Management Practices to Reduce P Index Values
In order to understand what strategies might reduce P Index values for a particular field, SNAP-Plus users should click on the details box on the cropping page for that field. The values for four components of the annual PI will appear under each crop year column. These components are Particulate PI, Soluble PI, Acute Loss (unfrozen) PI, and Acute Loss (frozen) PI. Planners can identify which components are high during each year of the rotation and design management strategies accordingly.
Click on the factors in each of the component equations below to learn about management strategies to lower that factor in the PI.
Particulate PI: Strategies to lower this component involve either reducing the amount of eroded soil leaving the field or reducing the P content in that soil.
Particulate PI = Soil loss from field x Eroded soil P concentration
Soluble PI: Strategies to lower this component decrease either the amount of runoff water leaving the field or the dissolved P content of the runoff. Dissolved P concentration depends on P concentration in the soil.
Soluble PI = Field runoff volume x Runoff dissolved P concentration
Acute loss (unfrozen) PI: Strategies to lower the non-winter acute loss index decrease the rate of unincorporated P applications, and the risk that unincorporated manure or fertilizer will run off.
Acute loss PI (non-winter) = Manure/fertilizer P applied to soil x Fraction at risk of loss in runoff
Acute loss (frozen) PI: Strategies to lower the acute loss index for manure P applications to frozen soil decrease the total P application rate and the risk that the manure P will run off during snowmelt or winter rains.
Acute loss P Index for frozen soils = Manure P applied x Fraction at risk of loss in snowmelt/winter rainfall runoff
Important Note: The P Index calculations assume that producers are already following all of the appropriate practices for minimizing entry of nutrients to surface and groundwater in the NRCS 590 Nutrient Management Standard, such as establishing grassed waterways in areas with recurring gullies or avoiding manure and fertilizer applications on concentrated flow channels. Adopting these required practices will not lower P Index values because the equations assume they are already in place.
