Plants need no phosphorus fertilizer above environmentally critical values

Optimizing phosphorus (P) application to agricultural soils is fundamental to crop production and water quality protection. Soil P tests and P sorption characteristics describing crop yield response to P application can also be used in predicting environmentally critical soil P status.

In an article recently published in the Journal of Environmental Quality, Norwegian researchers report on P response to P application in barley grown in pot experiments with 45 soils of different P status. The soil P status was measured by four soil P analyses, and the P sorption properties were determined by three methods.

All soil P analyses and two of the P sorption characteristics showed a nonlinear and significant relationship with yield response to P application and manifested a threshold value above which no P response was observed. Readily releasable P in soil reflects the risk of P loss to runoff and also had a nonlinear relationship to soil P analyses and P sorption characteristics. The threshold for yield response coincided with the environmentally critical values determined from the degree of P saturation.

The results support the conclusion that soil P levels for which no P application is needed also have an elevated risk of P loss to runoff.