So many questions have come in after the recent article about secondary macronutrients and micronutrients that I decided to write a follow-up. I think it's human nature, if we see our neighbor taking action to supposedly prevent problems, to get worried about being affected or not "measuring up," even if the likelihood is very rare. The following information describes what we have seen in Illinois with respect to various nutrients and offer guidance on determining whether there is something to worry about on your farm.
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, which will also adjust soil pH. The soil test for calcium is a reliable measurement. Test levels of at least 400 lb/acre in sandy soils and at least 800 lb/acre in silt loam soils are considered adequate for crop production. That said, since calcium 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 in Illinois such deficiency has been observed only in isolated cases in sandy soils with very low organic matter. The test for magnesium is fairly reliable. Test levels of about 60 to 75 lb/acre in sandy soils and about 150 to 200 lb/acre in silt loam soils are considered adequate for crop production. Keep in mind that some soils may have low magnesium levels in the sampling depth of 7 inches but have high concentrations below that depth. Applying magnesium to those soils is very unlikely to increase yield because of the ample supply below the top 7 inches.
Sulfur (S). Alfalfa is the crop most likely to respond to sulfur application. There are more occurrences of sulfur-deficient corn in the Midwest, probably the result of increased use of sulfur-free fertilizer; decreased use of sulfur as a fungicide and insecticide; increased crop yields, resulting in higher requirements for all essential plant nutrients; and decreased atmospheric sulfur supply. Despite the growing frequency of sulfur deficiency reports, crop responses to sulfur applications in Illinois have been inconsistent.
Early this year I wrote an article about corn in the Bulletin showing that many places had no response to sulfur applications, while others were very responsive. For this reason, I don't recommend routine application of sulfur fertilizer. Soil testing for sulfur is not very reliable, but should you decide to test, evaluate whether a sulfur response is likely by considering the results along with organic matter level, potential atmospheric sulfur contributions (maps are available at nadp.sws.uiuc.edu), subsoil sulfur content, and soil-water conditions just before soil samples were taken. Since soil organic matter is the primary source of sulfur, soils low in organic matter are more likely to be deficient than soils with higher organic matter (>2.5%).
Early-season sulfur symptoms may disappear as rainfall contributes some sulfur (especially downwind from industries emitting significant amounts) and as root systems develop to cover greater soil volume. Sulfur is also a very mobile nutrient. In sandy soils under excess precipitation, leaching may result in low test values of samples collected from the soil surface. On the other hand, if the soil surface is dry and hot at the time of sampling, test results can overestimate the soil's capacity to supply sulfur during the entire growing season. For these reasons, if a soil test is unexpectedly low, use sulfur only on a trial basis.
Boron (B). The crop that is sometimes affected by boron deficiency is alfalfa. Deficiency of boron has not been observed for corn or soybean in Illinois. In fact, these crops need boron in such small amounts that high boron applications to alfalfa can result in toxic levels for corn planted afterward. Deficiency symptoms in alfalfa typically appear on the second and third cuttings, and they can be more pronounced during droughty periods. Application of boron 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 observed to be deficient in Illinois. Since potassium fertilizers have about the same amount of chloride as they do potassium, but chloride is required in much smaller amounts than potassium, there is virtually no chance for chloride deficiency to occur.
Copper (Cu). Copper deficiency has not been observed in Illinois. The only instances in the United States where copper deficiency has been seen are in sands and soils with high organic matter (peats and mucks).
Iron (Fe). In Illinois, because their deficiency symptoms are similar, what is called iron deficiency in soybean is often manganese deficiency. Both nutrients when deficient cause yellow leaves with green veins. The difference is that iron deficiency causes leaves to eventually turn white, while manganese deficiency causes brown dead spots in the leaves. The only time one should be concerned about iron deficiency in Illinois soils 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 manganese deficiency. When a deficiency develops, the best alternative is to make a foliar application soon after symptoms appear, but only in the affected zone. There are some claims indicating that glyphosate applications can result in less availability of manganese in soybean. Research conducted in Illinois has provided no evidence to support such a claim.
Molybdenum (Mo). Deficiency of molybdenum is very rare in Illinois. This micronutrient is different than others in that it decreases in availability as soil pH becomes more acidic. Deficiency symptoms are limited to soybean when grown in soils with pH below 5.0. Rather than trying to correct the problem by applying molybdenum, it is far better to adjust soil pH with limestone applications.
Zinc (Zn). Though not a widespread problem, deficiency of zinc has been reported for corn in Illinois. Deficiency may develop in sandy soils and soils low in organic matter. Also, soil pH above 7.3 or tsoil excessively high in phosphorus can induce zinc deficiency. In most cases, however, high phosphorus levels are the result of manure applications. Since manure also contains zinc, it is unlikely that a deficiency will develop in fields where manure is applied.--