Fall Fertilization Strategies

Minimizing yield-limitations caused by crop nutrient deficiencies is top of mind for soil testing and fertilizer decision makers every autumn. Collecting soil samples after harvest is a good place to start. There are several ways to collect soil samples, from random sampling (collecting multiple cores from different areas of the field and creating a composite) and grid sampling (based upon GPS location) to zone sampling (based upon delineated management zones). There are pros and cons to each method.

The soil test report is only as good as the sample provided to the lab for analysis, so no matter the approach — ensure soil samples are administered correctly. Collect samples to the same depth when creating a composite, ensure adequate sample volume to accurately represent average soil nutrient levels, and mix composites thoroughly before collecting samples and depositing to soil bags. These are critical first steps along the nutrient management decision chain.

After submitting the samples to a reputable lab and receiving the results, the next step in the decision chain is to determine which nutrients and what quantity should be applied to ensure that soil levels maximize productivity. The natural place to start is to evaluate the soil pH level. Make certain that soil pH falls within optimum ranges for the crops to be grown. In general, legumes require a slightly higher soil pH to maximize production than grasses. If your soil pH level is below the optimum range, your soil may require neutralization.

When applied to soil, lime neutralizes soil pH, but is among the most expensive soil applications to be made in a single production year. Since adding lime to the soil can be critical to maximizing crop yields next spring, the expense should be considered a good investment.

Soil pH readings only indicate whether or not lime is needed, but lab-conducted buffer pH or lime index tests will determine how much lime is required to optimize soil pH. These tests will also determine the quality and lime grind fineness appropriate for application to fields. University specialists and local experts familiar with lime quality are the best consultative resources for lime application.

Evaluating P And K Levels

Table 1
Grain and stover/straw nutrient removal rates for common agricultural crops.
Crop Grain*  Stover/Straw*
P2O5 removal   K2O
Soybeans 0.84   1.30  8.80    37.00
Barley 0.40   0.32  —    —
Canola 0.91   0.46  —    —
Cotton 14.00   19.00  —    —

After assessing lime needs, evaluating phosphorus (P) and potassium (K) levels is essential to determine the soil’s ability to supply nutrient levels throughout growing season.

P is required to ensure adequate energy for crops and is critical to the formation of genetic material as the crop grows and develops. K is essential for drought tolerance, multiple enzyme reactions and crop disease resistance. As different crops have different critical P and K soil test levels, lab results will also reveal information about where test results are relative to crop needs.

P and K behavior in soil is dramatically different than mobile nutrients like nitrogen (N) as they can actually be built up in most soils. While this tendency can result in fertilization for more than one crop year, it does not mean continual crop production without nutrient replenishment should be the norm. This is an unsustainable practice that can dramatically decrease crop productivity.

Significant depletion of P and K levels can also be quite costly when attempting to bring soil test levels back in line for crop production, as equivalent or greater levels of these nutrients will have to be applied to see a noticeable reserve accumulation in the soil.

Beyond Soil Testing

Other factors can also be monitored to determine how much fertilizer will be required to maintain P and K soil test levels. Crop removal can either be monitored in a site-specific manner with the use of a yield monitor, or by simply monitoring and logging load weights when delivering grain. All that is required is a crop yield estimate to provide insight into the amount of P and K removed from the soil.

Crop P and K removal can be estimated using crop yield and crop removal values from Table 1 above. For example, a 200-bushel corn crop, where only grain is removed, will remove 76 (200 x 0.38) and 54 (200 x 0.27) pounds of P2O5 and K2O per acre, respectively. Corn and wheat growers who remove stover and straw, respectively, must make sure an estimate of nutrient removal in the stover/straw is included when determining the total P and K removed. Estimates of stover and straw tonnage, as a function yield, are provided in Table 2.

Table 2
Stover/Straw yiuelds based upon grain yields and an assumed harvest index of 0.55 for corn and wheat.
Crop Grain yield,
Stover/Straw yield,
Corn 150 3.4
Corn 200 4.6
Wheat 60 1.5
Wheat 90 2.2

Fall N Application Considerations

N applications planned for this fall also require consideration. Soil temperatures should be below 50°F to minimize conversion of supplied ammonia to nitrate. The same guideline surrounding soil temperature also applies to the application of DAP or MAP. Recent research has shown a significant amount of the N supplied with MAP, DAP or ammonium sulfate may be unrecoverable in the next corn crop-based on soil temperature and rainfall pattern in early spring. Nitrification inhibitors are more likely to reduce N loss in fall-applied systems because fall N application has the highest N loss potential. The use of N in the fall should therefore be considered.

Remember — the key to optimizing crop production is ensuring adequate nutrient availability. Spending a little time evaluating soil test reports and determining crop nutrient removal when making fertilizer decisions will go a long way in ensuring favorable production for years to come.

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