Effect of Sources of Phosphorus on Phosphorus Sorption in Cambisols and Ferralsols of Western Kenya

The influence of two organic materials (OMs); Tithonia and FYM and inorganic materials; triple superphosphate (TSP) and Minjingu phosphate rock (MPR) and Busumbu phosphate rock (BPR), when applied alone or in combination, on P sorption at two sites, Kakamega and Bukura, in western Kenya was investigated. The soil at Bukura was a Ferralsol while that at Kakamega was a Cambisol. A randomized complete block design with 3 replications was used. All the P inputs were uniformly broadcast on the plots and then incorporated into the top soil (0-15 cm) in April 2007. Soils were sampled at 4 and 9 weeks after incorporation of phosphorus inputs (WAI) from each of the treatments and the soil P sorption characteristics determined. The equilibrium P concentration which represents the amount of P in the equilibrium solution at zero P sorbed was < 0.2 mg P L-1regardless of the input combination used. The P sorption capacity of the soil at Kakamega was low compared to that at Bukura and was generally not affected by the treatments. None of the P sources significantly reduced the amount of P sorbed at Kakamega. However at Bukura, the amount of P sorbed (q) was significantly lower than the control for only Tithonia when applied alone and FYM applied in combination with TSP at 4 and 9 WAI. Tithonia applied with TSP significantly reduced q at 4 WAI but not 9 WAI while FYM applied alone was able to lower q at 9 WAI. None of the inorganic P sources, when applied alone, reduced the P sorption capacity of the soils. The results demonstrate that OMs have the potential to reduce P sorption in soils that are high in P sorption, but where the P sorption is relatively low, the application of OMs to reduce P sorption may not be useful.