Whitesburg KY

Sampling soil improves plant growth

Maintaining the proper pH and fertility in soil is important for higher crop yields. An important tool to assess fertility levels is to sample soil and have it tested. Kentucky is experiencing an unusually dry fall.

Soil samples taken during dry falls tend to result in soil pH and soil test K being unusually low. This results in recommendations that overestimate lime and K fertilizer that may be needed.

In January of 2010, the University of Kentucky soil test laboratories began a new test for soil pH to solve the problem occurring with soil samples taken in dry autumns.

High levels of salt accumulate in the soil when plants remove minimal amounts of nutrients from fertilizer application or organic matter mineralization. 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. Soil pH increases to normal values after adequate rainfall leaches out the salts.

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.

The soil pH the lab measures in a solution of high salt will be lower than normal. The pH that is reported is the familiar soil pH in water using a known relationship between soil-salt pH and soil-water pH from samples tested in the early spring.

The new test for pH will solve the problem with unusually low pH measurements on samples taken during dry falls. Low soiltest K during dry falls is still a problem with values that can be 100 lbs/acre lower than from spring sampling.

The reason for low soiltest K is related to the nutrient accumulating in plant tissue or locked up in clay minerals. For accurate soiltest 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 fertility of the soil and obtain a more effective lime and fertilizer recommendation.

For more information on soil testing and dry falls, contact Shad J. Baker at the Letcher Cooperative Extension Service, 478 Extension Drive Whitesburg.

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