The following is a good article on using iron chelates to correct iron deficiencies in greenhouses and nurseries.
Plants take up nutrients only when they are in solution, which can be a problem with iron.At pHs above 6.5, interactions with calcium and other ions in the media solution cause iron to precipitate out of solution and become unavailable to plants.Using iron chelates can keep plants green and growing until you can drop the pH to proper levels.
Chelates can best be visualized as a lobster’s claw (appropriate since the word chelate comes from a Greek word meaning claw) made of carbon and hydrogen atoms holding an iron ion. The more bonds, called ligands, that form between the iron ion and the carbon atoms, the stronger the iron ion is held within the chelate.The strength of the chelate’s hold on the iron ion determines, as pH increases, how long the iron ion will continue to be in solution and therefore available to plants.
There are four commonly used chelates: citric acid, EDTA (Ethylenediaminetetraacetic acid), DTPA (Diethylenetriaminepentaacetic acid) and EDDHA (Ethylenediamine di(o-hydroxyphenylacetic acid)). According to data presented by Norvel (Equilibria of Metal Chelates in Soil Solution, in Micronutrients in Agriculture, Soil Science of America, 1972) citric acid does not strongly bond with iron and is not effective at pHs above 6.0. EDTA strongly holds iron in solution up to pH 6.0, but by pH 6.5, almost one-half the iron is precipitated, and by pH 7.0, almost none of the iron is available to plants.DTPA is an excellent iron source up to media pH 7.0; however, 60 percent of the iron is precipitated and unavailable by pH 8.0. EDDHA is the strongest chelate of any of the commonly used materials and maintains iron availability to plants past pH 9.0. These chelates are ranked in the same order of effectiveness by Drs. Bill Argo and Paul Fisher in Understanding pH Management, Meister Publications.
Chelates prove the maxim “You get what you pay for.” The cheaper the chelate, the less useful it is.You may have to sit down when you write the check for iron-EDDHA, but remember you use very little per plant and you are assured that iron is available to your crop no matter the pH.
Sometimes growers are puzzled by plants showing iron deficiency symptoms when the media pH is only slightly higher than desired.Phosphorus ties up iron and the combination of a weak chelate, slightly high pH and high phosphorus levels can cause iron to be unavailable.Another reason not to over apply phosphorus.
Reprinted from "Selecting which iron chelate to use" by Dean Krauskopf, MSU Extension, Southeast Michigan in the May 10, 2007 Greenhouse Alert newsletter from Michigan State University.