Manganese is an essential micronutrient, meaning plants require it in small concentrations. Its presence is mandatory for several fundamental biological processes, including those related to growth and development. A shortage of manganese (Mn) can reduce a plant’s ability to grow, leading to drops in crop yield and overall quality. Understanding the functions of manganese and the environmental conditions that limit its availability is the first step toward preventing crop damage.
Manganese’s Critical Functions in Plant Life
Manganese plays a key role in photosynthesis, specifically within the light-dependent reactions, acting as a central component of the oxygen-evolving complex (OEC) in Photosystem II (PSII). The OEC is responsible for splitting water molecules, which generates the electrons and protons necessary to convert light energy into chemical energy. Without sufficient manganese, this initial step of photosynthesis is impaired, limiting the plant’s ability to produce sugars for energy and growth.
The element also functions as an activator for enzymes involved in various metabolic pathways. These enzymes regulate processes such as nitrogen assimilation, carbohydrate metabolism, and the synthesis of lignin. By contributing to lignin synthesis, manganese strengthens the plant’s physical structure and helps form a barrier against invading pathogens. Manganese is also a cofactor for the enzyme superoxide dismutase (Mn-SOD), which helps neutralize harmful reactive oxygen species, enabling the plant to manage environmental stressors like drought or temperature extremes.
Identifying the Visual Signs of Deficiency
Manganese deficiency is characterized by interveinal chlorosis, where the tissue between the veins turns pale yellow or light green while the veins remain a darker green color. Because manganese is immobile within the plant, it cannot be relocated from older leaves to newer growth. This means that the symptoms of deficiency first appear on the younger, recently expanded leaves.
As the deficiency progresses, the yellowed areas may develop small, scattered spots of dead tissue, known as necrotic spots, which can appear gray or tan. The affected young leaves may also become reduced in size, and the overall growth of the plant can be stunted. While other nutrient deficiencies can cause interveinal chlorosis, the location of the symptoms—primarily on the youngest leaves—is the most reliable diagnostic clue.
Soil and Environmental Triggers
Manganese deficiency is rarely caused by a lack of the element, as it is naturally abundant in most soils. The primary cause of unavailability is high soil pH, an alkaline condition typically above 6.5. Under these conditions, the soluble, plant-available form of manganese (Mn²⁺) is converted into insoluble manganese oxides that plants cannot absorb.
Cold, wet, or poorly drained soils often lead to temporary deficiencies because these conditions slow microbial activity necessary for manganese conversion. Nutrient antagonism can also occur, where excessive levels of other elements, particularly iron, zinc, or calcium, compete with manganese for uptake by the plant roots. High organic matter soils can also tie up manganese by forming stable, unavailable complexes, even if the pH is not excessively high.
Treatment and Prevention Methods
The most effective short-term solution is the foliar application of a manganese fertilizer. Spraying the nutrient directly onto the leaves ensures the new, affected growth can absorb and utilize it. Manganese sulfate is the most common source for foliar sprays, typically applied as a dilute solution when symptoms first appear.
Long-term management requires addressing the underlying soil pH. For alkaline soils, the pH can be lowered through the application of elemental sulfur, which soil bacteria convert into sulfuric acid. Using acidifying fertilizers like ammonium sulfate provides nitrogen while contributing to soil acidification. Soil applications of manganese fertilizers are unreliable in high-pH conditions, as the applied manganese reverts to its unavailable oxide form; therefore, foliar treatment remains the preferred corrective measure.