On top of limiting the yield potential of crops, dry soils also have other important impacts that need to be considered when making nutrient-management decisions for future years.
There are a few tips to keep in mind when taking soil samples and interpreting soil-test results this fall to help make decisions for the 2022 growing season.
1. Pay attention to soil-sampling depth.
With dry soils, soil-sampling depth can be an issue. A standard-series soil test is calibrated for the top 6 inches of soil, so it isn’t a good idea to scrape off the soil surface or go deeper than 6 inches. There are other measurements that call for a deeper sample, such as for nitrogen analysis, but for most purposes producers want to stick to six inches. It also may be difficult to take the appropriate number of soil cores needed for a good representative soil sample. Sticking to the appropriate sampling depth and number of cores per sample is important to receive representative results.
2. Account for the nitrogen left in the soil, so producers don't overapply.
It is a good idea to sample for residual nitrate this fall because it has been so dry during the Midwest growing season. Much of the nitrogen fertilizer producers previously applied may still be available for the next crop. Accounting for that nitrogen credit could help producers cut nitrogen fertilizer costs for the 2022 growing season, which should be especially welcome given the current high cost of fertilizer. The main challenge with nitrate sampling is that the sample should be collected to a depth of two feet, which can be extremely challenging in dry years. Analysis of nitrate on shallower sampling depths is not suggested.
3. With increasing fertilizer prices, consider phosphorus and potassium soil-test levels.
The most relevant soil-test data are phosphorus and potassium test results. Lower-than-expected yields in dry years can leave unused fertilizer near the soil surface, which will be picked up by soil tests. Using phosphorus and potassium tests to adjust rates accordingly can help save on fertilizer costs for the following crop. Remember the potassium soil test can be impacted by dry soils, so lower-than-expected results may occur following drought conditions, especially if sampling after corn. With a potentially early harvest, it might be tempting to sample early. But it is best to sample about the same time as in previous years to help with the consistency of the test results. That’s especially important for potassium. A substantial amount of potassium is released into the soil as rain washes it out of crop stover. Allowing that process to take place will ensure potassium is accounted for in the test results. Also soils like to maintain nutrient equilibrium. After crops are no longer taking up nutrients and water, allowing some time for chemical and physical processes to occur will ensure test results are more representative of what will be available for crop uptake the next growing season. Testing the soil for zinc is also a good idea, but testing for other micronutrients is not recommended.
4. Keep an eye on soil pH.
If pH levels are less than 6.0, liming can help bring them within the acceptable range of 6.0 to 6.5-plus. Having the correct pH ensures greater availability of nutrients already in the soil and may mean less fertilizer needs to be applied. Dry soils can have a marginal effect on soil pH, resulting in values 0.1 to 0.3 units less than if the soils are wet. Many crops can tolerate low pH values, but keep in mind that crops like alfalfa and some clovers will not do well when soil pH is less than 6.5, so knowing crop tolerance to low pH is important because liming costs can accumulate.
5. Do not overreact if soil tests seem out of the ordinary.
Dry years can present problems in situations where soil samples are collected in extended intervals, such as a four-year sampling cycle. If soil-test values seem too high or too low, producers may want to consider sampling the field again sooner, and weigh soil-sample results previously taken. Having multiple years of data is critical to interpret what the long-term trends in a field are. Remember producers do not need to be exact in how much fertilizer is applied in a maintenance-based fertilization program, and under-applying fertilizer is not likely to cause yield loss unless soil tests are low. Simply put, there is a lot of flexibility when it comes to how much fertilizer to apply, and in many circumstances some is better than none.
Dry soil conditions can present challenges when collecting fall soil samples, but careful planning can help producers receive the most out of soil-test data. If taken correctly at the proper sampling depth, most soil tests should not be impacted by dry conditions. Knowing which tests can be impacted is critical when making plans for the upcoming growing season.
Dan Kaiser and Fabian Fernandez are soil-fertility specialists for the University of Minnesota-Extension.