QBARS - v23n1 Iron Deficiency on Acid Soil

Iron Deficiency On Acid Soil
Eldred Green, Chicago, Ill.

A question arose at a recent meeting of the Midwest Chapter concerning some ailing rhododendrons. The problem was the decline and death of several established plants in one planting of a garden. The older plants seemed to be affected.
The symptoms were: mottling of the leaves with the green being definitely lighter in the areas between the veins so the veins stood out as dark streaks on the surface, and a reduction in the new growth with the foliage being smaller although buds were often formed. Death occurred by a failing root system and death of the stems.
The leaf showing the symptoms was checked against slides from the National Plant Food Institute which showed deficiency diseases of related plants such as azaleas and blueberries. The decision was that the leaf showed signs of iron deficiency. This was a puzzler. The plants had been fertilized with a well-known brand of azalea food. The soil had an abundance of humus and good drainage, and the soil reaction was slightly acid. Iron deficiency would have been suspect in an alkaline soil but this condition did not exist here.
A book was consulted to seek to verify the diagnosis made from the slides. The book, Hunger Signs in Crops, is a compilation of articles dealing with deficiency studies on major crops. Unfortunately this book has long been out of print. On page 9 a significant statement was found. It stated that iron deficiency could occur in acid soils if a large amount of soluble phosphorus is used. This causes insoluble iron phosphates to form. A check revealed that soluble phosphoric acid had been used in addition to the fertilizer on the affected plants. The extra phosphorus had been applied to aid in winter hardiness and to induce better blooming.
The diagnosis seemed to fit, iron deficiency resulting from soluble phosphorus locking up the available iron. All the necessary elements were there. The fact that the older plants were most affected was due to their having been treated for several years while the new plantings were not treated the first year.
This seems to be one of those very rare occurrences where the plants were literally killed with kindness. Inasmuch as iron deficiency in acid soils is almost un-thought of it is hoped that this bit of information may be useful to others who might be tempted to be too good to their plants.