Tips for Beginners: Chelated Fertilizers
The soils in southeastern Michigan for the most part have a pH of 7.0, which is too high for acid loving rhododendrons. I had been using pelletized sulfur to lower the pH. However, the underlying bedrock here is limestone and acidifiers do not have long lasting effects.
In 2001, I started using chelated fertilizers which the rhodies can intake at higher pH. I have had excellent results using them as a foliar spray, e.g., dark green healthy looking leaves, with good flower bud set. The two products I am using are: MICREL TOTAL™ (6-0-0) with nine chelated micronutrients (Hummert catalog # 07-0029-1 at approximately $30 per gallon of concentrate), and X-XTRA IRON™ 9% (Hummert catalog # 07-0041-1 at approximated $15 per gallon of concentrate). The X-XTRA IRON formula is 6% urea nitrogen, 4% sulfur, and 9% chelated citric acid iron. Both products can be obtained from Hummert International, which will now accept retail orders. Their website is user friendly: Hummert International, 4500 Earth City Expressway, Earth City, MO 63045; phone (800) 325-3055; website: www.hummert.com. or e-mail: firstname.lastname@example.org.
I apply both together as a foliar spray at the rate of two tablespoons each per gallon of water about two or three times early in the year (May and June) with a backpack sprayer. (Weldon Delp often said, "Rhodie leaves are ten times more efficient at absorbing nutrients then are the roots.") Foliar spraying also minimizes over or under feeding. Large plants with more leaf area receive more fertilizer; smaller plants receive a lesser amount.
I also started misting my seedlings with the MICREL TOTAL™, at the rate of one tablespoon per gallon of water with excellent results.
Harold Berg has been a member of the Great Lakes Chapter since 1976. He is a retired manager from the automotive industry, and currently tends 450 rhododendrons on five acres at his home in Rochester Hills, Michigan. He is also a hybridizer and receives seed from the A.R.S. Seed Exchange.
About Chelated Fertilizers
In the useful book Botany for Gardeners by Brian Capon, the author defines and describes the use of chelates in gardening: "At the other end of the pH scale [the alkaline end with high pH], toxic amounts of molybdenum are released in alkaline conditions; both phosphates and calcium are rendered unavailable to roots when they combine to form insoluble calcium phosphate; and iron and manganese become tightly bound into chemical complexes. Since the resultant iron deficiency is fatal to many species, gardeners who work with alkaline soils must supply the metal in chelated form. Chelates (soluble organic compounds to which iron is bound) make the metal available to plants without toxic effects. The chelate is eventually broken down by micro-organisms. Two commonly-used chelating agents are known by the acronyms EDTS and EDDHA."
Several authors of books on rhododendron culture mention the use of iron chelates in combating iron deficiency. Ed Reiley in Success with Rhododendrons and Azaleas says that iron deficiency is often the result of high pH (alkaline soil) and that iron chelates may be used as a quick, temporary solution to the problem but recommends acidifying the soil as a long-term solution. Kenneth Cox in Rhododendrons, A Care Manual suggests that when leaves turn yellow there may be iron deficiency and suggests trying a small amount of iron chelates on a few plants to see if the problem disappears. Clement Gray Bowers in Rhododendrons and Azaleas also mentions that iron chelates may help with iron deficiency as does David Leach in Rhododendrons of the World. Peter Cox in The Cultivation of Rhododendrons also recommends iron chelate for iron deficiency but warns that overdoses may cause withered and curled leaves.
The product that Harold Berg recommends, MICREL TOTAL™, is a fully chelated product. It is described by the manufacturer, Hummert's: "A complete fully chelated micronutrient package with an agronomically correct ratio of 8 micronutrients including Mg, S, B, Cu, Fe, Mn, and Zn. Used to correct deficiencies in greenhouse and nursery crops, trees and ornamentals. Apply to foliage for immediate response or to growing medium."
The other product mentioned by Mr. Berg, X-XTRA IRON™, is described by Hummert's: "A citric acid chelated iron with 20% more chelated iron than other liquid iron products. Excellent for foliar feeding in which the iron and nitrogen citric chelate is quickly absorbed by leaf tissue. Also buffers alkalinity in water (potential acidity of 872 lbs. CaCO3 equiv.)"