Linking phosphorus sorption and magnetic susceptibility in clays and tropical soils

Abstract

Maghemite (Mh) and magnetic susceptibility have been little studied in relation to phosphorus (P) sorption, despite the fact that tropical soils – particularly those derived from mafic rocks – may contain substantial amounts of this iron oxide. In this work, we investigated the relationship between P adsorption and magnetic susceptibility in tropical soils, and determined the maximum adsorption capacity of P (MACP) and P desorption in seven pedogenic clays from magnetic soils with contrasting parent materials and three synthetic Mh samples. Considering the heterogeneity of the soil dataset in this study, the exclusive adoption of magnetic susceptibility as an indicator of P adsorption potential in soil remains uncertain. The relationship between magnetic susceptibility and adsorbed P was more evident in the B horizon of red soils from basic igneous rocks. In this group, soils with magnetic susceptibility above 20 × 10−6 m3 kg−1 had high adsorbed P. Although the pedogenic clays exhibited lower MACP values (1353–2570 mg kg–1) than the synthetic Mh samples (3786–4321 mg kg–1), P desorption exhibited the opposite trend (~14% vs ~8%). The substantial P adsorption capacity of synthetic Mh confirmed the adsorption data for pedogenic clays, which were strongly influenced by magnetic susceptibility, Mh and gibbsite contents, and specific surface area.