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Dry weather can affect results of early fall soil tests

Posted Monday, October 17, 2011, at 2:22 PM

The exceptionally dry fall conditions due to low rainfall, especially since corn and soybean physiological maturity may affect the quality of soil samples and soil-test results. The first concern is with obtaining the actual soil sample. Sampling under very dry conditions may increase soil sampling error because of the difficulty of controlling sample depth and proper soil collection. This may be a problem for P and K due to nutrient stratification, which usually is more, pronounced in reduced till, no-till and pastures. Both P and K tend to concentrate at or near the soil surface and therefore the depth control for core collection is very important. With the dry soil it is often difficult to push the soil tube in to a uniform depth and cracks need to be avoided. Also, when the top inch of soil is very dry and powdery it is very easy to lose this soil portion, which will affect the soil-test result significantly.

Very dry soil conditions may result in more acidic soil pH. Differences from of 0.1 to 0.3 pH units are common with very dry soils.

Short-term nutrient recycling from plant residues and the equilibrium between soil nutrient pools also may be affected by rainfall, especially for potassium (K). In moist soils, some slowly exchangeable K can become exchangeable when easily exchangeable K is depleted by plant uptake or leaching. With dry soil at the end of the growing season, this replenishment of the solution and easily exchangeable fractions, which is what soil tests measure, is limited or does not occur. These processes also occur for phosphorous (P) but to a much lesser extent. Preliminary research data from Iowa State suggests that with little or no rainfall from crop physiological maturity until soil sampling there is less K recycling from plant tissue and crop residue and lower soil-test K values than with normal rainfall.

With these dry conditions, taking a soil sample becomes more important. If possible, try to delay soil sampling until meaningful rainfall has occurred. This may take at least one inch or more but this may vary by other environmental conditions and the previous crop. This will result in a better sample and more reliable soil-test results.

This time of year may not allow you to wait to soil sample. If you take soil samples under these dry conditions:

1.Be careful with sampling depth control and that you get the complete soil core in the bag.
2.Soil pH results may be a bit more acidic than it would be in normal conditions.
3.Soil K results may be lower than what they would be under normal conditions.
4.Soil P test results probably will be affected little by the recycling issue.


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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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