Organic matter increases availability of phosphate in tropical soils: a phosphate diffusion experiment

It is well established that the application of organic amendments (OA) to weathered soils increases the availability of soil or fertilizer phosphorus (P). It is often assumed that organic anions derived from OA compete with P for sorption sites on iron and aluminium oxides. Alternatively, the short-term effect of OA amendment on raising the pH has also been proposed as a mechanism. These potential mechanisms have never been tested in a P diffusion experiment that indicates the P mobility. In this study, a P diffusion experiment was set up with a Vietnamese Ferralsol treated with compost, lime, or a combination of both, in addition to an untreated control. Phosphorus adsorption isotherms were constructed for each treatment. Soils were incubated in Petri dishes with triple superphosphate (TSP) fertilizer granule placed at the center, while an additional compost treatment was incubated without TSP. Phosphorus diffusion was visualized at 1, 5, and 14 weeks using the diffusive gradient in thin films (DGT) technique; the P concentration in the DGT was visualized via colorimetry. Concentric soil sampling was performed after DGT deployment to measure P concentrations at increasing distances from the fertilizer granule. Results confirmed a strong positive effect of OA application on the availability of added P in time and distance. At 5 and 14 weeks, the +OA +TSP treatments increased soil P concentrations compared to -OA +TSP treatments up to twofold near the fertilizer granule (< 0.8 cm) and up to ninefold at distances of 0.8–1.6 cm. In the control and limed treatments, P concentrations showed a consistent downward trend in time. In contrast, they first increased (week 5) and then slightly decreased (week 14) in the OA and Lime+OA treatments, but always remained above the threshold value of 0.2 mg P/L necessary for plant growth. Soil 0.01 M CaCl₂ extractable P concentrations in the different treatments increased (Control < Lime < Lime+OA < OA) with decreasing solid-liquid distribution coefficients (K_D) recorded in  the P adsorption isotherms, indicating that reduced P sorption was the mechanism for increased P availability. The OA treatment incubated without TSP showed no detectable P at any time point, confirming that the OA itself was not a direct P source. Additionally, the increased P availability in compost-amended soils was not attributable to changes in soil pH, as all treatments raised the pH by 1.5 ± 0.5 units. We conclude that OAs enhance P availability in weathered soils primarily by modifying P sorption  rather than directly supplying P or altering soil pH. This mechanism underscores the potential of OAs to improve fertilizer P use efficiency and sustain crop production in nutrient-poor soils.