By George Silva, Senior Educator
Farmers usually apply phosphorus (P) and potassium (K) fertilizer and lime in the fall when there is more time and equipment available and soil compaction is less of a concern. This simplifies spring operations and streamlines planting. On soils with optimum fertility levels, field research has shown that fall applications of P and K would be equally effective compared to a spring application prior to corn and soybean planting. For winter wheat, all the P and K requirements are best applied at fall planting.
Dry fertilizers can be safely and quickly applied in the fall. Some tillage will help ensure nutrients are placed below the soil surface. This will help reduce stratification and lower the concentration of dissolved P in the runoff water. This practice is particularly important in areas close to rivers, drainage ditches and tile inlets. Runoff events are more frequent in late fall, winter and early spring, so incorporation of fall fertilizer is environmentally desirable. Although K is not an environmental risk, fall K fertilizer application on sandy soils with low cation exchange capacities is not recommended because of potential leaching losses.
The two most commonly available dry P fertilizers, diammonium phosphate (DAP) 18-46-0 and monoammonium phosphate (MAP) 11-52-0, contain some nitrogen (N). This N, although small in quantity, will readily convert to the nitrate form and potentially be lost to the environment before being utilized by corn and soybeans in the following year. Some will debate that this N will help to accelerate the decomposition of surface residues. For wheat, a small amount of N (25 -40 pounds per acre) at fall planting is beneficial to early development.
Fall P rates should be based on a reliable soil test and realistic yield goal. Michigan State University Extension P and K fertilizer recommendations utilize a build-up, maintenance and drawdown approach. The MSU Extension bulletin E2504, “Nutrient Recommendations for Field Crops in Michigan,” provides additional information on this approach and environmental risks associated with P application.
P and K content of fall-applied manure should be taken into consideration to determine if and when more synthetic fertilizer is required. On average, 80% of P and 100% of K in manure will be available in the first year of application. 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 high and resources are tight, a short-term strategy would be to apply only the crop removal rates. This temporary approach will provide adequate nutrients for near optimum production at a lower cost.
The soil test should indicate the soil pH and if lime is needed to rectify the acidity. Fall offers the best opportunity to apply lime as it provides more time to neutralize soil acidity. Long-term experiments in Michigan have shown that liming will improve nutrient availability and generate a good return for investment. Refer to the MSU Extension bulletin E1566, “Facts About Soil Acidity and Liming,” for additional information.