Effects of fertilizer and soybean residue management on nitrate and sorghum yields under intercropping in kanhaplic haplustvlts of western kenya
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Date
2015
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Sokoine University of Agriculture
Abstract
Grain sorghum {Sorghum bicolor L. Moench) is among the cereals produced under
smallholder agriculture and serves as a source of food fbr most of the world's
population. In Kenya, sorghum is ranked the third most important staple food after
maize and wheat. Despite the importance attached to the role sorghum plays in food
security, its production among the smallholder farmers is low with a fluctuating
and/or unstable trend. In Busia County, Western Kenya, sorghum production is
below optimum and this is a tlireat to food security among the smallholder farmers
in the region. Demographic factors and farmers9 perceptions on the native soil
fertility status and its improvement among others has implications on crop yields
and determines the level of yield gaps for the crops at the farm level. Further to this,
prevailing limitations on soil resource base of well characterized soils in this county
prove to be a challenge in forecasting the agricultural potential of the varied soil
population and has remained
setback in development of appropriate soil
management practices needed to improve soil nutrient status and its sustenance. Low
inherent soil fertility has also been reported as the major setback to improved yields
in Western Kenya, Busia County inclusive. Cereal-legume intercropping coupled
with mineral fertilizer application are among the research interventions that have
been developed to improve soil fertility and enhance yields. Despite the research
interventions that have been put across through research, sorghum yields are below
optimum. Therefore, the main aim of this study was to establish the underlying
socio-economic factors and soil fertility management aspects governing sorghum
production, conduct pedological characterization of the soils in the study sites, and
assess the effects of nitrogen fertilizer application rates and soybean residue
management options on nitrate nitrogen and sorghum yields under intercropping
system in some typical soils of the County. Thus, to capture socioeconomic factors and
farmers' perceptions on soil fertility management practices with regard to sorghum
production, a cross-sectional study was carried out using structured interviews and
observations. Gender disparity, social norms, literacy, fertilizer use, accessibility to advisory
services and farmers' perception on soil fertility management options were concluded to
impact on sorghum production. Women were predominant (57.3%) sorghum producing
fanners in the County. Literacy level revealed majority of the farmers (49.3%) had primary
education as optimum suggesting sorghum production to be through hands-on experience.
Individual land ownership was the norm with most farms being 1.5 to 2.0 hectares. To
establish representative research sites on the basis of landforms and other physiographic
attributes, standard soil survey was carried out in Western Kenya. Soil profiles were
characterized at Einaloniba (N 00° 25'28.8" E 034° 15' 51.9n) (Nambale District) designated
(EMA-P1) and at Bukhalalire N 00° 19'10.l" E 034° 16r 26A" (Butula District) named
BUMA-P1. Both pedons formed from in-situ weathering of granitic rocks under ustic
moisture and iso-hyperthermic temperature regimes. Both soils are highly weathered with
BUMA-P1 depicting more advanced stage of weathering. The two pedons differed
noticeably in terms of physico-chemical characteristics emphasizing the need to characterize
soils before fertilizer recommendations are made. In the USDA Soil Taxonomy, EMA-P1
was classified as Kanhaplic Haplustults and BUMA-P1 as Typic Kandiustults9 both
corresponding to Haplic Cutanic Acrisols in WRB. To gain a better understanding on soil
nitrate supply and enhancement of sorghum yields, field experimentation was carried out at
Emalomba in the year 2012 and 2013 to assess the effects of nitrogen fertilizer rates and
soybean residue management options on nitrate nitrogen (NO3-N) under intercropping
system. Six soybean residue management options were considered: sole sorghum, sorghum
+ soybean left to maturity, sorghum + soybean mulched, sorghum + soybean incorporated,
sorghum + soybean exsitu and sorghum + soybean exsitu and plot tilled. Three levels of
nitrogen (0 kg N ha", 40 kg N ha', and 80 kg N ha") as urea were applied as top-dress and
treatments arranged in randomized complete block design. Nitrogen fertilizer application at
40 kg N ha'1 with respect to on-farm soybean residue management in relation to NO3-N
supply was in the order: sorghum + soybean left to maturity > sorghum + soybean mulched
sorghum + soybean incorporated. Experimental plots with legume residues removed had
significantly (P < 0.001) low NO3-N. Leaf NO3-N concentration increased with
progressive sampling periods with a decrease noted as the crop leaf senesced.
Sorghum intercropped with soybean left to maturity with nitrogen fertilizer applied at 40 kg
N ha'1 reflected high (P < 0.001) soil NO3-N in comparison to the other management
options. In the year 2013, a study was carried out in the previous experimental plots with
respective treatments to establish the contribution of soybean residues under varied soybean
residue management options and nitrogen fertilizer rates on nitrogen supply to the
subsequent sorghum crop and on yield. Soybean left to maturity at 40 kg N ha'1 indicated
significant (P < 0.001) increase of 56% soil NO3-N. At 40 kg N ha'1, treatments with
soybean residues mulched and incorporated indicated an increase in soil NO3-N but
were not significantly different and the concentration levels were lower than that of
soybean residues left to maturity. Treatments in which soybean residues were removed
had significantly (P < 0.001) lower soil NO3-N contribution. The control treatments and sole
sorghum indicated the lowest soil NO3-N accumulation irrespective of nitrogen fertilizer
rates. Application of nitrogen fertilizer at 40 kg N ha'1 to sorghum intercropped with
soybean left to maturity indicated 43% significant (P <0.001) increase in leaf NO3-N
accumulation while treatments with soybean residues mulched showed 39% increase and
residues incorporated, 25% at the same application N fertilizer rate,
Treatments with
soybean residues ex-situ and ex-situ and till indicated decrease (- 6% and -7%) in leaf NO3-
N accumulation respectively. Sole sorghum had a unifbrm NO3-N increase of 4%
irrespective of nitrogen fertilizer rates. Preceding insitu soybean residues and nitrogen
fertilizer application had no significant (P <0.05) influence on sorghum yield. Sorghum
intercropped with soybean left to maturity with nitrogen fertilizer applied at 40 kg N ha4 is
therefore recommended as a possible optimum rate and legume residue management option
to improve soil NO3-N among small scale households with stretched socio-economic status.
Further research on nitrogen related nutrient: sulphur, be carried out to optimize yields and
soybean residue N contribution to the soil.
Description
Dissertation
Keywords
Grain sorghum, Cereal-legume, sorghum production