The ability of the apparently inadequate root system of the orchid to procure its necessary life-giving minerals from the most im­probable sources is a minor but happy miracle. These minerals are nitrogen, potassium, calcium, magnesium, sulphur, iron, and carbon. Combined, by photosynthesis, with proper amounts of hydrogen and oxygen, these minerals sustain all plant life. The areas where orchids are usually found seem to provide these min­erals only in the most minute quantities, which are slowly re­leased. The exact sources vary with the type of orchid.

Orchids, as we have seen, are divided into groups defined largely by the method of securing food. The two major groupings are terrestrial and epiphytic. There are sub-divisions into semi-terres­trial and semi-epiphytic, but in a consideration of feeding we need be concerned only with the major groupings.
The terrestrial group, as the name indicates, grow in soil. Many North American orchids are found in shadowy, marshy, rich ground or in springy mountain meadows near streams. Cypripe-diums, Spiranthes, and Habenaria are among this group. They are exquisite even when tiny, but resist all efforts to domesticate them. Tropical Cypripediums are also terrestrial but are successfully grown under greenhouse conditions. Cymbidiums, native to many areas of the Far East, make up a large and valuable portion of the terrestrial group. They are often found on trees or logs, a position that really classifies them as semi-terrestrial. Wherever found, they send out long, searching roots into the leaf mold at the base of the tree or under the log. When fed richly, their heavy, round, corky roots attain great size and length. The soil in which they thrive varies exceedingly and it is comparatively easy to satisfy them under cultivation. Growers who disagree on all other points of culture see eye-to-eye on the advisability of supplementary feeding of Cymbidiums.

The epiphytic group includes Cattleyas, Phalaenopsis, Vandas, Dendrobiums, Laelias, and Oncidiums. Large numbers of epi­phytic orchids, which obtain sustenance from chemicals in the moisture of the air, are found perched like nesting birds in the crotches of jungle trees. They usually send out two types of roots: one flat and straplike, that wind around the host for support and cling so tightly that they must be broken to release the hold; and the other, more flexible, that dangle in the air or penetrate the accumulated debris of dust, rotting leaves, and bird lime in the tree crotch. Frequent tropical rains slowly release available food materials and frequent drying winds aerate the roots and prevent any unhealthy chemical development. Plentiful sunshine steps up the process of photosynthesis and the orchid thrives until the dry months bring an enforced rest period. Not all epiphytic orchids are good subjects for supplementary-feeding experiments. Cat­tleyas grow so well in greenhouses on a simple diet of Osmunda and Polypodium that, as long as they are properly repotted every two or three years, any need of extra nourishment is unlikely. Phalaenopsis, on the other hand, grow and bloom so prolifically that they soon exhaust their food supply. Since they do not re­spond well to repotting, there is a real need for artificial feeding.

The slowness of orchid growth, the long periods of plant in­activity, and the difficulty of raising orchids from seed long ago led growers to experiment with artificial feeding. The results were often confusing and, more frequently, discouraging. There were hopes that the growth rate could be speeded up, the rest period dispensed with, and the production of flowers increased if a proper
adjustment between minerals, moisture, aeration, and light could be worked out. Although if some of the wilder hopes have been dimmed, there is still reason to believe that much can be accom­plished.

The peculiar roots of the orchid permit absorption when food and moisture are available, and prohibit an excess of transpiration during drought. The adjustment of artificial feeding to these roots seems to be a problem for the scientist with a knowledge of chem­istry and a feeling for growing plants. But amateurs have success­fully invaded the field. They have shown that one of the most important factors of artificial feeding is an intelligent application of conclusions drawn from close observation of the effects of such feeding. Some of these conclusions are that a plant that has plenty of light but whose leaves remain yellow probably needs iron; that too much nitrogen produces a beautiful green growth but may slow down flower production; and that precipitation of chemicals in the medium will injure the plant after a short period of well-being.

As has been shown, the feeding of terrestrial orchids is not complicated. Food may simply be added to the potting material as needed. Foods commonly used are bone meal, blood meal, leaf mold, rat's nest, manure (well-rotted and carefully used to prevent burning), and any of the nutrient formulas (such as Knudson's 'B' or 'C') or the commercial preparations specifically recom­mended for orchids, such as Plant-Chem. Mr. Robert Casamajor of Southern California has worked out an elaborate calendar for Cymbidium feeding.1 He feeds regularly every two weeks, using a stronger solution during active growth than after flower growths appear, and maintaining a pH 4.9-5.0.

Artificial feeding of epiphytes is a much more complex problem. After first being discouraged by the great casualty rate among tropical orchids that were treated like earth plants, early growers hit upon the very successful method of potting them in Polypodium, Osmunda, tree fern, or English peat. It is doubtful if they knew why these media were good. The alternate watering and drying out of the potting material released the stored food and dried out the roots, preventing rotting. In other words, the roots were aerated and could breathe. The conditions of their native habitat were sufficiently approximated.

As it is the nature of man never to be satisfied, however, growers still felt that supplementary feeding might accelerate growth and flower production, but found in the experiment that Osmunda and Polypodium are no longer ideal media. They break down rapidly and become sour. Roots begin to rot and the plant suffers. Eventually there is a concentration of chemicals, which even frequent flushings with clear water will not wash away.

The very special technique of gravel or Haydite culture may be the answer to this problem of feeding epiphytes. Both are inert and form a good base for the roots that support the plant. They have the added advantage of providing excellent drainage and aeration. Well-washed quartz gravel has been used in many sizes. Many growers use large gravel at the bottom of the container with a layer of fine gravel on top to provide a better clinging surface for roots. Haydite is a shaly deposit that has been subjected to heat at high pressure and has expanded and become porous. Both gravel and Haydite have their advocates.

No conclusive results have been published on gravel culture of adult Cattleyas. Growers who have experimented do not seem sure of the results and are inclined to hedge when asked about them. Many commercial growers have tried gravel with Cattleyas and then abandoned the idea. It is not known whether this is because the plants did not thrive or because the practice did not bring results commensurate with the labor and expense involved. Cattleyas do so well in Osmunda that further experiment may be pointless. More positive results have been noted with Cattleya seedings and Phalaenopsis. The latter, with their flat strap-like roots and spreading habit of growth, do particularly well in gravel or Haydite.

Recent experiments, performed under the direction of Dr. R. A. Davis at the University of California at Berkeley, have been under­taken to clarify the needs of the orchid plant under cultivation. The orchid, while highly evolved in many respects, is singularly weak in two important factors—the seed and the roots. The orchid root is very inefficient as compared to that of plants like rye or wheat. These inadequate roots must not only provide nourish­ment but also air.

Controlled experiments revealed the following:
Average growth
Experimental condition  after 5 weeks
Roots placed in aerated tubes with 1" of solution,
root tips just above the solution    21.5 cm.
Roots in tubes filled with solution and vigorously
aerated. Roots immersed 17 cm.
Root tips just above solution level in filled un-
aerated tubes 4.4 cm.
Roots in tubes filled with solution.  Roots im­ mersed, unaerated   3.4 cm.

The conclusion is that the root needs air as much as food, and that a medium allowing access to both is ideal.2 Haydite or gravel culture may be the answer.
Many other state universities, as well as the Missouri Botanical Garden under Dr. David Fairburn and Orchid Research under Dr. Henry Eversole, have been doing interesting and valuable work on the problem of orchid nutrients. The results of the work of Dr. Fairburn may be summarized as follows:3

1. Hybrid Cattleya seedlings grow well in most nutrient solu­tions.
2. Haydite was superior to cinders and Meramec gravel as a rooting medium.

Seedlings damped off or rotted if the gravel, cinders, or Hay­ dite were kept too wet (the need of the root to breathe, again).

The greatest success in Dr. Fairburn's experiments was with seedlings, particularly Cattleyas. Adult Cattleyas were found to make good root growth but poor top growth. Seedlings made two, three, and even four growths per year, apparently taking no rest and with the plant healthy and vigorous. Roots were extended and strong. Seedlings bloomed one to two years earlier than check groups in Osmunda. Pests did not bother the gravel-cultured plants. Perhaps most important was that root breakage, seemingly unavoidable in potting and repotting plants in ordinary media, was entirely lacking in gravel potting, and this seemed to be a major factor in the uninterrupted growth of the plants.

The formula used at the Missouri Botanical Garden was that for flask culture of orchid seed, except that the iron content was increased to 0.25 gm. to each liter and a pH of 5.0 was maintained by the addition of phosphoric acid. Plants were placed in a metal tank carefully painted with two coats of asphaltum (Grade A) paint, as chemical reaction on metal in the tank or from pipe or fittings can upset the balance of the formula and may harm the plants. Tanks were filled with Haydite, one-fourth- to one-sixth-inch grade. Cattleya seedlings, flooded with the solution every other day, responded magnificently. Gravel, tried later, was felt to be better because it was less absorbent than Haydite.

Later work at the Garden has shown success in transplanting seedlings directly from flasks into five-inch bulb pans with coarse, well-washed Haydite in the bottom of the pans and a topping of very fine granite or Haydite put through a one-eighth-inch mesh screen. Filled pans are sterilized in a pressure cooker to kill any fungus that might attack the tiny seedlings. Plantlets are placed in the fine gravel about a half-inch apart. These community pots are then placed in a ventilated Wardian case and sprayed whenever the gravel becomes dry. Water should be acidified, if no peat is used, to pH 5.0 by adding phosphoric acid. Pans are watered once a week with nutrient solution. After remaining in the community pot for six months to a year, they can be transplanted to either gravel or Osmunda.

W. B. Olsen of Berkeley, California, an experienced grower who has been particularly successful with artificial feeding of Phalaenopsis, reports that he does not add iron to the original formula, but, if plants need it, one dram or teaspoon of soluble ferric phosphate is added to fifty-five gallons of the formula.4 Usually the formula picks up enough iron from the pipe or fit­tings. In order to keep his formula balanced, once a month Mr. Olsen adds one-half of the chemical formula to the solution in the storage tank. Every three months he empties gravel from the containers, repaints with two coats of asphaltum paint, replaces gravel and plants (choosing a bright day to insure no set-back to the plants), and flushes the containers to within one inch of the top of the gravel. The solution is left in the containers for three or four hours and is then drained off and returned to the storage tank. Water is added to the storage tank to compensate for evapo­ration. The formula is flushed through the containers whenever the gravel seems dry—about every seven to ten days in spring, every three days in summer, and every two or three weeks in late fall and winter. This time would vary according to climatic con­ditions in the area. This grower has used Knudson's solutions, the Missouri Botanical Garden formula, and Plant-Chem, but modi­fies all of them by using calcium sulphate instead of calcium nitrate and ammonium phosphate instead of potassium phosphate.

Success in the gravel culture of orchids requires:

1. A well-balanced nutrient solution.
2. A well-aerated inert medium such as gravel or Haydite in which plants can find firm footing.
3. Inert containers. Clay pots or cement or metal tanks may be used. Metal must be painted with inert material such as asphaltum paint to prevent chemical reactions between metal and nutrients.
4. Proper account must be taken of the possibility of pipe or fittings freeing too large a concentration of iron, copper, man­ ganese, or other metal, thus upsetting the balance of the formula.
5. Means of washing out the gravel at regular intervals must be provided to prevent the building up of chemical concentrations harmful to the plant.
6. Ability to adjust pll and balance of the nutrient to suit light
   conditions. This ability will have to be based on observation and experience.
7. Careful checking of pH and formula balance at regular in­ tervals.
8. Heat and moisture modulated by air conditions to the needs of the species involved.
9. Close observation of the effect of the solutions on the plant as evinced by texture and color of leaves and strength and growth of the roots. The grower who watches the response of plants to the methods employed will be well rewarded.

Success in growing orchids depends on finding the method of culture best suited to utilize the inadequate root system to the highest possible degree. To date, gravel culture seems to be that method of culture.

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