Inhibitory Effect of Soil Micropores and Small Mesopores on Phosphate Extraction From Soils

Abstract

Slow release of added phosphate in soils is of fundamental importance for plant nutrition and pollution in aqueous environments. The diffusion of phosphate in micropores and mesopores after desorption from the inside surface of pores is the most likely mechanism for the slow release. There are limited experimental data on the effects of micropores and mesopores, and differences in the effects of these pores among various soil types have not been reported. Phosphate extractability was characterized using the Hedley fractionation method and sequential anion exchange membrane extraction for soils with widely different acid oxalate-extractable Al and Fe (Alo and Feo) contents (1.7-87 g kg-1 of Alo + 1/2Feo). We measured the total specific surface area (SSA), the SSA of micropores and mesopores, and the decrease in SSA after phosphate sorption. The proportion of labile phosphate relative to added phosphate and the rate of phosphate release in sequential extractions were negatively correlated with Alo and Feo and the porosity of the soils (the 0.7- to 4-nm pore SSA relative to the total SSA). The soil samples in which the 0.7- to 4-nm pore SSA decreased with added phosphate showed low proportions of labile phosphate and slow release rates. We demonstrated that phosphate release from soils with high Alo and Feo contents was more difficult because of phosphate sorption into micropores and small mesopores. Availability of fertilizer phosphate might be low in soils with a high porosity, but the effects of aging and competition with organic molecules should be further investigated.