Managing Iron Deficiency Chlorosis in Soybean

High lime IDC is quite common for soybean, peanut, grapes, citrus and peaches. Soil tests generally won't show these problems. High lime IDC is more severe in soils with finely divided lime (calcium carbonate). The fine calcium carbonate particles contact the soybean root and slowly neutralize the excreted acid meant to solubilize iron in the soil. The effect is that the plants cannot take up iron that is in the soil.

Wet soils also aggravate IDC. When soils are wet, there is limited air exchange with the atmosphere which causes a buildup of carbon dioxide in the soil. Roots and soil microbes produce the carbon dioxide through respiration. The amount of bicarbonate in the soil is proportional to the amount of carbon dioxide. As carbon dioxide increases so does bicarbonate. This increase will rapidly neutralize the acidity around the soybean root. Research has shown that soil moisture can increase IDC severity, especially at low temperatures. The amount of bicarbonate in the soil correlates with IDC in soybean in the field, so the more bicarbonate in the soil the more IDC will occur. In wet soil conditions, it may be useful to consider cultivation to speed the release of carbon dioxide from the soil.

Decaying organic matter, whether it is from crop residues or manures, adds to the amount of carbon dioxide in the soil. The microbes that are breaking the organic matter down release carbon dioxide as part of the process. Wet soils will limit diffusion of the carbon dioxide out of the soil. This increases the risk for bicarbonate buildup in the soil and therefore increases the severity of IDC.

Soil nitrate in the field can increase the chlorosis intensity. Field and greenhouse studies have shown the addition of nitrate increases the severity of chlorosis in soybean (Table 1). When a plant root takes up nitrate it must exchange with a bicarbonate ion. In addition, the plant needs to convert nitrate to ammonium within the leaves. This increases pH in leaf sap and decreases the rate of reduction of Fe III to Fe II that is necessary for leaf cells to have usable Fe. Iron in leaflets can be greater in the chlorotic plants than non-chlorotic plants because of an accumulation of iron in the leaves and the plant's inability to reduce the iron III into an available form. Both cases increase IDC.

Differences in soil nitrate may explain why we often see less chlorosis in the wheel tracks of the secondary tillage in a soybean field, Figure 1. The soil under the tractor wheels is more compacted. This causes the compacted area to be more anaerobic (low air), but not low enough to reduce much Fe III to Fe II . These slightly anaerobic soil conditions can cause denitrification of nitrate to N2, reducing the amount of nitrate in the soil and the amount of IDC.

Finally, any production practice that can cause stress to the plant also will cause problems with iron uptake by the plant. Examples include herbicide injury, reduced with the use of glyphosate, plant disease pressure and nematodes.

Soybean variety selection is important. If you have a field with a large amount of IDC in it, choose the most tolerant variety possible. Research shows that tolerant varieties have greater yield where IDC is problem, but even higher yields when combined with treatments used to decrease the severity of IDC (Table 2). A tolerant variety, however, should be the first line of defense against IDC.

Table 2 shows effects on IDC susceptible (V1) and tolerant (V2) soybean varieties grown on soils with low, moderate, or high IDC pressure.

Use a companion crop, particularly where soil nitrate can be high. The companion crop in these situations can use the excess soil nitrate and also dry a moist soil to reduce bicarbonate build up. For example, oat seeded at a 1.5 bushel per acre rate at or before soybean planting can increase soybean grain yield in IDC affected areas of the field. This practice does require extra management. You need to kill the oat by the time it is at the 10 to 12 inch height. Later than this causes drought and earlier does not allow for the nitrate uptake or soil moisture use (Table 3 and Figure 3).

Reduce stress to the plant as much as possible. Glyphosate herbicides will be less stressful than some of the preplant and in-season choices used in the past. Reduction of root damage from cultural practices such as excessive and deep cultivation is also useful. Beware of soil conditions that cause compaction during the tillage and planting operations. Compaction of surface soil at planting can stress a plant for the whole growing season. Be sure to treat plant diseases and pest according to accepted thresholds.

Use a seed placement of an iron chelate product that has a majority of its iron in the ortho-ortho form. Research has shown great success with the use of ortho-ortho chelated iron with the seed, Table 4. Use of other products and application methods has not produced consistent results. Most research data suggests a rate of one to three lbs of dry product per acre of the ortho-ortho chelate will be enough to increase yield. These chelates provide the best return on investment in areas with moderate or high IDC.