The relationship between amount of P in each site source and in runoff water is determined by studies using soil types similar to that of the field being evaluated. Once the evaluation is done, the ratings for the different source characteristics are summed to produce a single rating value for site source characteristics. Site Transport Characteristics. The potential for phosphorous to move offsite is also affected by the amount of water that is likely to flow from the site and the amount of material that is likely to go with it. A P Index, therefore, will evaluate such things as surface runoff class, yearly soil erosion, and leaching potential.
Surface runoff can carry solubilized P directly into surface waters.
Eroded soil may carry bound phosphorous which may be released after entering surface waters. Leaching potential involves the susceptibility of a soil to percolation of water carrying dissolved P down into ground water which could then move to surface water bodies. The ratings for site transport characteristics are summed to give a composite rating value for the group. Best Management Practices. Actions can be taken by a farmer to reduce the likelihood of phosphorous being removed from a site, for instance, maintenance of buffer strips between application sites and surface waters, incorporation of manure into soil, or manure application when there is minimal probability of surface runoff.