Revising the Maryland P Index: Current Statewide Distribution of Field Characteristics.

The Water Quality Improvement Act was passed in 1998 in Maryland, mandating nitrogen (N) and phosphorus (P) based nutrient management planning on nearly all Maryland farms. The Phosphorus Site Index (PSI) is the tool currently used by nutrient management planners to determine risk of P loss on a field-by-field basis. While the PSI was tested and determined to be a useful tool in determining P loss potential, research has acknowledged a need to further test and refine the index to more accurately reflect P loss on the variety of field conditions which exist in Maryland. In the current study, 391 fields were analyzed, inclusive of all counties in Maryland, to test how the PSI functions based on the unique characteristics. For the purpose of this research, Maryland counties were divided into 5 regions: Mountain, Piedmont, Southern, Upper Shore, and Lower Shore. Slope, slope length, buffer vegetation type, and buffer width were observed on site. Online resources, such as mapping programs and soil survey data, and nutrient management plans were used to research other variables. Soil was collected and analyzed from every field site. Each region has distinct concerns with regard to P; especially considering slope, distance to surface water, soil type, depth to groundwater, and permeability. For example, the mountain region has a mean slope of 7.8 and a median slope of 7.0, while the lower shore has a mean slope of 0.1 and a median slope of 0. However, the lower shore has a mean depth to ground water of 7.9 feet and median of 0 feet, while the mountain region contrasts with a mean depth of 45.4 feet and median of 1.7 feet. Understanding the differences between field characteristics by region is critical in revisions to the P index to increase accuracy of the P loss potential.