Traditionally, farmers apply phosphorus (P) and potassium (K) fertilizer in the fall when there is more time and equipment available, and depending on the weather, soil compaction may be less of a concern. This year, with delayed planting and maturity, farmers will be preoccupied with fall harvesting operations. For corn and soybeans, field research has shown that spring application prior to planting would be equally effective. For winter wheat, the P and K requirements need to be applied at planting. Dry fertilizers can safely and quickly be applied in the fall. Some light tillage may be helpful to ensure that nutrients stay where they were placed and reduce risks of P surface runoff.
P and K fertilizer rates should be based on a reliable soil test, especially at a time of high input costs. Michigan State University recommendations utilize a build-up, maintenance and drawdown approach for P and K. With this approach, a critical soil test level has been established where the optimum yield (95 to 97 percent of maximum yield potential) is attained. Applying sufficient P and K to build toward the critical level and maintaining nutrients at that level by applying yearly crop removal rates is the preferred management option. Please refer to Extension publication E2904 for MSU fertilizer recommendations and crop removal rates.
Based on the soil test recommendation, P and K rates can be applied every year or every two years. If it is done every two years, then the application rate should be sufficient for both crops in the rotation. Overall, corn removes more P and less K than soybeans. A 150 bu/A corn crop removes 56 lbs/A P2O5 and 41 lbs/A K20, while a 40 bu/A soybean crop removes 32 lbs/A P2O5 and 56 lbs/A K20. A combined two-year removal rate for a corn and soybean rotation is 88 lbs/A P2O5 and 97 lb/A K20. Especially for P, this does not mean that the soil test levels drop by this much in two years. If you have a soil test level in the excessive range, P fertilizer will not be warranted. These soils take many years of cropping for soil test P to drop to maintenance levels.
The P and K content of manure applications should be taken into consideration along with soil tests to determine if and when more purchased fertilizer nutrients are required. On average 80 percent of the P and 100 percent of K in the manure will be available in the first year of application. A nutrient analysis of the manure will provide the proper nutrient credits. Further allowances to the fall application rates should be made accounting for the starter P and K fertilizer banded at planting. There has been a recent increase in the use of P and K as starter fertilizer to counteract slow growing conditions early.