Must I Supplement Calcium & Magnesium?

Most fertilizer formulas do not contains calcium or magnesium, primarily because they are usually found in our water supplies. But is it enough? Must I supplement calcium and magnesium? And how about if I use reverse osmosis (RO) water or collect rainwater?

First, why are we concerned about calcium and magnesium in the first place?

Calcium (Ca) plays a wide range of roles in plant health and growth (see my “Fertilizer Basics” article), but we must pay attention to supplying it because – unlike most other nutrients – once deposited in plant tissue, calcium cannot easily move to other plant tissues. Without a sufficient, continuous supply of calcium, our plants may display stunted or stopped growth, distorted new growth, black spots on leaves, or yellow leaf margins. If you grow sympodial orchids like cattleyas, you may notice new growths dying and rotting before they are fully formed.

Magnesium (Mg) is a critical structural component of the chlorophyll molecule and is necessary for several other plant functions. Magnesium-deficient plants show yellowing between veins of older leaves, and may appear limp. Fortunately, magnesium can easily be shared among plant tissues, so a deficiency tends to be less debilitating.

The next question is “How much calcium and magnesium do my plants need?” This is yet another case in which there has been little research specifically with orchids, but based upon tissue analyses and professional growers’ recommendations, generally, our “fertigation” solutions should contain about 40-80 ppm Ca and 20-50 ppm Mg.

Comparing some fertilizers, when applied at 100 ppm N – a reasonable target for weekly feeding – these are the corresponding calcium and magnesium concentrations:

Fertilizer Comparison
Formulappm Nppm Cappm Mg
K-Lite Orchid/Epiphyte Fertilizer1007723
Greencare Orchid Special for RO (MSU RO)1006115
Greencare Orchid Special for Well Water (MSU WW)100100
Peters Excel Cal-Mag1003313

Does my water supply have enough calcium and magnesium in it already? Your municipal water provider is required by law to keep tabs on the water quality and to make that information available to its consumers. Their primary concern is your health, not that of plants, so there will be a lot of useless info, but look for “hardness” minerals like calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Based upon the elemental content of the dissolved minerals, water containing 100 ppm CaCO3 will contribute 40 ppm Ca and 100 ppm of MgCO3 will carry 25 ppm Mg.

If you use reverse osmosis (RO) water, distilled, or collected rainwater, you must provide all nutrients, including calcium and magnesium. If you haven a well or municipal water supply, I strongly suggest you get a water analysis done. While you may find a local lab, it is probably best to have your analysis done by the J R Peters Laboratory in Allentown PA. They are a “plant-centric” lab, so their analysis will be in directly applicable terms and units. For a nominal fee, they will send you a sample collection kit (you can request it online) and email the results back to you within 24-48 hours of receipt.

Can I give my plants too much calcium or magnesium? The answer is “yes”, especially for calcium, as a high concentration in the plant, the potting medium, and in the fertilizer solution can block the uptake of other nutrients. Stick to the guidelines above and it should not be a problem. An excess of magnesium has not been shown to be a particular problem, but again, stay within the guidelines to be safe.

Using Science & Logic to Improve Orchid Growing