11/12/2012 - By Marshall Horsman

The Often Overlooked Secondary Nutrient, Magnesium

In the next three months I will be covering the three secondary nutrients of plants, magnesium, calcium and sulfur. By "secondary" nutrient, we are describing nutrients not needed in such quantities as the major nutrients, but substantially more than the minor nutrients, the cation magnesium being one of them. Magnesium plays several roles in plants, including aid in sugar or carbohydrate transport through phloem and CO₂ fixation. But if one looks at the complex structure of a chlorophyll molecule, you will see that the nucleus atom is in fact a magnesium atom. No magnesium, no chlorophyll. No chlorophyll, no photosynthesis. It is that simple.

Magnesium plays a major role in chlorophyll synthesis, the process of plants taking light energy (photons) and using it to assimilate other nutrients into food. That is right, fertilizer is not plant food, it is a package of essential nutrients that plants convert into food with the aid of sunlight energy. This is what makes plants unique and as complex as any other organism on the face of the Earth. And elsewhere?! I'll save that for another newsletter.

Typically Magnesium comes in the form of sulfate of potash-magnesia, magnesium sulfate (Epsom Salts), magnesium oxide and less commonly magnesium nitrate. A good granular fertilizer should have at least 1-1.5% elemental magnesium. For magnesium loving plants, such as palms, we like to see at least 2.5-3%. In soil or leaf tissue analysis, it is usually not a bad thing to see excess magnesium. As a cation it can interfere with calcium or potassium uptake. But usually those nutrients are responsible with interfering with magnesium uptake, as they are often higher. In soil analysis, we do not want to see the calcium to magnesium ratio greater than 10:1. Dolomite is often preferred to straight lime as a soil amendment as it has magnesium carbonate as well as calcium carbonate. Mg readily translocates in plants' vascular systems. Therefore deficiency symptoms are usually found in lower leaves. As newly formed leaves will "steal" it from old leaves.

Plants deficient in Mg will often respond favorably to Epsom Salt sprays of 1.5-2lbs. per 100 gallons of water. The magnesium in magnesium nitrate however is considerably more available in tissue absorption than magnesium sulfate. However magnesium nitrate is uncommon these days and epsom salt is cheap. As a helpful "green up" foliar spray, I often recommend 1.5 lbs. epsom salts + 1.5 lbs potassium nitrate (or 20-20-20) + 12 ounces chelated iron (if needed) per 100 gallons water. You do not need to acidify as solution is already acidic. You do need to add spreader sticker.

Agronomically tomatoes, green peppers, mangoes and lychhe nutshave have high magnesium demands. In southern Dade county soils are high in limestone, fertilizer companies will attempt to put at least 2-3% Mg in granular package. In the ornamental world aroids, palms and ferns have high magnesium demands, as well as Hydrangeas and Roses. As mentioned in a previous newsletter (covering phosphorous), magnesium and phosphorous in solution will precipitate or crystallize. Therefore are often kept separate. And finally magnesium does leach readily from soils, so it needs to be provided systematically through regular fertilization.