Amending soils of different pH to decrease phosphorus losses

Context: Soils irrigated with wastewater are generally phosphorus (P)-enriched. P losses from these soils may impair surface water quality. However, wastewater applications also alter soil pH and P availability. Aims: We investigated if amending soils with aluminium (Al), iron (Fe) or calcium (Ca) sorbents could decrease the potential for P losses despite altering soil pH and potentially increasing soil P availability. Methods: Seven soils (pH 5.3–6.9) were incubated with lime, gypsum, hydrotalcite, alum sulfate, ferric sulfate, and ferric chloride at rates of 0:1, 0.25:1, 0.5:1 and 1:1 molar ratios of Al/Fe to P, and 0:1, 0.5:1, 1:1 and 5:3 for Ca to P, respectively. After 21 days pH and water extractable P (WEP) were measured. Key results: In most cases the application of Al, Fe and Ca amendments decreased WEP in proportion to the rates applied. However, poor performance was noted when amendments were mismatched to soils altering their pH into the range where high soil P availability was expected. Of the amendments used, alum and iron sulfate were the most cost-efficient. However, even when optimised and applied to critical source areas the estimated cost-effectiveness of these amendments is still poor and may only be effective in the short term. Conclusions and implications: We therefore recommend that other strategies such as inversion tillage bringing low P topsoil to the surface (and decreasing the potential for P loss by surface runoff) together with changes in the farm system to extract more P from the topsoil are the only strategies that will decrease the potential for P loss cost-effectively and in the long-term.