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

Abstract

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