Soil fertility status and moisture effects on fertilizer responses in pure and intercropped maize in selected ecozones of Kenya

Field fertilizer trials with pure maiz.e and maize I cowpea intercropping systems were conducted for a period of four years (1985 ¬1989) at four trial sites in some selected ecozones of Kenya. The aim was to derive site and crop specific fertilizer recommen¬dations in the different soil and climatic regions. This paper discusses how soil fer¬tility and water availability index influ¬enced response of maize yields to nitrogen and phosphate when planted as a sole and in mixed cropping with cowpeas. Nitrogen (N) and Phosphate (P) were tested in a fac¬torial experiment combining 0, 25, 50 and 75KgN/ha with 0,11,22, and 33 kgP Iha. Yield data for maize was collected from both cropping sequences and summarised in response equations. Maize control yields in both sole and intercrop increased with increase in moisture availability index that ranged between 40 and 60%. Intercropping lowered maize yields at all sites in Eastern Kenya due to strong competition for mois¬ture and nutrients where clayey rhodic Ferralsols predominate but increased maize yields at the coast site with sandy chromic Luvisols. These soils are deficient in both nitrogen and phosphate while organic mat¬ter is moderate to low. Chemical fertilizers improved maize yields in three out of four sites but was recommended in only two sites due to low returns to P application at Gachoka. Nitrogen is the only recommend¬ed nutrient where returns to every kg of nutrient was higher than lO.5Kg of maize per ha and good returns are obtained when nitrogen fertilizer is applied at 75 kgN Iha. Response to P application was insignificant and uneconomical. The single site at the Coast benefitted from intercropping and fertilization. Maize benefitted from cow peas in intercrop possibly due to Biological Nitrogen fixation (BNF) or reduced leach¬ing of N in the sandy texture due to a dense ground cover. Some studies could be done in the future to determine the extent of effective nodulation for cowpeas in these regions. At the three sites in Eastern Kenya, crop production can only be improved by irrigation or external water harvesting, while at the Coast, intercropping is a good alternative to nitrogen fertilization when cowpea straw is incorporated.