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Soil Fertility and Nutrient Management Considerations Following a Drought

Categories: GROWING, CORN, SOYBEANS
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  • Drought conditions can influence soil properties. Soil testing can help to accurately estimate nutrient availability.
  • For every 1 bushel of corn produced, 0.38 lbs. of phosphorus (P2O5) and 0.27 lbs. of potassium (K2O) are removed.
  • Drought can reduce nitrogen (N) and sulfur (S) mineralization and plant availability, potentially influencing availability the following year.
Alt text: Figure 1. Drought-stressed areas affecting the Midwest on Sept. 28, 2021
Figure 1. Drought-stressed areas affecting the Midwest on Sept. 28, 2021
Source: https://droughtmonitor.unl.edu/

Droughts and Soil Fertility

Many areas of the Midwest experienced periods of drought throughout the 2021 growing season (Figure 1). Drought conditions not only influence plant growth and crop performance but can also influence soil properties and nutrient availability. As farmers consider their fertility plan for the coming year, increased fertilizer prices can also be a cause for concern or even require farmers to adjust their plans. There are a few methods to monitor the soil-water interactions to make the best fertility recommendations for the coming year.

Alt text: Figure 2. Historical Nitrogen mineralization rates in north central IA from 1984-2021
Figure 2. Nitrogen mineralization rates in north central IA
Source: Iowa State University FACTS, https://crops.extension.iastate.edu/facts/soil-n-mineralization

Drought Impact on Soil Mineralization and Nutrient Interactions

Drought conditions reduce soil mineralization, decreasing the availability of N and S from organic matter. This predominately influences the current crop. Figure 2 shows the average rate of mineralization (black line) in north central IA in 2021, and the drop in the red line correlates to the lack of precipitation during that time. Under these dry conditions, plant nutrient uptake is also reduced for N and S since they translocate via water.

Without water, N and S leaching is reduced, potentially improving their availability for the next crop. As a result, soil nitrates accumulate in soils in drought conditions. These can remain until the following year, but heavy rainfall in the spring can cause leaching and make the N unavailable to the new crop.

Immobile soil nutrients, such as phosphorus (P) and potassium (K), do not leach under wet soil conditions and are predominately removed from the soil through soil erosion and plant uptake. Both of these nutrient loss mechanisms are often reduced in drought conditions, potentially resulting in above average K and P availability the following year. The exception to this can be soil availability from windblown sediment if there is not sufficient residue cover, such as from heavy tillage management. These water-nutrient interactions can be complex and unpredictable, but there are some management solutions.

Crop Nutrient Removal Considerations

The amount of nutrient uptake under drought conditions can vary based on the timing and the severity of the drought. If dry weather occurs early in the season or for only a short period of time, there may be no negative impact on yield. In this case, nutrient uptake may be consistent with previous years. In a severe drought, crop growth may be stunted and productivity reduced. In these scenarios, the crop likely extracted fewer nutrients from the soil, meaning those nutrients may then be available for the following year.

Using the yield of the field, the amount of nutrients removed from the soil can be estimated. For every 1 bushel of corn produced, 0.38 lbs. of P2O5 and 0.27 lbs. of K2O are removed. Using this method when yields are significantly different than previous yields can help make more accurate fertility recommendations.

Figure 3. Soil sampling with soil probe
Figure 3. Soil sampling with soil probe

Soil Testing Following Drought

Soil sampling for nutrients is one of the most reliable ways to know the nutrient availability of a field (Figure 3). When soil sampling, always collect the depth of the sample that the soil lab recommends. Fertility recommendations are based on nutrient calibration curves for a specific sample depth, typically 0-6 in. or 0-8 in., so anything shallower or deeper than those recommended depths will skew results. In dry soils, taking a deep enough sample can be especially challenging. For instance, in dry conditions, nutrients are often concentrated in the top few inches of the soil profile without water to move them downward. As a result, a shallow soil sample would likely have high soil test values and underestimate fertility recommendations.

In a drought, just as there is increased variability in yield potential, there is also increased soil variability across a field. The different landscape positions, water-holding capacity, nutrient uptake and mineralization will vary across a field, resulting in extreme nutrient concentrations. Often soil test values in dry conditions can seem out of the ordinary or may have samples that are outliers compared to the rest of the field. To combat this, consider increasing the density of soil samples collected in a field following a drought. It may also be beneficial to compare to previous soil sample results. Patterns of results from normal precipitation years compared to sample results from dry years can show trends and help make accurate fertility recommendations.

Summary

In a drought, mobile nutrients such as N and S may be less available to the crop. However, with reduced leaching potential, there is often increased concentration of these nutrients for the following year. Immobile nutrients such as K and P may also be more available following a drought if yields were reduced and there was less crop nutrient uptake. The nutrient availability may be unpredictable, so use yield to estimate the crop removal and proper soil sampling techniques for nutrient management planning.

To learn more about how nutrient removal rates impact rotational crops, read this article.

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