Elucidating the causes of low phosphorus levels in ferralsols of Siaya County, Western Kenya

Abstract

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.