Changes in inorganic phosphorus following incubation of some Zambian soils

Abstract

Phosphorus (P) deficiency is a major problem of many Zambian soils and annual applications of the recommended 60 kg P₂O₅ ha⁻¹ (26 kg⁻¹ ha⁻¹) do not seem to reverse this trend. In order to better understand the dynamics of P in some of these soils, a laboratory incubation study was done to determine the changes in inorganic soil P when fertilizer application is made. Two P rates, 0 and 13 mg P kg⁻¹ (0 and 60 kg P₂O₅ha⁻¹, respectively) were applied to one Alfisol (Chakunkula series) and two Ultisols (Misamfu Red and Mufulira series) which were incubated for 14, 28, 56 and 100 days. Extractable Bray-1 P, acid extractable Ca-P and non-occluded Al+Fe-P were determined. Addition of soluble P to these soils increased extractable Bray-1 P levels, although the contribution of organic P was important. As the time of contact between soil and fertilizer increased, extractable Bray-1 P levels decreased. The soil with more Fe and Al concentrations showed lower extractable Bray-1 P levels, indicating the probable role of these colloids in P retention. Following an initial decreasing trend in Al+Fe-P levels, this fraction increased at 100 days of incubation, pointing to this as the pathway for P transformation in the soils. Acid extractable Ca-P showed better correlations with extractable Bray-1 P, but the amounts of P in this fraction were generally lower than those of the non-occluded Al+Fe-P fraction. It was concluded that non-occluded Al+Fe-P is an important store for P in the soils included in this study, therefore the extractable Bray-1 P test method in use may need to be modified to better capture this pool.