Iron is a micronutrient integral to the formation of chlorophyll, the green pigment necessary for photosynthesis, and is also involved in cellular respiration. Without sufficient available iron, a plant cannot effectively convert sunlight into energy, leading to a decline in health and vigor. Correcting an iron deficiency requires a two-part approach: providing an immediate iron source and implementing long-term strategies to ensure the soil makes iron available. This guide provides practical methods for identifying and correcting iron deficiencies.
Recognizing the Need: Identifying Iron Deficiency in Plants
The most recognizable sign of iron deficiency is chlorosis, a yellowing of the leaves due to a lack of chlorophyll. Symptoms appear first on the newest or youngest leaves because iron is immobile and cannot be moved from older growth. This results in interveinal chlorosis: the tissue between the leaf veins turns yellow, while the veins remain dark green. Severe cases may cause the entire new leaf to turn white, potentially leading to reduced leaf size or branch dieback.
Iron deficiency is usually an availability problem, not a lack of the element in the soil. In alkaline soils (pH greater than 7.0), iron reacts chemically to form insoluble compounds that plant roots cannot absorb. Iron is most soluble and available when the soil pH is slightly acidic, typically between 5.0 and 6.5.
A soil test is the most reliable way to confirm the underlying cause before treatment. The test provides the soil’s pH level, the primary factor determining iron availability, and confirms the actual iron content. Correct identification prevents misapplication of amendments, as other nutrient deficiencies can show similar yellowing symptoms.
Choosing the Right Iron Amendment
Selecting the correct iron product depends entirely on the soil’s pH level. For neutral or slightly acidic soils, iron sulfates (ferrous sulfate) are a cost-effective choice. Ferrous sulfate is readily soluble in water, but when applied to alkaline soils, the iron rapidly reacts and becomes unavailable to the plant.
For alkaline soils, especially those above pH 7.0, iron chelates are the superior option. Chelation surrounds the iron molecule with a protective organic coating, keeping the iron soluble and preventing it from being locked up by the high pH environment. Different chelates offer varying degrees of stability.
For slightly alkaline soils, cheaper chelates like EDTA or DTPA can be effective. For very high-pH soils (exceeding 7.5), the more stable chelate EDDHA is recommended. Although EDDHA is more expensive, it maintains iron solubility up to about pH 9, making it highly effective for long-term soil application. Organic sources like compost or blood meal supply small amounts of iron and act as natural chelating agents, but they are too slow for correcting a severe deficiency.
Application Techniques for Maximum Absorption
Iron amendments can be applied directly to the soil or as a foliar spray to the leaves. Foliar application offers the quickest relief because the iron bypasses the soil and is absorbed directly through the leaf surface. This method is useful for an immediate fix when plants are severely chlorotic or when soil application is ineffective due to high soil pH.
For foliar spraying, the amendment (chelated iron or ferrous sulfate solution) should be mixed according to instructions and applied as a fine mist. To maximize absorption, consider adding a wetting agent or spreader-sticker to the mixture. Apply during cooler parts of the day, such as early morning or late evening, to prevent leaf burn (phytotoxicity) that occurs when concentrated solutions dry rapidly in hot sunlight.
Soil application involves dissolving the iron product, typically a chelate or ferrous sulfate, and applying it as a drench around the base of the plant into the root zone. This provides a longer-lasting supply of iron where the product remains soluble. After drenching, a light watering helps move the solution deeper for root uptake. Granular forms are spread on the surface and watered in, providing a slow-release effect.
Long-Term Soil Management to Improve Iron Uptake
Addressing the underlying cause of iron deficiency, frequently high soil pH, is necessary for a lasting solution. Sustained pH reduction can be achieved by adding elemental sulfur to the soil. Soil microbes convert the sulfur into sulfuric acid, which slowly lowers the pH over time, making existing iron more soluble and accessible. Since this process is gradual and can take up to a year, it functions as a preventive measure rather than a quick fix.
Incorporating organic matter, such as compost, aged manure, or sphagnum peat moss, also improves iron availability. Organic matter naturally chelates iron, holding it in a form that is less likely to become insoluble and more easily taken up by the plant. Organic materials also improve soil structure and drainage, which is important because poor drainage and compacted soil limit root function and exacerbate iron deficiency.
Proper watering habits prevent the recurrence of iron deficiency. Overwatering or planting in poorly drained areas reduces soil oxygen levels, limiting the root’s ability to absorb nutrients, including iron. Ensuring the soil is moist but not waterlogged helps maintain healthy root activity, allowing the plant to utilize available iron.