By Ron Honig- Horticulture Agent
April 2, 2024
Soil Amendments for Iron-Deficient Trees
Southwest Kansas homeowners are no strangers to iron deficiency symptoms in trees and shrubs. They may not know what the cause of the problem is for sure, but they have all seen the symptoms in their own yards or on a neighbor’s property.
Yellowing leaves, sometimes almost white, with green veins, is the textbook symptom of iron deficiency, or what is known as iron chlorosis. Homeowners with ornamental pear trees or silver maples most likely know this look well. Iron deficiency, when severe, prevents photosynthesis from taking place in the leaves and a shortage of food production leads to branch die back and sometimes complete decline of a tree. Early spring, is the recommended time to address this common issue and apply a soil amendment.
The cause of the problem is rarely a lack of iron in the soil, but rather high soil pH which causes the natural iron to be unavailable to certain plants. Scientists are not all in agreement as to if the problem caused by the high soil pH is a tie up of the iron to soil particles or an increased iron molecule size that is not absorbable by some plants.
Horticulturists and agronomists often feel a soil pH of 6.5 to 6.8 is a very workable level to grow most garden and field crops. Numerous soil tests taken in lawns in the district show a soil pH range of generally 7.9 to 8.0. This high of soil pH is a problem for several tree species we have growing in southwest Kansas, most notably maples and pears.
So, how do we deal with or remedy this problem? The addition of a chelated iron supplement can certainly help if we choose the right product. Most chelated iron products are not designed for a soil pH as high as ours in southwest Kansas. Several different iron formulations are available, but the formulation recommended for use in our area would be the EDDHA formulation.
The EDDHA chelated iron formulation will remain available to trees and other plants despite the high soil pH in this region. The other formulations will eventually tie up or become unavailable just like the natural iron. Ask your garden store professional for the EDDHA iron formulation if you plan to make an iron soil treatment to a tree or shrub.
The bad news is that iron is a not a mobile nutrient in soil, meaning if you apply it to the soil surface it is not likely to move down to the plant roots naturally on its own. Thus, creating a hole in the soil around your trees with a soil probe, drill bit, or even just a sharp rod or stick, opens up a channel for the iron product to move down to the tree roots very quickly.
Generally, horticulture experts recommend creating holes ten inches deep around a tree and in a grid pattern every two feet by two feet. This creates a field of treated spots with each being two feet from each other. The treated area should extend from the base of the tree trunk out to beyond the “drip line” or edge of the branch and leaf canopy about an additional six to ten feet in all directions.
The directions on the EDDHA iron product package will tell you how much of the product to use for the size of your tree, usually based on the diameter of the tree trunk. The product is normally dissolved in buckets of water and then dipped out and pored down the holes. How much one pours down each hole will vary from person to person but I usually pour two to three cups of the diluted solution down each hole and then water the area with a hose to help move the iron down to the bottom of the holes. It is not a precise application process.
Another school of thought is to add sulfur into the iron solution you are pouring down the holes to create a weak sulfuric acid solution and attempt to lower the soil pH in those spots and allow the natural soil iron to become available to the tree. This sulfur treatment can work but it is considered a slow conversion and may take three to five years to actually change the soil pH significantly.
Sulfur products such as elemental sulfur and iron sulfate dissolve slowly and sometimes incompletely in water, so give them time once mixed with the water and iron supplement. If adding iron sulfate, which does seem to dissolve better than elemental sulfur, use one pound of iron sulfate per inch diameter of the tree trunk, placed in holes in the yard around the tree as described above.
Iron injections directly into the tree’s trunk often give the quickest green-up. Unfortunately, there are not good injection kits available to homeowners. Generally, the K-State Extension Service recommends homeowners hire a lawn and garden professional to perform iron injections into their tree trunks, if homeowners wish to go that route.
For more information contact the Wild West Extension District office at 620-544-4359.
