Plants rely on iron for the most basic aspects of photosynthesis. Without this essential nutrient, plants can't create chlorophyll and in turn, they can't produce flowers and fruit. Fortunately, most U.S. soils contain enough iron for healthy plant growth. Excess iron can cause iron toxicity and affect other plant nutrients. Before you add iron to your garden soil, make sure a deficiency exists and understand its cause. Identifying the culprit helps put your garden back on track.
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Symptoms of Iron Deficiency
When plants lack nutrients, the leaves often reveal the problem. Unable to produce chlorophyll, iron-starved leaves lose their green color and turn yellow. Because iron doesn't move readily through plants, the youngest, newest leaves -- farthest away from nutrient pipelines -- turn yellow first. In iron deficiency, the veins of the leaves stay green, but the areas between veins turn yellow. Older leaves, closer to the main stem, stay green except in severe cases. This differs from nitrogen deficiency, where the entire leaf turns yellow, including the veins, and old leaves get hit first. With iron lacking, plants show weak growth, poor health, small flowers and few or small fruits, in addition to yellowing leaves.
When iron-deficient plants reveal themselves, the soil isn't necessarily low in iron. Yellowing leaves, called chlorosis, can be caused by other factors that keep the plants from absorbing iron. Iron must stay soluble for plants to use it. This happens best in soil with a low pH, also called acidic soil. In alkaline soil, the high pH limits access to iron even when it's abundant. In addition, anything that inhibits root activity interferes with iron uptake. Soggy, poorly drained soil slows root activity down, as does cold soil. Iron deficiency symptoms are most pronounced in cold soil with poor water management. High temperatures and intense light also lessen a plant's ability to take up iron.
In soil with a high pH, additional iron won't reach plants unless it takes a different form. Sprays that deliver iron directly to leaves give iron-chlorotic leaves relief. Use a water-soluble, all-purpose, acid-boosting fertilizer at a rate of 1 tablespoon fertilizer to 1 gallon of water, and spray affected foliage and soil. At temperatures above 85 degrees Fahrenheit, spray the soil only. Alternatively, use liquid iron at a rate of 1 1/4 ounces of liquid product per 1 gallon of water, or according to label instructions. Spray the leaves for a fast, but short-lived fix. Always wear protective eyewear and gloves when working with chemicals, and check the label before spraying specific plants, especially edibles. Spray iron carefully. All iron products can stain concrete and stone.
For longer-lasting results, modify your soil. If a soil test confirms high pH and adequate iron, add aluminum sulfate to acidify the soil. Application rates vary depending on soil type and pH, but a loamy soil may need 2.4 pounds of aluminum sulfate for each 10 square feet to lower the pH from 8.0 to 6.0. Clay soil can take twice as much aluminum sulfate as sandy soil to lower pH one unit. Alternatively, add iron to your garden soil in a form that stays available. Iron chelate powders containing EDDHA or EDDHMA, used according to product directions, provide long-term availability in high-alkaline and iron-deficient soils. For example, 2 ounces of iron chelate powder worked into soil around a small shrub's base provides a sufficient iron boost. You'll need to work in any of these amendments thoroughly into the soil.