For corn and soybeans, field research has shown that fall application of phosphorus (P) and potassium (K) would be equally effective as a spring application. Traditionally, farmers apply P and K fertilizer in the fall when there is more time and equipment available. For winter wheat, all the P and K requirements need to be applied in the fall at planting.
Dry fertilizers can safely and quickly be applied in the fall. Some light tillage or fall cover crops may be helpful to ensure that nutrients stay where they were placed and reduce risks of P surface runoff. Even though K is not an environmental risk, fall K fertilizer application on sandy soils with low organic matter is not recommended because of potential leaching losses.
Michigan State University Extension 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 – 95-97 percent of maximum yield potential – is attained. Applying sufficient P and K to build toward the critical level and maintain nutrients at that level by applying yearly crop removal rates is the preferred management option. See the MSU Extension article “Nutrient removal rates in grain crops” to determine the crop removal rates of major Michigan grain crops. The four “rights” of nutrient stewardship –right rate, right amount, right time and right placement – should be used as a guide for fertilizer practices.
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 bushels per acre corn crop removes 56 pounds per acre P2O5 and 41 pounds per acre K20, while a 40 bushels per acre soybean crop removes 32 pounds per acre P2O5 and 56 pounds per acre K20. A combined two-year removal rate for a corn or soybean rotation is 88 pounds per acre P2O5 and 97 pounds per acre A K20.
The P and K content of manure applications should be taken into consideration, along with soil tests, to determine if and when more synthetic 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 to be banded at planting.
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. Michigan soil test data has shown that nearly 70 percent of Michigan farm fields contain adequate P levels. However, only 20-25 percent of fields contain adequate K. 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 that are most in need – below the critical level – and then allocate the remaining fertilizer to fields above the critical level.
Seasonality of fertilizer prices is another consideration. Purchasing fertilizer in the fall is generally cheaper compared to spring. The current trend in declining fertilizer prices is discussed in “Fertilizer prices continue to stumble.”
One drawback for fall P application is that the most common P fertilizers, diammonium phosphate (DAP) 18-46-0 and monoammonium phosphate (MAP) 11-52-0, have some nitrogen (N) in them. This N could readily convert to the nitrate form and lost from the root zone before being utilized by spring planted crops. Some of this N can be helpful to accelerate the decomposition of crop residues. Also, new granular fertilizer products are now available with multiple nutrients and flexibility in timing. Please consult with your fertilizer dealer about options available to you. For winter wheat, a small amount of N (25-30 pounds per acre) is recommended at fall planting to promote early vegetative growth.
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 shown that lime will improve nutrient availability and crop yield, and generate a good return for investment. Refer to MSU Extension Bulletin E1566, “Facts About Soil Acidity and Liming.” On rented land, the landowner and farmer should both agree on who will pay for liming, if needed. Correcting pH often resolves soil micronutrient deficiencies.