The phosphorus saturation degree as a universal agronomic and environmental soil P test

Summary

This critical review argues that conventional soil phosphorus tests inadequately inform fertiliser recommendations that balance agronomic productivity with environmental and resource-conservation goals. The authors identify the oxalate extraction method and derived phosphorus saturation degree as a superior agri-environmental soil test, capable of quantifying both the readily available P reserve and the soil's maximum P sorption capacity. They conclude that agronomic target P levels should be contextualised by soil properties: lowered in soils with low sorption capacity to mitigate leaching risk, and lowered in high-capacity soils to conserve finite P reserves.

UK applicability

The framework is directly applicable to UK agricultural soil testing and fertiliser policy, as UK soils exhibit variable P sorption capacities depending on parent material and management history. Adoption of PSD-based recommendations could help UK policy align intensification with water quality objectives (nitrate and phosphate directives) and reduce reliance on imported phosphate rock.

Key measures

Oxalate extraction method; phosphorus saturation degree (PSD); amorphous iron- and aluminium-(hydr)oxide content; soil P sorption capacity; reversibly bound phosphorus pool

Outcomes reported

This review evaluates the phosphorus saturation degree (PSD) as a soil phosphorus test that measures both the pool of reversibly bound P and soil P sorption capacity. The study demonstrates how PSD can guide agronomic target P levels to balance crop yield, water quality protection, and judicious use of finite phosphorus reserves.

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