Effect of Manure Application on Soil Nitrogen Availability to Intercropped Sorghum and Cowpea at Three Sites in Eastern Kenya

A trial was conducted where sorghum and cowpea were intercropped at three sites with different natural concentrations of soil phosphorus (1.0, 7.0 and 26.3 mg kg–1 Olsen-P) and with biannual cropping. There was a total of five field treatments, namely goat manure applied at rates of 0, 5 and 10 t ha–1 annually for 6 years and at 5 and 10 t ha–1 annually for 4 years followed by 2 years without manure. Continuous manuring at 10 t ha–1 created the same amount of soil N as 5 t ha–1 manure so the residual effects were the same for 5 and 10 t ha–1 manure and the effects were the same at all sites. In the season studied, between 1.8 and 4.1% of the native soil N was mineralized and taken up by the crops. Eleven percentage of the manure residual N (applied between 2 and 6 years previously and remaining in the soil) was taken up and this fraction was the same in all soils. Recently applied manure N contributed significantly to crop N in the most nutrient-deficient soil. On the soils with Olsen-P ≥ 7 mg kg–1, cowpeas obtained a significant extra 32 kg N ha–1, attributed to biological N fixation (BNF). If Olsen-P was < 6 mg kg–1, BNF was negligible.