Fall fertilizer practices

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.

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