Iron Deficiency in Rhododendrons
P. B. Orr, Jr., Knoxville, Tennessee
As any new seriously interested rhodie fancier must do, I have avidly devoured all the literature on the subject I could find in this area. One of the first things learned was the recognition of the symptoms of iron chlorosis, as shown by the yellow leaf with dark green veins.
For the last three years I have tried my hand at raising rhodies under lights, using A.R.S. Seed Exchange products, and hopefully each year using improved techniques and better results. This past spring, several species and hybrid seeds were planted. some of these were the Smith-Mossman R. occidentale seeds which germinated and have done well. Periodically, all the seedlings were treated with a homemade fertilizer concoction, for growth inducement.
When the plants were about one and a half inches high, the symptoms of iron deficiency in the leaves of the occidentale seedlings became apparent, before it showed up in the evergreen rhodie seedlings. It progressed rapidly enough that a few plants died before the treatment with chelated iron was effective.
Reflection on this chlorosis made the fertilizing technique suspect, since it appeared rather suddenly, and since it would seem that there should have been enough trace iron in the peat and river sand mixture to amply supply these little plants.
The fertilizer was a solution approximately two and a half percent nitrogen (ammonia form), phosphorus, and potassium, with the pH on the acid side. This was diluted to a very weak solution before being used on the tender plants. Of the three elements contained in the mixture, only the phosphorus appeared capable of precipitating (tying up) the iron so that it was unavailable for plant assimilation.
Some of the information available to the amateur has indicated that besides the usefulness of ferrous sulfate as an acidifying agent, it was reported to be also useful in the treatment of iron deficiency. This was questioned when a small portion of the rhodie garden was treated with ferrous sulfate to rectify an acidity problem where leaves and brush had been burned a year or two previously, and where a few 4 or 5 year old plants have turned up with chlorosis. These plants also had received an annual super phosphate treatment each October for bloom stimulation.
As the result of these experiences, a few simple tests were made in the laboratory to determine whether commercial 5-10-5 fertilizer and triple super phosphate would precipitate iron from solution. Ten grams of the 5-10-5 fertilizer and ten grams of triple super phosphate were boiled in separate beakers of water to leach out the soluble components, then filtered from the insoluble filler. The pH of the 5-10-5 solution was 4.0, and the super phosphate 2.5. The insoluble filler from the 5-10-5 fertilizer contained small eighth inch pebbles which turned out to be calcium carbonate (either limestone or marble granules).
Two 1 gram portions of ferrous sulfate were dissolved in small amounts of water. One 1½ gram portion of the filtered solution of the 5-10-5 fertilizer was mixed with one gram of ferrous sulfate, resulting in the formation of a heavy precipitate of ferrous phosphate. When the filtered super phosphate was mixed with the ferrous sulfate. a similar precipitate was formed.
The above tests were made assuming that the iron in the soil was the ferrous form (II valence) and not ferric (III valence). This assumption was made due to the presence of much organic matter in the soil which can act as a reducing agent. However, since ferric iron is very stable and since much of the iron ores found in nature are ferric, the previously described tests were repeated using ferric sulfate instead of ferrous sulfate. The results were the same, with copious quantities of ferric phosphate being precipitated.
As a final test, an iron chelate (FeDTPA) was investigated under the same conditions, replacing the ferrous and ferric sulfates. No iron was precipitated from solution, showing no unusable phosphate of iron was formed.
From the rhododendron iron deficiency experience and from these described tests, it is my conclusion that when a chemical fertilizer containing phosphorus is used on rhododendron plants, a subsequent treatment of iron chelate should also be used to prevent these plants from becoming chlorotic. One phosphorus treatment may not cause a deficiency, but it is my belief that problems will arise sooner or later. Most garden centers stock a FeDTPA chelate which will do the job nicely.
The fact that the insoluble filler of the 5-10-5 "acid form fertilizer suitable for azaleas and camellias" turned out to also contain calcium carbonate will cause me to investigate future fertilizers more thoroughly. Although the soluble portion of the fertilizer was acidic, the long term effects were the opposite, due to the calcium carbonate granules. The acids in the soil will slowly leach the pebbles, actually raising the pH of the soil. If a fertilizer label does not indicate what the filler is, the salesman is not apt to know either. In areas of the country where soil alkalinity is already a problem, there is no advantage to compounding difficulties. The proper filler for a good "azalea" fertilizer should be silica sand, which has neither acid or alkaline long term effects.