Soil Fertility: Short Course in Secondary Macronutrients and Micronutrients from Boron to Zinc

Soil Fertility: Short Course in Secondary Macronutrients and Micronutrients from Boron to Zinc


It's human nature to worry about not "measuring up" to our neighbors, says Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition. However, he says just because your neighbor is taking action to prevent nutrient problems that may or may not occur this growing season is not a good reason for you to jump on board.

Fernandez shares a quick course in secondary macronutrients and micronutrients to help growers determine what they should and should not worry about when it comes to nutrient deficiencies in their fields.


Calcium (Ca):Calcium deficiencies in Illinois have not been observed when soil pH is at or above 5.5. In acid soils, calcium deficiency can be corrected with limestone applications that will also adjust soil pH. The soil test for Ca is a reliable measurement, but because Ca deficiency has not been observed in the state, there is no need to recommend testing.

Magnesium (Mg):Both corn and soybean are sensitive to magnesium deficiency, but such deficiencies in Illinois have been observed only in isolated cases in sandy soils with very low organic matter. While the test for Mg is fairly reliable, in some soils Mg levels may be low within the sampling depth of 7 in., but concentrations below the sampling depth are high. Applying Mg in those soils is very unlikely to produce a yield increase because of the ample supply of this nutrient below the top 7 in.  

Sulfur (S):Alfalfa is the crop that most likely responds to sulfur application. The number of incidents with S-deficient corn in the Midwest has increased due to less incidental S and greater removal with higher crop yields. Despite the increasing frequency of S-deficiency reports, corn responses to sulfur applications in Illinois have been inconsistent. For this reason, routine application of S fertilizer is not recommended.

Because soil organic matter is the primary source of S, soils low in organic matter are more likely to be deficient than soils with higher organic matter (greater than 2.5%). Sulfur availability is affected temporarily by many environmental and crop factors, and partly because of that soil tests for S are not very reliable. Still, if you decide to conduct a soil test, evaluate whether a S response is likely by considering the test results along with conditions of crop development, soil organic matter level, potential atmospheric S contributions (maps available), subsoil S content and soil-water conditions just before soil samples were taken. If a soil test is unexpectedly low, once you have considered these factors use S only on a trial basis.

Boron (B):The crop that is sometimes affected by boron deficiency is alfalfa. Deficiency of this nutrient has not been observed for corn or soybean in Illinois. In fact, these crops need B in such small amounts that high B applications to alfalfa can result in toxic levels for corn planted afterwards. Deficiency symptoms in alfalfa typically appear on the second and third cuttings and can be more pronounced during droughty periods. Application of B can be beneficial for high-yielding alfalfa production on soils with less than 2% organic matter, strongly weathered soils in south-central Illinois, soils with high pH or soils with sandy texture.

Chloride (Cl):This nutrient has not been deficient in Illinois. Since potassium fertilizers have about the same amount of Cl as potassium, but Cl is required in much smaller amounts than potassium, there is virtually no chance for chloride deficiencies to occur in Illinois. 

Copper (Cu):Copper deficiency has not been observed in Illinois. The only places in the U.S. where Cu deficiency has been observed include sands and high-organic-matter soils (peats and mucks).

Iron (Fe):In Illinois, what is called iron deficiency in soybean is actually often manganese deficiency.  When deficient, both nutrients cause yellow leaves with green veins. The difference is that Fe deficiency will cause leaves to eventually turn white, while manganese deficiency will cause brown dead spots in the leaves. The only time growers should be concerned about Fe deficiency in Illinois is if soil pH is above 7.3.

Manganese (Mn):Soybeans are susceptible to manganese deficiency when soil pH is above 7.3. Corn and alfalfa have not shown Mn deficiency. When deficiencies develop, the best alternative is to do a foliar application soon after symptoms appear, but only in the affected zone. There are some claims indicating glyphosate applications can result in less availability of Mn in soybean. Research conducted in Illinois has provided no evidence to support such a claim. 

Molybdenum (Mo):Deficiency of this nutrient is very rare in Illinois.  Deficiency symptoms are limited to soybean when grown in soils with pH below 5.0. Rather than trying to correct the problem by applying Mo, it is far better to adjust soil pH with limestone applications.

Zinc (Zn):Although it's not a widespread problem, deficiency of zinc has been reported for corn. Deficiency may develop in low-organic-matter soils and sandy soils. Also, soil pH above 7.3 or soils excessively high in phosphorus can induce Zn deficiency. In most cases, however, high phosphorus levels are the result of manure applications. Since manure also contains Zn, it is unlikely that a Zn deficiency will develop in such fields.

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