Effects of Fertilizer Phosphorus and Crop Residues on Crop Biomass, Soil Carbon and Phosphorus in a Ten year Field Trial in Semi-Arid Kenya.

Abstract

The possibility of improving soil fertility with a “recapitalization" dose of phosphorus (P) fertilizer in conjunction with crop residues was assessed in a severely P-deficient soil. The experiment comprised three treatments cropped to sorghum (sorghum bicolor): (i) crop residues returned to the plot, (ii) residues returned plus additional residues, (iii) residues removed and added to treatment (ii); and bare and grass fallows. All treatments were repeated with and without an initial triple superphosphate application of 250 kg Plha. After 17 (8.5 years), soil organic carbon (SOC) increased significantly under grass and declined under sorghum and bare fallow. Residue transfer treatments gave no significant effect on sorghum biomass or extractable (Olsen) P. Removal of residues caused decline in SOC. P fertilizer increased crop biomass only in five of the first eight seasons and SOC was unaffected. In the 18th to 20th seasons, sorghum was replaced by maize (zea mays) in an N x P factorial design within each of the initial P treatments. Biomass increased only if both N and fresh P were supplied and initial P had no effect. P recapitalization was inefficient because P recovery was low, so a "little and often" regime for fertilizer P appears better.