Iron Deficiency Chlorosis

Identification

**Figure 1.** Soybean trifoliate leaf showing interveinal chlorosis, where leaf tissue turns yellow and leaf veins remain green.

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 (1^st trifoliate), but plants will often recover from this condition. These symptoms should not be confused with potassium or nitrogen deficiency (Figure 2).

**Figure 2.** A) Potassium deficiency in soybeans appears as yellowing of outer leaf margins followed by necrosis, beginning in oldest leaves and progressing upwards; B) Iron deficiency chlorosis results in yellowing of new soybean leaves but veins remain prominently green.

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.

Yield Impact

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, MPSG’s Agronomist-in-Residence, at the University of Manitoba. Results have shown that for each 0.1 unit increase in IDC score, soybean yield is estimated to be reduced by 1.5 – 2.8 bu/ac, on average (Figure 3).

**Figure 3.** Effect of average IDC score on soybean yield from the variety evaluation site in 2017, 2020, 2021 and 2022 (each data point is the mean of three replicates). Source: Kristen MacMillan, Soybean and Pulse Agronomy Lab.

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 at V5-6 (Figure 4). In this study, even low IDC scores resulted in yield loss.

**Figure 3.** Relationship between soybean yield and chlorosis rating (IDC score), where 1 = green leaves and 5 = severe chlorosis and a stunted growing point. Source: Goos and Franzen, 2016.

Within a field, IDC can be highly variable. Often, only 10-25% of the field is affected. In 2021 and 2022, the Soybean and Pulse Agronomy Lab began evaluating the performance of varieties in IDC and non-IDC soil conditions in the same field. Do IDC tolerant varieties yield as well in the non-IDC areas of the field? This research is on-going and preliminary results can be found in the 2021-2022 Annual Report.

Management Options

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, largely by selecting tolerant soybean varieties. 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 varieties 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 5). Foliar sprays have proved to be ineffective, as Fe is unable to translocate within the plant (Figure 6).

**Figure 5.** Yield response of resistant, intermediately resistant and susceptible soybean varieties to IDC with and without Soygreen (FeEDDHA) applied in-furrow. Source: Goos and Franzen, 2016.

**Figure 6.** Soybean trifoliate leaflet half-covered with a Post-It note, then treated with foliar Fe spray (left) vs. untreated trifoliate leaflet (right). Source: Dr. Jay Goos, NDSU.

Refer to the MPSG Soybean Variety Evaluation Guide or Seed Manitoba for information on variety tolerance to IDC.

Sources

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.
MacMillan, K.P. 2022. 2021-2022 Annual Report. Soybean and Pulse Agronomy Lab, University of Manitoba.
Yield Impact of Yellow Soybeans and Management Strategies