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Fred Hulme, Ph.D., Technical Services - ICL Specialty Fertilizer
Why is magnesium important?
Magnesium (Mg) is a mobile, secondary essential element that is often ignored by growers. Mg plays a key role in a number of important plant structures and metabolic processes and is a critical part of the chlorophyll molecule which produces sugars for the plants and also makes foliage appear green. Mg is often present below optimal levels in most agricultural systems and this is true especially in container production. If it is not supplied at sufficient levels throughout the growth cycle, plant quality will definitely suffer
When Mg is deficient, it can result in classical deficiency symptoms of interveinal yellowing that begins on older leaves (Fig. 1). While this can occur quite frequently, many growers experience Mg deficiency symptoms of a less dramatic nature. When Mg is slightly low in the root zone, this can result in stunting, delayed growth and pale green foliage color. These symptoms may be less obvious, since they may occur across the entire crop. The crop might appear uniform, but lack of sufficient Mg has cost you lost time and money.
How much Mg should I supply?
Irrigation water is the first place to look for Mg. Growers should always test their irrigation water especially for Calcium (Ca) and Mg baseline levels. This will help in developing a good fertilizer program that includes plenty of Mg. ICL Testing Lab’s general recommendations for irrigation water are: 40-70 ppm Ca and 30-50 ppm Mg. Higher levels than this are fine as long as Ca and Mg remain in balance – the ideal ration of Ca: Mg is 2:1. From my experience most irrigation waters are low in Mg. When Mg levels fall below the general recommendation (30 ppm Mg), additional Mg should be supplied to the plants in the fertilizer.
Where can I get Mg?
Growing media or fertilizer program are the primary sources of Mg:
• Most growing media contains dolomitic limestone which can supply Mg to plants. Since lime is sparingly
soluble, this will only provide low levels of Mg during the early crop phase.
• The growing media might also contain a starter nutrient charge which may contain some Mg, but again this
small amount also will not support an actively growing crop for long.
• If water is low in Mg, you should select a fertilizer high in Mg or supplement your base Water Soluble
Fertilizer (WSF) product.
• Many growers use Epsom salts (magnesium sulfate) to supplement Mg; four oz. of Epsom salts mixed
into 100 gallons of hose-end solution will provide 30 ppm Mg. Magnesium sulfate is generally compatible
in the same stock tank with most WSF formulations that do not contain Ca. Note: there is a current
worldwide shortage of magnesium sulfate, so Epsom salts may be harder to source than usually.
• Magnesium nitrate is an alternate source of water soluble Mg and is highly compatible with other
fertilizers.

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