Lime and Gypsum as Source Measures to Decrease Phosphorus Loss from Soils to Water

Murphy, Paul N. C.; Stevens, R. J.

Lime and Gypsum as Source Measures to Decrease Phosphorus Loss from Soils to Water

2010

Murphy, Paul N. C. | Stevens, R. J.

Phosphorus (P) loss from agricultural soils to water is a major contributor to eutrophication. In an incubation experiment with five contrasting soils, lime and gypsum showed potential as source measures to decrease P loss risk, as assessed by water extractions. Soils were incubated with lime to achieve a target pH of 6.5 and with gypsum at equivalent Ca rates for 108 days. P was added (17 kg P ha⁻¹) as KH₂PO₄ in solution. Gypsum appears to have greater potential as a source measure, decreasing molybdate-reactive P (RP) solubility by 14-56% and organic P (OP) by 10-53% across all soils. RP and Ca may have precipitated or co-sorbed, and OP may have been stabilised in organic matter complexes due to the bridging effect of Ca and the flocculating effect of increased ionic strength. Greater effectiveness of gypsum may be due to its higher solubility and the fact that it increases Ca concentrations without increasing pH. Lime decreased RP solubility in two soils (by 4% and 20%) but increased solubility in two and had no effect in a third. The overall effect of lime may depend on several competing effects of raised pH and Ca concentration that may vary in importance depending on soil characteristics such as base cation and P status. For the highest P status soil, both lime and gypsum were effective, decreasing RP solubility by 4% and 15%, respectively. Targeted treatment of high P soils may be an effective and economical strategy to minimise P losses.

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