Sept. 15, 2016 - For some parts of the province, the 2016 growing season has been exceptionally dry. As we move towards fall, harvest, and soil sampling, there are some unique soil fertility considerations if you have experienced particularly dry weather this season.
Figure 1. Departure from average precipitation across Ontario from April 1-Sept. 12, 2016 can be viewed here.
If you follow a build-up and maintain approach to fertility and your crop yields are significantly reduced due to growing conditions this year, you may be able to give credit to some of the phosphorus (P) and potassium (K) applied this year to next year’s crops. It’s little solace, surely, but here is how it could work. For example, if you applied P at a rate of 63 lbs/acre P2O5 and K at a rate of 45 lbs/acre K2O to account for crop removal from a 180 bu/acre grain corn crop and only yielded 90 bu/acre, you applied 31.5 and 22.5 lbs/acre P2O5 and K2O, respectively, above actual crop removal. On the other hand, if you harvested grain corn as silage, there may not be a significant difference in nutrient removal. In this case, the best way to know for sure is to submit a whole plant sample for analysis.
Dry soils can also have an impact on soil test results themselves. In a dry year, soils may not replenish easily available pools of nutrients as quickly throughout the season. Even if plant growth and nutrient uptake are less than normal, soil test readings may not reflect this. It may take some time once soil moisture conditions improve in the fall for less easily available pools of nutrients to be converted to forms that are measured by the soil test.
Soil test potassium, in particular, may be affected by dry soil. A lack of soil moisture this season resulted in an increased incidence of K deficiencies (see Figure 2). If fall continues to be dry, it may also take longer for K to leach from crop residues and return to the soil. Also, soil test values for soils low in K may be overestimated, whereas fields high in K may test lower. If you have experienced a very dry season, you may want to delay sampling until a bit later in the fall once you have moist soil conditions for an extended period.
Figure 2. Potassium deficiency in soybean can be viewed here.
Soil pH can also be affected during a dry season. If precipitation has been well below normal, it is possible that applied fertilizer has not have moved down below 6 inches. In these cases, pH may be reported as lower than it actually is. Also bear in mind that if you applied lime in the spring and have received very little rainfall, there may not have been sufficient moisture to fully activate the neutralizing reactions.
When it comes to nitrogen (N), generally speaking, this season did not present a significant risk of N loss from denitrification (saturated soils) or leaching. Therefore, if you anticipate harvesting a much lower corn yield than was fertilized for, there will likely be a substantial quantity of nitrate nitrogen left in the soil. If this is the case, consider seeding a cereal rye cover crop, which is an excellent scavenger of excess nitrogen in the soil profile. The Midwest Cover Crops Council has a very useful online Cover Crop Decision Tool that provides information on seeding rates and timing.
Finally, samples of dry and hard soils can sometimes be difficult to properly collect. If you cannot obtain a proper number of samples (minimum of 20 cores per 25 acres for a standard composite sample) consistently to a six inch depth, your test results will be likely elevated due to nutrient stratification. In this situation, you can try using another type of soil core sampler. Ultimately, if you are not able to sample to the proper depth, consider delaying sampling and using your latest soil test results.
Putting it all together
- Nutrient removal may have been less than normal this season and should be accounted for
- Excess nitrogen may be present as nitrate in the soil if crop yield did not match nitrogen fertilization rate. Consider using a cover crop to capture nitrogen that is vulnerable to being lost over the winter.
- Take caution when interpreting soil sample results acquired in dry soil conditions. Dry conditions may alter values, in particular potassium and pH levels, and may also make it difficult to obtain proper sample depth.
Camberato, J and B Joern. 2012. Nutrient management related to dry soil conditions and poor crop yields. Agronomy Department, Purdue University. https://www.agry.purdue.edu/ext/soilfertility/droughtnutrients.pdf.
Sawyer, J. 2003. Drought impacts on soil fertility management. Department of Agronomy, Iowa State University. http://www.ipm.iastate.edu/ipm/icm/2003/10-6-2003/droughtsoilimpact.html.
Jongeneel, S. 2012. Soil testing after a dry growing season. Department of Crop Sciences, University of Illinois. http://cropsci.illinois.edu/news/soil-testing-after-dry-growing-season.