( Originally Published 1942 )
THE climatic and soil conditions which have molded members of the Cactus Family into fantastic forms and clothed them with protective vestments have done more than modify their external appearances. Their internal structure has been considerably changed, and even their method of digestion differs radically from our common leaf-bearing plants.
About one-half of the solid matter of most plants is carbon which enters the leaves through countless window-like openings in the under side of the leaves. This carbon is the same carbon with which we are familiar in the form of soot and charcoal, but it is in the form of an invisible gas, carbon dioxide (CO2), in the air all about us. Mixed with air and moisture, the gas floats in through these tiny windows where the plant cells with the aid of sunlight, break down the molecules of the gas into separate atoms of carbon and oxygen. The important carbon is then recombined with hydrogen and oxygen secured from water broken down in much the same way. The new combination is starch which is useless to the plant as such, but must again undergo a shuffling of atoms into a form of "sugar which is soluble and can be assimilated by the plant. The mind staggers in its efforts to conceive the amount of air a tree, for instance must "refine" in order to secure a hundred pounds of carbon.
But leaves with sufficient windows to take in the required carbon dioxide food are also open for the escape of moisture, and cacti, which get a drink of water normally only a few times a year have been compelled to check that waste of moisture by the simple method of discarding leaves which contained the windows. But the leaves also contained the digestive cells, so the cactus might be said to have thrown away its stomachs. They seem to have tried to make up for this loss by producing the same type of cells under the bark of the stem and even inserting a few windows, but the development of even this substitution could not be carried out sufficiently to meet the food needs of the plant without permitting fatal evaporation.
No one knows how many groups of plants, in the past, facing far less discouraging problems, have succumbed in their struggle for existence and now lie buried in the forgotten limbo of fossilized rocks. But not the Cacti with characteristics which almost deserve to be personified as "indomitable courage," "unbeatable American spirit," "irrepressible will to survive." They met successfully the threat of extinction.
Cacti share with many other plants the ability to form such organic acids as malic acid, which gives the sour taste to many plants and fruits, and oxalic acid which, in a pure state, is a violent poison. Botantists tell us it is this poisonous acid, apparently so useless to other plants, which has been used by cacti to solve their acute food problem.
Oxalic acid is composed of hydrogen, car-bon, and oxygen (H2C204) . The plant se-cures calcium carbonate, which is composed of carbon, calcium, and oxygen (Ca CO3) , and is to some extent soluble in water from the soil. These two compounds, oxalic acid and calcium carbonate, are broken up in the plant, and the atoms reshuffled forming calcium oxalate, composed of carbon, calcium, and oxygen (Ca C2O4) There are some atoms of hydrogen, carbon, and oxygen left over in the reshuffle, and these atoms combine to form water and the all important carbon dioxide which supplements the carbon dioxide secured from the air through the skin of the stem.
Limestone (calcium carbonate) in the soil is beneficial to a very large number of leaf-bearing plants. It encourages chemical reactions which change certain important plant foods already in the soil but useless to the plant because they will not dissolve in water, and plants can eat only gas and liquids. The new combinations of chemicals brought about by the limestone are soluble in water, and, therefore, become good available food for the plant even though the limestone itself is not used as food by these leaf-bearing plants.
Limestone also functions as a very efficient "chef" in preparing food for the use of cacti; but the purpose of this lengthy and more or less technical discussion is to emphasize the double importance of limestone to these leafless plants since the calcium carbonate itself is used in securing sufficient carbon dioxide for their growth and well being.
Not all species of cacti are equally dependent on limestone. There are species -which seem to get along fairly well in soil suitable for other plants. There are many others which manage to live in such soil but neither grow nor thrive—they merely exist. Some species, however, have become so de-pendent on calcium carbonate that the residue from the digestive process, white, insoluble crystals of calcium oxalate, accumulate in the cells of old specimens until they are not only conspicuous, but constitute a considerable portion of the weight of the plant itself.
Limestone has been purposely emphasized as a source of calcium carbonate. It is, of course, not the only possible source of this compound, but the author feels that, for the amateur, at least, it is the safest, the most easily obtained in reliable form, and sufficiently effective for satisfactory results.
So-called "limestone" picked up in the country may contain minerals which are not only useless but may even be detrimental to plant growth. Nearly all seed stores which handle fertilizers have ground limestone of dependable quality.
Good limestone is so mild in its chemical action there is no danger of "burning" either the plant or the soil though considerably more is used than really needed.
In many of the written discussions of this subject, it seems to be implied that calcium is the important factor, but it should be re-membered that it is the calcium which is largely discarded by the plant in the calcium oxalate and the carbon which is assimilated to help form the plant protoplasm and tissues.
Chalk is even a purer form of calcium carbonate than limestone usually is since it was made by tiny sea animals many ages ago. Although it does not differ chemically from limestone calcium carbonate, there may be some slight advantage in its very finely divided state which may be useful in obtaining some especially desired mechanical condition of a certain type of soil, but there is little to indicate that there is appreciable difference in the efficiency of the two forms other than may be attributed to the difference in their effect on soil.
"Lime" is one of those words in common usage which is applied to so many different substances that it might be called a "shot-gun" term. A cactus enthusiast reads in some publication that lime in the soil is recommended. "Lime" is associated in his mind with plaster and building materials, so he hastens to the lumber yard and asks for five pounds of it. He receives a package of rock-like lumps which he carries home, pulverizes and mixes in the soil of his various plants. In a short time his plants will probably die with about the same completeness as would his pet dog if given a dose of strychnine. The "lime" he got was "quick-lime" calcium oxide (Ca O) which is violently caustic when mixed with water and deadly to vegetation.
Instead of quicklime, the dealer might have given him a package of white powder as fine as flour which would have been "lime" but "airslaked lime" (Ca 02) . This form is not quite as disastrous as the use of quicklime. It is sometimes used in soil for plants less delicate than cacti, but it is often somewhat caustic, and it has a tendency to ruin the soil by releasing plant food stored in the soil much more rapidly than the plant can use it, and the released food is lost by being washed away by the water.
Had our enthusiast stopped at a house under construction and asked the plasterers for "lime" they would have given him soft, wet, white material of about the consistency of soft soap. This, again would have been lime, but "slaked" lime, calcium hydroxide (Ca (OH)2), which is not a desirable addition to a soil for cacti because it is still more or less caustic, is inclined to "burn," and has a tendency to ruin the texture of the soil.
It should be pointed out that all these forms of calcium, quicklime, air-slaked lime, water-slaked lime, and fresh plaster, all change into the desirable form of lime-stone with age. After months of exposure to air, soil and moisture, carbon is absorbed from these sources forming the friendly calcium carbonate or limestone. The rate of the chemical processes required to transform a dangerous combination to a desirable form is dependent on so many conditions that it is offered as justification for the recommendation that amateurs play safe and make use of accredited limestone only. It is fun to experiment with some of these other forms of calcium if the experimenting is done on such a small scale and with common species of plants so that failure will not mean tragedy.
Ten years ago our neighbor built an out-side chimney of limestone rock. One of these rocks, brought in from a ranch pasture, has a little cactus growing in a cavity about an inch and a half across and two and a half inches deep. Although the neighbor is not particularly interested in cacti, he so much admired the pluck of this one in trying to make a success of life under such hardships that he ordered the mason to lay up that particular rock so as not to disturb the plant. The soil in the cavity is estimated to be about one tablespoonful. No additional food or soil has ever been added. It has had no water but natural rainfall, from which it is largely protected by the over-hanging roof, and an occasional extra drink when the lawn was watered. Today, the plant is apparently as happy and healthy as any plant to be found, but, and note this carefully, after ten years of growth, this brave little cactus is only three inches tall, having added one inch to its stature in the ten years. And it has never bloomed. Countless similar examples have led to the widely accepted belief that such poverty of food and water is essential to the well being of these plants and that prosperity for them means death. Such a conclusion is illogical and scientifically unsound. Just because a donkey, for instance, can subsist on the desert with a paucity of food and water which would mean death to most other animals of its size does not prove that bigger, finer looking, and more valuable donkeys cannot be produced under more favorable conditions of food and water.
There is abundant evidence to indicate that cacti will respond favorably to a much more abundant supply of food and water than is usually apparent in their native environment. It should be emphasized, however, that nourishment must be selected and administered with careful regard to the plant's needs and eccentricities. In this respect, they are little different from other forms of life both animal and vegetable. A baby fed a calcium-deficient ration develops soft bones and teeth. Hens given food de-void of calcium lay eggs without shells. A pear tree which has abundant nitrogenous food and water but no phosphorus may make a very beautiful shade tree but produces little or no fruit.
During their growing season, cacti greatly appreciate a constant film of moisture surrounding the fine soil particles in which they grow, but quickly drown if the tiny spaces between these soil particles are kept filled with water so as to exclude air for any considerable length of time. Cacti are definitely fresh-air babies even down to their roots and not water babies. Hence the necessity of frequent watering to maintain the water film, but with good drainage to keep the inter-soil spaces clear for access of air.
This group of plants seems to be particularly allergic to acids. So it is wise to avoid them in the soil as much as possible. Of course, limestone tends to neutralize acids, but charcoal is also very valuable because it has remarkable ability to "sweeten" soil by absorbing the undersirable acids, and it has a favorable effect on the general condition of the soil. Since charcoal is pure carbon and cacti use carbon in their digestive processes, it would seem that the charcoal could be fed directly to the plant. This is not possible, however, because the carbon does not dissolve in water and the plant eats only soluble food. The charcoal is particularly valuable, however, when, vegetable matter in which decomposition has not been entirely completed has been used, because acids are formed in this breaking down process which are detrimental to the plants if al-lowed to accumulate.
Nitrogen is probably the best known and most discussed plant food among cactus growers. It is common knowledge that humus and barnyard manure are the two most common sources of this important food. It should be borne in mind that many cacti are compelled to extract the last tiny particle of plant food from its poverty stricken soil in order to survive. To accomplish this extraction, the plants have developed root-hairs so delicate they are susceptible to the "burning" tendency of concentrations of nitrogen which other plants can assimilate without harm. These root-hairs are not unlike some blonde persons on the seashore who sunburn terribly under conditions which may not affect some brunettes.
It is this tenderness of the root-hairs which makes it good practice to give the plants a liquid diet instead of mixing the manure directly into the soil as is done with plants more used to "high living." But even this liquid manure should not be too concentrated and should be prepared with consider-able care.
Preparation is very important. Place the dry decomposed manure in a large container, about one half full, wooden by preference, add water and let stand several days until the liquid becomes dark in color. Dip out about a cupful of liquid, pour into a two or three gallon bucket of water. Water plants with this diluted liquid. Best results are obtained by placing the pots in a shallow tray containing the liquid, and letting the plants soak it up. A safe rule is to wait until the soil becomes moist at the top. This takes more time but watering can be spaced farther apart.
In order to maintain the proper balance in this enriched food, it is essential that we supply additional potassium, but much care must be exercised not to over do the matter since it is powerful in its action, and, in excess, destroys the crumb structure of the soil so essential to cacti. Potassium is available in several forms of commercial fertilizers which may be used by the more experienced growers, but unleached wood ashes are probably safer and more satisfactory for most of us to use if they available. Although wood ashes vary considerably in the amount of potassium carbonate they contain, it is suggested that two level tablespoonfuls to a six-inch pot will prove a satisfactory dosage in most cases if the ashes have not been exposed to rain. Additional applications of small amounts may be made during the growing season. Potassium has a stimulating effect on plants somewhat similar to nitrogen. It improves the color of both plants and bloom, but it also has a tendency to prolong the period of growth and to postpone the production of bloom. It is claimed that when used on fruit trees, garden, and farm crops, it builds up in the plants quite a definite resistance to diseases. It is logical to expect this same effect on cacti, but there is no experimental data available either to confirm or disprove such a supposition.
Phosphorus is another one of these chemicals whose importance to plants is out of proportion to the amount they actually use. It stimulates flower and fruit production. If there is a deficiency of phosphorus in the soil but an abundance of nitrogen, plant growth will be strong, but flowers will be few or entirely lacking. Soils vary greatly in the amount of phosphorus present, but rarely is a soil found with sufficient phosphorus to stand a large increase of nitrogen without applying additional phosphorus to maintain a proper balance. One soil may be available with plenty of nitrogen; another may have more than average potassium; and a third may be strong in phosphorus. It is quite possible to make a combination of these soils and obtain the balanced ration required. In fact, there are some clever growers who do just this thing with excellent results. Other growers have discovered that certain combinations of soils produce wonderful plants and bloom without knowing why they get such results.
Most of the commercial forms of phosphorus are too high-powered for use in the cactus garden. The commercially prepared pulverized bone-meal, however, is rich in phosphorus, and may be used with safety if not applied in excess. A tablespoonful to a six-inch pot applied two or three times dur ing the growing season should be sufficient for most soils.
A word of caution concerning the application of these fertilizers may not be amiss. It is wise not to place fertilizers in the soil close to the stem of the plant. They should be dug into the soil near the outer edge of the pot. Even when the fertilizers are mixed with the soil before planting, it is well to place a small amount of normal planting soil such as loam, sand, and charcoal, but without the special fertilizers, in the center of the pot in which to plant the roots and stern of the cactus. Soluble plant food from the outer edge will be carried in to the newly formed feeding rootlets with each successive watering, but not in sufficient quantity to cause indigestion. Some successful growers even insist that instead of pouring the liquid manure in the top of the pot, it is worth the extra trouble to set the pots in the pans of the diluted liquid so it may permeate the soil from the bottom.
The purpose of this discussion is not to set forth a complete and final settlement of the problem of feeding cacti. Instead, it has been an attempt to bring into focus some of the general principles of plant feeding and to suggest possible diets which may be adapted to the eccentricities of cacti. The discussion is presented as a starting point rather than a destination.
The frequent publication of apparently contradictory advice in this field by experienced and successful growers results in such confusion it is little wonder many amateurs become so discouraged they quit our ranks in disgust. This is especially true when one "authority" brands as "sheer nonsense" recommendations of other growers who, so far as the amateur knows, have had just as much experience and success as the first.
Until some unbiased and scientifically minded individual, with a knowledge of chemistry and a broad understanding of the intricate physiology of plant life, undertakes and completes an experimental study of the food requirements of various species of cacti which will form a reliable basis for conclusions, the amateur will do well to keep in mind four fundamental principles of diet which, at this time, seem to be generally applicable to practically all cacti.
First: A water supply during the growing season which will maintain a moisture film surrounding the soil particles but which does not completely fill the spaces between the particles for any considerable length of time.
Second: Cacti will not grow on imagination----they require food, but their "digestions are delicate and the food should be given in dilute form.
Third: Cacti seem to require about the same balance in their diet as do other plants. Nitrogen, potassium, and phosphorus are required with the addition of more calcium carbonate than demanded by most leaf-bearing plants.
Fourth: Cacti need a "sweet soil obtained by applying limestone and charcoal and by avoiding much humus, such as leaf-mold, and manures in which fermentation has not been completed.