East Everglades
Orchid Society
 15220 SW 232 St.
Miami, FL 33170
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Care & Feeding of Orchids
by Dr. Bert Pressman
Many orchid enthusiasts pass along various unsupported cultural myths as if they were divine revelations of truth. Their zeal is not diminished by the hearsay nature of their advice. Thus a recent guest speaker at a Miami club meeting claimed that orchids have no need of the nutrient MAGNESIUM. This motivated me to compile a rational basis for advice about orchid fertilizer application.

A convenient starting point is a recent (1998) technical book with the jazzy title "The Physiology of Tropical Orchids in Relation to the Industry." It provides chemical analyses of several orchid tissues. Without going into the specific function of the various components of orchid tissues, it reports that different orchid species and tissues have essentially equivalent mineral composition, and it follows that every ingredient listed in the table below, must be supplied by fertilizer, if orchids are to grow, prosper and flower.


TYPICAL COMPOSITION OF DRY ORCHID TISSUE BY PERCENT
 
Nitrogen
Phosphate
Potassium
Calcium
Magnesium
Iron
Leaves
1.8
0.2
4.2
1.3
0.5
0.01
Roots
2.0
0.3
2.2
0.8
0.8
0.04


We are familiar in general with the three number designation of commercial fertilizers for Nitrogen, Phosphate and Potassium, e.g., 20-20-20. In a balanced fertilizer, the phosphate would seem disproportionately high, but it is not absorbed as efficiently as the other elements. We needn't be concerned with calcium, since our tap and well water percolates through limestone before it hits the orchids, and picks up just enough calcium to satisfy our plants. Orchids also need low levels of the trace elements iron, manganese, zinc and copper, but these are amply supplied as impurities in the grey-brown, technical grade chemicals used in fertilizers. That accounts for everything except the major component MAGNESIUM.

The "Leibig Principle", a concept developed by the father of agricultural chemistry during the 19th century, states that the lack of any essential nutrient is equally capable of limiting plant growth. Thus if you don't want lack of magnesium to limit the growth and flowering of YOUR orchids, I recommend adding ½ teaspoon Epsom Salts [magnesium sulfate, get it at your pharmacy] to each gallon of fertilizer solution, EVERY time you fertilize.

Other, more succinct cultural recommendations:
1. There is no theoretical or experimental support for the myth that high phosphate fertilizer favors flower initiation. Kerry's Bromeliads has checked this possibility exhaustively with negative results.
2. Minerals are absorbed by orchid LEAVES as well as by roots. Accordingly, a detergent should be added to fertilizer solutions so that they coat leaf surfaces entirely and evenly without beading. This will also minimize spotting by calcium residues as the beads concentrate and dry.
3. While orchids absorb nutrients better under acidic conditions, and ground filtered water is usually alkaline, do not be concerned, since all commercial fertilizers are designed to maintain optimal acidity, even if alkaline water is used to dissolve them for application. This is true for all of a half dozen commercial fertilizers I have personally tested.
4. Your water can be too pure! If you use rainwater, RO water, or your tap supplies surface water, you may not be providing enough calcium or magnesium for optimal growth. Consider using a Cal-Mag fertilizer.
5. If you fail to flush your orchids periodically, as water evaporates, salts will concentrate and may become toxic. Conversely, if flushed too soon after fertilizing, orchids may not absorb optimal amounts of nutrients.
6. While most of the salts, which occur naturally in water, are beneficial to orchids, beware of salt incursion showing up as elevated SODIUM. A second source of detrimental sodium levels could be commercial water softeners, which exchange desirable calcium for toxic sodium.



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