Iron deficiency chlorosis (IDC) is most commonly observed in soybean crops, but dry beans, faba beans and field peas can also be sensitive to IDC.
Symptoms of IDC include interveinal yellowing (chlorosis) of new growth, where leaf veins remain green (Figure 1). In extreme cases, interveinal browning (necrosis) can occur. Symptoms show up in soybeans as early as V1 (1st trifoliate), but plants will often recover from this condition. These symptoms should not be confused with potassium or nitrogen deficiency (Figure 2).
Causes and Assessment
Prior to V1, cotyledons supply stored iron (Fe) to the soybean plant. Once this source of Fe is depleted, plants must acidify their root zone to access it in a plant-available form from the soil. Despite the abundance of Fe in Manitoba soils, factors such as excess calcium carbonates, soil moisture, soluble salts and/or high nitrate levels can impede Fe uptake into the plant and increase the risk of IDC. Wet years can bring soluble salts to the upper soil profile. If dry conditions follow, it is expected that these salts remain in place, increasing the risk of IDC.
Monitor the patterns, persistence and severity of IDC in your fields. It can occur in large patches and is most likely to occur at the tops of eroded knolls or in field depression areas. In-season management options are often not effective, but it is important to accurately diagnose the problem and adjust management strategies for future years. Visual diagnosis, tissue testing and knowledge of soil characteristics can help you diagnose this condition.
The risk of IDC development can be predicted based on the combination soil test calcium carbonate and soluble salt levels (Table 1). For example, a combination of high soluble salt (>1.0 mmhos/cm) and carbonate (>5.0%) concentrations in the soil indicate an extreme risk of IDC.
If chlorosis is present early on and plants recover by the V5 to V6 stages, yield loss will be minimal. If symptoms persist beyond these stages, expect more significant yield loss.
Research has been underway in Manitoba, looking at the relationship between IDC score and soybean yield of Manitoba-grown varieties, led by Kristen P. MacMillan (Soybean and Pulse Agronomy Lab, University of Manitoba). Results have shown that for each 0.1 unit increase in IDC score, soybean yield is estimated to be reduced by 2.4-2.8 bu/ac, on average (Figure 3).
Older research from NDSU (1998-2000) reported soybean yield to be reduced by 9-19 bu/ac for each one-unit increase in chlorosis rating (Figure 4). In this study, even low IDC scores resulted in yield loss.
For more on the latest Manitoba-based IDC information, check out the Soybean and Pulse Agronomy Lab 2019-2020 Annual Report or the article, Yield Impact of Yellow Soybeans and Management Strategies.
In-season rescue options for IDC are limited, so waiting until the plants recover on their own is the most common practice. The best method for control is prevention, in which selection of tolerant varieties is the primary option. Other options for control include heavier seeding rates, improved drainage, and practices that reduce soil N levels such as cover cropping and N management in other crops.
Soybean cultivars vary in their ability to acidify their root zone and take up iron. If your soil test carbonate and soluble salt levels meet at a “high” risk category in Table 1, aim to select a tolerant variety.
Iron chelate products are also on the market for IDC prevention. Research from North Dakota State University has shown that in-furrow iron chelate products, such as Soygreen (2-3 lb/ac of FeEDDHA) can offer some protection. However, significant yield loss can still occur when varieties are susceptible to IDC (Figure 3). However, foliar sprays have proved to be ineffective, as Fe is unable to translocate within the plant (Figure 2).
Goos, R.J. and D. Franzen. 2016. How much does IDC reduce soybean yield? NDSU Extension Service article. ag.ndsu.edu
MacMillan, K.P. 2020. 2019 and 2020 Annual Report. Soybean and Pulse Agronomy Lab, University of Manitoba.