Almost every week, and sometimes several times a week, we work through some cultural problems with people via email or phone calls that resolves back to calcium and/or magnesium deficiencies. These usually result from the use of reverse osmosis, deionized, distilled, rain or tap water sources low or devoid of calcium and magnesium. We have long advocated good water quality as an important aspect to getting the most out of your slipper orchid culture, but have also always pointed out that you must couple it with a complete feeding program. By a complete feeding program we mean all of the appropriate minerals in a reasonable balance and a form readily available to the plant, and at a proper pH.
Many fertilizers contain little or no calcium and magnesium. These are not, as commonly stated, micronutrients but rather macronutrients for our plants. Deficiencies of one or both of these nutrients can lead to poor growth, deformed leaves, chlorosis, blackened areas at leaf tips and new growths with a leading yellow edge, stunted, shortened roots and dead root tips and necrotic spotting. The plants may be more susceptible to fungal and bacterial infections. Since most of us also grow our orchids in mixes that release little or no calcium and magnesium, the use of pure water sources can lead to deficiencies. Adding back tap water is one possible solution, but you do need to know what the content of that water is. It may still leave you deficient in one or the other, or it may have other components in high enough levels to be detrimental to your plants. There are also potential problems with precipitation of insoluble carbonates in your mix (see http://ladyslipper.com/fert.htm ).
A second solution is to feed regularly with calcium nitrate and magnesium sulphate (Epsom Salts). These are very soluble salts and any excess that precipitates out in the mix with drying will quickly leach out with the next good watering. The downside is that this requires more work, and greenhouse grade calcium nitrate can be hard to obtain in quantities of less than 25 pounds. If you do decide to use either of these every few weeks in rotation with an otherwise complete fertilizer rotation, typical greenhouse grade calcium nitrate (15-0-0) weighs about 4.8 grams for a level teaspoon, so ˝ teaspoon per gallon of pure water would give you a TDS of approximately 635 ppm with approximately 137 ppm calcium (taking into account typical hydration) and approximately 95 ppm nitrogen at a nearly neutral to slightly acidic pH, depending on the level of dissolved CO2 in the water. A teaspoon of typical Epsom Salts ( MgSO4.7H2O ) weighs approximately 4.5 grams, so ˝ teaspoon per gallon yields approximately 54 ppm magnesium, again at a nearly neutral to slightly acidic pH, depending on the level of dissolved CO2 in the water.
The third solution is the easiest, use a fertilizer with adequate calcium and magnesium included. As I mentioned earlier we have used several such fertilizers over the years. When our collection was smaller we used Dyna-Grow products with success, although we did supplemental calcium and magnesium feedings periodically as we did feel the levels were lower than we wanted. We used and still use part time Miracle-Gro Excel Cal-Mag (15-5-15) in our program which at 100 ppm nitrogen provides 33 ppm calcium and 13 ppm magnesium, again we felt an occasional supplementation of calcium and magnesium was in order when using this. Excel Cal-Mag does drastically lower the pH of low alkalinity water so pH adjustment of the final solution is necessary. This can be accomplished with the use of Dyna-Grow Pro-Tekt, potassium hydroxide or potassium bicarbonate. Be careful of using sodium bicarbonate (Baking Soda) as was recently suggested on one of the online forums, there is no need to purposely introduce extra sodium when good alternatives exist. Our mainstay for all but the summer months now is Miracle-Gro Excel 13-2-13 Plus Ca and Mg Plug Special. At 100 ppm nitrogen this provides 46 ppm calcium and 23 ppm magnesium (this would be just a little less than 3/4 teaspoon per gallon which for us yields a pH around 6.0 in RO water). These last two fertilizers are available generally in 25 pound bags only. A very similar fertilizer to the Plug Special appears to be the Michigan State University formula fertilizers, and these are available in repackaged small quantities, one formula for low TDS water and another for tap water. We have not personally worked with the MSU formula fertilizers, but based on their analysis I expect they would behave and perform very similarly to the Plug Special. Both the Plug Special and the MSU formula fertilizers are high nitrate fertilizers, suitable for cooler lower light conditions, so it might be advisable to rotate them with fertilizers with other nitrogen sources in warm and bright conditions. Of course, here in Candor, New York we only have to “worry” about warm and bright condition about four months of the year!
Some further material relevant to this topic:
Water quality - http://www.ladyslipper.com/waterq.htm
Mineral nutrition - http://www.ladyslipper.com/minnut.htm
Notes on fertilizer use - http://www.ladyslipper.com/fert.htm
Limestone supplementation - http://www.ladyslipper.com/calsub.htm
Product Sources. This certainly is not an exclusive list, you may find these products at many locations not listed.
Dyna-Grow products- available at lots of orchid retailers, home page is http://www.dyna-gro.com/ .
Calcium Nitrate - greenhouse supply stores - you want greenhouse grade, not spray grade which may be what you find at farm supply stores - we get it from http://www.griffins.com/Company/Company.html .
The Excel fertilizers and potassium bicarbonate are also available from greenhouse supply stores - we again use http://www.griffins.com/Company/Company.html .
The MSU fertilizers are available in small packages from Roberts Flower Supply. They may not yet be on the web, email or call and ask - http://www.orchidmix.com .
Regards, Bob & Lynn
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