The primary symptom of iron deficiency is interveinal chlorosis – the development of a distinctive yellow leaf with a network of dark green veins. When severe, entire leaves turn yellow or white and the outer edges may scorch and turn brown as plant cells die.
Symptoms do not show up on cotyledon (seed leaves) or unifoliate (single leaf) leaves. Initial chlorosis symptoms will occur on the trifoliate leaves, beginning as early as the first trifoliate stage.
Symptoms may increase or decrease in intensity during the season depending on growing conditions. Iron chlorosis in a soybean field occurs in spots and usually in a random pattern, depending on chemical and physical soil differences in the field.
Facts on Iron Chlorosis
Iron deficiency severely limits growth of soybeans in several regions of North America, particularly in poorly drained calcareous (heavy lime) soils in parts of Minnesota, the Dakotas, Nebraska and Iowa.
Iron deficiency chlorosis is a complex plant disorder often associated with high pH soils and soils containing soluble salts where chemical conditions reduce the availability of iron.
Environmental and soil conditions including compaction, excessive soil moisture and low soil temperatures can contribute to iron chlorosis severity.
The calcareous soils where iron chlorosis frequently occurs are also referred to as alkaline soils and are characterized by their high pH values (>7.5). However, chlorosis of soybeans does not occur on all high-pH soils. Soil tests of the surface soil in chlorotic and non-chlorotic areas of a field often are similar; however, the subsoil may be greatly different.
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. Most soils contain abundant levels of iron; however, deficiencies develop because soil chemical reactions render the iron unavailable to plants.
At high soil pH, iron is not soluble and cannot be absorbed by plant roots. Oxygen is needed in the root zone for plants to take up iron. Soil compaction, excessive rainfall or irrigation can result in poorly aerated soils and reduced iron uptake. Also, lower soil temperatures reduce soil microbial activity, which in turn reduces iron uptake.
Finally, plant genetics can be an important factor influencing the plants’ ability to take up iron in iron-deficient growing environments.
Management Considerations of Iron Chlorosis
Selection of varieties with good iron chlorosis tolerance is required to overcome iron chlorosis-related losses.
In recent years there has been a substantial improvement in the development of varieties that are tolerant of conditions leading to iron chlorosis.
In addition, the following management practices can help reduce the severity of iron chlorosis symptoms in soybeans:
Test soil for soluble salts in all fields suspected of having an iron chlorosis problem. Also consider testing for soybean cyst nematode (SCN), which can mimic iron chlorosis symptoms.
Use higher seeding rates (200,000 – 250,000 plants/ acre) in chlorosis-prone areas
When applying herbicides to chlorosis-prone fields, follow label instructions carefully to avoid stressing soybean plants. Avoid using aggressive postemergence herbicides
Under severe chlorosis stress, apply foliar iron chelate or iron sulfate treatments to problem areas beginning at the second-to-third-trifoliate (V2-V3) stages of growth.
Improve soil drainage to aid in getting rid of excess moisture in high salt areas.