IRON DEFICIENCY OF LANDSCAPE PLANTS
J. R. Hartman, M. L. Witt, K. W. Wells, and W. O. Thom
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 both the
black and white oak groups, white pine, rhododendron and sweetgum. The
problem is most commonly observed on pin oak, azalea and rhododendron.
Not all mineral deficiencies are iron-related, however. In these same high
soil pH sites, maples may develop manganese deficiency and pecans may show
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, necrotic 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 terminals, may occur. In addition, terminal 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.
Correcting Iron Deficiency
Start corrective action when you first
observe and confirm the deficiency. If treatment is delayed until most
of the terminal growth becomes chlorotic and twig dieback is prevalent,
it may be too late to effect a cure. After treatment and for the next year
or two, keep the plant growing vigorously by timely watering and fertilization.
(1) Acidifying the soil
Although it would appear that adding
more iron to the soil or tree would help, changing soil pH can make available
iron already present. Other mineral elements may also be present in excess
or be deficient in a high soil pH. For instance, chlorotic oaks may be
especially high in phosphorous, potassium and nitrogen, while low in iron,
manganese, zinc and/or molybdenum. On the other hand, chlorotic red maples
may be low in manganese, but high in potassium and iron. Consequently,
altering soil pH by making it more acidic usually gives the best long-term
results. Based on research and observations, the following procedures are
Elemental sulfur treatments:
* Spread (broadcast) sulfur evenly
on the sod surface over the root zone. Beginning 1 ft from the trunk and
working out, cover a distance equal to twice the distance from the trunk
to the dripline. We recommend using as much as 2 to 10 lb/100 sq ft of
soil surface area, depending on the pH change you want. See Table 1.
•In extreme circumstances where more
than 10 lb sulfur/100 sq ft is to be used, you need to remove the sod and
replace it with wood chips. This amount of sulfur is excessive for quality
•Avoid heavy applications of sulfur
to poorly drained soils to prevent formation of toxic hydrogen sulfide.
Sulfuric acid treatments:
Although sulfuric acid has been used
experimentally with mixed results, handling problems and potential danger
of such treatments do not warrant their use. Do not use sulfuric acid
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.
Pre-plant bed and planting hole treatments:
Acidifying soil is much easier when
you do it before you plant acid-loving plants. Have soil tested to get
information on its current pH. The pH can be lowered using elemental sulfur,
which should be incorporated into the soil at least 2 inches deep before
planting. Refer to Table 1 for the amount needed to bring about the desired
pH change. These amounts will effectively treat about 55 cu ft of soil
(100 sq ft x 6-7 inches deep).
If the tree or shrub planting hole
is deeper than 7 inches, you may need to treat the back fill soil. A bare
root tree planted in a 2 ft diameter hole, 2 ft deep, may need 5.5 cu ft
of back fill soil, so about 1/10 the amount of sulfur needed to treat 100
sq ft as suggested in Table 1 is needed per hole. Balled-and-burlapped
trees generally need little back fill soil, so the treated soil in the
surrounding landscape should suffice. Acidifying only the soil in the planting
hole is not enough for most woody plants. Roots will grow far beyond the
Acid peat may be mixed with bed and
planting hole soil at a rate of 20 percent of the soil volume when only
a slight pH correction is needed. This will help for 1-3 years until long-term
pH correction with sulfur can be done.
Table 1. - Amount of sulfur needed to lower soil pH.*
*Since pH is usually reduced on a per plant or small bed basis, pt/100
sq ft is useful. For ease in measuring, pints rather than pounds of sulfur
are used here. One pint can approximate one pound, depending on how fluffy
or granular the sulfur used. Although aluminum sulfate often is recommended
to gardeners for increasing the acidity of the soil, it has a toxic salt
effect on plants if large amounts are used and small amounts are not very
effective. You need about 7 lb of aluminum sulfate to accomplish the same
effects as one lb of sulfur.
|Original pH of Loam
Soil (based on
water pH value)
Pints of sulfur/100 sq ft needed to reach pH of
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
•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
•Phosphorous and potassium fertilizers.
(2) Applying iron to the soil
Iron can be added to the soil as water-soluble
iron chelates. Chelated iron is sold in many forms and most brand names
have the words "Fe" or "iron chelate" in them. You can apply material to
the soil of the root zone either dry or in solution, any time during spring.
Follow label directions for the amount to use.
Chlorotic foliage should become green
within a month and the treatment may last several months. Results, however,
may be inconsistent. Soil treatments for iron deficiency will not work
where high soil pH is also inducing other nutrient imbalances.
(3) Applying iron to the plant
Iron can be applied directly to the plant in one of these ways:
1.Spray soluble forms of iron, like
iron sulfate or chelated iron, on foliage in late spring. This method gives
quick but short-term recovery from chlorosis.
2.Implant iron in the tree trunk (Medicap
implants) via capsules containing iron citrate or related materials. Green
color may be restored to the foliage in a few weeks and may last a year
3.Inject iron solutions into the trunk
via feeder tubes (Mauget system).
Although implanting or injecting iron
does give short-term benefits, long-term results are not always possible
and tree injury remains a problem. Injection treatments are normally done
by professional arborists.