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Soil, plant tissue tests: If it ain't broke, don't fix it

The real danger in "going by the numbers" is that farmers who interpret them incorrectly may apply unnecessary and expensive nutrients.

Assistant professor of crop sciences Fabian Fernandez thinks that, in the last few years, there has been a trend toward more micronutrient testing in the soil as well as in plant tissues.

"While I think there is value in regularly checking the progress of the crop, it is important not to become compulsive or over-reliant on having hard numbers to tell us something that we could discern simply by looking at the crop, or that we know from experience," he said.

The real danger in "going by the numbers" is that farmers who interpret them incorrectly may apply unnecessary and expensive nutrients.

"Recently, I received a call from someone who farms a very good prairie soil in central Illinois," said Fernandez. "This person wanted to know how to apply copper to the corn field because the tissue test indicated a deficiency. The farmer also wanted information on how to obtain soil-test values for copper."

Fernandez was surprised because copper deficiency is extremely rare and has not been observed in Illinois. In the United States, copper deficiency has been observed only in sands and high-organic-matter soils, such as peats and mucks. After further discussion, Fernandez realized that the farmer had not seen any signs of distress in the corn plants, but the test results had worried him.

The Illinois Agronomy Handbook (available at rates soil tests on a scale of 0 to 100, where 100 is a reliable and cost-effective test and where 0 is a test with little usefulness.

Soil tests for micronutrients (boron, chloride, copper, iron, manganese, molybdenum, and zinc) and secondary macronutrients (calcium, magnesium, and sulfur) are not very reliable tools to detect potential for crop response and soil scientists and agronomists do not have much confidence in their results.

Fernandez notes that soil testing for secondary macronutrients and micronutrients is most useful when accompanied by an understanding of the specific crop requirements and factors that impact availability, such as soil and environmental conditions.

May not get crop response

A high test result indicates that yield loss due to the nutrient in question is unlikely. However, when the test result is medium or low, it does not automatically mean that applying the nutrient will cause a crop response.

Fernandez notes that tissue nutrient analysis is usually more reliable than soil testing for secondary macronutrients and micronutrients, but some of the same issues mentioned for soil analysis apply.

Sufficiency ranges shown in reference tables are quite large for most nutrients. This is due in part to the large variability inherent in these measurements. As with the soil test, having a tissue test value below the sufficiency range does not necessarily mean that applying that particular nutrient will cause a yield response.

Fernandez said it is important to remember that, while the tables in the Agronomy Handbook or other such tables that may be used to obtain the standard value, tissue test levels are specific to a certain growth stage and plant part.

Both plant part and growth stage have to match the table; otherwise, the numbers are meaningless.

Because micronutrient and secondary macronutrient deficiencies normally occur only in parts of the field, tissue testing can be a useful diagnostic tool when it is used to compare healthy plants to those showing a problem.

However, Fernandez recommends caution and suggests using this information while taking into account the issues mentioned for soil testing.

"If, after considering all the evidence, one determines there is indeed a nutrient problem and an application of that nutrient can solve it, one should make the application only in the problem area and not on the entire field," he said.

Finally, he said, "If it ain't broke, don't fix it."

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