Iron deficiency chlorosis (IDC) is an issue that plagues many farmers in regions where the soil contains high amounts of calcium carbonate, particularly over much of the western Great Plains. The excess calcium carbonate converts iron from the highly available Fe (2+) into unavailable Fe (3+). Soybeans and sorghum are two crops that are particularly affected by this malady. There are many places in the western Plains where farmers have given up trying to grow soybeans because of IDC, and have adopted the mindset that “beans just won’t grow around here”. Many remedies have been tried to help with IDC, such as chelated iron fertilizer placed in the furrow at planting, or foliar iron sprays, but they tend to give at best temporary relief of symptoms.
But there may be relief on the horizon. We have understood that IDC always occurred on soils with excess calcium carbonate, but it has been a mystery why it occurred on some soils, but not on others with equal calcium carbonate, and why it occurred on one soil in some years but not in other years. But recent research has found a clue to this mystery. It seems that IDC occurs where there is a combination of excess calcium carbonate and free nitrate in the soil. Yes, that is the same nitrate that plants use as a source of nitrogen fertilizer. Understanding why this occurs requires an understanding of the chemical interactions that takes place in the soil immediately surrounding the roots of a plant, called the rhizosphere.
Plants must remain electrically neutral; if they take up a nutrient with a positive charge, they must give off a positive charged ion to maintain neutrality. This particle is called hydronium. If plants take up a negative charge, they must give off a negatively charged ion to counteract this charge. This negatively charged ion is bicarbonate, which will convert iron into unavailable forms. Since nitrate has a negative charge, and is taken up in large quantities when available in soil, it results in a large amount of bicarbonate in the rhizosphere, and unavailable iron.
To work around this phenomenon, it is necessary to first reduce the amount of free nitrate in the soil, and then provide nitrogen in another form rather than nitrate. In the case of soybeans, providing another source of nitrogen is easy, since soybeans are a legume and capable of fixing their own nitrogen. It is only necessary to use a grass cover crop like rye, oats, or barley planted prior to the soybean crop to soak up the nitrate and convert it into protein in the residue. This protein will later decay and slowly release the nitrogen back first into amino acids, then into ammonium, and, if it has not been taken up by plants by this time, it will finally be converted into nitrate, a little at a time. Contrary to what many believe, plants can take up not only ammonium as a nitrogen source, but also amino acids and many other organic compounds. If plants did not take up organic compounds, herbicides would never work. Sucking up all the nitrate forces the soybeans to fix more nitrogen of their own, which is in the form of ammonium and does not result in bicarbonate production.
Sorghum provides a bigger challenge, since it is not a legume. However, it is also possible to use cover crops to solve IDC in this crop as well, though it requires different cover crops to achieve the goal. The cover crop prior to sorghum should be a legume, like yellow blossom sweetclover or spring field peas. A legume will first use up any soil nitrate, then nodulate and begin to fix more nitrogen. This fixed nitrogen will also be in the form of protein that decays slowly like the grass cover crops ahead of soybeans, but since the legume also fixed additional nitrogen, and decays more rapidly, it will be able to add substantial amounts of biologically produced, slow release nitrogen to reduce the amount of fertilizer nitrogen needed. Fertilizer nitrogen sources like urea, or liquid urea-ammonium nitrate, or anhydrous ammonia, can convert into nitrate in just a few weeks and make IDC worse. There are nitrification inhibitors available to keep fertilizer in the ammonium form longer, and there are also slow release forms of urea available to ration out the nitrogen over time.
Although this is a rather new technique, both research and practical experience have demonstrated great potential for this method. One research trial showed a 40 bushel soybean crop where an oat cover crop was used, compared to a 4 bushel crop without a cover crop.
Another new tool to prevent IDC is inoculating with mycorrhizal fungi. Mycorrhizal fungi (MF) are a beneficial fungi that colonizes plant roots and then extends root-like structures up to two feet past the root zone that absorb water and nutrients and bring it back to the plants. MF are of particular help in obtaining immobile nutrients like phosphorus, iron, and zinc, and also function to make the plant more drought tolerant. Spores for mycorrhizal fungi are now available from Green Cover Seed.
Manure and compost can also be great aids in preventing IDC. They contain nitrogen in organic forms that release slowly over time as first protein, then amino acids, then as ammonium, just like cover crop residues. They also contain plant available iron in organic compounds that are not readily tied up by calcium carbonate, and contain organic acids that make the iron already present in the soil more available. Since IDC usually occurs in spots in fields, these areas should be targeted for manure or compost applications. A combination of manure and legume cover crop residue can provide most of the nitrogen needs for a sorghum crop on IDC prone soils.
Finally, don’t abandon the old remedies for IDC such as resistant varieties, in-furrow iron and foliar iron. Though they are seldom sufficient by themselves, in combination with cover crops, mycorrhizal fungi, and manure they can provide a synergistic effect that can recover most of the yield potential lost to IDC. Another piece of good news: these benefits seem to accumulate over time, making the soil less and less prone to IDC each year these practices are used, because the pool of actively decaying soil organic matter laden with available iron increases every year.
The bottom line is that there are effective actions we can take to manage IDC, and we no longer have to accept miserable performance of sorghum and soybeans on those areas of our fields susceptible to this condition.
Dale Strickler is an agronomist for Green Cover Seed. He can be reached at firstname.lastname@example.org.