IDC in Soybean Prevelant in 2023

Soybean iron deficiency chlorosis (IDC) is a nutrient deficiency with general symptoms of chlorosis (yellowing) of the soybean foliage and stunting of the plant. IDC most frequently develops in high-pH soils and soils containing soluble salts, where chemical conditions reduce iron availability. Several enzymes require iron (Fe) to form chlorophyll; a lack of active iron in leaves leads to chlorosis. Soil normally contains large amounts of iron, but it is not in the soluble form the plant needs. The most soluble form in oxidized (aerated) soils is Fe(OH)3, where iron is in the Fe(III) form. This iron becomes less soluble at higher soil pH, especially when the soil has large amounts of calcium carbonate. 

IDC Symptoms

You can typically spot soybeans affected by IDC by leaves that turn yellow while the veins of the leaves stay green (known as interveinal chlorosis). Iron is an important constituent of enzymes essential for producing chlorophyll. An iron deficiency will limit chlorophyll production, resulting in yellowing of plant tissue. If conditions are severe, the entire leaf may turn yellow, and the leaf margins may turn brown, a condition known as necrosis.

Symptoms will not be visible until soybeans begin to develop trifoliate leaves. Cotyledons and unifoliate leaves do not exhibit IDC symptomology. Depending on growing conditions, symptoms may increase or decrease in intensity during the season. Iron is not mobile within the plant, so symptoms will appear on the youngest leaves first. Iron chlorosis in a soybean field typically appears in spots, often with no apparent pattern, due to differences in the chemical and physical properties of the soil. 

Key Points
  • Iron deficiency chlorosis (IDC) is a challenge for soybean farmers in several regions of North America, particularly in poorly drained calcareous soils in Minnesota, the Dakotas, Nebraska, and Iowa.
  • Soil properties can influence the severity of IDC in a field, including carbonate levels, salts, and drainage.
  • Environmental conditions such as soil moisture, temperature, and compaction can also influence IDC, resulting in variability of symptoms from year to year.
  • Selecting soybean varieties with good iron chlorosis tolerance is the most important management strategy.
  • Corteva Agriscience soybean breeders continually implement new methods for understanding and evaluating soybean response to IDC.
Three Factors Contributing to IDC

Soils typically have abundant iron levels, so IDC is not caused by a lack of iron but by conditions limiting iron availability for plant uptake. The factors that may cause chlorosis are complex and interact with each other to intensify the level of chlorosis. The most dominant factors affecting IDC occurrence are carbonate levels, salts, and depressional field areas with poor drainage. IDC severity can vary from year to year within the same field depending on the environmental conditions of the growing season.

1. Soil Properties

Soybean IDC frequently occurs in calcareous (lime-containing) soils. These soils are often called alkaline soils and have high pH values (>7.5). At high soil pH, iron is less soluble, making it less available for uptake by plant roots.

However, chlorosis of soybeans does not occur on all high-pH soils. The pH of surface soils in areas where IDC symptoms occur and areas where they do not are often the same, but there can be differences in both the chemical and physical properties of subsoil. The subsoil in a chlorotic soybean area is generally poorly drained, higher in pH, contains soluble salts and excess lime (carbonates), and may have a higher sodium concentration.

2. Weather Conditions

The interaction of weather conditions with soil properties causes differences in IDC severity from year to year and field to field.

Growing seasons with excess rainfall and cool soils typically result in a higher incidence of IDC. Soils with high calcium carbonate levels near the soil surface often have significant IDC symptoms. Biological activity in the soil converts calcium carbonate into carbon dioxide and bicarbonate (HCO3). Wet conditions limit air exchange between the soil and the atmosphere, causing bicarbonate ions to accumulate in the water in the topsoil. Bicarbonate interferes with both uptake of iron and the mobility of iron within the plant.

 

Continual rainfall and saturated soils also reduce oxygen in the root zone. Oxygen is needed for plant uptake of iron. Soil compaction, along with excess rainfall can be contributing factors in the reduction of iron uptake. Cool springs with lower soil temperatures reduce microbial activity within the soil. The reduction of microbial activity leads to less iron uptake and increased severity of IDC.

3. Nitrate Levels

According to field and greenhouse studies by University of Minnesota researchers, higher nitrate levels in the soil are also a contributing factor to IDC (Kaiser et al., 2011). Differences in IDC driven by soil nitrate levels are commonly seen when wheel tracks through a chlorotic area of the field remain green.  The soil under the wheels is slightly more compacted, creating a lower oxygen environment and increasing denitrification. The compacted soils under the wheels are not excessively compacted, just enough to account for differences in nitrate in the soil.

Lower oxygen levels in the soil can reduce IDC severity due to a reduction in nitrates in the soil and increase IDC severity in saturated soils by limiting iron uptake, which exemplifies the complexity of factors and interactions that contribute to IDC occurrence.

Six Management Options for IDC

A survey of soybean producers in areas affected by soybean IDC found that the selection of IDC-tolerant soybean varieties was the most common management tactic (employed by 70% of respondents), followed by planting practices (42%), field drainage (33%), tillage (16%), fertility practices (11%), and herbicide selection (6%) (Hansen et al., 2003).

1. Select a Tolerant Soybean Variety

Soybean varieties vary widely in their tolerance to IDC, making variety selection the most important step in managing this problem.

2. Increase Seed Density

University and research studies have shown that higher seeding rates can reduce iron chlorosis symptoms and increase yield in areas of fields with a history of iron chlorosis (Goos and Johnson, 2001; Naeve, 2006). Soybean roots excrete acids as they are growing, which increases the availability of iron. Higher plant density increases the amount of this acid in the root zone.

Variable rate seeding allows farmers to increase seeding density in areas of the field with a history of iron chlorosis and reduce it in areas that are not prone to iron chlorosis. Reducing the seeding rate in areas of the field that do not exhibit iron chlorosis can help reduce pressure from white mold.

3. Improve Soil Drainage

Soils with poor drainage often have higher accumulations of soluble salts and carbonates that reduce iron solubility in the soil. Wet soils also lead to lower oxygen levels in the soil and reduced root growth and health. The reduction in root health and the lower solubility of iron in wet soils are major contributors to IDC symptoms. Practices that improve soil structure and water infiltration can reduce issues with IDC. Field tile drainage is also important to consider where applicable to help with soil moisture levels.

4. Consider Your Herbicide Selection

Foliar and soil-applied herbicides may increase plant stress, accentuating IDC’s symptomology. Research has shown increased potential for greater yield loss when applying some post-emergence herbicides to soybeans under chlorotic stress. Reduce stress from herbicides by following manufacturer recommendations for weather and application conditions.

5. Use a Companion Crop

In fields with high levels of nitrates, a companion crop of oats may reduce iron chlorosis symptomology. This companion crop must be terminated when it is 10 to 12 inches tall. 

6. Iron Chelate

Iron chelate products have been evaluated as seed, foliar, and in-furrow treatments. The benefits of seed-applied and foliar-applied treatments have been inconsistent. Research by both Universities and agronomists has shown a more consistent yield response to in-furrow applications of iron chelate.

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