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Monitoring and Correcting Magnesium Deficiency in High Tunnels

About Magnesium Deficiency

tomato plant leaves with green veins and yellowing centers

Written by Geoffrey Njue, former SDSU Extension Specialty Crops Field Specialist.

This point in the growing season is a good time to monitor and correct magnesium (Mg) deficiency in high tunnel crops. Magnesium is the most common deficiency in high tunnels especially on tomatoes.  Deficiency symptoms normally appear mid-season on plants with heavy fruit load. Magnesium deficiency is common in sandy soils that are easily leached. Excessive levels of potassium can also induce magnesium deficiency, and fertigation with high rates of ammonium nitrate can also contribute to magnesium deficiency. Magnesium deficiency can also occur in the field but is most common in high tunnels.

tomato plant with yellowing leaves


Magnesium is highly mobile in the plant and deficiency symptoms first appear on the lower leaves. Symptoms are more severe on the lower leaves because magnesium is moved to the new growth. Deficiency symptoms consist of interveinal chlorosis (leaf veins stay green while the regions between them turn yellow). Older leaves lose their green color except in the veins. Interveinal chlorosis can lead to necrosis (death of tissue) of the affected areas. On tomato leaves advanced magnesium deficiency leads to purpling of the affected areas. It may also lead to the defoliation of the lower leaves. Magnesium deficiency does not affect the fruit but severe deficiency can stress the plants leading to reduced yield.


The most common method of correcting magnesium deficiency is applying Epsom salts (magnesium sulfate). Do not to apply Epsom salts unless you see symptoms of magnesium deficiency. Epsom salts can be applied either as a side dressing or through the drip system. You can apply Epsom salts through the drip system at the rate of 60ppm or dissolve 1-2lb in 100 gallons of water and apply as a drench. Do not mix Epsom salts with another water soluble fertilizer. When applying as a side dressing, apply 1 tablespoon (about ½ dry oz) per plant.