SOIL-PHOSPHORUS FRACTIONS AND ADSORPTION AS AFFECTED BY ORGANIC AND INORGANIC SOURCES

The effect of organic and inorganic sources of phosphorus (P) on soil P fractions and P adsorption was studied in a field without plant grow th on a Kandiudalf in western Kenya. A high-quality organic source, Ti thonia diversifolia (Hemsley) A. Gray leaves, and a low-quality source , maize (Zea mays L.) stover, were applied alone or in combination wit h triple superphosphate (TSP). The P rate was kept constant at 15 kg P ha(-1). Soil extractable P (resin, bicarbonate and sodium hydroxide), microbial biomass P and C and P adsorption isotherms were determined during 16 weeks after application of treatments. Application of tithon ia either alone or with TSP increased resin P,bicarbonate P, microbial P, and sodium hydroxide inorganic P. Tithonia alone reduced P adsorpt ion at 2-16 weeks. Maize stover had no effect on any of the P fraction s or P adsorption. At 8 weeks, the application of tithonia reduced mic robial C-to-P ratio (20) as compared to maize stover, TSP and the cont rol(31-34). The reduction in P adsorption by tithonia was accompanied by increases in all measured P fractions, the sum of P in those fracti ons (resin, bicarbonate and sodium hydroxide) being larger than the P added. The reduction in P adsorption apparently resulted from competit ion for adsorption sites, probably by organic anions produced during d ecomposition of the high quality tithonia. Integration of inorganic P (TSP) with organic materials had little added benefit compared to sole application of TSP, except that combination of tithonia with TSP incr eased microbial biomass, The results indicate that a high quality orga nic input can be comparable to or more effective than inorganic P in i ncreasing P availability in the soil.