Nutrient availability is subject to biological, geological, and chemical processes.
Specifically, organic P depletion was observed under long-term row cropping with conventional tillage and organic P restoration was observed under continuous alfalfa. Robertson, G. P. 1998. Field crop ecosystems. Pages 1-16 in M. A. Cavigelli, S. R. Deming, L. K. Probyn, and R. R. Harwood, eds. Managing Biological Processes for Productivity and Environmental Quality .