Do You Need Secondary Macronutrient and Micronutrient Analyses?

Whenever crop prices are high, a lot of interest is generated in products and services that most producers don’t normally consider. Typically the product or service does very little to nothing for the crop, but persuasive advertising can create thoughts like “I have to have it” or “I will forfeit profit if I don’t have it” or “It costs so little–it makes sense to get it, just in case.” Some services are even promoted as a bonus “at no additional cost to you” or as “a package deal that saves you money.” A service receiving a lot of publicity this year is testing for secondary macronutrients (Ca, Mg, and S) and micronutrients (B, Cl, Cu, Fe, Mn, Mo, and Zn).

While it might be nice to have numbers for the various nutrients, it is important to understand the true value of those numbers. When nutrient numbers are used in isolation, soil testing does not do much in predicting response. The Illinois Agronomy Handbook lists a rating for 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 some of that information in Table 2.

Table 2. Ratings of soil tests for secondary and micronutrients.
Test Rating
Boron: alfalfa 60
Boron: corn and soybeans 10
Iron: pH > 7.5 30
Iron: pH < 7.5 10
Calcium 40
Magnesium 40
Sulfur 40
Zinc 45
Manganese: pH > 7.5 40
Manganese: pH < 7.5 10
Copper: organic soils 20
Copper: mineral soils 5

As the table makes clear, most soil tests for these nutrients are not very reliable. If a result is high, you will know that your crop won’t suffer a yield loss related to that particular nutrient, but a medium or low test result doesn’t necessarily mean that applying the nutrient will produce a crop response.

When test levels are low to medium, that’s when most people start spending money on fertilizers that are not likely to produce much difference for the crop; the farmer’s loss becomes someone else’s gain. Table 2 also shows that these tests are affected by crop and soil conditions. Soil testing for secondary macronutrients and micronutrients is most useful when accompanied by an understanding of crop requirements and of the crop, soil, and environmental factors that impact availability.

An additional point to remember is that for some nutrients, the extraction and analysis procedures used by testing laboratories may differ. Different procedures can result in vastly different reported values. For example, zinc levels obtained by one method are considered adequate for crop production if they are above 7, while for another method the value considered adequate is anything above 1. Make sure the testing laboratory provides the adequacy value or range for the test being done; otherwise the results are useless.

Tissue test analysis is another service being heavily promoted for secondary macronutrients and micronutrients this year. While tissue test analysis is generally more reliable than soil testing for these, some of the same concerns apply. For most nutrients, the sufficiency range is quite large, partly because of the sizable uncertainty about what the value really reveals. Having a value below the sufficiency range does not automatically mean that applying the particular nutrient will cause a yield response.

In addition, tissue test levels are specific to a certain growth stage and plant part. Test information can be of little value, and can even be misleading, if applied to a stage or plant part other than what is recommended. I see the greatest value of tissue testing as a diagnostic tool to distinguish “good plants” from “poor plants,” and then only when it is considered along with the other factors I described for soil testing.

A secondary macronutrient or micronutrient deficiency is usually a localized problem within a field. If there is evidence that a nutrient problem does exist and an application of that nutrient can solve it, make the application only in the problem area and not to the entire field.

In deciding whether to test for or apply a secondary macronutrient or micronutrient, consider this: If it’s not broken, why fix it?–Fabián G. Fernández

Fabián Fernández

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