Adsorption and risk of phosphorus loss in soils in Amazonia

Abstract

Low availability of phosphorus in Amazonian soils is the result of both naturally occurring low levels and adsorption processes in the soil. These factors encourage the use of intensive phosphate fertilizations to increase productivity, which, in turn, increases P saturation and contamination risk. This study was designed to determine the adsorption capacity, as well as the risk of P loss and contamination of water bodies by P in Amazonian soils. Eleven soils located in zones free from any alteration were selected in order to set parameters for natural conditions for the local soils. Adsorption experiments were carried out in CaCl2 solution 0.01 mol L−1, containing 0 to 200 mg L−1 of P in the form of KH2PO4. The phosphorus sorption index (PSI) was determined, and soluble and available forms of P were extracted in water, Mehlich-1, Olsen, and CaCl2 solutions. The phosphorus saturation degree (PSD) was obtained by extraction with acid solution of ammonium oxalate. PSI was lower in the soil GL (Eutric Gleysol) and higher for NT (Rhodic Nitisol), while the maximum adsorption capacity (Smax) was lower in the soil AR (Ferralic Arenosol) and higher in AC1 (Haplic Acrisol 1). The PSD was higher in AT (Hortic Anthrosol) which also showed higher values of soluble and available P, while lower saturation values were observed in FR2 (Xanthic Ferralsol), FR4 (Rhodic Ferralsol), Haplic Acrisol 2, and Eutric Gleysol (< 5%). These soils presented low to medium maximum adsorption capacity and low P saturation; however, Ferralic Arenosol soil showed a high risk of P loss, making P management very difficult. Similarly, Hortic Anthrosol showed high susceptibility to P loss.