Investigation of the soil properties that affect Olsen P critical values in different soil types and impact on P fertiliser recommendations

Summary

This pot experiment investigated how soil properties influence critical Olsen P values—the phosphorus soil-test threshold used to guide fertiliser recommendations—across 10 low-phosphorus soils. Whilst phosphorus availability was affected by soil pH and metal oxide contents, the critical Olsen P values for optimum ryegrass yield varied substantially between soils and could not be predicted from the soil properties measured. The findings indicate that one-size-fits-all phosphorus fertiliser recommendations based on Olsen P are inadequate, and soil-specific recommendations are needed for more sustainable and economically efficient phosphorus use.

UK applicability

The Olsen P test is widely used in United Kingdom agricultural advisory systems to guide phosphorus fertiliser recommendations. These findings suggest that current UK fertiliser guidance, which typically applies uniform critical Olsen P thresholds across soil types, may be suboptimal and that developing soil-specific recommendations could improve fertiliser efficiency and reduce environmental losses.

Key measures

Olsen P values, phosphorus additions, ryegrass yield response, soil pH, soil metal (hydr)oxide contents (manganese oxide, crystalline aluminium oxide, amorphous iron oxide)

Outcomes reported

The study measured relationships between added phosphorus, resulting Olsen P values, and ryegrass yield response across 10 soils with low available phosphorus. It identified soil properties (pH, manganese oxide, crystalline aluminium oxide, and amorphous iron oxide) that affect phosphorus availability but found that critical Olsen P values for optimum yield varied by soil and could not be fully explained by the measured soil properties.

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