Phosphate sorption isotherms of selected soils of Kenya and maize response to phosphate fertilizer in the greenhouse

Abstract

Phosphate sorption characteristics of eight selected soils of Kenya were determiner order to establish the influence of previous manure and phosphate (P) fertilizer applications. The soils were two ando-humic Nitisols, orthic acrisol, humic Nirisol, dystromollic Nitisol, mollic Nitisol and a ferralo-chromic Acrisol. A greenhouse experiment with maize was performed to determine the external P requirement for maximum dry matter yield (DMY) of eight-week old plants. Phosphate sorption data were generated by equilibrating soil samples at room temperature with O.OlM KCI solutions containing various concentrations ofP (0.05 to 3.0 mg P/kg soil). P sorbed was taken as the difference between the initial P concentration and the final P concentration in the equilibrium solution. Sorption curves were plotted using: h,.· data obtained. The ando-humic Nitisols gave the steepest curves and the highest Standard Phosphate Requirement (P required to raise the soil to 0.2 mg PI kg soil). They had a Standard Phosphate Requirement (SPR) of 380 mg P/kg soil (760 kg P/ha) and 340 mg P/kg soil (680 kg/ha). The lowest sorbing soil was the orthic acrisol with zero SPR. Phosphate characteristics of the soils were mainly influenced by soil pH (r = 0.619***), organic carbon (r = 0.581 ***), clay content (r = 0.438*), Base Saturation (r = 0.596***) and iron and aluminium extracted by both dithionite citrate bicarbonate (Fed' Aid) and ammonium oxalate (Fe., Alo)' Phosphate sorption was positively correlated to the Fe and .'.j contents: AIJ (r = 0.822***), Alo (r = 0.794***), FeJ (r =:: 0.740***) and Fe, (r = 0.512"'*). *, ** and *** means significant at 0.05, 0.01 and 0.001 level respectively. viii Manure increased P sorption in four soils and reduced in the other four. Effect of manure on P sorption was indirectly through its influence on the Fe and Al levels. Manure could probably increase or decrease the Fe and Al levels and hence reduce or increase P sorption respectively. Phosphate application reduced P sorption through its effect on the soil sorption complex by narrowing the ratio ofR20):P20s (where R stands for Fe or AI) which led to reduced P sorption. Results from the greenhouse showed that maize dry matter yields increased to a maximum (42.17 to 53.3 g/pot) with increasing P concentration up to 0.6 mg P/kg soil after which the yields started decreasing except in the andosol where the maximum was not achieved with the P levels tested. For soils tested in.this study, maximum dry matter yield can be obtained by fertilizing the soils to 0.6 mg P/kg soil equilibrium solution P level though the amounts of P required will depend on the phosphate buffering capacity of the soils. Phosphate sorption of the soils may be reduced by application of manure but increased sorption may result in some soils and, therefore, a need for further research is necessary. Phosphate applied in previous applications is useful in that it reduces subsequent P sorption and hence P requirement.