Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209708
07/10/97
Secondary and Micronutrients for Vegetables and Field Crops Extension Bulletin E-486, Revised August 1994
By M.L. Vitosh, D.D. Warncke and R.E. Lucas Department of Crop and Soil Sciences Michigan State University Extension
Iron is a constituent of many organic compounds in plants. It is essential for synthesizing chlorophyll, which gives plants their green color. Iron deficiency can be induced by high levels of manganese. High iron levels can also cause manganese deficiency.
Iron Deficiency Symptoms
Deficiency symptoms are marked and show up first in terminal leaves as a light yellowing. The symptoms are very similar to those of manganese deficiency (Vis. 26). A lack of iron in field and vegetable crops is not common in soils with pH below 7.0.
Iron deficiency is common in the western states, where the soils contain considerable sodium and calcium. In Michigan, woody plants such as pines, pin oaks, roses, certain ornamentals and acid-demanding plants such as blueberries, azaleas and rhododendrons may need iron. Lawns, particularly putting greens on golf courses, sometimes show a lack of iron because of high pH and high levels of phosphorus.
Iron deficiency in many woody plants appears when they are grown in soils low in organic matter and high in pH. Mixing in organic materials such as manure or acid peat will help increase the availability of the iron.
Sphagnum peat moss in mixtures with sand, perlite or vermiculite intensifies the need for iron fertilizer in the production of petunias, snapdragons, tomatoes and other bedding plants.
Correcting Iron Deficiency
Soil treatments usually require applications of iron chelates at a rate equivalent to « to 1 pound of iron per acre. Often it is difficult to correct iron deficiency with soil applications when soils are alkaline. Soil applications are effective if soils are acid or neutral in reaction. Under alkaline soil conditions, foliage sprays are recommended. Use iron sulfate, iron chelates or iron citrate according to the supplier's recommendations. Wet foliage thoroughly. Iron chelates, though more expensive than iron sulfate, persist longer.
Sometimes the best cure for Fe deficiency is to grow varieties that are not sensitive to Fe deficiency. For instance, some soybean varieties are more sensitive to Fe deficiency than others. For bedding plant production, use 1 to 2 ounces of elemental iron per cubic yard of soil mix.
To help prevent an iron problem, avoid using excessive amounts of lime or phosphate. Apply chemicals or fertilizers to increase the soil acidity and add organic matter.
Iron Toxicity
Injury due to high soil iron concentrations is not common under neutral or high pH soil conditions. Toxic situations occur primarily on acid soils (< pH 5.0) and where excess soluble iron salts have been applied as foliar sprays or soil amendments. The first symptoms of iron toxicity are necrotic spots on the leaves.
An unusual form of iron toxicity has been observed in Michigan on organic soils and high organic sands. Some iron-rich, low pH, low manganese soils create an environment in which an interaction between the iron and manganese in the soil reduces manganese uptake by plants. The symptoms observed on the plants are of manganese deficiency, but the low plant uptake of manganese is caused by excessive available iron in the soil. The addition of iron chelates or manganese chelates, which rapidly convert to the iron form under these soil conditions, aggravates the situation by increasing the amount of available iron and without solving the manganese deficiency problem.
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| Iron-deficient corn - 31K |
