The Micronutrients & Plant Growth - Manganese's Role
Manganese (Mn) acts as an enzyme activator for nitrogen assimilation. It is essential for the manufacture of chlorophyll. Low plant manganese, therefore, reduces the chlorophyll content causing leaves to turn yellow (chlorosis). Organic soils usually have low to intermediate amounts of manganese. Due to the acidic nature of organic soils, manganese deficiency is rarely observed even when soil manganese is less than 4 ppm.
The manganese deficiency is typically characterized by interveinal chlorosis (dark green veins with yellow discoloration between the veins), but symptoms vary depending on the crop. Small grain crops may have grey specks and stripes that run parallel to the leaf blade. Known as "grey speck", this symptom occurs at random across a field. Leaves of deficient corn plants have yellow stripes that run parallel to the blade. Leaves of tobacco and soybean seedlings exhibit a rusty, flecked appearance similar to ozone damage. In acute situations, tobacco leaves turn brown, disintegrate, and fall off. Older soybean plants show the typical interveinal chlorosis pattern, and plants are stunted. Peanuts exhibit an interveinal chlorosis that generally occurs in spots across the field.
Manganese deficiency is due to one of two factors: the soil contains less than 4 ppm manganese; or the soil contains more than 4 ppm manganese, but the pH is above 6.2. Deficiencies occur most frequently in sandy coastal plain soils that have been over limed. In such cases, the deficiency becomes less severe as pH declines and may disappear when pH drops below 6.2. Banding acid-forming fertilizers increases the amount of manganese available for plant uptake. However, in soils with low manganese content, correct the deficiency by applying a minimum of 4-5 kg Mn/acre as a broadcast application. Manganese sulfate (~28% Mn) is a good source. Chelated and foliar fertilizers are also effective when applied at the appropriate rate. Sources composed of oxides or oxy-sulfates are generally less effective in building the soil manganese content and correcting a deficiency. In cases where manganese deficiency occurs due to lime application, growers may often associate the problem with lime and ignore the need for applying manganese to the soil. In such cases, both lime and manganese need be applied to prevent crop losses due to soil acidity, on the one hand, and manganese deficiency, on the other. The yield response from applying lime pays more than for the additional cost of applying manganese.