Why Iron Fertilizers Don't Work

Q. You mentioned on your blog that chelated iron won't green up yellow leaves when it's applied to the soil outside of the spring months. Why does this happen? We are a major tree company and see this sometimes. Also, how long should it take to see leaves become green again after we treat the tree with iron?

A popular iron fertilizer that takes the shotgun approach is Kerex. It contains EDDHA chelate in small amounts along with iron oxides, iron sulfate, Milorganite and other ingredients that are a hodgepodge of fertilizers. 

A. Two methods are used here to green up yellow leaves. It has a lot to do with the chemistry of the soil, more specifically how alkaline it is, whether it is an iron problem or not, and the type of chelate used in the iron fertilizer. It can get hard to predict if it will work or not. You will see later I will sometimes “shotgun” it if I am confident the yellowing is due to iron and the problem can be corrected.

This, Sprint 138, manufactured by BASF, is an excellent EDDHA iron chelate and works great but it is expensive. One 5 lb bag is typically over $100. Less expensive EDDHA iron chelates are manufactured by China but I wonder sometimes about their quality. I have seen mixed results with off brands.

            If I have the time, I will start with a spray bottle of iron fertilizer to see if an iron application will work or not. Spraying this liquid mixture on yellow leaves will tell me if the yellowing is due to iron or not. But it takes about 24 hours to find out.

This is a 1 lb cannister of EDDHA iron chelate but manufactured by an unknown source and marketed by an American company. It costs considerably less than Sprint 138 and works sometimes, and sometimes not.

            If the leaves change to a darker green in 24 hours, then an iron fertilizer application will work. Otherwise it’s a guessing game. For some people, 24 hours is too long to wait for an answer.

This is another iron chelate labeled Sprint 330 and contains a different chelate called DTPA. It works IF the soil alkalinity is measured less than a pH of 7.6. Most desert soils are not that low in pH and so it does not work in many soils. It costs about half the amount as Sprint 138 so its tempting to buy.

            Here’s where it can get tricky. If it is early enough in the year and the tree is still producing new leaves, I can apply an iron fertilizer to the soil, and it should work. If it is later in the growing season and the tree has, for the most part, stopped growing and preparing for next years growth, then the only thing that will work is spraying the leaves with a liquid iron fertilizer.

This is another iron fertilizer made from acidified mine tailings that contain iron. If the pH of a soil is low enough then iron will be available to the plant.

            If an iron fertilizer is applied to the soil, then the choice of iron fertilizer to use can become critical. If the soil is in rough shape and neglected, then use an iron fertilizer containing the chelate EDDHA. I would, in fact, use that iron chelate fertilizer whenever trees and shrubs are surrounded by rock. Because this can be a guessing game, I only recommend iron fertilizers that contain the EDDHA iron chelate. Its more expensive but I am more confident it will work under most circumstances.

Another way of correcting yellowing due to unavailable iron...iron sulfate. It will work as a foliar spray if the iron spray solution has a pH of about 7 or a little above. Works great on lawns. But it will stain fruit, concrete walks and pools.

            If the soil is in good condition, then usually any iron fertilizer containing an iron chelate should work. If you are price sensitive and always buy the least expensive product then, because of the soil chemistry, your selection may or may not work. I understand the reasoning, but it is a gamble. Sometimes you win, sometimes you lose.

Iron sulfate staining grapes

Iron sulfate staining concrete

            If you see leaf yellowing about halfway through the year or later, then soil applications wont usually work. You must spray the leaves with a liquid iron fertilizer to cause the leaves to change from yellow to green. This can mean multiple applications of an iron spray. Please read my directions on how to make liquid iron applications to the leaves on my blog to improve your chances of success.

Iron fertilizer containing HEDTA chelate plus 3% water soluble nitrogen and potassium for foliar spraying. If the yellowing isn't due to iron, then the nitrogen will green it up.

            What to do? I usually do both treatments, shotgun it, if there is a problem and I want the best chance of success. Two applications can be cheap insurance. If spraying works, it will cause the leaves to turn green overnight. Soil applications might take up to a week to see a change in leaf color and then it will be only in the new growth.

            But applying the right iron fertilizer before the leaves come out in late winter or early spring can pay some dividends if you suspect leaf yellowing might occur.