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Best of soil tests

Confused by results? Look at what’s important and ignore the rest.

Soil scientist George Rehm of the University of Minnesota Extension Service says some of the information in soil test reports is important and some is not. Here's a summary of his recommendations for Minnesota growers. While most of these hold true for other corn-growing regions, some may not. Always double-check with unbiased local experts before making a fertilizer decision based on soil test results. Rehm's comments serve as a good starting point for anyone who's puzzled by all those numbers, or concerned about specific recommendations for particular nutrients.

Soil pH is usually the first piece of information that appears on a soil test report. The soil pH is a measure taken when the soil is mixed with water. If the soil pH is less than 6.0, the soil sample is placed in a buffer solution and the buffer pH is measured. The buffer pH is the basis for the recommended amount of lime to apply.

Residual or carryover nitrogen in the form of nitrate-nitrogen is a key piece of information when planning fertilization of sugar beets, corn, wheat and other crops. The amount of carryover nitrogen is usually reported as pounds per acre. An accurate measure of carryover nitrogen requires soil from depths below 6 in.

Phosphorus. Most labs use the Olsen procedure if the soil pH is 7.4 or higher. At lower values, they use Bray and Kurtz or Mehlich III. The Bray and Kurtz #1 procedure has also been referred to as the "weak" Bray test. Potassium test results can vary substantially over two or more years. This is due to soil moisture at the time of sampling rather than the laboratory analytical procedure. Soluble salts combined with a measure of calcium carbonate can predict iron chlorosis severity in soybeans, Rehm says. However, there are currently no firm recommendations for interpreting these measurements.

Less Useful
Strong Bray value. The results of the "strong" Bray test have no relationship to crop response to phosphate fertilizer, Rehm says. He tells Minnesota growers to ignore the "strong" Bray test results when determining fertilizer needs.

Sulfur. Rehm says soil test sulfur recommendations are not a good way to predict crop need for sulfur. "Considerable research in Minnesota and neighboring states has led to the conclusion that there is no analytical procedure that will accurately predict the need for sulfur fertilization," he says. Rehm says soil texture is a more accurate predictor of the need for sulfur. Sulfur added to soils with a loamy sand or sandy loam texture will usually increase yields for alfalfa, corn and small grains, he says.

Nutrient ratios. Rehm recommends ignoring test information about ratios for potassium/calcium, potassium/magnesium and calcium/magnesium. He says extensive research has shown these ratios have no relationship to crop fertilizer needs. Magnesium itself can be important for crops on acidic, sandy soil, but the magnesium measurement is not important if soils aren't sandy.

Cation exchange capacity is another figure to ignore, Rehm says. It's an indicator of soil texture and has no relationship to fertilizer recommendations in Minnesota and similar areas.

DTPA tests for the micronutrients zinc, copper, manganese and iron. Rehm says the test is reliable for predicting crop needs for zinc. However, it will not predict the severity of iron chlorosis in soybeans. And except for the organic soils in northern Minnesota, the copper measurement has no relationship to crop needs for copper. The need for manganese fertilization has not been documented in Minnesota.

The boron soil test has little value in Minnesota, Rehm says. However, he thinks a boron test is worthwhile for farmers who plan to grow alfalfa on the coarse-textured soils in east-central Minnesota.

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