Iron Deficiency Chlorosis in Soybeans

• Soil properties and other environmental factors are responsible for iron deficiency in soybeans.
• Iron deficiency chlorosis (IDC) is a complex issue for soybean farmers, especially in calcareous soils (soils with excessive lime).
• Selecting varieties more tolerant to IDC is the best available management practice, although other management practices can help lessen severity.

Iron (Fe) is an essential nutrient and an important component of nodulation, nitrogen fixation and enzymes that form chlorophyll. A lack of iron within soybean plants is often referred to as iron deficiency chlorosis (IDC) and is easily recognized due to reduced leaf chlorophyll, chlorosis and subsequent yellowing of leaves. IDC symptoms begin to appear within a few weeks after soybeans emerge, with interveinal chlorosis showing on the first trifoliate. Iron does not readily translocate within the plant, causing new growth to be impacted first when deficiencies continue. Unlike visible symptoms of other deficiencies, soybean leaf veins remain green with IDC as the remainder of the leaves begin to yellow. Under severe IDC, edges of leaves will become necrotic and start to die (Figure 1).

IDC symptoms tend to appear in irregular shaped areas across fields, causing significant reduction in yield potential of affected areas. Substantial yield reductions have been reported across many areas where soybeans are grown but are more prevalent in western Minnesota, northwestern Iowa, Nebraska, North Dakota and South Dakota, where calcareous or sodic (alkali) soils are most common.

Common Causes for IDC

Although the name IDC implies it is caused by low soil iron levels, it is the result of soil conditions that decrease iron uptake by soybean roots. Most soils likely have sufficient iron concentrations; however, not all of it is readily available to plants. Soybean IDC is mostly observed in areas with high calcium carbonate and/or high salinity soil levels.

Calcareous soils developed from calcium carbonate parent material commonly have pH levels that range from 7.0-8.5, making them highly conducive to IDC symptoms. Within fields predominately having calcareous soils, IDC symptoms will often appear first in wetter areas where parent calcium carbonate more readily dissolves into a solution that releases carbonate (C0₃^-2). This acts as a strong base that increases soil pH. If soil nitrate levels are also high, soybeans will preferentially uptake soil nitrogen and subsequentially release additional carbonates that further increase pH within the soil root rhizosphere (root zone) and exacerbate IDC symptoms. Although IDC symptoms are commonly observed in high pH soils, this can be a poor predictor. Symptoms are not always seen in elevated soil pH areas with lower carbonate and salinity levels.

Soils with high pH levels oxidize iron into a ferric state (Fe³⁺), which binds iron tightly to soil components, making it less soluble and able to move to nearby roots. Soybeans depend on iron being in a ferrous state (Fe²⁺) for uptake and transport into the plant. Acidification from plant roots helps reduce iron from the Fe³⁺ to Fe²⁺ form, making it more available for plant uptake. IDC symptoms in high pH soils can be worsened by other nutrient deficiencies as well as cool growing conditions that slow growth and development. If Fe deficiency is not severe, and environmental conditions improve, resumed root growth will normally allow plants to absorb sufficient Fe and recover.

Management Practices

1. Variety Selection

   • Golden Harvest has significant research efforts to characterize varieties for IDC tolerance (Figures 2 and 3). Soybean variety tolerance is the most important strategy in managing this complex issue. Varieties characterized as having high tolerance to IDC are generally more “iron efficient” or better able to reduce Fe³⁺ to the Fe²⁺ form in soil around roots, making it more available for plant uptake.¹
   • Although some IDC symptoms may be visible on tolerant and susceptible varieties in severe situations, tolerant varieties will have less symptomology and yield loss. Susceptible varieties can sometimes provide a better option in fields not prone to IDC, making it important to map areas that have shown IDC symptoms and use this insight for making future variety selection decisions.

2. Apply Iron Chelates to Soil

   • Iron chelates are often added as a soil amendment to increase the solubility of iron in the soil and deliver it to the plant to minimize IDC symptoms. Chelated forms of Fe have shown to help correct IDC and protect yield potential with varying levels of effectiveness in different soil pH.² Fe-EDDHA fertilizer is considered to be the most stable of the chelates and has shown to increase grain yield of soybeans on calcareous soils.¹ Research has also shown that the most effective chelate application timing and placement for reducing IDC is Fe-EDDHA chelate fertilizer in-furrow at planting, even though no iron chelate treatment completely eliminates chlorosis.¹ The success of chelate application relies heavily on the buffering capacity (ability to maintain stable pH) and pH of the soil.³ Return on investment (ROI) of applying an iron chelate should always be considered before using on a large scale.
   • Foliar Fe applications have shown to “regreen” chlorotic symptoms in some studies but were less effective in severe IDC field conditions. While leaves appeared less chlorotic, previous research showed that foliar Fe may not reach the plant roots and, therefore, yield potential may decrease later in the season.³ Again, ROI should be evaluated for foliar Fe products before using broadly.

3. Manage Areas With High Soil Nitrate Levels

   

   • Excess soil nitrates in IDC-susceptible soils have been shown to increase the severity.² Soybeans commonly use symbiotic relationships with rhizobia to form nitrogen-fixing nodules on roots, but when soil nitrates are available in soil, they will take up nitrogen directly from the soil. When taking up nitrogen directly from the soil, soybean roots release bicarbonates, which further increases soil pH and reduces Fe uptake. This can be highly visible in fields with tire track compaction (Figure 4). Previous research showed that soil nitrates were lower in compacted tracks than in adjacent uncompacted soil.² It is believed that the compaction decreases soil porosity, thus increasing soil saturation, which increases soil denitrification. Denitrification within compacted areas helped minimize IDC in those areas. Using oat companion crops interseeded within soybeans has also been found to help manage high nitrate soils since these crops scavenge soil nitrogen and excess soil moisture, thus decreasing bicarbonates that increase pH in the soybean root rhizosphere.³

4. Adjust Seeding Rate and Row Spacing

   • Increasing soybean seeding rate has shown to minimize IDC symptoms in some cases. Increased seeding rates result in more plants, which develop additional roots. Because soybeans acidify the rhizosphere, the increased root mass helps to further acidify the root zone, reducing more iron from the Fe³⁺ to Fe²⁺ form, which is a more plant-available form. Research shows that increased seeding rate can reduce chlorosis, although yield responses also depend on other environmental conditions.^4,5
   • Narrow row spacings (<30-inch) generally increase seed-to-seed spacing, producing a similar outcome to reducing seeding rates. Due to this, much larger seeding rate increases may be needed to help minimize IDC symptoms when planting narrow-row soybeans.

5. Minimize Additional Plant Stress

   • Any additional stress will only exasperate IDC symptoms. Help minimize additional plant stress with specific management practices as needed to avoid the following:
     • Nutrient deficiencies
     • Diseases
     • Nematodes
     • Herbicide injury
     • Severe compaction, which damages soybean roots

Summary

IDC is a complex field issue that requires a robust management strategy. Since it generally occurs due to various stresses and not simply due to low soil Fe, it is challenging to mitigate. In areas where IDC is a concern, selecting a soybean variety with tolerance to IDC is one of the best methods to protect yield potential.