Soil Testing After a Drought

While we have not experienced a drought in 2020, there are areas of our trade area that have been abnormally dry. This article from 2012 does a great job explaining how dry weather conditions can impact soil test results.

There has been a substantial amount of information and speculation published recently regarding how the drought of 2012 may affect soil test results of samples collected during the dry period. This article is an attempt to summarize these facts.

Soil pH: Water pH readings may be 0.1 to 0.6 pH units lower than expected. This is due to a slight increase in soluble salts in the soil solution that haven’t leached into the soil profile. This condition, though, does not alter the buffer pH result so the amount of lime recommended for most samples will not be affected. An exception to this would be sandy soils where the water pH determines the lime recommendation. However, sandy soils are leached more easily so the amount of soluble salts in solution may be much lower than a heavier soil.

Potassium: Soil test levels for potassium may be lower than normal. When soils remain extremely dry for extended periods of time, the moisture that normally keeps the clay latticework open for potassium exchange retracts, capturing the available potassium from solution. This will show up as a reduction in the soil test level. Also, potassium is easily leached from crop residue following harvest. With little rainfall, this potassium reserve could remain in the tissue. One caveat of this, though, is with inadequate moisture to produce normal yields, less potassium may be removed from the soil reserve.

Phosphorus: Soil test levels for phosphorus may be slightly lower than normal. The affect of the dry soil on phosphorus levels isn’t as dramatic as potassium, but less moisture in the soil may lower the soil test readings. The same situation of reduced crop yields, though, may result in less phosphorus being removed from the soil.

Soil sampling technique: It is extremely difficult to sample dry soils. Many times the top one or two inches of the core are compressed enough that some of this material may spill out of the probe. In minimum tillage situations, this could have a dramatic affect on the soil test readings. As of the publication date of this newsletter, hurricane Isaac has deposited a substantial amount of rainfall on much of the Midwest. Soils in the lower half of Illinois, Indiana, and Ohio may have enough time to equilibrate moisture levels prior to fall sampling so that some of the drought effects will be negligible. Reduced yields, though, will still be a remnant of decreased nutrients being removed from the soil. This year is one where soil sampling should occur in order to assess the affects of this unusual growing season. Soil sampling technique: It is extremely difficult to sample dry soils. Many times the top one or two inches of the core are compressed enough that some of this material may spill out of the probe. In minimum tillage situations, this could have a dramatic affect on the soil test readings. As of the publication date of this newsletter, hurricane Isaac has deposited a substantial amount of rainfall on much of the Midwest. Soils in the lower half of Illinois, Indiana, and Ohio may have enough time to equilibrate moisture levels prior to fall sampling so that some of the drought effects will be negligible. Reduced yields, though, will still be a remnant of decreased nutrients being removed from the soil. This year is one where soil sampling should occur in order to assess the affects of this unusual growing season.