Hydroponics is the art and science of growing crops without soil, and its application. The word is derived from the Greek and means literally "water working.'* It is thus distinguished from agriculture, "care of the field." Hydroponics is based on the theory that all the factors of plant growth naturally supplied by the soil can be coordinated artificially by the use of water and chemicals into a crop-production method capable of competing with agriculture.
With few exceptions, such as the Eskimos, man in the past has been completely dependent upon the soil for his food supply. The course of human civilization has been determined largely by this dependence. Racial migrations and the opening of new frontiers have dramatized man's historical need for fresh and fertile soil. In recent years chemists have tried to create food by converting indigestible plant material, such as wood cellulose, into edible products. So far, they have had only slight success. Efforts have also been made to reproduce photosynthesis the natural process by which plants use sunlight to manufacture food material out of carbon dioxide and water. But hydroponics is agriculture's first real competitor.
Soilless crop production has captured world-wide attention. Thousands of inquiries have been received concerning it. Today proof of its worth is being provided by growers in all major agricultural states. It seems strange that soilless crop-production was not developed long ago. The immediate scientific basis for other great technological developments has been laid by a few talented men, in some cases by only one individual, but the theoretical basis tor hydroponics has been known to many. More research has been carried on in the fields of soil science and plant nutrition than in any other branch of agricultural scientific endeavor. Soon after 1868, the conditions were as auspicious for the birth of hydroponics as they were in 1929.
Scientists failed to realize the true value of a principle they applied in laboratory experiments. The development of water culture as a means of studying the life processes of plants is covered briefly later in the chapter. It is enough to point out that plants have grown in nutrient solutions under experimental conditions for over a century. Modern scientific agriculture has been greatly aided by information obtained through these studies. By no means do I wish to disparage their value.
The fact remains, however, that laboratory hobby culture has been aimed at but one objective, that of making better use of the soil. Not until 1929, when the theory of hydroponics was presented, was it pointed out that crop production needs no longer be chained to the soil, that some commercial crops could be grown in larger quantities without soil in basins containing solutions of plant food. Indeed, it is obvious that since hydroponics requires a larger expense per unit of area than does agriculture, either yield must be larger, or there must be other compensations if the method is to succeed commercially. And experience has already shown that it can succeed.
Some scientists who failed to realize the import of natural and field conditions have compared yields from small hydroponic basins with those from basins of fertile soil, and also those of sand treated with nutrient solutions, using the same number of plants each. In using the same number of plants in the hydroponic basin as in the soil, these experimenters have made the mistake of limiting the productive capacity of hydroponics to that of soil. Comparison can be only by growing as several great plants in each case as the fertility of the culture medium can support.
A greater mistake was to consider the yield from a few square feet of soil in a basin as representative of that of an equal sector of the field. How large a hydroponic basin must be to represent the conditions which will be encountered in large-scale production is not known at present. The established yields of agriculture are known, and comparison between the two systems can be made only under conditions representative of practical production and not by small experiments in a laboratory. It was the laboratory point of view and method in studying crop production that circumscribed the potentialities of water culture in the minds of plant physiologists familiar with nutrient solutions. The basin is capable of nourishing a much larger number of plants than is an equal area of soil because it can provide more water and nutrients. To utilize these fully, it is necessary to provide as many plants as light conditions will permit, regardless of species, as will be shown later on in the chapter on multiple cropping.
Hydroponics is not an exact science at present. It is still in the experimental stage. The most universal of all arts that of growing plants cannot be changed overnight simply by following the directions on a package of chemicals. Do not believe all the exaggerated statements you may hear. Results have been extraordinary but, unfortunately, they seem to have convinced many people that tremendous yields of vegetables and flowers can be produced with little trouble and without any real knowledge of the problems involved.
The idea is widely held that the nutrient solution will take care of everything and that, like Topsy, the crops will just grow. Unthinking enthusiasts by the hundreds have been misled by promoters selling tank equipment and "magical" formulas at exorbitant prices. The vendors of this equipment had no valuable information to offer, nor could their customers obtain the necessary knowledge from any publications. Because the buyers were not warned concerning the limitations of hydroponics, most of their expensive projects have failed.
The problems involved in growing plants without soil are many. Supplying the proper food elements through a well-balanced solution is only one. Once the plants have started to grow, other difficulties come up. The questions of light and heat, as well as of protecting the crop from pests and diseases, must be met and solved. For example, some plants will not grow in dimly lighted basements, nor even in well-lighted houses, though certain publications have stated that they will. You must realize that the theory from which hydroponics has grown is based more on field observations, which cannot be expressed in neat tables of figures, than on laboratory measurements, which lend themselves to such statistical treatment. The successful farmer's instinctive ability to coordinate all the growth factors of plants is indispensable. Thus, your success or failure in hydroponics will depend more upon skill in working out a proper technique indescribable in textbook language than upon possession of a simple chemical formula. You must combine to some extent the knowledge of the chemist, the botanist, and the farmer, arming yourself with an understanding of the fundamental requirements of plant life and developing through your alertness and insight a sure sense of the technique required.
The productive powers of hydroponics dwarf those of agriculture. Yields far outstripping those obtained from some of the richest farming sections of the nation have been produced on experimental plots. These yields, large as they have been, by no means exhaust the possibilities. A later chapter will show how several different crops can be grown simultaneously from the same basin, each of them providing larger harvests than can be taken from the soil. This has already been done in large-scale experiments. It promises to overcome one of the major objections to hydroponics: the high cost of equipment for some crops. Nevertheless, caution should be employed in selecting the crops to be grown; some will always be grown more economically in soil.
Soilless crop production presents a challenge to the amateur. The main purpose of this book is to aid him in mastering it. As the rules of operation become standardized, the ^wage earner with a small plot of ground at his back door may regain a measure of economic independence. His food supply will be more under his control so that his livelihood will no longer depend solely upon national philanthropy or the weekly pay check. As a means of providing subsistence to those thrown out of employment by recurring economic depressions, hydroponics deserves the utmost consideration from the government.
In the commercial field, soilless crop production is now being employed successfully. Experiments indicate that it will soon invade new regions and new fields of agricultural production. Hydroponics can be used wherever good climate prevails. Thus, states like New Mexico and Arizona, lacking in soil resources but blessed with mild temperatures and plentiful sunshine, will find it ideal.
Hydroponics offers much to those who are interested in solely in growing flowers for their enjoyment and the beautification of their homes. Daisies, snapdragons, begonias, and dahlias are but a few of the garden flowers which can be grown to their natural size and beauty in neatly concealed tanks.
Nations such as Italy and Japan, which are worried by crowded populations and inadequate agricultural land, could easily use it to multiply their production of foodstuffs manifold. Once their hunger is satisfied from within their boundaries, the reasons for seizing the rolling wheat fields of their neighbors might be swept away.
The field we now call hydroponics. Instead, they were diverted to a relatively lesser endeavor. In his first experiments in 1859, Knop grew plants in natural water without mineral nutrients. Seeds were sprouted in sand or fiber netting. The seedlings were then inserted in holes made in rigid support. Usually cork stoppers held tightly by a cotton wadding, and suspended in glass or earthenware containers filled with liquid.
Thus, Knop established the technique now used universally for laboratory experiments. By this method, Knop also found that a plant can make an appreciable gain in weight simply by using the food contained in its seed and that the seed provides nourishment to those parts of the plant which form first. From this Knop concluded that the growth of vegetal tissue of plants is proportional to the nutrient content of their seeds. This theory has since been accepted by plant physiologists.
By this time it had been established that, if soil nutrients were to be used by plants, they must be present in soluble form. It was also known that the amount of soluble plant food in the soil was very small compared to that which was insoluble. This information provided a scientific basis for Knop's future work. However, methods had not yet been devised for measuring such properties of the solution as osmotic pressure. Nor did scien- tists have any clear idea as to what these properties might be.
So, while Knop knew that solutions which were too concentrated might prove harmful, he did not know how this harm was done. Nevertheless, in 1860 he succeeded in growing plants weighing many times more than their seeds and containing a much larger quantity of nutrients. In 1868 buckwheat weigh- ing 4,786 times and oats weighing 2,359 times more than their original seeds were produced by others using Knop's method. This established beyond doubt the fact that normal plants could be grown without soil. Knop had a fairly good idea of what elements were necessary. As early as 1842 another investigator had compiled a list of nine elements which he believed were the essential one's provided by the soil.
The first concern of agricultural chemists and botanists was to determine which elements were needed and Introduction 9 which were not. There was no unanimous agreement on this point, nor is there today. From 1860 to about 1920 most scientists thought nitrogen, calcium, magnesium, phosphorus, potassium, sulfur, and iron were the only essential elements from soil. But during the past twenty years, as purer materials have become available for laboratory research, we have found that the ''trace elements" boron, copper, zinc, and manganese are also required. From a wide variety of compounds, Knop finally selected calcium nitrate, mono-potassium phosphate, and magnesium sulfate as the chief ingredients of the nutrient solution. Each of these supplied two of the essential elements.
Consequently, he was able to keep the concentration of the solution at a low level, at which plants grow best. Nevertheless, Knop's choice of chemicals was not a good one. The compounds contained elements which were not used in the same quantities by the plants. As one was absorbed, an excess of the other was released and entered into another com- bination in the solution. In time the acid-alkaline reaction of the liquid changed. This was contrary to the pattern of nature, for the soil solution from which plants derive nourish- ment in agriculture changes very little if at all. Knop's nutri- ent solution, on the other hand, became progressively more alkaline. Knop realized this and specified that a good nutrient solution should be slightly acid.
Knop made this recommendation before the theory of elec- tric dissociation of molecules was even dreamed of. It was known in his time that plants exercise "selective absorption" preferring some elements to others. On this principle it might be argued with some validity that it makes no difference how, or in what quantities, the various elements are supplied in the solution. The plants simply absorb what they want and leave the rest. Knop, like the other scientists of his time, had no way of knowing what effect this residue would have on the properties of the solution and on the plants themselves.
Today the theory of electric dissociation of molecules tells us that salt molecules in a solution split up into particles, or ions, carrying positive and negative electrical charges. The positive ions can- not exist unless an equal number of negative ions is also pres- ent. As it happens, plants prefer nitrate ions (NO 3 ~) above all others of negative charge. For this reason, nitrogen is ab- sorbed quickly and, unless an equal number of positive ions is also absorbed, the solution will turn alkaline. The most pre- ferred of the positive elements is potassium. Consequently, there being no other modifying factors present, a good nutrient solution must have more nearly equal portions of available nitrogen and potassium than of any other elements. Each of the major elements in the solution must be considered in rela- tion to another of the opposite electrical charge.
In Knop's formula of 1868, he added, to one liter of water, one gram of calcium nitrate, .25 gram of magnesium sulfate, .25 gram of mono-potassium phosphate, .12 gram of potassium chloride and a trace of iron chloride. In this mixture, the ratio of positive potassium ions to negative nitrate ions is about two to eight. No wonder, then, that his solution turned alkaline shortly after plants began feeding on it.
Looking back upon Knop's experiments, we see that they threw considerable light upon the question of salt proportions in the solution. Before taking up this important point, how- ever, let us consider the influence of modern chemical analyses upon water culture. New developments have made it possible to measure the osmotic pressure of a solution. Osmotic pressure is a highly important physical property of the liquid. It derives its name from the process called osmosis by which liquids pass through the permeable membranes or tissues of plants. The movement takes place from the region of high water con- centration into that of the low.This is because water, like gas, always flows from a high-pressure area into a low-pressure area. A solution high in solutes has fewer water molecules per unit volume than one low in solutes.The force of this flow acting on the dissolved substances in the solution and measured as the pressure they exert on a membrane through which they cannot pass is the osmotic pressure. If roots are immersed in a solution which is too concentrated, this pressure may cut down their intake of water or even draw water out of them. In this way the plant's life processes are deranged.
The osmotic pressure is also a measure of the num- ber of molecules and ions in a solution. It is one of the three ways of expressing the concentration of the liquid. The other two are by parts per million, or the ratio of chemicals to water by weight, and by molecular concentration. To return to the question of salt proportions. Many other investigators have tried various combinations of chemicals in nutrient solutions. The most extensive work in adapting some of the newer concepts of physical chemistry to the use of such solutions was done at the Laboratory of Plant Physiology, Johns Hopkins University. The pioneer work done there on salt proportions in solutions of various molecular concentrations and osmotic properties played an important part in laying the foundation for hydroponics.
Water culture supplied the answers to such important ques- tions as what, when, how, why, and how much of certain ele- ments are necessary for plant growth. Many scientists in all parts of the world have contributed to the knowledge now amassed on these points. The technique used to determine what elements are essen- tial was quite simple. A mixture was made from which one certain substance was missing, and the plant was then studied to see what effect lack of this element had upon it. The absence of those elements needed in large quantities usually had a more pronounced effect than that of those required in small amounts. This did not hold, however, for those elements which, like iron, play a specific role in plant processes. It has long been recognized that the composition of a plant does not remain constant throughout its existence. This raises the question: Does the food requirement of plants vary ac- cording to the conditions under which they grow? To explain the variation in a plant's composition, we consider that it is composed of two parts: (i) the food which it actually needs, and (2) that which it does not need but stores up in its tissue. It is the second part, absorbed during the latter part of the plant's growth, which causes variations in composition. It has been possible, by the use of water culture, to withhold varying amounts of certain elements from plants during their latter growth stage.In this way scientists have determined how much growth a plant can make from any given quantity of nutrients absorbed during its early growth. In other words, it has been possible to find out just how much food the plant requires to grow normally at any age. From this we have learned that there is a period late in life when the plants absorb only a very small amount of nutrients.
The question of how plants absorb their food has provided the basis for a most intriguing study. We know that they have the power of selective absorption, being able to take one ele- ment from a compound and leave the other. The theory that some certain combination of chemicals would ultimately prove to be the best under all conditions had to be considered in the light of this fact. Now an element taken up separately by the plant is absorbed as an ion. Two ions of opposite charge taken in together will have the same effect on the solution as if they were united in the molecule of a chemical compound. This has a most important bearing on the composition of nutrient solutions. For the reaction of the liquid remains most con- stant when elements are absorbed as if they were complete molecules. Therefore, the elements should be paired in the so- lution.
Each has an opposite which should be used at the same time so that they will be absorbed as a unit. Water culture experiments not only opened the way to this discovery but also provided the knowledge as to which elements should be used together. From a great amount of such study, formula was finally evolved which incorporates the chief theoretical features as well as the evidence derived from physiological studies and plant analyses. In this way, the physiological basis for hydro- ponics was laid. The basic formula will be discussed more fully in the chapter on nutrient solutions. How much of each element does the plant need? To answer this question we must first answer another: How much growth can a plant make from a given quantity of any one nutrient absorbed? By multiplying the weight of the plant by the per- centage of each element it contains, we can determine how much of each has been absorbed. The plant may take in more of some foods than it needs, so that composition is not always an accurate answer to our query.
Nevertheless, we must know its composition not only at maturity but also during any of its various growth stages. The elements within the plant stand in a complementary relationship to each other. A heavy intake of one will lower the intake of others. Consequently, the amount of any certain element contained in the plant may vary over a wide range. This fundamental complementary relation- ship between the various food factors must be considered. If we know the range of variation in composition among the dif- ferent plant parts, and the causes thereof, we can forecast how much growth a plant can obtain from a given quantity of food. It was this knowledge which made it possible to compound a chemical formula which would insure the most efficient use of all nutrients. The question of why various elements are needed has re- ceived a vast amount of study and undoubtedly will continue to draw attention for years to come. There is still much to be learned. At present, our knowledge is limited to those elements which are constituents of specific chemical compounds or per- form some definite function in plant life.
Nitrogen is required as a raw material for proteins manufactured by the plant. It is the only element which we find fixed in a specific chemical product in practically the same amount that is absorbed. For this reason, analysis of a plant for its nitrogen content will also reveal the amount of protein it contains.
Phosphorus plays a part in the formation of new cells. It is particularly abundant in the growing parts of the root tips and enlarging shoots. At maturity, large amounts of this element are stored in the seeds after having performed their specific function in the formation of new cells.
Sulfur is also a constituent of proteins. Magnesium is used in the synthesis of chlorophyll, the green coloring matter of plants. Calcium is a binding material which holds together the cells of various plant tissues. So vital is this function that the absence of calcium causes more profound disturbances in many species of plants than does lack of any other element. Potas- sium seems to act more or less as a helper to other elements. It does not enter into any specific chemical compounds inside the plant. The amount of nitrogen absorbed, hence the amount of protein that can be manufactured is related to the absorption of potassium. The actual synthesis of protein by the plant appears to bear a closer relationship to the amount of calcium rather than potassium, which is present. Iron is needed for the manufacture of chlorophyll but is not a constituent of the pigment.
The function of the trace elements boron, manganese, zinc, and copper has not been established. It seems to vary with the amount of light provided to the plant. Still, this can be said for all elements, since light affects growth and is thus reflected in the nutrition of plants. There is no doubt that the data accumulated through water culture experimentation facilitated the birth of the soil-less method of crop production in 1929. It was certain to be discovered in time. No insuperable barrier to discovery re- mained once the general precepts had been established and it became known that crop production required a proper coordi- nation of all the various growth-affecting factors.
For three-quarters of a century before hydroponics, water culture was used solely as a laboratory method of studying plant nutrition. The scientific contributions mentioned in the pre- ceding section had established its value as an aid to experiment- tation. Supposedly, this was the extent of its potentialities. The phrase "crops are grown without soil" was never used to de- Introduction 15 scribe water culture experiments, nor were scientists interested in the possible crop yields of water culture as compared to agriculture.
They were interested solely in growth processes. Tiny plants weighing only a few grams supplied all the experimental material needed. The feeling prevailed that plants could be grown in nutrient solutions only under rigidly controlled con- ditions, with pure water, pure chemicals, pre-sprouted seeds, glass bottles, and a meticulous technique. Had any of these pure laboratory features proved indispensable, hydroponics would never have been possible. It jvas necessary to prove that none of them was needed to grow plants without soil. The preliminary work in hydroponics was devoted to finding satisfactory substitutes for water culture methods and to an- swering one all-important question: Can nutrient solution com- pare favorably with soil in production per unit area If not, the whole idea would be worthless.
The first step in hydro- ponic development, then, was to determine what yields might be obtained from a given area of nutrient solution exposed to good light under the normal conditions of field or greenhouse. To do this, it was necessary to grow plants on a rather large scale. Basins had never been used before. They presented several new problems. They had to be constructed from materials such as cement, metals, and certain woods which were sup- posedly toxic. In practice, it was found that the effect of harmful elements in these materials had been accentuated by the unfavorable conditions of laboratory climate.
Some way must then be found to introduce plant food into the water. In laboratory tests, this had always been done by mak- ing a separate stock solution of each salt and adding quantities of each solution to the water separately. The small containers were then agitated to obtain a thorough distribution of chemicals. This could not be done with large basins. Various methods of mixing small amounts of chemicals into the basins were tried.
Ultimately, such mechanical means of obtaining a thorough distribution were found to be unnecessary. The different solutions were simply poured into the water at various points and natural diffusion did the work of distribution. It made no difference if the chemicals were not distributed uni- formly, so long as the plants all received their minimum food requirements. They were able to grow equally well under a fairly wide range of concentration in the nutrient solution,Thus, a blow was struck at one of the established concepts of water culture.
Constant conditions and concentrations in the solution were shown to be unnecessary. The corks used in water culture held the plants in their proper relation to the solution but could not perform the functions of soil. In hydroponics it was hoped to reproduce the natural conditions of growth as accurately as the use of artificial means would allow.
The first support used in large basins was composed of stiff paper board with holes cut in it for the insertion of rose plants. Some of the flowers failed to grow well because of the high reflection of light and heat from the paper surface. In another experiment a variety of different plant species were wedged between laths nailed to the top of the basin. The solution was not covered. Again some of the plants failed; this time because the air immediately above the solution was too dry.
Next, burlap was laid on a wire netting attached to the basin and sand was poured on top. Seeds were sown in the sand. This arrangement was not designed for large-scale production or for ease of handling. Today it has been replaced bythe litter seedbed. But in these early experiments the plants did well. For the first time seeds were sown in the same material used to support crops in the nutrient solution. This established the fact that pre-rooted plants were not necessary. Soon two more pillars were swept from under the old water-culture theories. It was proved that neither pure chemicals nor pure water was needed. One by one the old concepts had begun to fall and hydroponics began to develop. Still another feature of water-culture technique found to be unnecessary was the practice of adding each chemical to the solution separately. To get away from this a mixture of dry chemicals was encased in gypsum and the resulting "plant pill" placed in the basin filled with water. The object was to insure a slow diffusion of the elements, thus doing away with the necessity for frequent additions to the solution. After gypsum, waterproof paper and glass bottles were used to hold the chemicals. Finally, all of these were found to be superfluous. The dry mixture was simply added to the water and allowed to dissolve.
Once the physical equipment such as basins and seedbeds had been developed, attention was turned to the cultural technique required for hydroponics. Experiments were conducted both out of doors and in the greenhouse. Some crops were not adapted to both habitats; it was necessary to grow them under diverse climatic conditions in order to see what differences in yield and quality might result. Consideration was also given to the nutritional and cultural requirements of the different plant species. Species are living evidence that varying requirements exist. Their growth together in the same soil and under the same natural conditions also proves that they have some characteristics in common. But hydroponics had eliminated the soil. It was necessary to find out how the various species reacted to their new environment. Observations revealed no general rule pertaining to this point. Two species which acted quite similarly in soil might differ markedly in their characteristics when grown by hydroponics. But the reverse of this proposition was also true. By this time it was evident that, while hydroponics could be a better cropping method than agriculture, it could also be a poorer one.
The margin of safety against the development of poor growing conditions was much smaller in soilless crop production. For example, it was found that the liquid solution has a much lower resistance to chemical change than does the soil. Thus, experiments were required to find out how poor growing conditions might develop and to discover methods of forestalling them. Different varieties of roses were grown in the greenhouse and also in outdoor basins. They differed much more widely in adaptation to water culture than to soil. It then became clear that a number of factors which afforded little or no difficulty in agriculture would be of importance in hydroponics. The adaptability of roses to hydroponics was governed by the type of root stock to which they were grafted or budded, by the age of the stock, the state of dormancy, the season of the year in which they were planted, and the methods of root pruning used. All of these factors were apart from the general features of cultural technique. Data were collected on the yield, composition, and general quality of a large variety of field, vegetable, and floral crops.
Experiments conducted in 1929 supplied the first insight into the tremendous productive capacity of the new cropping method. They showed that multiple cropping would be prac- tical in hydroponics and would make possible yields per unit area running from ten to fifty times as large as the average obtained by the single-crop, field system of agriculture. Such yields, it appeared, could be obtained over large areas of the earth's surface. It was simply a matter of deriving the utmost benefit from available sunlight by maintaining as much green vegetation as possible on each unit of area. Most of the research on plant composition was done with wheat. By withholding certain nutrients, or otherwise radically changing the composition of the solution during the latter growth stage, one could alter the composition of the grain. From these experiments it was possible to determine the minimum and maximum amounts of each food element that could be absorbed. It was then an easy matter to find out generally the amount of chemicals required to produce a unit gain in weight of the plant and how much the chemicals would cost. The data showed that this cost would be too high to make wheat production feasible but that crops notably high in water, starch, or sugar (such as potatoes) could be grown at a surprisingly low cost for chemical food elements. . Another discovery, perhaps more obvious than some others, was that a cultural technique would have to be worked out for each separate plant species. Plants differ from each other in the complexity of structure and life processes. The more complicated the plant, the more spedalized must be the technique for handling it. Annual plants propagated from seed are easiest to grow. Some bulbs are also easy; others present great difficulty. Perennials, with their complex life machinery and period of dormancy, usually require the most painstaking care.
Probably the outstanding revelation of these experiments was that study of plants, of their adaptability to varying conditions, and of individual differences is more vital to success in hydroponics than the ability to handle physical equipment and provide the proper mixture of nutrients. It is by studying plants as crops rather than as test materials for the laboratory that you will master the principles of soilless crop production. The evolution of water culture into hydroponics has been the story of the use of ordinary materials and methods to reproduce natural growing conditions. The soil is a vast reservoir of water. Therefore, a basin is needed. Soil provides vegetation with eleven food elements in solution.* Thus, these must be added to water made available to plants grown in the basin.
The soil provides support by anchoring plant roots which are then immersed in the soil solution. Hence, a seedbed must be provided to support the plants in their proper position relative to the nutrient solution. Hydroponics is an artificial but not -in unnatural crop-production method, based upon those same principles which nature has set up as the pattern of plant life. Plant physiology has provided the foundation for hydroponics. Hydroponics has outgrown the older science in that it is concerned with more than simply the study of the fundamental relationship of the various plant processes. Hydroponics coordinates the other sciences which deal with plant growth and use into a system of production that is independent of fertile soil, the very foundation of agriculture.