Do You Need Micronutrient Soil Tests?

Some soil test laboratories now offer secondary (Ca, Mg, and S) and micronutrient (Zn, Mn, Fe, Cu, and B) soil tests at no additional cost beyond the charge for the standard soil analysis.

While it is always nice to get something for nothing, keep in mind that seldom is there a "free lunch." When used alone, these secondary and micronutrient test results don't do a very good job of predicting response.

If your test results are high, you can rest assured that yield will not be limited by that nutrient, but a low or medium test result may or may not mean that you will get response to application of the particular element.

Using the results of the secondary and micronutrient soil tests will best be done when they are tempered with an understanding of the soil and environmental factors and the differences in crop requirement that affect availability of each element.

SECONDARY NUTRIENTS

Calcium

Calcium (Ca) deficiency has not been seen in Illinois when soil pH was greater than 5.5. Calcium deficiency associated with acidic soils should be corrected by using limestone.

The laboratory procedure used for Ca is easy and reliable — probably more accurate than the K test — but since the deficiency does not exist, there is no reason to recommend the test. Even though the database is very limited, suggested critical levels have been provided in the Illinois Agronomy Handbook (Table 1).

Magnesium

Magnesium (Mg) deficiency occurs on both corn and soybean, but in Illinois is limited to sandy, low organic matter soils. As with Ca, the laboratory procedure for Mg is accurate, but caution must be taken in interpreting the results.

Southern Illinois University research has shown no response to applied Mg even when the Mg test from the surface soil was below the critical level (Table 1). They observed that Mg levels below the surface 7-inch level was adequate and apparently met the needs for optimum crop production even when surface levels were considered deficient.

Sulfur

Alfalfa is the crop most likely to respond to sulfur (S) application in Illinois. Corn has been shown to respond in a few experiments, primarily in northwestern Illinois.

Organic matter is the primary source of sulfur in soils, so soils low in organic matter are more likely to be deficient than soils high in organic matter. Because S is very mobile and can be readily leached, deficiency is more likely on sandy soils.

Experiments conducted across much of Illinois showed a response to sulfur at five of 85 locations. The correlation between yield increases and measured soil levels in the soil was very low, indicating that the soil test does not reliably predict sulfur need.

The test inaccurately predicted that 43 of 80 sites would respond to S. When soil test levels are above the level of 22 lb S/acre (Table 1), it is very unlikely that a response to applied S will occur. However, when tests are below that level, response might or might not be observed.

Micronutrients

Boron, chloride, copper, iron, manganese, molybdenum and zinc are the seven nutrients known to be needed for crop production.

Some suggest that sodium and cobalt are also essential, but evidence that the latter two are essential is limited.

Deficiencies have been confirmed on four of these seven micronutrients in Illinois, namely boron, manganese, molybdenum, and zinc. Guidelines for interpreting some of the micronutrient tests are provided in Table 2. Caution must be exerted in using any of these tests.

Boron

Boron (B) deficiency is a common occurrence on alfalfa in many Illinois soils. Characteristic symptoms of the deficiency are yellowing of the upper leaves, eventually turning to a purpling color, along with stunting of the upper stems.

Deficiency symptoms for B are very similar to leaf hopper damage. The difference between the two problems is that leaf hopper damage will be characterized by a V-shaped discoloration that starts at the leaf margin. Boron deficiency is often confused with drought, as it occurs when plants are under moisture stress.

If B deficiency has previously been observed, it will likely occur whenever alfalfa is grown in that field. In that case, apply B after the first cutting in the seeding year. Do not apply B prior to seeding.

The soil test does a reasonable job of predicting B deficiency for alfalfa, but since it is so easy to detect the deficiency symptoms on the plants, its use is of relatively low priority. The correlation between B soil test and corn or soybean response to B has been low. In fact, there are no confirmed B deficiencies on either corn or soybean in Illinois.

Chloride

Chloride (Cl) deficiency has not and will not likely be observed on any Illinois crop. The Cl requirement is much less than that of K, and each time that K is applied as 0-0-60, there is as much Cl applied as K. Chloride deficiency of wheat has been observed in states where potassium deficiency does not exist. There is no reliable soil test for Cl in Illinois.

Copper

Copper (Cu) deficiency is rare in the U.S. and has not been observed in Illinois. Sweet corn and wheat are two of the crops most sensitive to the deficiency. Limited reports of the deficiency have been reported in Michigan and Wisconsin on high organic matter soils (mucks and peats).

Iron

What is called iron (Fe) deficiency of soybean by many in Illinois is often manganese. The symptoms are similar. In both cases, the leaves turn yellow and the veins stay green. The difference is that Fe-deficient leaves will eventually turn white and manganese-deficient leaves will develop brown necrotic spots on the leaves. Soil pH is the best predictor test of Fe deficiency. Unless soils have a pH greater than 7.3, Fe deficiency is unlikely. The Fe soil test is not well correlated with crop response to fertilizer Fe application.

Manganese

Soybean grown on high pH soils (pH>7.3) often exhibit manganese (Mn) deficiency symptoms. Suggested treatment is to spray the affected area soon after the symptoms appear. Oats are the other crop sensitive to Mn deficiency. The problem is rare on corn or alfalfa. Neither of the two tests (Table 2) currently available is well correlated to crop yield response to applied Mn.

Molybdenum

Molybdenum (Mo) differs from most of the other micronutrients in that it increases in availability with an increase in pH. The deficiency has been limited almost exclusively to legumes, including soybeans grown on very acidic soils (pH< 5.0). In nearly all cases, it is more economical to apply limestone to correct the problem than to apply Mo.

However, if you must grow soybeans on very acidic soils, be sure to use a seed treatment that includes molybdenum. Soil pH is the only soil test that detects the potential for Mo deficiency.

Zinc

Zinc (Zn) deficiency, while not widespread, is much more likely to occur on corn than on soybean. Documented response to Zn has been limited to low organic matter soils and sandy soils in northwestern Illinois.

High pH (greater than 7.3) will enhance the potential for Zn deficiency, as will high P soil test. However, if the high P soil test was derived from the application of manure, Zn deficiency is unlikely, as the manure probably supplied ample Zn to correct the problem.

Note that there are two commonly available Zn soil test procedures (Table 2). Interpretation of the results requires that the user know which procedure has been used in the laboratory, as the levels that indicate deficiency are markedly different for the two test procedures. Neither of these tests is well correlated to grain yield response to the application of Zn.

SUMMARY

None of the secondary or micronutrient soil tests are very reliable for predicting crop response to the applied element. If the test levels for any of these are high, the potential for response to the element is very low.

However, if the test level is low to medium, the potential for response to the applied element may be high, or it may be low.

A decision as to whether or not to apply secondary or micronutrient fertilizers should take into account the sensitivity of the crop to be grown to the element as well as other soil characteristics that affect the availability of the element, such as soil pH, organic matter, soil texture, and soil P level.

Use a combination of all of the above factors, along with plant analysis, in deciding the probability of a deficiency. If both soil test and plant analysis indicate the potential for a deficiency, apply the element on a trial basis.