What Happens During a Soil Test

What Happens During a Soil Test

A closer look at how a soil testing lab processes soil samples to obtain recommendations

Ever wonder what happens when you take a soil sample and send it off to be tested? David Dunn, a University of Missouri (MU) Extension soil testing lab associate, helps process about 10,000 soil samples each year at the MU Fisher Delta Research Center in Portageville, Mo.

“We can tell you what is going on below your feet,” he says. “We are all about giving recommendations for farmers to achieve the yields they want. To ensure the best results and recommendations, we suggest taking at least about 10 to 15 different sub-samples. Look at gathering a sub-sample from at least every acre. Even on small farms, the soil changes at different spots in the field.”

In Missouri, soil organic matter tests are used to estimate nitrogen availability in the soil. A general rule is every 1% of soil organic matter will release about 20 lb. of nitrogen per acre for crop availability. Less than half of the samples tested by MU in 2013 had medium levels of soil organic matter of around 2% to 3%.

Upon arrival at the Delta Research Center, soil samples are sorted and placed in marked containers before undergoing a 16-hour drying session.

Extracting solutions are added to simulate the plant’s ability to obtain each nutrient in question from the soil. To find potassium, calcium and magnesium, the samples are tested in an atomic absorption spectrometer that burns the soil and solution in an acetylene-fired flame that changes color based on nutrient levels. To find phosphorus, for example, a solution that turns blue is added. To determine pH, soil is mixed with a diluting solution and analyzed with an electrode.

Kansas State University researchers say the return on investment for soil testing is higher when grain prices are lower.

“This is because potential returns to inputs are tighter at lower crop prices,” says nutrient management specialist Dorivar Ruiz Diaz.

If actual soil test levels of nitrogen or phosphorus are higher than expected, producers can realize significant savings by reducing or eliminating unnecessary inputs, he says. On the other hand, if producers overestimate how much nitrogen or phosphorus is in the soil, they could miss out on higher yield opportunities by underfeeding the crop.

Don’t forget to evaluate micronutrients either, adds Steven Tillman, territory manager for Compass Minerals.

“Be sure to request micronutrient levels from the soil test lab, too,” he says. “Even if you have not previously tested for them, micronutrients can drag yield if they are deficient. Don’t let a micronutrient like zinc or boron determine whether you get the most from your seed and fertilizer investments.”

In general, farmers should test each field every two to four years, or more often when introducing a new crop 
rotation, Tillman says. 

To see a more in-depth, behind-the-scenes look at soil testing at the University of Missouri Delta Research Center, visit www.FarmJournal.com/soil_lab


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