Soil testing after dry conditions

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Dry weather conditions this past summer may lead to lower soil pH and potassium levels in fall soil samples according to Frank Sikora, UK Extension Soil Testing Coordinator. 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 the producer to miscalculate 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 occur during both wet and dry soil conditions. It is okay to test 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’s predicted that the clay soils don’t 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.

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

Remember, soil tests are free of charge to all Trimble County home owners, gardeners and farmers. We also have soil probes available for free check-out to speed up the process of soil sampling.

For more information about collecting soil samples and other questions regarding soil testing, visit the Trimble County Cooperative Extension Service at 43 High Country Lane in Bedford or call us at 502-255-7188. While at the Extension Office, ask for the following publications, AGR-57: Soil Testing: What It Is and What It Does, AGR-16:

Taking Soil Test Samples and AGR-5: When to Apply Lime and Fertilizer.

Michael Pyles is Trimble County’s Cooperative Extension agent for agriculture.