Agriculture is a major contributor of phosphorus (P), the primary cause of eutrophication in Wisconsin’s fresh water streams and lakes. Most Wisconsin agricultural land, however, probably contribute comparatively low levels of P to surface waters. Changing practices on fields with already low P loss is not an effective way to improve water quality. Wisconsin’s P Index is a planning tool that farmers can use to demonstrate when there is low risk of runoff P from their fields contaminating nearby waterways. Use of the P Index, rather than a set of required management practices, also allows producers to meet runoff P standards using management techniques that suit their operations.
The P Index estimates average annual runoff P delivery to surface water from a given field based on field conditions such as soil test P, manuring or P fertilization, tillage, crop rotation, and other common crop production and conservation practices. It integrates long term weather patterns and the Revised Universal Soil Loss Equation 2 (RUSLE2) into the calculations. If there is very little potential for erosion or runoff from a field, then the P Index can be relatively low even with comparatively high soil P and manure applications.
The P Index is calculated by the SnapPlus nutrient management planning software, which allows a farmer to assess the risk of runoff P loss based on their planned crop management, fertilizer, and manure applications. If farmers have crop rotations and tillage operations that meet the basic soil conservation standard (Tolerable soil loss or T), apply nutrients to UW recommendations, and follow the other requirements of a 590 nutrient management plan, they are usually already maintaining rotational average P Index values below six. If this is not the case, producers can quickly evaluate alternatives with the P Index to determine how modifying their crop management or manure and fertilizer application programs will lessen potential runoff P losses.
The basic framework for the Wisconsin P Index runoff P delivery calculations.