Soybean Fertility/Inoc

Iron Deficiency Chlorosis

Iron deficiency chlorosis (IDC) is a common sight in soybean crops, but dry beans, faba beans and field peas can also be sensitive to IDC. It can occur in large patches and is most likely to occur at the tops of eroded knolls or in field depression areas.

Symptoms

Symptoms of IDC include interveinal yellowing (chlorosis) of new growth, where leaf veins remain green. 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.

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

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.

Scouting

Monitor the patterns, persistence and severity of IDC in your fields. 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 IDC.

See Table 1 to determine your risk of IDC based on soil test carbonate and soluble salt levels. 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.

Impact on Yield and Management

Yield loss experienced in plants that have a small degree of chlorosis early and recover by the V5 to V6 stage should be minimal. However, yield loss can occur if symptoms persist to the V5 to V6, even if plants recover later.1

According to NDSU research from 1998-2000, soybean yield was reduced by 9-19 bu/ac for each one-unit increase in chlorosis rating (Figure 1). In other words, more IDC means greater yield loss. However, even low IDC scores resulted in yield loss.

Jay Goos (Professor, Dept of Soil Science, NDSU) highlights the 1998-2000 research results in Iron deficiency chlorosis in soybeans… causes and control measures.

In 2017, an IDC test site near Winnipeg was taken to yield for the first time in order to demonstrate the effect of IDC rating on yield. Based on the 2017 trial, soybean yield was reduced by 20 bu/ac with each 1-unit increase in IDC rating at V5/R1.

Kristin P. MacMillan (Research Agronomist, University of Manitoba) highlights the 2017 trial results in Yield Impact of Yellow Soybeans and Management Strategies.

Control and Prevention

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. Variety selection is the primary control 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 more tolerant varieties.

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).

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

Resources


1 Goos, R.J. and D. Franzen. How much does IDC reduce soybean yield? NDSU Extension Service. ag.ndsu.edu