( Originally Published 1920 )
AN intelligent understanding of the soil is of paramount importance to the success of the greenhouse. There are three important points that we must consider in the study of a healthy soil. They are : (1) texture, (2) fertilizers, (3) soil flora.
Texture deals with the character of the particles which make up a soil, and with their arrangement in relation to each other. Clay soils are generally made up of very minute particles. Silt is made up of large grains. Coarse sand or gravel is composed of the largest grains. A soil is said to be porous when air or water can circulate through it freely. The porosity depends on the various proportions of clay, silt and sand which that soil contains. Plant growth, and incidentally plant health, is closely interwoven with the soil structure. Compact, sticky clays will be far more unfavorable to greenhouse crops than a clay loam.
The greenhouse man has the advantage over the ordinary farmer because he can modify the texture of his soil so as to make it ideal for his crops. By combining the proper amounts of clay, silt, sand and humus, he may give to the plants a most congenial place to thrive in. To obtain such a result the gardener must exercise his best judgment. By varying the texture of the soil we may often. influence the plants unfavorably. Flowering plants may be made to produce excessive foliage and few blossoms, while others may be differently affected.
Crops require certain food elements to make growth possible at all, and they further require specific substances to enable them to accomplish definite purposes. The carnation, for instance, requires peculiar food elements to attain maximum growth. It further requires special nutritive elements to enable it to produce flowers and to avoid going altogether to foliage. The four leading plant foods needed by greenhouse crops are nitrogen, phosphorus, potassium and lime. All the other plant food elements are present in nearly all soils.
The effect of nitrogen is to stimulate leaf and stem growth, and to add green color. An overdose of it, however, may result in soft plant tissue, and thus retard fruiting. Acid phosphate stimulates root growth, and an overdose of it encourages an excess of root formation over foliage. Phosphorus also stimulates earliness in fruiting. The effect of potassium is to help the plant in assimilating other plant food, and indirectly in the manufacture of starch. It also encourages the production of finer plant tissue, thus increasing the plant's resistance to disease.
The aim of the greenhouse man is to produce early truck crops or cut flowers and this is directly concerned with feeding. Because the four plant food elements above mentioned are of extreme importance, their application cannot be indiscriminate. The greenhouse man must know how much of them to use in combination or separately. He must know also which element will especially benefit the particular crop with which he deals. In his investigations with the fertilizer requirements of lettuce, Stuart reached the following conclusions : Potash when used in any considerable amount either alone or with nitrate of soda is unfavorable for growth (fig. 1, D.). Acid phosphate alone, in combination with nitrate of soda, or in combination with muriate of potash, stimulates growth (fig. 1, A, B, C.). For lettuce the use of chemical fertilizers proved slightly superior to stable manure, while nitrate of soda was found to be superior to dried blood. Wheeler and Adams, in their work with radishes, found that an application of partially composted horse manure at the rate of 75 tons per acre gave better results than any other combination of fertilizers used. Working with carnations, Darner and his associates found that the use of nitrogen and acid phosphate caused an increase in the quantity and quality of the blossoms, but that the excessive use of potassium sulphate and dried blood would act injuriously on the plants. In his work on roses, Muncie concluded that nitrogen in the form of farm manure, liquid manure or blood is very beneficial. The same seems also to be true for acid phosphate when used at the rate of 4 to 8 tons per acre. Lime should be added only when necessary to sweeten the soil. In this case, finely ground limestone may be used as a top dressing at the rate of 10 pounds per loo square feet of bench space.
From the above discussion, it is evident that the proper handling of fertilizers underlies the success or failure of greenhouse crops. The cattleman, the poultryman, and others who deal with live stock now fully, appreciate the importance of a properly balanced ration. Plants are similarly living organisms and consequently they too derive most benefit from a balanced ration (fig. 1, A.).
Aside from a consideration of the relation of the fertilizer to plant growth, its relationship to the soil must not be overlooked. Certain fertilizers, such as nitrate of soda, yield a residue of sodium, the accumulation of which sweetens the soil, and in the long run makes it alkaline. In clay soils serious physical effects may be the consequence. On the other hand, muriate and sulphate of potash, and sulphate of ammonia leave an acid residuum, the accumulation of which may render the soil sour. It therefore becomes imperative to so use or to so mix these fertilizers that their residues will combine and thus neutralize each other. One reason perhaps why greenhouse men favor the use of manure is that they have experienced the bad effects of the residue of improperly mixed fertilizers.
By a soil flora is meant the bacteria or fungi, whether beneficial or harmful, which thrive in that soil. Science has proved definitely that a soil can no longer be regarded as a conglomeration of dead, inert particles of rock. The soil teems with life which to a large extent determines its fertility. The more numerous the beneficial bacteria and fungi it contains, the more fertile it will be. On the other hand if the beneficial micro-organisms are absent, or perform their work imperfectly, or if the soil is overridden by harmful parasitic bacteria or fungi, we speak of it as a sterile or sick soil. In the green-house, the soil flora is often entirely different from what it is outdoors. This is due to the fact that the soil is artificially made up of a mixture of various ingredients with the object of making it ideal for plant growth. It is imperative that the greenhouse manager possess some knowledge of bacteria and fungi, and that he understand the functions and the requirements of the soil micro-organisms, if he wishes to secure proper control of his soil and to make it ideal for plant growth.
A. BACTERIA. Bacteria are minute microscopical plants that consist of a single cell. They are composed of a cell wall of protoplasm and average about 1/25000 of an inch in length. These simple organ-isms multiply by fission, that is, the original mother cell divides in two equal parts, which may separate or remain united, giving the appearance of a thread. It has been estimated that a single bacterium divides about every twenty minutes. Granting that this rate of division is uninterrupted for twenty-four hours, the descendants of a single one within a day would be in round numbers 1,800,999 trillions. These when placed end to end would make a string two trillion miles long, or a thread long enough to go around the earth at the equator 70,000,000 times. However, multiplication at such a rate cannot occur because food conditions are restricted. The three main types of bacteria are : 1. the cocci, 2. the bacilli or rods, 3. the spirilla or spirals (fig. 2, c.). The greater number of the soil bacteria are beneficial, the most common being the saprophytes, or those which help to decay the dead organic matter from either animal or plant. The parasites on the other hand are those which produce disease.
B. FUNGI. Fungi are low forms of microscopic plants, of a slightly higher type than bacteria. Fungi are made up of colorless feeding threads technically known as hyphæ or mycelium. The spores which correspond to the seed of the higher plants are borne either in sacs, known as pycnidia (fig. 2, d) or on free stalks, known as conidiophores, meaning stalk bearing spores (fig. 2, e.). Fungi, like bacteria, depend on animals or plants for their food. Like bacteria, they are differentiated into saprophytes and parasites.
RELATIONSHIP OF MICRO-ORGANISMS TO THE FERTILITY OF A SOIL
Bacteriologists are continually engaged in discovering the possible function of numerous groups of the soil organisms. A recent exhaustive study * of Actinomyces, or thread bacteria, in the soil, for instance, seems to show that they serve to decompose grass roots, being more numerous in sod than in cultivated land. Other groups of bacteria undoubtedly perform other important functions.
The mere presence of friendly micro-organisms in the soil, however, would be insufficient to assure the welfare of our cultivated lands. These minute organisms must find the conditions necessary to induce a maximum activity in the performance of their work, which is to act as chief cook in the dietary of the plant. Most of the plant's food, as it is found in the soil, is in a crude and unavailable form. The bits of mineral matter, the manure, or fertilizer added to the greenhouse soil, all contain plant foods, but in a form which plants cannot readily use. They must be softened and predigested, and this work is done by the friendly micro-organisms. The supply of plant food is therefore directly de-pendent on the work of these minute scavengers. An intimate relation exists between the higher and the lower form of plant life, the one depending on the other for sustenance.
A. NUMBER OF MICRO-ORGANISMS IN SOIL.
Investigations by Waksman and others clearly show that micro-organisms are present in soils everywhere (see Table I and fig. 2, b).
It should be remembered that differences in the physical and chemical nature of the greenhouse soil, the sort of fertilizers used and the amount of temperature and moisture will all be important factors in determining the number of micro-organisms present.
NATURE AND FUNCTION OF A HEALTHY SOIL FLORA
The function of a normal soil is to provide avail-able plant food. About 95 per cent of the weight of a growing plant is made up of carbon, hydrogen, oxygen and nitrogen. The remaining 5 per cent constitutes the mineral or the non-combustible part or ash of the plant. Carbon, hydrogen, and oxygen are absorbed in the form of carbonic acid and water; nitrogen is usually derived from nitrates produced by micro-organisms out of organic matter in the soil. Neither the organic nor the mineral elements are in a form which plants can use. They must at first be acted upon by certain definite micro-organisms in the soil.
A. THE TRANSFORMATION OF CARBON.
Cellulose, which is but a form of carbon, constitutes a large per cent of the woody tissue of plants. Soils contain large amounts of cellulose and this undoubtedly helps to maintain their proper physical condition. It is found in large quantities in straw, manure, or in green vegetable matter. But because of its complex form, plants cannot make use of it, until it undergoes a certain decomposition. This is accomplished by a group of soil bacteria known as Amylobacter, which, feeding on the dead vegetable cellulose, break it up, and reduce it to carbon dioxide, hydrogen and fatty acids. The carbon dioxide either returns to the air to replenish the atmospheric supply, or it unites with water to form carbonic acid and soil carbonates. The carbon dioxide is taken by the plants either directly from the air through the leaves, or from the soil in some carbonate form. Thus we see that it is not the cellulose nor the product of its decomposition that furnishes plant food, but certain inorganic elements which are set free in its decomposition.
B. ELABORATION OF AVAILABLE NITROGEN.
From the viewpoint of plant nutrition, nitrogen is unquestionably the most important of all elements. The nitrogen of the air, although totaling about 79 per cent of it, is not in an available form. In the transformation of proteids into available nitrogen in the soil, three definite processes take place, all thanks to the work of certain soil micro-organisms.
1. Ammonification. In this process, the soil bacteria attack the complex proteids and convert them into ammonia. The odor of ammonia from decomposed urea, manure, or any other organic matter is always an indication that ammonification takes place. According to Sackett * and others the ability to bring about this change is attributed to the following soil bacteria: Bacillus mycoides, Bacillus proteus vulgaris, Bacillus mesentericus vulgatus, Bacillus subtilis, Bacillus janthinus, Bacillus coli communis, Bacillus megatherium, Bacillus fluorescens liquefaciens, Bacillus fluorescens putidus and Sarcina lutea.
Recent investigations by Waksman f and others indicate that certain classes of fungi are even stronger ammonifiers than are bacteria. Trichoderma Koningi and the Mucorales fungi were found to be strong ammonifiers. Fungi, too, are very strong cellulose decomposers. Further extensive investigations on soil fungi will no doubt more strongly establish their relationship to ammonification.
2. Nitrification. In order to be readily available for plants, ammonia and ammonia compounds must be changed still further into simpler compounds or, as the process is known, must undergo nitrification. The ammonia is first oxidized into nitrous acid and nitrates. This is accomplished by soil bacteria, Nitrosomonas and Nitrosocoscus. The nitrates are then oxidized into nitric acid and nitrates, through the work of the bacterium, Nitrobacter. The nitrates are the only forms of nitrogen which plants can use.
C. ACTION OF SOIL FLORA ON MINERAL SUBSTANCES.
Inert mineral substances, like the organic matter in the soil, must first be acted upon by certain soil bacteria to be converted to a form which plants can readily assimilate.
1. Changes of Phosphates. Phosphates as they commonly occur in nature are but little soluble in water. This is why they cannot be used in their first form, although they are required by most plants. Soils deficient in this element may be improved by such fertilizers as superphosphate of lime, ground bone, phosphate rock or Thomas slag. In the process of decomposition of organic matter a large quantity of carbon dioxide is liberated, which unites with the water in the soil to form carbonic acid. This acid attacks the insoluble phosphates, transforms them into superphosphates—the only form soluble in water,—and renders them available to plant life.
2. Changes in Potassium, Sulphur, and Iron. The carbon dioxide and other organic acids produced during the fermentation of organic matter, attack the potash feldspar which occurs in the soil. The product is potassium carbonate which is soluble in water and hence readily taken up by plants. The nitric acid which is formed during nitrification may also combine with the raw potash in the soil, forming potassium nitrate which is a form available for plants.
As a result of the activity of soil bacteria, hydrogen sulphate is evolved from the decomposition of proteids. The sulphur may be further changed into sulphur dioxide, and when combining with water and oxygen, into free sulphuric acid. The latter readily combines with calcium or magnesium, forming calcium or magnesium sulphate, from which the plant obtains sulphur for the construction of its proteids.