The Secondary Nutrients & Plant Growth - Magnesium's Role
Magnesium is a constituent of the chlorophyll molecule, which is the driving force of photosynthesis. It is also essential for the metabolism of carbohydrates (sugars). It is an enzyme activator in the synthesis of nucleic acids (DNA and RNA). It regulates uptake of the other essential elements, serves as a carrier of phosphate compounds throughout the plant, facilitates the translocation of carbohydrates (sugars and starches) and enhances the production of oils and fats. Magnesium deficiency is most prevalent on sandy coastal plain soils where the native magnesium content is low.
Magnesium deficiency is most prevalent on sandy-textured soils, which are subject to leaching, particularly during seasons of excess rainfall. The predominant symptom is interveinal chlorosis (dark green veins with yellow areas between the veins). The leaves at the bottom are always the first to get affected. As the deficiency becomes more acute, the symptoms progress upwards the plant. Chlorotic leaves generally turn red and then develop spotted necrotic areas.
Crops that commonly exhibit magnesium deficiency include tobacco, corn, small grains, forages and vegetable crops. About grain crops like the smaller grains and corn, magnesium-deficient leaves have light green to yellow stripes that run parallel with the blade. In severe cases, the entire leaf turns yellow. Magnesium deficiency is the common cause of "grass tetany" in ruminant animals.
Deficiency symptoms begin on the tips of the lower leaves and progress across the entire leaf. In acute situations, the entire plant becomes Chlorotic. Deficiency symptoms generally appear after topping when the plant is growing most rapidly. The symptoms are commonly referred to as "sand drown" since they occur most frequently on very sandy soils during periods of excessive rainfall. Magnesium deficiency occurs if soil pH is low or if only calcitic lime has been applied. Depending on the stage of growth and crop, magnesium deficiency can be corrected by soil application of lime or fertilizer. However, once a deficiency symptom has appeared nothing much can be done to correct the affected leaves.
Application of a soluble magnesium fertilizer may prevent upper leaves from developing symptoms. Foliar applications of magnesium are effective in emergency situations where immediate response is required to salvage a crop. Usually, magnesium is applied to the soil through use of commercial fertilizers or dolomite lime.