Long‐Term Phosphate Desorption Kinetics of an Acid Sandy Clay Soil from Mpumalanga, South Africa

Abstract

Growing input cost in extensive and intensive agriculture production systems, as well as growing concern about environmental pollution in intensive production systems (e.g., the growing cut flower industry in southern africa) will force farmers in developing countries to reevaluate conventional approach to fertilization programs. Phosphate (P) desorption characterisation of soils under cultivation, to determine the time frame of applied P release, can be used to optimize fertilizer P programs. The desorption kinetics of residual and applied P to an acid sandy clay soil were investigated over 56 days using hydrous ferric oxide in dialysis tubes as a specific P sink, followed by a sequential P fractionation. P desorption kinetics were described with a two‐component first‐order model. Assumptions made were that two discrete P “pools”, (a labile P pool [SP A ] and less labile P pool [SP B ]), participated in the desorption process and that P release from the two soil pools follow first‐order kinetics. Soil P desorption kinetics were described (R2=0.97 for the control and R2=0.95 and R2=0.99 for the P treatments of 75 (R75) and 150 mg kg−1 (R150), respectively) by the two‐component first‐order model. The P treatments reached no desorption maximum in the 56 days. P released from the control treatment in the 56 days was 10 times more than the Bray‐extractable P. Only 8% of P applied in R150 and 6% in R75 were recovered in the 56 days. P applied increased desorption rate of P from SP A and the contribution from SP A to total P released in the 56 days. P applied had less of an impact on desorption rate of P from SP B . However, the 150 mg kg−1 P treatment increased the half‐life of SP B of the studied soil with ±13.5 days.