Assistant professor of crop sciences Fabián Fernández says, 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 notes. The real danger in going by the numbers is 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," Fernández says. "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."
Don't Get Obsessed With Micronutrient Numbers
Fernández was surprised because copper deficiency is extremely rare and has not been observed in Illinois. In the U.S., copper deficiency has been observed only in sands and high-organic-matter soils, such as peats and mucks. After further discussion, Fernández realized the farmer had not seen any signs of distress in the corn plants, but the test results had worried him.
Unreliable tests
The Illinois Agronomy Handbook 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.
Fernández notes 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.
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.
Tissue testing reliability
Fernández notes 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 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.
Fernández says it is important to remember that, while the tables in the Agronomy Handbook 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, Fernández 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 that 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 adds.
Lastly, he reminds producers, "If it ain't broke, don't fix it."