Some locations in our trade area have experienced drought conditions and many of these areas continue to remain drier than normal. If you are faced with sampling under drought conditions, the following information may assist you in your planning and the interpretation of the results that you receive.
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 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 effect of the dry soil on phosphorus levels isn’t as dramatic as potassium, but less moisture in the soil may lower the soil test results. The same situation of reduced crop yields may result in less phosphorus being removed from the soil.
Soil sampling technique: It is extremely difficult to sample dry soils. Often 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 effect on the soil test readings. Also obtaining the correct depth of soil sample maybe difficult, auger soil probes tend to work better in extremely dry conditions.
Many of the areas that were suffering under dry conditions earlier in the season may have had 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 to assess the effects of this unusual growing season.