Low available soil phosphorus (P) is one of the factors limiting crop production in western Kenya. Availability of soil P depends on amount of apatite, addition of P fertilizers, P fixing potential of a soil, soil water content and P removal by crops. Ferralsols are one of the major soil groups in Siaya County in western Kenya with low available P. We evaluated the possible causes of low available P in Ferralsols in the region. Composite soil samples (0-20 cm depth) were collected from various sites in Siaya County and analyzed for physical, chemical and mineralogical properties as well as P sorption characteristics to determine the cause(s) of low availability of P in these soils. The soils were weakly (pH-water 6.02) to moderately (pH-water 5.06) acidic and had aluminum (Al) saturation range of 1 to 69 g kg-1. There were variations in the amounts of Al2O3, Fe2O3 and MnO ranging from 19.9 to 26.5%, 10.77 to 21.36% and 0.09 to 1.24%, respectively, in the soils. Boro, Malanga, Ndere, Wagwer, Ukwala and Yala soils had moderate organic carbon (OC) ranging from 1.48 to 2 g kg-1, while Rangala, Sidindi, and Ugunja soils had low OC levels ranging between 0.5 and 1 g kg-1. Phosphorus sorption by the soils increased with increasing amounts of P concentration in the medium. The P-sorption isotherms of the soils were characterized by two maxima. Oxides of Al and Fe together with clay and free Fe3+ and Al3+ were the main factors responsible for P-fixation in the studied soils. Phosphorus fixation in these soils should be reduced through limimg and/or increasing organic matter content of the soils so as to increase P availability and thus crop productivity of the soils.
Key words: Clay, ferralsols, isotherm, phosphorus fixation, sesquioxides, soil profile.