The P Index equations estimate long term average annual delivery based on field conditions and management for all of the years in a crop rotation. SnapPlus displays annual P Index values for each crop year. It also reports the annual Particulate P Index, accounting for the component of the runoff P losses attached to eroding sediment, and the annual Soluble P Index, accounting for dissolved P released to runoff from soil, manure, and fertilizer. The annual P Index values are averaged across the rotation for the rotation average P Index value.
Although the P Index delivery estimations are calculated in units of pounds P per acre per year, in SnapPlus the P Index is reported as a whole number without units. This is to avoid giving users a false sense that the P Index is an accurate prediction of annual P delivery from the whole field in any particular year. The P Index has to have a limited data entry requirement to be usable by farmers, and therefore contains assumptions designed to avoid under-estimating risk for the most vulnerable parts of the field. For example, the RUSLE2 erosion calculations use the characteristics of a single slope in the field. Following NRCS conservation planning guidelines, the slope used for soil loss calculations for nutrient management planning is one that represents the most erodible soil map unit (a combination of soil type and slope) that makes up at least 10% of the field. Another assumption designed to avoid under-estimating risk is the use of the most aggressive representative for each class of tillage tools in RUSLE2; for example, if the chosen tillage is chisel plow, a chisel plow with a twisted shank rather than a straight one is used for estimating erosion.
For cropland in Wisconsin, the Particulate P Index is often the largest component of the rotational P Index. It increases with the estimated erosion rate. Below are graphs that illustrate the P Index value by component for a Marathon county field in either a corn grain-soybean rotation or in continuous corn silage. The field has a soil test P of 47 ppm (the average for soil samples analyzed in Marathon County, 2005-2009) and 10,000 gallons per acre dairy slurry is fall-incorporated.
Continuous corn silage has a much higher estimated average soil loss rate on this field, 5.8 ton/acre/yr compared to 3.1 ton/acre/yr, and therefore has a proportionately higher Particulate P Index. Average annual runoff is also estimated to be higher with corn silage, resulting in higher estimated average dissolved P losses.
The proportion of the P Index that is Particulate and Soluble P will vary across soils, even under the same management. Fields with soils that are less erodible but with a higher runoff potential than the above example will have a higher proportion of the estimated losses as dissolved P in runoff. A comparison of P Index for different Wisconsin soils under the same management scenarios can be found at this link: http://wpindex.soils.wisc.edu/managing-runoff-p/site-effects/soilgraphs/.
While the P Index standard in nutrient management planning refers to the total P Index, looking at the particulate and soluble components of the P Index can be important for trying to figure out how best to reduce runoff P losses for individual fields. It also may be important for determining how best to target reductions to improve local water quality. The dissolved P represented by the Soluble P Index is in a form that is immediately available for algal uptake, while the particulate P that is carried with eroded sediments is unlikely to be completely converted to algal-available forms in a stream or lake. Therefore some water improvement projects may want to emphasize reducing the Soluble P Index.
Figure – Rotational P Index for two row crop rotations on a fall-chisel plowed Loyal silt loam field with 4% slope, 47 ppm soil test P, 2.5% organic matter, and 10,000 gallons fall-incorporated liquid dairy manure applied to corn. The P Index is shown by component: Particulate P I is P in eroded soil, Soluble P I soil is dissolved P from soil, Soluble P I manure is dissolved P from manure.