August 23, 2006 | By: Laura Skillman

Fall is a popular time for soil testing and with today’s high prices for fertilizer the practice can save money by aiding in a farmer’s decisions on the type and amount of fertilizer he will need for his 2007 crops.

Sampling in the fall offers the advantages of good weather, allows time to plan for coming crops and gives lime, if needed, time to react with the soil prior to spring planting. However, it is not without some disadvantages. Seasonal fluctuations mean the pH and potassium levels are at their lowest during this time. Rainfall and crop nutrient uptake are factors in these fluctuations.

Understanding these seasonal fluctuations can aid in understanding and interpreting soil test results that vary from year to year or within the same year, said Lloyd Murdock, University of Kentucky College of Agriculture soils specialist.

The pH levels in soils generally are lower during the summer and early fall. The reduction generally is attributed to soil drying, root and bacteria activity and nitrification of nitrogen fertilizers. The process is reversed as soil moisture increases.

Soil tests for potassium often have the lowest levels in the fall for grain crops. The difference is most profound if samples are taken immediately after harvest in a dry fall. As potassium from crop residue is washed back into the soil, levels will increase. 

Taking samples from beside the row may provide a more accurate representation of potassium levels because rains move the potassium from the crop residue to the soil near the row faster than to the row middles. At the very least, Murdock said, samples should be taken in equal numbers from both the row and row middle positions to help reduce the seasonal effects.

Soil samples can be submitted to your local UK Cooperative Extension office for testing. The UK College of Agriculture has two soil testing facilities that provide farmers with professional test results. The labs - one in Lexington and one at the UK Research and Education Center in Princeton - test thousands of samples each year to aid farmers and homeowners in determining their fertilizer needs. Since 1988, the two labs have annually handled between 43,000 and 60,000 samples.

Chemical analyses and recommendations from the UK Agricultural Testing Labs are specifically made for Kentucky conditions, said Frank Sikora, UK soil test coordinator. Nutrient needs and fertilizer responses are determined by research conducted through UK on crops and soils in Kentucky.

Soil tests should be compared to previous tests to help determine if seasonal fluctuations are great enough in a year to make adjustments in fertilizer and lime recommendations, Murdock said.

It is also important to compare apples to apples. Different labs use different testing methods as well as different units. UK lists levels in pounds per acre while other results may be listed in parts per million. So, farmers need to make sure they are comparing similar testing methods from one year to the next, wherever the testing is done, Sikora said.

Spring soil sampling can provide the most accurate information on what is available in the soil for that growing season, but getting samples taken and the results interpreted during this busy season often is not practical. Fall sampling is a popular practice and one that can provide farmers with the information they need to determine their fertilizer needs, Murdock said. However, it is important that they understand these seasonal fluctuations as they make their decisions.


Lloyd Murdock, 270-365-7541, ext. 207, Frank Sikora, 859-257- 2785