By George Silva, Senior Educator

Mid-Michigan was adversely affected by early season drought this year. Corn yields in particular were reduced. Reduced yields have implications for fertilizer practices in the following year. Instinctively, farmers recognize that substantially reduced yield lead to less removal of nutrients from the soil. Estimating the nutrient carryover or credit in a drought year is particularly relevant to the two relatively immobile soil nutrients: phosphorus (P) and potassium (K). Nitrogen carryover, however, is not considered significant in Michigan because of potential leaching losses in fall and spring.

Two methods are available to estimate the P and K credits in a drought year. The first is soil testing. Routine soil testing remains the best and most scientifically sound approach to determine rates of nutrients needed. Soil tests can be used to measure what’s left in the soil and make adjustments to future recommendations. This method is useful for detecting carryover credit when large amounts of nutrients were applied preceding a drought.

It is necessary to point out that soil tests after a drought may produce some unexpected variances in K compared to a normal year. Shortage of rainfall will reduce the natural leaching of K from decomposing plant tissues and decrease soil test levels. Additionally, soil mineralogy can contribute to an increase or decrease in available K under drought conditions.

The second method is to estimate the nutrient credit based on a yield deficit formula when soil testing is not possible.

Example: A corn field had phosphate and potash applied based on a 180-bushel-per-acre yield goal. The drought-induced yield was 100 bushels per acre. Use the figures shown in the table to estimate the drought nutrient credit which is equal to: (Expected yield – Drought-induced yield) x nutrient removal per bushel harvested.

Nutrient Removal Rates in Pounds per Bushel of Grain Harvested
Crop Phosphate (P205) Potash (K20)
Corn 0.37 0.27
Soybean 0.80 1.40

Drought phosphate credit: (180 bu/A – 100 bu/A) X 0.37 lbs P205/bu = 30 lbs phosphate/A

Drought potash credit: (180 bu/A – 100 bu/A) X 0.27 lbs K20/bu = 22 lbs potash/A

Accordingly, the 2017 phosphate recommendation can be reduced by 30 pounds per acre and potash recommendation by 22 pounds per acre. This represents a potential savings of about $20 per acre in P and K fertilizer. It is necessary to point out that the nutrient removal rates used in this formula are really meant for normal, “unstressed” plants. The grain nutrient concentration of drought-stressed corn may differ from unstressed plants depending on the timing and severity of drought. This year, farmers encountered drought-induced K deficiency symptoms on corn, implying reduced K uptake. Therefore, some reports point out that tissue testing for grain P and K, and using those figures will improve the drought credit estimate.

Other fertilizers practices following a drought season are similar to normal seasons. On soils with optimum fertility levels, fall applications of P and K would be equally effective compared to a spring application. Dry fertilizers can be safely and quickly applied in the fall. Some incorporation is necessary to 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 incorporating 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.

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, about 50% of P and 100% of K in manure will be available in the first year of application.

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 where needed will improve nutrient availability and generate a good return for investment. Please refer to the Michigan State University Extension bulletin E1566, “Facts About Soil Acidity and Liming,” for additional information.

MSU 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. Previous soil test data indicate that K provides a higher return to investment compared to P fertilizer on most Michigan soils.