Phosphorus deficits by long-term organic farming?

Summary

Long term studies (1989- ) on an organic dairy farm show decreasing topsoil concentrations of extractable phosphorus (P-AL), and a low P surplus on farm level. The average P-AL value, 100 mg kg-1 dry soil is still above optimal (70 mg), but has decreased by more than 50% since 1989. Perennial ley yields tend to decline, and variations in soil P concentrations contribute to explain ley yields. This may call for additional P application on fields with low P status. However, for soils with medium (30-70) or low (< 30) P-AL, no relationship was found with yield level. Further, in a field experiment located on this farm, on low P soil with low and high applications of slurry over 5 years, applied P increased P concentrations in plant material rather than in soil. Future farm management should utilise locally available P sources to close the P gap by bringing back P removed in farm products, to avoid long-term P deficiency in soil.