How Much Magnesium Does a Plant Need?

Plants and Magnesium

• Magnesium has many roles in plant growth. As the central element of each molecule of chlorophyll, it is vital to photosynthesis and deficiencies lead to stunted growth. Magnesium also plays a part in enzyme activation, sugar synthesis, oil and fat formation, control of nutrient uptake, and other processes. Plants differ in their need for magnesium, blueberries, broccoli, cabbage, celery, corn, cotton and cucumbers being among the crops that need the most. Since it is available to plants as a positively charged ion, magnesium in the soil needs to be in balance with other positively charged ions, such as calcium, potassium, sodium, iron and aluminum.

Sources of Magnesium

• Magnesium is one of the more abundant elements in the earth crust and is part of the structure of many minerals. The weathering of rock into soil releases magnesium and makes it available to plant roots. The positive ion of magnesium is held on the surface of particles of clay and organic matter. Though it is readily available to the roots, magnesium does not leach easily through the soil.

Amounts Needed

• A professional soil test is the best indicator of a deficiency of magnesium. A crop such as corn, for instance, needs over 150 parts per million in the soil for best growth and an amount less then 50 ppm is considered to be low. Most fruit and vegetable crops, however, do well with 100 ppm or more. You can also check with your local extension agent to see if soils in your area are generally well-supplied with magnesium.

Symptoms of a Deficiency

• One of the best indicators of a magnesium deficiency is the loss of color between the veins in the leaves with the veins themselves staying green. However, there are other causes of yellowing such as herbicide damage and disease. Only a soil analysis could pinpoint the cause as magnesium deficiency. As soil acidity increases, minerals become less available to plant roots. Dolomite lime can raise pH to near neutral, the best for most crops. Since dolomite is 8 to 10 percent magnesium, it also supplies adequate amounts of this essential nutrient.