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.
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
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