Threshold for labile phosphate in a sandy acid sulfate soil

Abstract

In acid soils, plant availability of fertiliser P is affected by phosphate binding to Fe or Al oxide minerals. However, little is known about the P pools in acid sulfate soils after phosphate fertilisation, or the capacity of the oxyhydroxy sulfate mineral jarosite (KFe3(OH)6(SO4)2) found in these soils, to bind or release added phosphate. Sulfuric acid sulfate soil (initial pH 3.5, adjusted to 5.5) was amended with phosphate at 0, 96, 385, 578 and 770 mg P kg−1 and incubated under submerged conditions. Soil phosphate pools (labile phosphate, moderately labile phosphate, non-labile phosphate and residual phosphate) and amorphous Fe/Al were measured after two and four weeks. Phosphate sorption and its release were determined in incubated soils and jarosite, separately. In soil, maximum phosphate sorption was about 350 mg phosphate kg−1. Labile phosphate was higher than the control (without phosphate addition) at phosphate rates above 350 mg P kg−1 and was more than 50% of added phosphate. The length of submergence had little effect on phosphate pools. Below the threshold of 350 mg phosphate kg−1, absorbed phosphate was strongly bound to the soil whereas 50% of sorbed phosphate was released above the threshold. The high phosphate binding capacity of jarosite (maximal 360 mg phosphate kg−1) confirmed the importance of Fe oxyhydroxy sulfate minerals in P binding in acid soils. However, at maximal phosphate binding, up to 20% of bound phosphate could be released, suggesting that phosphate bound to jarosite may become plant available. The results of this study will help inform fertiliser requirements for crop growth or remediation of acid sulfate soils using plants.