Using Quantity/Intensity Relationships to Determine Phosphorus Losses.

Anthropogenic eutrophication of surface waterways is major environmental concern as it leads to the decline of aquatic resources and degradation of water. Phosphorus is the most limiting nutrient in freshwater systems and therefore an important factor in the control of eutrophication. Identifying the natural P thresholds in soils using Quantity/Intensity relationships can help determine areas with a high risk of P loss to waterways. Forty-nine sites covering a range of land uses were sampled in the Potomac Watershed in the northeast United States by the USDA-NRCS. Four pedons described and sampled at each site were used in this analysis. Samples from horizons in the plow layer (0 to 30cm) were analyzed for Mehlich III and water extractable P. A threshold in the relationship between water extractable P and Mehlich III P was identified using segmented linear regression (change point analysis) for each land use type.  The change point relates to the P level in soil above which an increase in soil test P will lead to a large rise in P loss to waterways.