Estimating Nitrogen Credit from Manure
Manure is a valuable source of nutrients offering agronomic and soil health value. Most manure nutrients (e.g., phosphorus) can be managed successfully with traditional soil analysis. However, nitrogen in manure requires some simple advance planning to insure that it is given proper credit for offsetting commercial fertilizer inputs.
Soil Samples Don’t Credit Manure N
Manure nitrogen comes in two forms, ammonium-N (or NH4+-N) originating from urine and organic-N originating from the feces. Soil samples typically only measure nitrate-N (NO3--N) although ammonium-N can be requested. Ammonium-N from manure will convert to nitrate-N in the spring as soils warm. Organic-N from manure is a slow release nitrogen that converts to crop available-N through the warmer summer months of the growing season. A late fall soil sample will completely miss any manure nitrogen and a spring soil test may only partially credit manure’s ammonium-N and not credit any of the organic-N.
Take Home Message
Don’t rely on your soil sampling program to credit nitrogen applied as manure.