Iron deficiency is a common problem of landscape plants in some Kentucky locations. This condition, also referred to as iron chlorosis or lime-induced chlorosis, occurs where soil is pH neutral or alkaline (pH 7.0 or above).
Even a soil pH of 6.5 will cause problems for some plants. Woody plants that are particularly sensitive to iron deficiency, and consequently high pH, include: azalea, some birches, blueberry, dogwood, American holly, magnolia, various oaks in the black and white oak groups, white pine, rhododendron and sweetgum. The problem is most commonly observed on pin oak, azalea and rhododendron.
Chlorosis of younger leaves is the most common distinctive symptom of iron deficiency. Early symptoms are green leaf veins with yellowish or whitish (chlorotic) color between veins. As the problem worsens, newest leaves may become nearly white because leaf veins and areas between veins lack any green or yellowish-green coloring. Affected leaves are generally smaller than normal. Brown areas may develop along leaf margins and between veins in severe cases. These symptoms may be similar to those caused by other nutritional disorders.
To positively identify the problem you need to have a soil test and, in many cases, a leaf analysis. When iron deficiency symptoms progress further, some older affected leaves may become tinted reddish brown. Leaf drop, beginning at the tips of branches, may occur. In addition, tip growth is stunted and twigs may die back. Over a period of years, unless treatment is given, branches will die back and the entire plant may succumb.
Start corrective action when you first observe and confirm the deficiency by a soil test and having leaf samples diagnosed at the Extension Office. Here are some ways to start correcting the problems, should tests come back positive for iron chlorosis:
>Acidifying the soil. Altering soil pH by making it more acidic usually gives the best long-term results. Spreading elemental sulfur evenly on the sod surface over the root zone can be done. However, the application of too much sulfur is detrimental to turf and other plant growth. Avoid heavy applications of sulfur to poorly drained soils to prevent formation of toxic hydrogen sulfide.
An acidifying fertilizer such as ammonium sulfate can be used to help maintain low soil pH levels. To avoid over-fertilizing turfgrass, apply 0.3 lb of ammonium sulfate/100 sq ft to the soil surface beneath the crown and 1.2 lb/100 sq ft beyond the dripline annually. Divide applications over fall, winter and early spring.
Acidifying soil is much easier when you do it before you plant acid-loving plants, such as azalea and rhododendron. Have soil tested to get information on its current pH. Just remember, acidifying only the soil in the planting hole is not enough for most woody plants. Roots will grow far beyond the original hole.
>Apply iron. Another method to help correct chlorosis is to apply iron. Iron can be applied to the soil, in the form of chelated iron, which can be found in many garden centers. This should be applied in the spring, and label directions need to be followed for application.
Iron can be applied directly to the plant in the form of sprays. This method is short term, but may be beneficial until other methods can be done. Also iron can be injected directly in the tree trunk. This method must be done by a trained, professional arborist for the best results. This is also a short-term fix.
>Keep pH proper. To keep your soil pH in line for acid loving plants avoid the use of: Alkaline hard water - frequent irrigation with water from a public water supply may cause an increase in soil pH and progressive chlorosis in affected plant(s). Use rain or similar water instead.
Limestone, as fertilizer, or materials containing limestone as mulch around the base of newly placed plants. Soil near buildings or masonry walls for plantings - pH in these areas may be higher than native soil in other locations.
Phosphorous and potassium fertilizers. Nitrate-containing fertilizers.
For more information on iron chlorosis in plants log on to http://www.ca.uky.edu/agc/pubs/id/id84/id84.htm or call and ask for ID-84, Iron Deficiency of Landscape Plants. Contact the Franklin County Extension Office, 695-9035 or e-mail Kim.Cowherd@uky.edu.