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.
On short-term rented land having low to average test levels, it may not always be economically justified to apply P and K fertilizer at the buildup rates. If fertilizer prices are very high and resources are tight, a short-term strategy would be to apply only the crop removal rates that will provide adequate nutrients for near optimum crop production at less cost. Michigan soil test data has shown that nearly 70 percent of Michigan farm fields contain adequate P levels. However, only 20 to 25 percent of fields contain adequate K levels. Therefore, K may provide a higher return to investment compared to P fertilizer. Another economic consideration is to apply P and K first to fields most in need (below the critical level) and then allocate the remaining fertilizer to fields above the critical level. On fields that have the same soil test levels, applying fertilizer at 70 percent of the recommended rate on all acreage will provide a higher economic return than applying the full rate on 70 percent of acres and none on 30 percent of the acres.
One drawback for fall P application is that the most commonly available P fertilizers are diammonium phosphate (DAP) 18-46-0 and monoammonium phosphate (MAP) 11-52-0. The nitrogen (N) in these products could readily convert to the nitrate form in the soil. At high P application rates, most of the N could be lost to the environment before being utilized by crops. A small amount of N (25 to 30 lb/A) applied to wheat at planting, however, can be beneficial to early development.
Unlike P and K, heavy N fertilizer applications are not recommended in the fall due to potential losses to the environment. If, however, fall applications are needed, anhydrous ammonia is the preferred form to apply when the soil temperature falls below 50°F. The use of a nitrification inhibitor applied with anhydrous ammonia would also help to reduce N losses.
Research conducted at the University of Wisconsin has shown that there is no benefit in applying fertilizer N to enhance the breakdown of corn residue. Even though strong cornstalks of the new hybrids have become a management issue, fall N applications were ineffective in changing the C:N ratio and accelerate microbial decomposition.
The soil test should indicate if lime is needed to rectify soil pH. Fall offers the best opportunity to apply lime. It provides more time for lime to neutralize soil acidity. Long-term experiments in Michigan have revealed that lime (when applied according to a soil test recommendation) will improve nutrient availability and crop yield, generating a good return for investment. (Refer to Extension bulletin E1566 Facts about Soil Acidity and Liming.) On rented land, both the landowner and farmer should share the cost of liming. Correcting pH often solves soil micronutrient needs.
The author wishes to thank Natalie Rector, Marilyn Thelen and Steve Wagner for reviewing the content.