The Effect of Dry Weather Conditions on Fall Soil Test Samples

Weekly News Article for The News-Democrat, Carrollton, KY

(Photo Courtesy of Tom Schultz & Iowa State University Extension)

It is still not too late to get in your fall soil samples for next year's growing season, and the recent rains make it an excellent time to collect samples.

An interesting result of the dry weather conditions of the past summer is that we may see lower soil pH and potassium levels in fall soil samples.  Measuring the accurate levels of pH and potassium in soil after harvest will provide the proper amount of potassium fertilizer and lime needed in the spring to supplement the soil's nutrients to provide suitable growing conditions.

Lack of rainfall causes the soil to retain salt, which leads to the lower levels of soil pH results.  There is approximately one-half of a pH unit difference between fall and spring soil samples.  Rainfall is the only way for soil to leach trapped salts.  Soil samples collected during dry conditions can produce lower than normal results.  Inaccuracy of pH measurements can cause miscalculations of the proper amount of lime needed  for the next planting season.



In January 2010, a new method of testing dry fall soils was implemented at the University of Kentucky soil test laboratory.  The previous method for soil sampling required a mixture of one part water and one part soil.  The new method replaces water with a high concentration of potassium chloride salt.  The high concentration of salt removes seasonal variability because lower than normal results if it is consistent throughout the year.  A normal pH is calculated from the lower than normal result.

Lower potassium soil measurements can also result after dry conditions.  Potassium levels can range from about 50 to 100 pounds lower per acre.  It is predicted that the clay soils do not release potassium until the soil becomes wet.  Unlike the new testing method for pH, there is not currently a method to resolve the lower potassium levels during the dry season.

To get accurate potassium measurements of your soil sample, collect soil after rainfall to obtain optimum results.

Other labs may still use the previous method of utilizing water to measure soil pH, so the lower pH measurements can still occur if you submit soil samples to other laboratories.  If dry soil samples are sent to a University of Kentucky County Extension Office for testing, the new method used will help generate a proper measurement of pH in the soil.

Soil probes are available for borrow at the Carroll County Cooperative Extension Office.  Soil tests cost $5 per sample.  Remember, soil sampling and testing are not just for large-scale crop producers, but also for those with pastures, hayfields, gardens, and lawns.

For more detail information about soil sampling and testing, check out the UK publications, Taking Soil Test Samples and Soil Sampling: What It Is and What It Does.