Influence of Soybean Residue Management on Nitrate Nitrogen Accumulation and Subsequent Sorghum Yield in Kanhaplic Haplustults of Western Kenya

Crop residues are overlooked when making fertilizer recommendations, yet have potential to contribute to soil nitrogen in addition to mineral fertilizer use. A study was carried out in western Kenya to establish the contribution of soybean residues under varied management options and nitrogen fertilizer rates on nitrogen supply to subsequent sorghum crop. Six soybean residue management options were evaluated; 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-1, 40 kg N ha-1, and 80 kg N ha-1) as urea were applied as top-dress and treatments arranged in randomized complete block design. Soybean left to maturity at 40 kg N ha-1 indicated significant (P < 0.001) increase (56%) in soil NO3-N. Removal of soybean residues resulted in significantly (P < 0.001) lower soil NO3-N increase while control treatment and sole sorghum indicated the lowest soil NO3-N accumulation irrespective of nitrogen fertilizer rates. Soybean left to maturity indicated significant (P < 0.001) increase (43%) in leaf NO3-N accumulation at 40 kg N ha-1 while treatments with soybean residues mulched showed 39% increase and those with residues incorporated, 25% increase. Soybean residues ex-situ and ex-situ and till indicated a decrease (-6% and -7%) in leaf NO3-N accumulation, respectively. Sole sorghum had a uniform NO3-N increase of 4% irrespective of nitrogen fertilizer rates. It was observed that insitu soybean residues and nitrogen fertilizer application had no significant (P < 0.05) influence on sorghum yield. In conclusion, soil and leaf NO3-N accumulation by soybean residues in addition to nitrogen fertilizer does not translate to optimum nor potential research sorghum yields. There is need to research on sulphur and nitrogen to establish their interactive effects on sorghum yields.