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 http://extension.cropsci.illinois.edu/handbook/pdfs/chapter08.pdf) 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.