Speciation and pH- and particle size-dependent solubility of phosphorus in tropical sandy soils

Growing demands for crops have expanded agricultural production to sandy soils with low available phosphorus (P) content and high P mobility. Knowledge of speciation solubility in such is poorly understood. Here we investigated: i) the clay size fraction soils, ii) pH-dependent soil orthophosphate (PO4–P), iii) particle size-dependent P. The studied loamy sand were organic matter (2.6–7.9 g kg−1) deficient (1.5–8.5 mg kg−1), kaolinite (21%) was only mineral identified using X-ray diffraction technique clay-sized particles. K-edge absorption near edge structure analysis revealed that associated Al phases, 45% adsorbed gibbsite 35% variscite (AlPO₄·2H₂O), dominated very PO4–P solubilities investigated bulk exhibited a pronounced positive pattern consistent Al3+ Fe3+. Geochemical modeling suggested maximum linked strengite (FePO₄·2H₂O), indicating controlled by Fe3+ as restricted phases Fe (hydr)oxides. fractions largest (1000–2000 µm), containing visible plant debris contributing least total mass (<0.126%), contained most stocks (50–60%). small factions (<250 µm) key PO4–P. Our data highlighted pivotal role Al/Fe (hydr)oxides controlling these tropical having limited contents risk loss.