A crop plant does not take up equal amounts of nitrogen (N) and phosphorus (P) from the soil as it grows. In most crops, greater amounts of N are needed than P. Therefore, at the same application rate of N and P to soil, P will accumulate in the soil because the plant uses less P during development.
To make matters worse, manure does not contain equal amounts of N and P. Again N is usually higher than P. If the N:P ratio of manure was equal to the N:P ratio of crop uptake, we would only need one application rate for both nutrients. Unfortunately, the ratio of N:P in manure is not the same ratio as N:P needed in the crop. The amount of P contained in manure is usually higher than the amount of P needed by the crop.
Because the ratio of N:P in manure isn't equal to the N:P ratio of nutrients needed by the crop, we need to have separate spreading rates for manure depending on whether we are limiting manure to the P needed by the crop or are applying based on the N needed by the crop and allowing P to accumulate in the soil.
Author: Richard Meinert, Extension Educator, University of Connecticut
