Assessment of soil fertility and crop yield changes under continues maize production in western Kenya

The per capita food production in the Kenyan highlands is on the decline due to continous cultivation with little or no external inputs, leaving the soils highly deficient nitrogen (N)and phosphorus (P. Recently, the use of improved fallows and minjigu rock phosphate (MRP)a cheap P source with long residue effects on acid soils has been promoted in western Kenya. However, the high rates recommended (250 kg P ha) for long time replenishment of soil p is prohibited for most small-scale farmers. This study was conduced in Busia district with the aim of evaluating changes in soil properties and maize yields over severalseasons o continuous maize production after a one-time application of MRP at low rates and incorporated fallow biomass. Minjingu rock phosphate was applied at O, 20, 40, and 60kg P ha' in the first season (2000 long rains (LR) while the fallow biomass was incorporated after the harvest of the first season crop. Maize was planted in the subsequent seasons with no fertilizer input, and samples soil were taken at the end of each season and analyzed for soil PH and available P. Results showed slight pH increases after the first season of MRP application. However in the subsequent seasons, significant decreases in PH were noted after incorporation of the fallow biomass alone but combination of fallow biomass in treatmnets with fallow biomass and residual MRP led to slight HP increases. This was attributed to the release of organic acids from the decomposing fallows resulting to dissolution of MRP and a subsequent inncrease in available P especially in treatment with 60 kg P ha MRP combined with 2t ha crotalia bimas. Nonetheless, after the third season (secong residual crop) available P decreased significantly due to the high P sorbing capacity of these soils. Significant grain yield increases occured in treatment with high P application rates (60 kg P ha) in all the seasons giving a cumulative yield increase of 5.4 t ha above the control over the five-season period. The yields increased progressivley up to the third season probably due to decline was noted from the fourth season probably due to decline in soil nutrient levels (aspecially available P.)Thus, at 60 kg P ha MRP, the residual benefits of MRP on available P and crop yields may persit up to two seasons, therefore, an addition of P fertilizer is necessary.