Manganese (Mn) is a micronutrient required in small amounts, but its role in plant health is significant, particularly in photosynthesis. It facilitates the splitting of water molecules within Photosystem II, the initial step in converting light energy into chemical energy. Mn also serves as a cofactor, activating over 35 enzymes that regulate metabolic processes like nitrogen assimilation and carbohydrate use. A sufficient supply contributes to stronger cell walls by playing a part in lignin synthesis, helping plants resist physical stress and disease. Even when present in the soil, manganese is often unavailable to plants. This article provides practical guidance on identifying deficiency and applying corrective measures.
Recognizing Manganese Deficiency
A manganese deficiency typically presents as interveinal chlorosis: the yellowing of leaf tissue between the veins while the veins remain green. This chlorotic pattern gives the leaf a net-like or checkered appearance, often pronounced on broadleaf plants. Since manganese is relatively immobile within the plant, it cannot be easily moved from older leaves to newer growth.
Deficiency symptoms first appear on the younger leaves at the top of the plant. If uncorrected, the yellowing progresses to include small, brown or black necrotic spots, and overall growth may become stunted. Visual symptoms can be confused with deficiencies of other nutrients like iron or magnesium, making a professional soil test or tissue analysis the more reliable way to confirm the diagnosis.
Factors Limiting Manganese Uptake
Manganese is absorbed by plant roots in its divalent form, \(\text{Mn}^{2+}\), but several soil conditions restrict its availability. The greatest limiting factor is high soil pH, common in alkaline or over-limed soils. When the soil \(\text{pH}\) rises above 6.5, \(\text{Mn}^{2+}\) ions quickly oxidize, forming insoluble compounds like manganese dioxide (\(\text{MnO}_2\)) that plants cannot absorb.
Soils high in organic matter, such as muck or peat, can also contribute to deficiency. In alkaline conditions, organic matter binds with manganese, creating complex compounds unavailable for plant uptake. Furthermore, cold temperatures slow the microbial activity necessary to convert manganese to its soluble form, while poor drainage impedes root growth and nutrient uptake.
The presence of other nutrients can also interfere with manganese absorption, a process known as antagonism. High levels of calcium, magnesium, or iron compete with manganese for uptake sites on the plant roots. Managing the soil environment to optimize for a slightly acidic \(\text{pH}\) is the best long-term strategy for ensuring manganese availability.
Effective Methods for Manganese Application
Addressing a confirmed manganese deficiency requires balancing immediate relief with a long-term strategy. The choice of application method depends on the soil \(\text{pH}\), as soil applications are largely ineffective in highly alkaline conditions. The most common and cost-effective product for soil application is Manganese Sulfate (\(\text{MnSO}_4\)), which is water-soluble and provides the plant-available \(\text{Mn}^{2+}\) form.
Soil Application
For long-term correction in soils with a \(\text{pH}\) below 6.5, \(\text{MnSO}_4\) can be broadcast and tilled into the soil or applied in a band near the seed row. Application rates typically range from 20 to 60 pounds per acre for broadcast treatment, but specific rates must be based on soil test results to avoid over-application. In high \(\text{pH}\) soils, chelated manganese products, such as \(\text{Mn}\)-EDTA, are a better choice because the protective chelate molecule keeps the manganese soluble and available for absorption.
Foliar Application
When the deficiency is acute or the soil \(\text{pH}\) is too high to allow for effective soil application, a foliar spray is the preferred method for rapid correction. This delivers the nutrient directly to the leaves where it is quickly absorbed. Manganese Sulfate or chelated manganese can be used for foliar sprays, with concentrations typically kept below 5% to prevent leaf burn. Apply foliar sprays during the early morning or evening when temperatures are cooler and leaf stomata are open, which maximizes absorption.
Preventing Manganese Toxicity
Manganese toxicity can severely damage plants and usually occurs in highly acidic soils. When the soil \(\text{pH}\) drops below 5.5, manganese becomes excessively soluble, allowing plants to take up toxic amounts. Symptoms appear on older leaves first, often presenting as small, dark brown or black spots, or general chlorosis that can be mistaken for a disease.
Excess manganese disrupts photosynthesis and impedes the uptake of other elements like iron and magnesium. The primary method to prevent toxicity is to carefully manage soil \(\text{pH}\) by avoiding excessive acidification. If toxicity is diagnosed, applying lime or dolomite to raise the soil \(\text{pH}\) above 5.3 will reduce manganese solubility and correct the problem. Always follow soil test recommendations, as over-fertilization can quickly lead to toxic levels.