility
decline through nutrient mining and degradation is
the main challenge to increased yields (Mango, 1999;
Jaetzold et al., 2009; Okalebo et al., 2005), alongside
post harvest losses (AFFA, 2016; USAID, 2012), and
pests and disease (Muvea et al., 2014).
Combined use of organic and inorganic fertilizers has
been proposed as a solution to reverse poor soil
health and low crop yields (Vanlauwe et al., 2002;
Sanginga and Woomer, 2009; Vanlauwe et al., 2015;
Ruganzu et al., 2015). For instance onion yields
resulting from combined use of organic and inorganic
fertilizers in a field experiment in India were
significantly (P ≤ 0.05) increased by 77% compared to
no fertilizer input, and up to 45% higher yields than
sole inorganic fertilizers application (Rai et al., 2016).
This study was thus premised on the need to increase
onion production in Kenya to meet growing demand
through integrated soil fertility management and
raise income to the local farmers.
Materials and methods
Study Site
The study was conducted in Ukwala division (N 0o 12’
37’’latitude and E 34o 7’ 21’’ longitude and at an
altitude of 1267m above sea level) in West Ugenya
Sub County, Kenya. It falls under agro-climatic zone
II, classified as sub humid (Jaetzold et al., 2009).
Ferralsols are the dominant soil type in the study area
(Jaetzold et al., 2009).
The mean monthly temperature is 21.7°C with March
being the hottest (22.6oC) and July the coldest (20.7o
C) months while rainfall is bimodal, with long rains in
March- June and short rains from September to
November (Jaetzold et al., 2009). The physiography of
the area presents a lower middle to level uplands
comprising of gently undulating slopes of between 2 and
8% (Mango, 1999; Jaetzold et al., 2009). The major land
use is intensive mixed farming accounting for 71% of the
Ugenya population (KNBS and SID, 2013). Main crops
grown include maize (Zea mays), beans (Phaseolus
vulgaris), sorghum (Sorghum bicolor), cassava
(Manihot esculenta) and sweet potatoes (Ipomoea
batatas). About 79% of the population own livestock
consisting of indigenous (small East African Zebu) and
hybrid cattle (Ayrshire and Friesian), goats, sheep, pigs,
rabbits and poultry (KNBS, 2009).
Experimental Design and Treatments
Experimental design was RCBD replicated thrice on
four treatments (T), namely: T1 (5Mgha1 cattle dung),
T2 (46kg P ha-1x 26kg N ha-1 inorganic fertilizers), T3
(unfertilized control), and T4 (half of T1 x half of T2).
The sources of Phosphorous (P) and Nitrogen (N)
were Triple Super Phosphate (TSP) with 46% P2O5
and Calcium Ammonium Nitrate (CAN) containing
26% N, respectively, while composted cattle manure
was obtained from a local cattle herder.
Table 1 shows the properties of the cattle manure
used in the experiment during the two cropping
seasons while table 2 shows the fertilizer application
rates for onion. The test crop was bulb onion (Allium
cepa), variety Neptune, directly planted at a spacing
of 20cm x 15cm, at 3.1Kgha-1 seed rate translating to
about 300,000 plantsha-1.
Table 1. Chemical properties of composted cattle
dung used in season I and II.
Parameter
Season I
Season II
Total Nitrogen
(TN), %
2.1
1.9
Phosphorous (P), %
0.5
0.7
Potassium (K), %
2.1
2.6
Organic Carbon
(OC), %
28.5
31.7
Carbon-Nitrogen
(C:N) ratio
13.6
16.7
Int. J. Agron. Agri. R.
Mbindah et al. Page
54
Table 2. Organic and inorganic fertilizer application
rates in season I and II.
Recommended
application rates
Reference
Nutrient source
26 kg N ha-1
Nguthi et al.
(1994) and
Muendo and
Tschirley, (2004)
Calcium
Ammonium
Nitrate (26% N)
46 kg P ha-1
Nguthi et al.
(1994)
Triple Super
Phosphate
(46% P2O5)
5 Mg ha-1
manure
Muriuki and
Qureshi, (2001)
and Jaetzold et al.
(2009)
Composted
cattle dung
Soil and Manure Characterization
Initial soil characterization involved taking 6 soil
samples from the experimental site with a soil auger
in a transect, at 0 – 20cm depth, and mixing them to
form a 1Kg soil composite sample that was collected
in a polythene bag for chemical analysis. Also, a 1Kg
cattle manure sample was collected in a separate
polythene bag for chemical analysis.
At harvest in the two cropping seasons, plant litter on
the soil surface was removed before post-soil
sampling was done. Similarly, 1Kg soil samples were
taken from each plot, replicated three times for better
representation and accuracy, and analyzed for
chemical properties separately. The samples were
first air-dried, crushed and then passed through a
2mm mesh sieve for physical and chemical analyses.
Soil pH was determined with a pH meter in a ratio of
1:2.5 soil/water suspension.
Soil texture was by hydrometer method as described
by Glendon and Doni (2002). CEC (cation exchange
capacity) and exchangeable bases (Ca, Mg, Na and
K) in the soil and manure samples were determined
from an ammonium acetate (NH4OAc) extraction
following procedures described by Okalebo et al.
(2002). Organic carbon in soil and manure samples
was determined following Walkley and Black (1934)
method as described by Nelson and Sommers
(1996). Total N was determined by micro-Kjeldhal
distillation method as described by Bremner (1996).
The Bray II (molybdate blue) method was used to
determine available P.
Undisturbed core soil samples were also collected in a
transect at a depth of 0-20cm using core-rings and
used for bulk density and saturated hydraulic
conductivity (Ksat) determination. Bulk density was
determined by calculating the weight of oven dried
soil at 105oC divided by the soil volume, equivalent to
the volume of the core rings. Porosity (P) was
calculated according to Landon (2014) from the
relationship;
100.1
s
b
P
Where;
b
= Bulk density,
s
= Particle
density, and 100 and 1 are constants. Ksat
determination was by the constant head permeameter
method as described by Klute and Dirksen (1982).
Agronomic Practices
Land was tilled using oxen plough, and hand hoes
used to prepare 40m x 1m raised beds at 10cm above
the ground with 1m boundary between the raised
beds. Onion seeds were sown directly along 5cm deep
furrows on the raised beds, and covered lightly with
soil at the beginning of September 2015 in season I,
and March 2016 in season II. The germinated seeds
were thinned to attain a spacing of 15cm within rows
and 20cm between rows 6 weeks after emergence.
Hand weeding was done after every 4 weeks or any
time the weeds emerged to avoid competition for
moisture, sunlight and nutrients. Diseases: Downey
mildew, purple blotch, and blight were controlled by
spraying with 20ml of Mistress 72 WPTM fungicide in
a 20liters knapsack at the onion vegetative stage,
while continuous visual inspection of plants in the
field was done for any signs of pest or disease attack.
Harvesting was 130 days after crop emergence when
80% of the crops in a 1m2 quadrant had their leaves
fallen over, by uprooting the onions from the ground
by hand and sun drying in the field for 7 days.
Crop Data
At physiological maturity, crop yield data was
collected in four evenly spaced sections in each of the
40m x 3m plots using a 1m2 quadrant. Bulb weight
was computed by weighing together the bulbs inside
the quadrant. The yield weight was extrapolated to
Mega grams (Mg) ha-1.
Int. J. Agron. Agri. R.
Mbindah et al. Page
55
Climate Data
Rainfall (mm), relative humidity (%), wind speed (ms-
1) at 2m above ground, maximum and minimum air
temperature (0C), and sunshine hours for the study
period were obtained from the Kenya Meteorological
Department from the closest station ~ 40km away
from the study site. The data allowed for calculation
of Reference Crop Evapo-transpiration (ETo) using
FAO-ETo calculator version 3.2 that utilizes the
Penman Monteith method (Allen et al., 1998). The
United States Department of Agriculture (USDA) Soil
Conservation Service method for estimating effective
rainfall in Allen et al. (1998) was used to calculate the
effective rainfall in the study area. Effective rainfall is
rain water remaining in the soil after losses from run
off and deep percolation (FAO, 1978).
Statistical Analysis
Soil and crop yield data were arranged in Microsoft
excel spread sheets and imported into Genstat
statistical software, 15th edition (Payne et al., 2009)
where they were subjected to analysis of variance
(ANOVA). Least Significant Differences (LSD) at 5%
level were used to detect differences among means.
Results and discussions
Initial Soil Fertility Status
The soil (Table 3) was coarse textured, exhibiting high
sand content (52%) and moderate clay, (38%)
implying low water retention capacity. Hence onion
crop failure was imminent in the event of a drought
due to high soil water percolation. Bationo et al.
(2012) indicated that soils of sub Saharan Africa
exhibiting ≥ 35% sand have low water holding
capacity and therefore prone to nutrient leaching by
percolating water. In addition, the high Ksat of 64.7
mmday-1 in the 0-20cm soil depth, categorized as
moderately rapid (Gaines and Gaines, 1994) and a
moderate rating of CEC of 17.4 meq 100g-1 (FAO,
2006) all implied significant nutrient leaching was
expected, such that limited nutrient availability would
hinder onion growth.
Table 3. Initial soil characterization of the study site.
Parameters
Soil
characterization
Very
high
High
Medium
Low
Very
low
Sand (%)
52
-
-
-
-
-
Silt (%)
10
-
-
-
-
-
Clay (%)
38
-
-
-
-
-
Texture class
Sandy clay - - - -
-
pH-H2O (1:2.5)
6.05
-
> 7
5.5 – 7.0
<5.5
-
CEC (meq 100g-1)
15.40
> 40
25 – 40
12 - 25
6 - 12
< 6
OC (%)
2.59
> 2.5
1.5 – 2.5
< 1.5
TN (%)
0.17
> 0.7
0.5 – 0.7
< 0.5
P (ppm)
15.00
> 46
26 - 45
16 – 25
10 - 15
< 9
K (meq 100g-1)
1.50
> 1.2
0.6 – 1.2
0.3 – 0.6
0.2 – 0.3
< 0.2
Ca (meq 100g-1)
12.60
> 20
10 - 20
5 - 10
2 - 5
< 2
Mg (meq 100g-1)
4.90
> 8
3 - 8
1 - 3
0.3 - 1
< 0.3
Na (meq 100g-1)
1.30
> 2
0.7 - 2
0.3 – 0.7
0.1 – 0.3
< 0.1
ESP (%)
7.50
> 25
20 - 25
< 20
Bulk density (g cm-3)
1.21
> 1.9
1.6 – 1.9
1.3 – 1.6
1.0 – 1.3
< 1.0
Porosity (%)
54.50
>50
50
< 50
Ksat (mmday-1)
64.70
Legend: CEC – Cation Exchange Capacity, OC – Organic Carbon, TN – Total Nitrogen, P – Phosphorous, K –
Potassium, Ca – Calcium, Mg – Magnesium, Na – Sodium, ESP – Exchangeable Sodium Percentage, Ksat –
Saturated hydraulic conductivity.
The soil was slightly acid with initial pH of 6.05 that
was within the 6 – 7 optimal pH range for onion
growth and development (Muendo and Tschirley,
2004). Initial organic carbon (OC) was 2.6%, and
adequate (≥1.5%) based on Bationo et al. (2012). The
mean bulk density of 1.28 gcm-3 and 1.21 gcm-3 in
season I and II, respectively was low according to
Hazelton (2007), probably due to the high OC content
(Alemayehu et al., 2016; Karuku and Mochoge, 2016)
in the 0 – 20cm depth. Low bulk density would imply
no hindrance to root penetration (Landon, 2014) by
the onion crop root system. Also, initial soil porosity
Int. J. Agron. Agri. R.
Mbindah et al. Page
56
was 55% which was within recommended range of ≥
50% (Landon (2014) that would not limit root growth
and extension.
Organic matter is an important source of soil N for
crop growth through gradual decay and
mineralization in the soil. Initial total Nitrogen (TN)
was low at 0.17% according to FAO (2006) that
classifies low N as < 0.5%. Deficiency of N would
result in reduced onion yields with respect to size and
weight of the bulb (Mohammad and Moazzam, 2012).
Initial available P was fairly low at ≤ 15 ppm as
confirmed in earlier experiments in the study area by
Mango (1999), Okalebo et al. (2005), Jaetzold et al.
(2009) and Owino et al. (2015). This implies that the
onion crops could experience poor root development,
stunted growth and delay in crop maturity unless P is
supplemented as either foliar spray or soil fertilizer.
Chacon et al. (2011) reported that inadequate P
inhibit cell division in the meristematic tissues and
encourage premature cell differentiation within the
root tip, resulting in inhibition of primary root growth
of young flowering plants.
Initial exchangeable K indicated high levels at 1.5 meq
100g-1 (FAO, 2006), implying high ‘luxury consumption’
whereby plants take up excess K than is required for
their physiological needs. In water stressed conditions, K
is important particularly for maintenance of turgor
pressure, accumulation and transport of metabolic
products in plants (Bationo et al., 2012) hence an
essential nutrient for optimal crop production and
yields. This is in agreement with Mageed et al. (2017)
who noted that application of higher levels of K
fertilizer in calcareous soils of Egypt where
environment is arid, improved plant water status as
well as growth and yield of soya beans. This implies
that onion crop will be highly resilient to water stress
and subsequent withering during dry spells within the
short and long rainy seasons in the study area.
Initial exchangeable Ca and Mg were also high at 12.6
and 4.9 meq 100g-1 soil according to rating by FAO
(2006) of > 10 and > 3 meq 100g-1, respectively. Ca
and Mg are important in plants for enzyme activation
and carbohydrate transport (Bationo et al., 2012). Mg
deficiency mostly results in leaf chlorosis (Hao and
Papadopoulos, 2004; Keino et al., 2015), while
stunting of new growth in stems, flowers and roots
occurs when Ca is limiting (Bationo et al., 2012). This
implies that photosynthesis in the onion crop would
not be hindered due to these macro nutrients.
Initial exchangeable Na was high at 1.3 meq 100g-1 in
the study site according to FAO, (2006) ranking of 0.7
– 2 meq 100g-1. Although small quantities of Na are
used in plant metabolism, it is not an essential plant
element hence deficiency does not appear to exhibit
any symptoms on onion crop (FAO, 2006). The
exchangeable sodium percentage (ESP) was below the
20 - 25% tolerance range for onions (FAO, 2006)
implying it was too low to inhibit the crop’s nutrient
mining ability in the soil of the study site. ESP greater
than 15 results in clay dispersion thereby affecting soil
permeability and consequently water transmission
properties (FAO, 1996).
Weather Conditions during Onion Development
Stages
In season I, the onion seed was planted 14 days after
½ETo equaled rainfall at 2.6 mmday-1 (Fig. 1)
depicting the start of the growing period as rainfall
was increasing (Karuku et al., 2014). The late planting
implied that potential yields would be reduced as the
rain-fed crop would not receive adequate water unless
supplemental irrigation was carried out. As the onion
development progressed it was accompanied by 85
days humid period where rainfall maintained above
the ½ETo. The humid period helped to store water in
the soil for crop use as water loss from crop
transpiration and evaporation from soil surface was
low (FAO, 1986). End of the growing period was
marked when rainfall reduced to ½ETo at 3.2 mm
day1. End of the growing period come 16 days early to
the 130 days requirement for onion (variety Neptune)
growing period; and the inherent coarse texture of the
soil could not store the water during the humid period
due to its high percolation rate, which would
necessitate supplemental irrigation for the crop to
meet its full water requirements.
Int. J. Agron. Agri. R.
Mbindah et al. Page
57
In season II, onion seed was also planted late 9 days
after ½ETo equaled rainfall at 2.9 mm day-1 (Fig. 1). The
humid period in this season lasted 67 days and the
growing period was shortened by 32 days compared to
season I causing premature senescence of the crop as
excess water stored in the soil during the humid period
was lost before crop use due to the low water holding
capacity of the sandy soil. The shorter growing period
compared to season I was due to reduced rainfall which
had the effect of increasing the yield reduction factor and
lowered onion yield, as crop water needs were higher
than available soil moisture.
Fig. 1. Rainfall, evapotranspiration (ETo) and half evapotranspiration (½ETo) during onion growing period.
In season I, effective rainfall was 85.2mm at initiation
stage and continuously increased up to the end of the
reproductive stage, then reduced as expected at
maturation (Table 4). In season II, the initiation stage
received 65.6mm effective rainfall and increased in
the vegetative and reproductive stages to 118 and
148mm, respectively. The maturation stage saw a
rainfall reduction to 53mm, which was necessary as
this stage requires relatively dry and warm weather to
attain high quality onion yields, otherwise rotting and
bulb splitting would occur. For instance, Karuku et al.
(2014) observed that an increase in precipitation in
the maturation stage of tomatoes affected yield
quality and quantity through fruit drops. Effective
rainfall was within the 300 to 500 mm water
requirement for onion optimal growth and yields
(FAO, 1986), where it was higher at 413mm
throughout season I compared to 384 mm in season
II. Reduced rainfall in season II compared to season I
(Table 4) would imply that crop yield in season II
could be lower as uptake of water and nutrients by
plant roots would difficult as water is held at higher
tension meaning more energy expended in water
uptake that could go to yield production.
Table 4. Weather and related crop data during onion growth stages.
Growth stages
Season I
Season II
Growth
length
(days)
R
(mm)
ETo
(mm
day-1)
ETonion
(mm
day-1)
Tmean
(oC)
R
(mm)
ETo
(mm
day-1)
ETonion
(mm day-
1)
Tmean
(oC)
Initial
15
85.2
5.1
2.6
24.6
65.5
5.4
2.7
24.1
Vegetative
25
107.9
4.9
3.7
23.2
117.5
5.5
4.1
23.4
Reproductive
70
170.4
5.0
5.3
23.6
147.7
4.8
5.0
23.5
Maturity
20
49.1
4.8
4.1
25.3
53.5
4.6
3.9
23.8
Legend: R – effective rainfall; ETo – evapotranspiration; ETonion – actual onion evapotranspiration; Tmean – mean
air temperature.
Int. J. Agron. Agri. R.
Mbindah et al. Page
58
In season I, ETonion was between 2.6 and 5.3 mmday-1
during the growing period and was largely outside the
5 to 6 mmday-1 range that would allow the onion meet
its full transpiration water requirements (FAO, 1986).
In season II, ETonion ranged between 2.7 and 5.0
mmday1, and due to lower rainfall, resulted in onion
water stress at reproductive stage of bulb formation
that could have led to high yield reduction factor (Ky)
in this stage. In both season I and II, supplemental
irrigation or other soil water conservation
management practice such as mulching was necessary
for the onion crop to meet its full water requirements.
Actual onion evapotranspiration, ETonion (i.e. ETo x
Kc) at the initial stage was low, 2.6 mmday-1 in season
I and 2.7 mmday-1 in season II because of a low onion
crop coefficient (Kc initial = 0.5, FAO, 1986). This
implied that moisture loss from the soil through the
plant atmosphere continuum was dominantly due to
direct evaporation from the soil surface as the crop’s
canopy cover was small to transpire a significant
amount of water. At the onion vegetative stage ETonion
increased due to increase in Kc to 0.75. ETonion was at
its maximum at the reproductive stage since Kc had
increased to its highest 1.05 value (FAO, 1986),
implying that onion canopy cover had spread
substantially to shade the underlying soil from the sun,
hence less moisture loss through leaf surface. At
maturity stage, ETonion was 4.1mm day-1 in season I and
3.9mm day-1 in season II. This decrease in ETonion was
due to a steady decline in Kc maturity to 0.85 (FAO,
1986) due to senescence. Mean air temperature was
between 23 and 25oC in both growing seasons which
was within the optimal thermal range of 15 – 25 oC for
onion germination and growth (Jaetzold et al., 2009).
Soil Nutrients Status at the End of Season I and II of
Cropping
There was a significant (P ≤ 0.05) increase in soil pH
in T1 and T4 compared to T2 and the control in season
I (Table 5). This pH increase could have been due to
high levels of lime-like materials such Ca and Mg
compounds in the applied organic cattle manure
(Table 2) in T1 and T4 that neutralized the
concentration of acidifying H+ ions from the soil
(FAO, 2006).
Table 5. Soil properties after harvesting the first and second season crop.
T
Season I
Season II
pH
(H2O)
OC (%)
TN (%)
P
(ppm)
K
(meq
100g-1)
pH
(H2O)
OC (%)
TN
(%)
P
(ppm)
K
(meq
100g-1)
T1
6.08c
2.98c
0.20b
16.1b
1.5a
6.08b
3.06b
0.21c
16.5b
1.5a
T2
6.04a
2.51a
0.18ab
17.6c
1.4a
5.98a
2.56ab
0.19b
18.1c
1.5a
T3
6.03a
2.47a
0.17a
15.1a
1.4a
6.04b
2.50a
0.17a
15.9a
1.4a
T4
6.06b
2.94c
0.19ab
17.2c
1.5a
6.06b
3.36c
0.21c
17.8c
1.5a
SE
0.01
0.09
0.02
0.34
0.06
0.03
0.14
0.01
0.25
0.10
Legend: T – Treatment, T1 - 5Mgha-1 cattle manure, T2 - 46kg P ha-1 x 26kg N ha-1 inorganic fertilizers, T3 –
control, T4 - half T1 x half T2, SE – standard error, Mean Fig.s followed by same letter down the columns are not
significantly different at P ≤ 0.05.
At the end of season II, soil pH ranged between 5.98
and 6.08 and was lowest in T2 compared to the other
treatments probably due to a net increase in protons
through nitrification process of NH4+ ions in CAN
fertilizer (CaCO3 + NH4NO3) applied, thereby
releasing H+ ions (Yan et al., 1996; Braos et al., 2015).
Comparison of soil pH in season I and season II
showed no significant difference between T1, T3 and
control. This was could have been due to vegetative
onion parts in season I being returned to the soil as
decomposing crop litter and acting as a buffer to pH
change in season II. Organic matter contains weak
acids having carboxyl group (-COOH); which
dissociates to attain a negative charge (-COO-) thus
buffering soil pH (FAO, 2005). However, T2 was
characterized by a significant pH reduction in season II
compared to season I probably due to a net increase in
H+ ions beyond the buffering capacity of the soil by
organic matter from leaf litter.
Int. J. Agron. Agri. R.
Mbindah et al. Page
59
The pH increase could also have been due to plant
nutrient uptake whereby attraction of soil nutrient
cations to the charged surface of root hair cells caused
the plant root hairs to release H+ ion which acidified
the rhizosphere (Henkel, 2015).
At the end of season I and season II, there was a
significantly higher (P ≤ 0.05) OC and TN in T1 and T4
compared to the control. This might be attributed to
decomposition of cattle manure in both T1 and T4, in
addition to organic residues arising from
decomposition of crop litter fall during the onion
growing period. Application of organic manure as well
as decomposition of crop litter can significantly
increase soil OC (Bedada et al., 2014; Cotrufo et al.,
2015; Novara et al., 2015; Mariaselvam et al., 2014)
and TN (Abbasi et al., 2015 and Mahmoud et al.,
2009). Despite inorganic fertilizer application that
indirectly increased OC by up to 2% above the
control, from organic matter arising from high
vegetative growth that was returned to the soil as
decomposing plant litter, T2 showed no statistical
difference to T1, T4 and the control. This may be due
to the sandy nature of the soil (Table 3) that
promoted leaching of salts in applied inorganic
fertilizer beyond the rooting zone of the onion crop.
Tropical soils with high sand content (≥ 35%) are
highly susceptible to leaching of nutrients (Bationo et
al., 2012). Leaching reduced vegetative growth vigor
in T2 compared to T1 and T4 that resulted in lower
litter fall that would have otherwise mineralized to
add to the soil OC and TN stock.
Due to inorganic P fertilizer application, significantly
(P ≤ 0.05) higher soil available P was observed in T2
and T4 in comparison to T1 and the control, at the end
of the two onion growing seasons. T1 with sole cattle
manure application had significantly (P ≤ 0.05) lower
soil available P compared to T2 and T4 in both seasons
because in contrast to inorganic fertilizers, the P
concentration in livestock manures is much lower
(Bationo et al., 2012).
There was no significant change in exchangeable K
across all treatments in season I compared to the
initial soil K status.
This is because Ferralsols, the soils of the study area,
are dominated by low activity clay minerals, mainly
kaolinite (WRB, 2006), which have inaccessible inter-
layers due to hydrogen bonding that prevents K
fixation (Tran, 2010; WRB, 2006). Similarly, no
significant difference in K across all treatments was
observed in season II due to the reason adduced
above on K fixation. There was no statistically
significant difference in K between season I and
season II probably due to luxury crop uptake (FAO,
2006). Also, K is prone to leaching especially in areas
with heavy rainfall (Keino et al., 2015), as is the case
in the study area, hence additions from cattle manure
application in T1 and T4 could have been lost through
moderate leaching in the study area.
Bulb Onion Yield at the End of season I and II
In season I, the yields ranged between 20.3 and 25.2
Mgha-1 (Table 6). T1 and T4 had significantly (P ≤
0.05) higher mean onion yields, 15% and 19% above
the control, respectively. This is attributed to the
nutrients retained on the soil surface being available
for plant uptake in the cattle manure that could have
reduced leaching (Bationo et al., 2012), unlike T2 where
much of the applied inorganic fertilizer could have
leached with increasing rainfall. According to
Vanlauwe et al. (2002), Okalebo et al. (2005), and
Ruganzu et al. (2015), addition of organic materials
improves the soil chemical, physical and biological
properties that enhance nutrient availability, retention
and uptake by crops.
This is also in agreement with Otinga et al. (2013) who
found application of composted cattle manure
increased maize yields compared to sole use of
inorganic P fertilizer.
Table 6. Onion yields (Mgha-1) as affected by the
different treatments.
Treatment (T)
Season I
Season II
T1
24.1b
16.2d
T2
20.7a
12.9c
T3
20.3a
12.3c
T4
25.2b
16.3cd
SE
1.2
1.9
Legend: T1 – 5Mg ha-1 cattle manure, T2 – 46kg P ha-1
+ 26 kg N ha-1 inorganic fertilizers, T3 – unfertilized
control, T4 - half T1 + half T2, SE – standard error,
mean Fig.s followed by same letter in the rows or
columns are not significantly different at P ≤ 0.05.
Int. J. Agron. Agri. R.
Mbindah et al. Page
60
While there were no significant differences between
onion yields of T1 and T4, mean separation data
indicated that T4 had 4% more yield than T1. Higher
yields in T4 could have been due to organic fertilizer
that gradually released its nutrients, further
supplemented with inorganic fertilizer that released
nutrients more readily, thus increasing T4 nutrient
status compared to T1. Studies have shown that
combined use of organic manures with inorganic
fertilizers significantly increase soil nutrients uptake
by plants and maximizes yields compared to sole
application of either organic or inorganic fertilizers
(Rai et al., 2016; Sanginga and Woomer, 2009;
Vanlauwe et al., 2015).
There was no significant difference between onion
yields of T2 and the control in season I mainly because
heavy rains in the initial growth stage dislodged and
damaged the young onions plants thus reduced plant
population and expected yield in a large section of T2
plots. Gaping to replace the destroyed onion seedlings
was not done as it would have required three more
weeks to sow afresh onion seeds, that would have
resulted in non-uniform growth as the gap
replacement onion crop would not have attained
maturity by the time yield of the remaining majority
onion crop that withstood dislodging and damage was
being determined.
In season II, trend in the results were similar to
season I and yields significantly (P ≤0.05) varied
between 12.3 and 16.3 Mgha-1. The highest onion
mean yields were recorded in T4 at 16.3 Mgha-1 with
the lowest in the control. Thus, the addition of
organic materials to soil improved the chemical,
physical and biological properties that enhance
availability of nutrients and their uptake by crops
(Otinga et al., 2013 and Ruganzu et al., 2015).
No significant differences were observed between T2
and the control. This is probably due to lower rainfall
in season II that could have caused low availability of
inorganic fertilizer in T2 which did not to fully
dissolve, and probably burnt the onion seedlings.
Hergert et al. (2012) found out that reduced crop
emergence and stand in maize, sorghum and soya
bean can occur when soil moisture is limited and
fertilizer is placed too close to the seed, as this
increases salt concentration which interferes with
root development.
Also, no significant difference was observed in mean
onion yields between T1 and T4, both at 24% higher than
the control. It would have been expected that T4 with
higher nutrient content from the combination of organic
and inorganic sources, would give higher onion yields
compared to T1. However, this was not the case because
most of the inorganic nutrients in T4 could have been
bound with organic cattle manure thus temporarily
immobilizing nutrients (Vanlauwe et al., 2002).
Mean yields comparison between seasons and
treatments showed that season I was higher at 22.5
Mgha-1compared to season II at 14.4 Mgha-1. The
difference in yield was due to the low rainfall of 286
mm in season II which was a limiting factor in
contrast to season I at 390mm (Table 4) as it
provided less water to the sandy soil with inherent
low water holding capacity. The soil moisture was
insufficient for optimal onion transpiration needs
leading to low yields as Zhang et al. (2004) found in
wheat yields under varying levels of soil water deficit,
as onions require 350 – 600mm rain for mean yields
of 17 Mgha-1 in the study area (Jaetzold et al., 2009).
Conclusion
The objective of this study was to evaluate the influence
of the four soil fertilizer treatments on soil properties,
bulb onion yields, and recommend the best fertilizer
option for small scale onion farmers in Ugenya, Siaya
County. From data of two growing seasons, treatment 4
(T4) consisting of 2.5Mgha-1 cattle manure in
combination with inorganic fertilizers (23kg P ha-1x
13kg N ha-1) gave the highest increase in onion yields
by 19 to 24% on bulb weight in comparison to the
unfertilized control. Thus, integrated soil fertility
management option of use of organic manure and
inorganic fertilizer resulted in highest yields due to
increased soil nutrient availability and crop uptake,
and should be recommended in the study area for
sustainable bulb onion farming.
Int. J. Agron. Agri. R.
Mbindah et al. Page
61
Acknowledgement
The authors are grateful to Christopher Edmonday,
the farmer who allowed this study to be carried out on
his farm.
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