Two factors affect this concern. First, current sources of granular fertilizer P are 100% water soluble. If runoff producing rain events occur soon after application the nutrients are exposed to runoff or via macro pore flow out tile systems which result in elevated losses. Second, in production systems with reduced soil disturbance the amount of P nutrient is disproportionately located in the upper soil profile through residue deposition and break down. Additionally, if surface applications of fertilizer are used the accumulate soil test value of P values at the surface are disproportional to the profile soil test values. This scenario results in highly soluble p without binding sites thus exposed to runoff events. Manure nutrient solubility is different than commercial fertilizer with organic bound and other less soluble P fractions. This tends to be a temporary situation and within a six week period manures tend to react similar to commercial fertilizer.
• Control rate- annual applications can reduce surface exposure of nutrients to loss
• Conduct a 2 inch soil test to assess nutrient levels in upper soil profile in comparison to 6-8 agronomic sample under reduced till conditions and surface applications have been used for extended periods.
• Through banded application equipment soil place nutrients 3-6 inches deep in profile to reduce exposure of nutrients to runoff risk.
• Raise DWM structures levels until sufficient rainfall to allow fertilizer/manure to solubilize and bind to soil so future runoff events will not results in elevated P discharges.