Are All Calcium / Magnesium Supplements Alike?

With all of the emphasis on N-P-K in agriculture, calcium and magnesium supplements are sometimes overlooked. Calcium and magnesium are essential macro-elements, used in relatively large quantities by plants. In fact, plants typically take up more calcium than phosphorus! Calcium builds strong stems and leaves. When calcium is taken up by the plant it forms pectin, the glue that binds cell walls together. So a steady supply of calcium is necessary throughout the life of the plant to strengthen the vascular system of the plant, produce thicker stems and build new cell walls at the growing tips. If the plant has a healthy vascular system, the plant is also able to take up water and all of the other essential minerals more efficiently, resulting in stronger, more disease-resistant plants.

Maintaining adequate levels of magnesium throughout the crop cycle is also essential. Magnesium is the central element in chlorophyll, essential for absorbing light energy and converting it to high-energy sugars for use by the plant. Magnesium also activates more than 300 known enzymes in plant cells, including the enzymes that “load” sugars for transport throughout the plant. So magnesium is especially important when the energy needs of the plants are highest, such as during periods of rapid vegetative growth or during heavy fruiting and flowering.

Most base nutrients provide enough calcium and magnesium to meet the needs of the plant under normal growing conditions. In hydroponics, calcium is commonly supplied as calcium nitrate and magnesium is supplied as magnesium sulfate. Apply them separately, and they are very water soluble. But combine them together in a liquid concentrate, and there’s a problem. In concentrated form, calcium reacts with sulfates and turns into calcium sulfate (gypsum). That’s what plaster is made from, and it is 98% insoluble! That’s why most hydroponic base nutrients come in a two-part formula. The calcium nitrate is in the part A and the magnesium sulfate is in the part B. Once they are diluted in enough water, however, the calcium and sulfates remain in solution and are readily available to the plant.

Diagnosing Calcium and Magnesium Deficiencies

Calcium is an immobile element. In other words, once it is locked up in the plant tissue, it can’t be translocated to other parts of the plant. So a calcium deficiency normally shows up in the new growth at the growing tips of the plant, causing deformed leaves and reduced root growth. There may be plenty of calcium in the nutrient formula, but anything that interrupts the flow of water in the plant could cause a calcium deficiency. For example, if relative humidity is too high, the plant may not transpire enough water to transport calcium to all of the cells of the plant. As a result, calcium deficiency may show up as tip burn in lettuce or blossom end rot in tomatoes. High temperatures, stagnant air and over fertilizing can make the calcium deficiency even worse. So before reaching for a bottle of calcium/magnesium supplements, make sure that your environment is under control, first.

Magnesium, on the other hand, is a mobile element. In other words, if there is a magnesium deficiency the plants will strip magnesium out of the lower leaves and transport it to the new growth where it is needed the most. So magnesium deficiency shows up first as interveinal chlorosis on the older, bottom leaves. Magnesium is the central element in chlorophyll, the green pigment in plant leaves. So if there is a magnesium deficiency, the veins of the lower leaves will stay green, but the tissue between the veins will turn yellow. Magnesium deficiencies are fairly common in indoor gardens. Indoor gardeners often use powerful HID grow lamps such as metal halide or high pressure sodium. As the light becomes more intense, the plants need more magnesium to efficiently utilize the light energy. That’s why the symptoms of magnesium deficiency are often more obvious in gardens that have more intense light, and magnesium supplements are sometimes required.

Cal/mag supplements are commonly prescribed to treat calcium and magnesium deficiencies. Since magnesium is so biologically active, calcium is often used as a buffer. The calcium slows down the uptake of magnesium so that excessive magnesium doesn’t accumulate in the leaves and become toxic to the plant. For best results, most cal/mag formulas will provide calcium and magnesium in about a 5:1 ratio. The calcium and magnesium compete with one another for uptake by the plant roots, helping to provide balanced uptake.

Not all calcium/magnesium supplements are alike! So make sure you carefully read the labels to find out what sources the calcium and magnesium are derived from. Calcium and magnesium supplements may be derived from carbonates, nitrates or sulfates. Each source has its pros and cons, so here is a quick overview to help you make the right choices for your garden:

Standard Calcium / Magnesium Supplements

Standard cal/mag concentrates are derived from calcium nitrate and magnesium nitrate. Since they contain no sulfates, the cal/mag can be bottled as a single liquid concentrate with no lockup. Nitrates are very water soluble, and the magnesium in magnesium nitrate will certainly treat a magnesium deficiency. Unfortunately, plants don’t always need the extra nitrates, especially during flowering. In fact, excessive nitrates at the wrong time may delay or even prevent flowering. Since about 30% of the energy of photosynthesis is used just to assimilate nitrates, energy may be taken away from the flowering process if you give your plants more nitrates than they need. So if you must use cal/mag supplements during flowering, check on the form of magnesium used, and try to stay away from too much magnesium nitrate.

Powdered Calcium/Magnesium Supplements

Powdered calcium/magnesium supplements are a good compromise since they reduce the amount of nitrates used. Just like in a base nutrient, the calcium is derived from calcium nitrate and the magnesium is derived from magnesium sulfate. But since it is in powdered form, the calcium and the sulfates don’t react with one another. Both the calcium and magnesium are 98% water soluble and available to the plant. Just make sure the powdered cal/mag is thoroughly diluted with water and mixed well. It is also recommended that powdered cal/mag is added to water first, before any other fertilizers or additives. Powdered cal/mag still contains some nitrates, however, although not nearly as much as liquid concentrates. Although an excellent source of calcium and magnesium when applied to roots, cal/mag supplements with nitrates should not be used as foliar sprays.

Organic Calcium/Magnesium Supplements

“Organic” cal/mag supplements are another alternative. Organic cal/mag is derived from calcium carbonate and magnesium carbonate. Although not nearly as soluble as calcium nitrate, calcium carbonate (limestone) retains its OMRI certification. One of the benefits of calcium and magnesium carbonate is that it contains NO nitrates. Therefore, it won’t throw the nitrates out of balance or interfere with flowering. It is also a good choice when using reverse osmosis (RO) water. The bicarbonates formed from calcium and magnesium carbonate act as a pH buffer and help avoid the wild swings in pH often associated with RO water.

The down side of organic cal/mag is that calcium carbonate and magnesium carbonate are only sparingly soluble. So in some ways, organic cal/mag may actually change clean water into “hard” water. The result is the potential for lime scale. In hydroponics, for example, when phosphoric acid is added to hard water to lower pH, the bicarbonates are burned off as carbon dioxide and water, but the phosphoric acid reacts with the calcium ions to form calcium phosphate. Calcium phosphate is what your bones are made of and it is only about 5% water soluble. So if you choose to use an organic cal/mag product, always use an amino acid supplement with it. The amino acids will chelate the calcium and magnesium ions, making them more water soluble and helping to prevent lime scale.

Magnesium Supplements

If you want a quick fix for a magnesium deficiency, without any extra nitrates or bicarbonates, the best solution may be to use a magnesium sulfate supplement without the calcium. Magnesium sulfate is fast acting, highly soluble, and may be used safely at the roots or as a foliar spray. Since it contains no nitrates, magnesium sulfate is an excellent choice during flowering. In fact, many “sweet” products designed specifically for flowering are loaded with magnesium sulfate. The magnesium helps keep the sugars flowing all the way to the day of harvest, and the sulfates are an added bonus. Sulfate compounds actually help turn on flowering genes in the plant, contribute to aromas, and hasten the ripening process.

One of the latest developments in magnesium nutrition is combining B-vitamins with a magnesium sulfate carrier. B-vitamins stimulate carbohydrate metabolism, producing abundant energy in the form of ATP molecules. Magnesium, on the other hand, positions the ATP molecules so that the energy can be released for plant growth and reproduction. In fact, the relationship between ATP and magnesium is so intimate that many biologists refer to ATP as magnesium-ATP. Since B-vitamins work at the microscopic level, just a small amount of B-vitamins (1%) can have a positive effect on the function of magnesium, especially when the energy needs of the plants are greatest.

The bottom line is that plants need a steady supply of water-soluble calcium and magnesium to reach their true genetic potential. Most of the time, a good base nutrient should provide adequate calcium and magnesium, but there are times when calcium/magnesium supplements may be beneficial. For example, coir is naturally high in potassium but low in calcium and magnesium, and calcium is easily locked up in certain soils. Magnesium, on the other hand, is highly mobile, but it can become a limiting factor when plants are pushed to their limits. Choosing the right calcium/magnesium supplements and using them judiciously can help keep your garden growing strong!

By Harley Smith