Because of the great deal of uncertainty, a more accurate estimate of P and K removal can be obtained by sending silage samples to a laboratory for analysis. The Iowa State University Soil and Plant Analysis Laboratory and all private laboratories are equipped to analyze plant tissue for P, K and other nutrients. You need to be sure the sample is representative by making a composite sample from material taken from different silage loads.
No matter the method used to estimate silage P and K concentration, the amount of silage harvested per acre has the greatest impact on nutrient removal. Therefore, it is important to have a good estimate of silage harvested and the moisture content.
For P and K, even if estimates of removal with silage harvest are off somewhat, soil sampling and testing in the fall can help with longer-term management in drought-affected fields. In extreme cases where no or very limited crop is harvested, then P and K will recycle from plants and be available for the next crop.