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When leaves turn a sickly yellow....

Suspect iron chlorosis. Here are causes and cures

SICKLY YELLOW leaves like the ones in the photograph above are often the first signs that a plant in your garden may be suffering from the nutritional disorder called iron chlorosis.

Plants need iron to make chlorophyll, the green color in plant cells necessary for photosynthesis. But in the West, many plants don't get enough iron because their roots are unable to obtain it from the soil. Roots may be damaged by a lack of oxygen in overwatered or poorly drained soils--typical after a rainy spring--or by extreme soil temperatures. High concentrations of limestone (calcium carbonate) make the soil more alkaline, which makes the iron less soluble.

In severest untreated cases, the leaves can even turn almost white, and the plant can decline and eventually die.

Iron chlorosis is easy to confuse with other nutrient deficiencies. If a plant is iron-deficient, its newest leaves are yellower than old ones. If it's nitrogen-deficient, the old leaves are yellow and the new ones green.

Just about any plant can become chlorotic, but a few--including citrus and gardenia--are more susceptible than others.

CURING IRON CHLOROSIS

Here are some remedies. You may need to try more than one--improve your soil and apply iron chelates, for example--to cure your plant.

Improve the soil. Dig about four breathing tubes--holes about 2 to 3 feet deep and a foot wide--evenly spaced around each tree's drip line; fill them with compost. If necessary to improve soil drainage, regrade the soil surface or install an underground drainage system. Or move plants to raised beds with good soil.

Acidify the soil. Reducing soil pH below neutral to slightly acidic is the single best way to remedy iron chlorosis (it also improves air and water movement through soil). You can apply powdered soil sulfur, but that may require a lot of sulfur and a year or more to produce results. More soluble forms of sulfur with iron and manganese (such as DisperSul Plus Fe Mg) are available in some nurseries. To lower pH from 8.5 to 8 before planting, and to increase iron availability to roots, work 5 pounds of this type of product per 100 square feet into the top 4 to 6 inches of soil (but no more than 6 pounds per 1,000 square feet over turf). Water heavily, let soil drain, and then plant.

Apply iron sulfate. This is the cheapest and most widely available iron fertilizer; it's fine as a foliar spray but becomes insoluble on contact with alkaline soil (pH greater than 7.0). It stains concrete, stucco, or anything that contains lime. To increase its effectiveness, blend it with organic matter--2 pounds of iron sulfate to 2 pounds of peat moss or any good compost--and concentrate the mix in soil near plant roots. It costs $5 for a 5-pound bag.

Make acid spots. Instead of amending a large area to lower pH, apply soil sulfur in just a few places around a plant. Around a mature citrus tree, for instance, make four holes each about a foot wide and deep. Into each hole, place a mixture of 5 pounds soil sulfur, 1 pound ammonium sulfate, and 1 pound iron sulfate. Refill holes with soil.

Apply iron chelates. These organic compounds keep iron soluble and available to roots. Scatter dry granules within the plant's drip line, then water thoroughly so the chelate soaks into soil around roots. Leaves should start to green up in two to three weeks. Or spray liquids on the leaves. Follow package directions.

Some iron chelates hold onto iron more tightly than others. The four commonest ones listed below range from those that hold iron the least (top) to those that hold it tightest (bottom). Generally, the latter are the ones that work best in alkaline soils.

Citric acid. Best as foliar spray (it oxidizes rapidly in soil). If you apply too much, it can burn leaves. Inexpensive, and widely available.

EDTA (ethylenediamine tetraacetic acid). Effective only at a soil pH of 6.8 or less, a rarity in the low-rainfall West. Deactivated upon contact with clay or calcium carbonate in soil.

DTPA (diethylenetriamine pentaacetic acid), sold as Sequestrene 330 and Sprint 330, is useful where soil pH is 7.2 or less and soils are not high in clay. But in any soil it is useful on lawn grasses. Cost is about $30 for 5 pounds of 6 percent chelated iron.

EDDHA (ethylenediamine di-o-hydroxy-phenyl acetic acid), which is sold as Sequestrene 138 and Sprint 138, is a powder you mix with water. It's effective in soils of pH 7 to 8.7 or higher, and is the most reliable chelate for Western soils (it does not fix to clay); it's not recommended for turf.

Cost is about $100 for 5 pounds of 6 percent chelated iron (to treat a mature citrus in heavy soil, use 1/2 to 1 pound).
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:iron chlorosis
Author:MacCaskey, Michael
Publication:Sunset
Date:Mar 1, 1993
Words:819
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