Phosphorous is generally deficient in highly weathered soils of western Kenya highlands because of their high P sorbing capacity. Unfortunately, though organic residues have been found to reduce P fixation, most studies on this subject have been carried under controlled conditions. The objective of this study was therefore to compare under greenhouse (incubation) and field conditions the effect of combining organic residues with Minjingu rock phosphate (MRP) on soil pH, available P, sorbed P and asseS'S the residual impact of MRP on maize yield in Busia district for five consecutive maize cropping seasons, Treatments consisted of Minjingu rock phosphate (MRP) at 0, 30 and 60 Kg Pha-1 in combination with chicken manure (2t ha'l), farmyard manure (land 2t ha'l) maize stover (land 2t ha'I) and sugar baggasse (1 t ha'l) which were applied during the first season only. MRP rates were selected based on the past work in the same area that rates between 20-30 Kg P ha.1 gave optimum yields while double this rate was meant to check the additive effects. Minjingu rock phosphate applied alone significantly (P0.05) increased pH from 4th to 12th week of incubation with interaction effect between the organic materials with MRP occurring at 4th (1'