Humic acid enhances phosphorus transport in soil and uptake by maize

Abstract Background Most of the phosphorus (P) applied to low fertility, acidic tropical soils with high iron and aluminum oxide contents end up adsorbed to soil colloids and not available to crops. Diffusion of P from fertilizers was found to be facilitated by coating with humic substances and by soil moisture. Aim However, there is still controversy on the effect of humic substances on soil P forms and diffusion, and there is a gap in the knowledge of how coating the fertilizer interacts with soil water. Methods We studied P diffusion and availability from a conventional single superphosphate and a humic acid‐coated superphosphate in Petry dishes as affected by soil moisture and the effect of the distance of the fertilizer to the roots using a root‐matt methodology. Results Coating single superphosphate with humic substances from leonardite (0.5%) and increasing water moisture resulted in improved P diffusion in the soil and plant P uptake. Maize dry matter increased by humic acid‐coated phosphate from 3% to 26%, depending on the distance from the fertilizer to the roots. Therefore, there is evidence of a lower P adsorption to soil colloids. The coated fertilizer increased labile P by 76% around the fertilizer granule, while higher levels of less labile P were observed with the use of conventional single superphosphate. Conclusion All this resulted in facilitated P transport from the soil to the roots, which is important mainly in double‐cropped systems where the second crop is usually exposed to temporary drought.