Now that information for the Source factors, Dissolved P Transport factors, and Particulate P Transport factors have been collected, a Dissolved P Index Score and a Particulate P Index Score can be calculated for the field.
The Dissolved P Index Score and a Particulate P Index Score are calculated similarly, as shown below.
To determine the risk of loss from the field, choose the greater of the two scores and categorize it as a Low, Medium, High, or Very High risk ranking.
Low Risk: If the field is in the Low risk category, then the risk of runoff and erosion losses is not great, so manure and fertilizer applications can be made to meet the nitrogen requirements of the crop.
Medium Risk: If the field falls in the Medium risk category, then manure and fertilizer can be applied to meet the nitrogen requirement of the crop, but best management practices (BMPs) should be employed to minimize losses. Such best BMPs could be filter strips, conservation tillage, cover crops, less erosive crop rotations, etc.
High Risk: If a field ranks in the High risk category, then manure and fertilizer applications should be limited to the amount of P that is removed by an average yield of the crop. For example, if a corn silage crop averages 18 tons/acre (35% DM) and corn silage contains 0.62% P2O5, how much P could be applied to the field?
18 tons/acre x 2000 lbs/ton x 0.35 x 0.0062 = 78 lbs P2O5/acre.
To find the P2O5 concentrations of other crops, visit the P Index User’s Manual.
Moving ahead, if a manure analysis has a P2O5 concentration of 5 lbs P2O5/ton, how many tons of manure could be applied to the field to meet the crop removal value?
(78 lbs P2O5/acre) / (5 lbs P2O5/ton) = 16 tons/acre.
Very High Risk: If a field ranks in the Very High risk category, then no additional P from manure and/or fertilizer should be applied. Instead, the crop should rely solely on P from the soil, thereby working to reduce the source of P over time. Fertilizers that do not contain P2O5, such as nitrogen or potassium fertilizers, may be used to support crop production. Why can’t manure be applied, similarly?
If we revisit the example watershed, including Field #2, we can see a how the overlap of high source and high transport ratings leads to an emphasis on tighter P management lower in the watershed (near the stream) and a balance of N management in areas where the risk of P loss is lower. Click on the watershed photo to illustrate this.