Iron Poor Plants
by Dr. Robert Black, Consumer Horticultural Specialist

What do you do for a sick, yellow azalea, ixora, gardenia, Japanese privet or hibiscus plant? Horticulturists and gardeners in the know would prescribe iron, the same thing physicians at times prescribe for run-down people. Yes, iron is a tonic for plants, as well as for people.

When plants lack iron, the leaves turn yellow between the veins and may eventually die and drop off. This happens even when plenty of fertilizer has been added to the soil. The iron may be in the soil, but the plant can't get it. Why not? Because in either slightly acid or alkaline soil iron often combines with another chemical and becomes "tied up" so that plants such as azaleas, hydrangeas, gardenias and others can't get it. No amount of fertilizer that does not make the soil more acid will help.

The solution to this problem would be either add iron or to add an acidifying agent to the soil. If iron is added, it has to be in a form plants can use. Such a form is called an iron chelate. This form of iron does not get locked up with other soil chemicals and is available to plants. They are sold under various trade names and in various formulations and can be applied to the foliage or to the soil.

The recovery of plants from an iron-anemic condition can be amazingly fast. A sick, yellow plant sprayed with iron chelates can be a lush green in 48 hours. When sprayed on plants, chelated iron is absorbed by the leaves. Some chelates are also mixed with water and poured over the soil and taken up through the roots. When this is done, green color will return to treated plants within a week if iron is lacking.

The other solution to iron deficiency problems is to treat the soil with an acidifying agent such as elemental sulfur or aluminum sulfate. These materials, when applied in the proper amounts, will correct the basic cause of iron deficiency which is soil alkalinity. Use sulfur at the rate of one pound per 100 square feet of area per application and do not use more than two to three applications per year. Sulfur must be washed into the soil immediately after application or it will severely burn grass or shallow plant roots. Aluminum sulfate can be applied at the rate of one-fourth pound per square yard of bed area. Do not apply aluminum sulfate more often than one time in a 12-month period. Repeated use of aluminum sulfate can lead to a toxic condition.

Iron sulfate can be used to supply iron to plants. This material is usually applied to the soil and serves not only to supply iron but also to reduce soil alkalinity.

For soil treatment around trees, shrubs, vines, and flowers, the solution is made by dissolving one pound of iron sulfate per gallon of water. For dormant trees, use one gallon of the solution for each foot of the diameter of the drip line. This rate should be cut in half during the growing season. Put solution in holes deep enough to hold one gallon of solution (one-half gallon during growing season) around the drip line of the tree at intervals of about three feet. The holes should be filled with water once or twice, letting the water soak away. Then refill the holes with soil.

Shrubs and vines can be treated by digging a trench four to six inches deep around the drip line of the plant and pouring in from two to five gallons of iron sulfate solution, depending on the size of the plant.

For lawns, iron may be applied in summer to provide dark green color without stimulating excessive grass growth. Apply ferrous sulfate (liquid iron) at the rate of 2 ounces in 3 to 5 gallons of water per 1000 square feet of lawn. Stronger solution could burn the grass.

Help your run-down plants back to health by giving them an iron tonic. Use one of the methods given above to supply needed iron.