Magnesium is required for crops to capture the sun's energy for growth and production. Chlorophyll, the green pigment in plants, is the site where photosynthesis occurs without chlorophyll plants could not manufacture food. Magnesium is an essential component of the chlorophyll molecule, with each molecule containing 6.7 percent magnesium.  Its functions include phosphate metabolism, plant respiration, and activation of enzyme systems.  Availability of Mg is often related to soil pH. Other situations also increase needs for Mg: sandy soils with low cation exchange capacity, low Mg-supplying power, and a high Mg leaching potential; application of calcitic limestone on low Mg soils; crops with high Mg requirements; high application rates of ammonium-N and K; soil test levels below 50 to 100 lb/A exchangeable Mg.

Plant Deficiency Symptoms

Magnesium is taken up by the plant as the divalent cation, Mg++. It is mobile within the plant and easily translocated from older to younger tissues. When deficiencies occur, the older leaves are affected first. The deficiency symptoms may include the following: (1) loss of color between the leaf veins, beginning at the leaf margins or tips and progressing inward. This can give the leaves a striped appearance. (2) Leaves may become brittle and cup or curve upward and they may become thinner than normal. (3) Tips and edges of leaves may become reddish-purple in cases of severe deficiency (especially with cotton). (4) Low leaf Mg can lead to lowered photosynthesis and overall crop stunting.

As a rule of thumb, most crops have a critical plant tissue Mg concentration of about 0.2 percent. Some species have a higher total requirement than others: forage legumes and grasses, cotton, oil palm, corn, potatoes, citrus, sugar beets and tobacco need lots of Mg. Some varieties and hybrids of crops such as corn, soybeans, cotton and celery may require more Mg than others.

If Mg deficiencies are detected in growing crops through plant tissue analysis, a soluble magnesium source may be applied and watered into the soil by irrigation or rainfall. This will permit root access and plant uptake. Small amounts of Mg can also be applied to growing crops through foliar fertilization to correct or prevent developing deficiencies. The preferred approach is to soil apply the required amounts of Mg before crops are planted or before they begin active growth.