Information required for using a P index varies. P indexes make numerous assumptions to reduce data requirements. This reduces the potential for accuracy, but poor estimates of difficult-to-obtain information are likely to have a more detrimental effect on accuracy. The most difficult and most important estimate is generally that of mean annual soil erosion and the amount of eroded sediments and nutrients that are likely to reach surface water. Erosion loss can be estimated with the USDA’s RUSLE2 which, itself, has a high data requirement. An alternative is to obtain an estimate from the local NRCS office.
Depending on the P index used, the following information may be needed for each field or management zone within a field. It may be obtained from the farm operator, records and reports and observation.
- The county in which the field is located and the predominant soil mapping unit.
- Soil test P (Bray-P1, Mehlich 3, or Olsen) from a soil test report with soil samples taken for the 0-8” depth.
- Application rates for P in fertilizer or organic materials, e.g., manure, compost and bio-solids. A P-analysis report is required for any organic P source.
- Time and methods of P application.
- Land use and crop rotation.
- Tillage practices.
- Width of vegetated buffer or filter strips, if present.
- Conservation practices, such as terrace systems and strip or contour cropping, if present.
- Irrigation practices, including furrow irrigation, typical flow rate, furrow slope, use of PAM, and the presence of a reuse pit.
- The average distance from the management zone to either concentrated water flow or to surface water.
- An estimate of mean annual erosion loss of soil for this field/management zone, which might be obtained from NRCS or by using the RUSLE 2 Program. Additional information may be needed to run RUSLE 2--field slope length and steepness, ridge height, ground cover and crop yield.
- An estimate of gully erosion may be needed. This also might be obtained from NRCS.