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 soil properties governing critical Olsen P values—the phosphorus availability thresholds used to guide fertiliser recommendations. Whilst phosphorus availability was significantly affected by soil pH and metal (hydr)oxide contents, critical Olsen P values for ryegrass yield optimisation varied widely between soils and could not be reliably explained by measured soil properties alone. The findings argue for soil-specific rather than generic Olsen P-based phosphorus fertiliser recommendations to improve sustainability and reduce environmental impacts.

UK applicability

These findings are directly applicable to UK soil conditions and fertiliser advisory practice, as Olsen P testing is a standard diagnostic tool in British agriculture. The research suggests current generic Olsen P thresholds used in UK fertiliser recommendations should be refined with soil-specific calibrations to improve phosphorus use efficiency and reduce unnecessary applications.

Key measures

Olsen P values, phosphorus additions, ryegrass yield response, soil pH, manganese oxide content, crystalline aluminium oxide content, amorphous iron oxide content

Outcomes reported

The study measured the relationship between added phosphorus, resultant Olsen P values, and ryegrass yield response across 10 soils with contrasting properties. It identified which soil properties (pH, manganese oxide, crystalline aluminium oxide, amorphous iron oxide) affect phosphorus availability and critical Olsen P values, though critical values themselves could not be reliably predicted from measured soil properties.

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