Critical soil phosphorus values for yield reduction in intensive agricultural systems

Phosphorus (P) is an essential element for agricultural production. Over-fertilization during decades caused an accumulation of P in soils leading to eutrophication in regions characterized by intensive agriculture. These environmental concerns together with the non-renewability of P resources have led to a more sustainable P use. Knowledge about the P need of crops is essential for a sustainable agriculture thereby minimizing P losses to the environment without lowering the yield substantially. Therefore, in this study, critical soil P values for yield reduction (PCrit) were determined based on fertilizer trials conducted between 1970 and 1988 and more recent fertilizer trials (2016-2017). At rotational level a common PCrit value of 109 mg P/kg dry soil (in an ammonium lactate and acetate extract) was determined. Crop specific PCrit values were also determined for seven crops (potato, winter wheat, barley, rye, maize, sugar beet and temporary grassland). These critical values ranged from 59 mg P/kg dry soil to 164 mg P/kg dry soil with winter wheat the least and maize the most sensitive towards P deficiency. The diversity in PCrit values among crops can mainly be explained by the root intensity but also rooting depth, exudation of organic acids and phosphatases may influence the PCrit value. The soil pH also influenced the P availability significantly. Soils with a favorable pH had a significantly higher availability (i.e., lower PCrit value) for all crops compared to soils with a suboptimal pH. Critical soil P values might help to set up new or to evaluate current soil P in target zones used for P fertilizer recommendations.