Issue No. 8, Article 5/May 25, 2012
Soil and Plant Tissue Testing: If Ain't Broke, Don't Fix It
The last few years have seen a new trend toward more testing for micronutrients in soil as well as in plant tissues. While I think there is value in regularly checking crop progress, it is important not to become compulsive or overreliant on having "hard numbers" to tell us what we could discern simply by looking at the crop or what we know from experience.
The real danger in looking at so-called hard numbers is that unless the numbers are interpreted correctly, they can lead to unnecessary anxiety and sometimes unwarranted--and expensive--nutrient applications. I recently received a call from someone who farms a very good prairie soil in central Illinois, wanting to know how to apply copper (Cu) to a corn field because the tissue test indicated a deficiency. The farmer also wanted information on how to obtain soil test values for copper. I was surprised that anyone would find a deficiency of copper, which is extremely rare and has not been observed in Illinois. The only places in the U.S. where copper deficiency has been observed are sands and high organic matter soils (peats and mucks). A little more discussion revealed that the farmer could not see any signs of distress in the corn plants, but that concern was generated by a few "hard numbers."
Let's talk first about soil testing. While it might be nice to have values for the various nutrients, it is important to understand the true importance of such values. Soil testing for micronutrients (boron [B], chloride [Cl), Cu, iron [Fe], manganese [Mn], molybdenum [Mo]), and zinc [Zn]) and secondary macronutrients (calcium [Ca], magnesium [Mg], and sulfur [S]) does not do much to help determine the potential for crop response to an application of the nutrient. The Illinois Agronomy Handbook (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 0 is a test with little usefulness. I show an abbreviated list of ratings here in Table 1.
Table 1. Ratings of soil tests.
Boron: corn and soybeans
Iron: pH > 7.5
Iron: pH < 7.5
Manganese: pH > 7.5
Manganese: pH < 7.5
Copper: organic soils
Copper: mineral soils
Most soil tests for micronutrients and secondary macronutrients are not very reliable; soil scientists and agronomists don't put a lot of confidence in the results, and neither should you. Table 1 also shows that these tests are affected by crop and soil conditions, and for some nutrients different extraction and analysis procedures are used by different testing laboratories. Make sure you know which test was used before trying to interpret the result.
Soil testing for secondary macronutrients and micronutrients is most useful when accompanied by an understanding of specific crop requirements and factors that impact availability, such as soil and environmental conditions. A test result of high indicates that there is very low probability for yield loss due to the nutrient in question. However, a result of medium or low does not automatically mean a crop response will occur after applying the nutrient.
Now let's address tissue nutrient analysis. While such analysis is generally more reliable than soil testing for secondary macronutrients and micronutrients, some of the same cautions apply. For most nutrients, when a sufficiency range is provided, it is typically quite large. This is in part due to the inherent wide variability in these measurements. As with soil tests, having a tissue test value below the sufficiency range does not necessarily mean that applying the particular nutrient will cause a yield response.
Please note in the Illinois Agronomy Handbook Table 8.2 (extension.cropsci.illinois.edu/handbook/pdfs/chapter08.pdf), or similar tables that may be used as the standard or "book value," that tissue test levels are specific to a certain growth stage and plant part. If the two do not match (meaning one is not comparing apples to apples), the interpretation will be meaningless, and even potentially misleading. Since micronutrient and secondary macronutrient deficiencies normally occur in parts of a field, not the entire field, tissue testing as a diagnostic tool to distinguish "good plants" from "poor plants" can be valuable, I believe. But even when tissue testing is used as a diagnostic tool, I recommend exercising caution and using the results only along with some of the factors mentioned for soil testing. If, after considering all of the evidence, you determine that indeed there is a nutrient problem and an application of that nutrient can solve it, make the application only in the problem area and not on the entire field.
Though sometimes things that aren't broken can be improved, in the case of testing for or applying a secondary macronutrient or micronutrient, the popular saying applies: "If it ain't broke, don't fix it."--Fabián G. Fernández