Soil Test Phosphorus Trends in the Chesapeake Bay Watershed in Pennsylvania.

Phosphorus from agriculture has been identified as an important water quality concern in the Chesapeake Bay.  This largely stems from the high intensity of animal agriculture in the Chesapeake Bay watershed, which has resulted in large imbalances of nutrients.  The animal density per acre in PA  has been steadily increasing.  As a result, the Mid-Atlantic Water Program indicates that there has been over 50 million pounds of P per year in excess of crop needs in Pennsylvania over recent years.  This raises questions about the impact this has had on soil P levels.  Soil test levels in PA have been averaging in the above optimum category (82 mg/kg Mehlich 3 P in 2009) for over 20 years.  This average has been increasing, but only by about 1 mg/kg/yr.  However, soil test P levels in PA are not uniform and reflect the uneven distribution of animal agriculture and consequently the potential variable impact of P on water quality across  the state.  It has been proposed by some that P application should not be allowed when a soil test  is above 200 mg/kg Mehlich 3 P.  On average in PA this would affect 7.2 % of the agronomic fields tested.  However this impact would be much larger in counties with substantial animal agriculture.  An alternative proposal is to restrict P on soil with greater than 20 % P saturation.  This would restrict P application on 6.8% of the fields tested. This is very similar to the 200 mg/kg  Mehlich 3 P cutoff because these two measurements are very highly correlated in PA.  While these soil P tests are correlated with areas of excess P and could be useful for guiding strategic decisions about where animal agriculture could be a having a negative impact on water quality from excess nutrients, these tests are not good by themselves to guide management on any given field.  A more comprehensive assessment of the risk of P loss that includes the major sources of P, including but not limited to soil P or P saturation, and the major transport mechanisms for P loss ie. the P Index approach.