Soil samples taken during dry falls tend to result in soil pH and soil test potassium being unusually low. This results in recommendations that overestimate lime and K (potassium) fertilizer that may be needed.
Recently, University of Kentucky soil test labs began a new test for soil pH to solve the problem occurring with lower pH levels in soil samples taken in dry falls.
High levels of salt accumulate in the soil when plants remove minimal amounts of nutrients from the soil. This effect can occur anytime but is particularly prevalent in drought conditions. Soil pH measured with a high level of background salt causes an unusually low pH measurement.
The new pH method involves measuring pH in a solution with a high salt concentration instead of with water. By adding a solution with high salt, any effect of varying amounts of residual salt on pH during the year is removed.
Low soil-test K during dry falls is still a problem with values that can be 100 pounds per acre lower than from spring sampling. These differences can be quite significant.
The reason for low soil-test K is related to the nutrient accumulating in plant tissue or locked up in clay minerals. For accurate soil-test K measurements, it is best to wait for rain to wash K out of crop residues and clay minerals.
Understanding the seasonal fluctuations of soil tests will allow a producer to take fall soil samples that better represent the actual fertility of the soil and obtain a more effective lime and fertilizer recommendation.
The cost of a routine soil test conducted through the LaRue County Extension Office is $5 per sample. There is also a two-sample for $5 option available.