The Nature of Soil Fertility
WHAT is this soil fertility? What exactly does it mean? How does it affect the soil, the crop, and the animal? How can we best investigate it? An attempt will be made in this chapter to answer these questions and to show why soil fertility must be the basis of any permanent system of agriculture.
The nature of soil fertility can only be understood if it is considered in relation to Nature's round. In this study we must at the outset emancipate ourselves from the conventional approach to agricultural problems by means of the separate sciences and above all from the statistical consideration of the evidence afforded by the ordinary field experiment. Instead of breaking up the subject into fragments and studying agriculture in piecemeal fashion by the analytical methods of science, appropriate only to the discovery of new facts, we must adopt a synthetic approach and look at the wheel of life as one great subject and not as if it were a patchwork of unrelated things.
All the phases of the life cycle are closely connected; all are integral to Nature's activity; all are equally important; none can be omitted. We have therefore to study soil fertility in relation to a natural working system and to adopt methods of investigation in strict relation to such a subject. We need not strive after quantitative results: the qualitative will often serve. We must look at soil fertility as we would study a business where the profit and loss account must be taken along with the balance-sheet, the standing of the concern, and the method of management. It is the 'altogetherness' which matters in business, not some particular transaction or the profit or loss of the current year. So it is with soil fertility. We have to consider the wood, not the individual trees.
The wheel of life is made up of two processes -- growth and decay. The one is the counterpart of the other.
Let us first consider growth. The soil yields crops; these form the food of animals: crops and animals are taken up into the human body and are digested there. The perfectly grown, normal, vigorous human being is the highest natural development known to us. There is no break in the chain from soil to man; this section of the wheel of life is uninterrupted throughout; it is also an integration; each step depends on the last. It must therefore be studied as a working whole.
The energy for the machinery of growth is derived from the sun; the chlorophyll in the green leaf is the mechanism by which this energy is intercepted; the plant is thereby enabled to manufacture food -- to synthesize carbohydrates and proteins from the water and other substances taken up by the roots and the carbon dioxide of the atmosphere. The efficiency of the green leaf is therefore of supreme importance; on it depends the food supply of this planet, our well-being, and our activities. There is no alternative source of nutriment. Without sunlight and the green leaf our industries, our trade, and our possessions would soon be useless.
The chief factors on which the work of the green leaf depends are the condition of the soil and its relation to the roots of the plant. The plant and the soil come into gear by means of the root system in two ways -- by the root hairs and by the mycorrhizal association. The first condition for this gearing is that the internal surface of the soil -- the pore space -- shall be as large as possible throughout the life of the crop. It is on the walls of this pore space, which are covered with thin water films, that the essential activities of the soil take place. The soil population, consisting mainly of bacteria, fungi and protozoa, carry on their life histories in these water films.
The contact between the soil and the plant which is best understood takes place by means of the root hairs. These are prolongations of the outer layer of cells of the young root. Their duty is to absorb from the thin films of moisture on the walls of the pore space the water and dissolved salts needed for the work of the green leaves: no actual food can reach the plant in this way, only simple things which are needed by the green leaf to synthesize food. The activities of the pore space depend on respiration for which adequate quantities of oxygen are essential. A corresponding amount of carbon dioxide is the natural by-product. To maintain the oxygen supply and to reduce the amount of carbon dioxide, the pore spaces must be kept in contact with the atmosphere. The soil must be ventilated. Hence the importance of cultivation.
As most of the soil organisms possess no chlorophyll, and, moreover, have to work in the dark, they must be supplied with energy. This is obtained by the oxidation of humus -- the name given to a complex residue of partly oxidized vegetable and animal matter together with the substances synthesized by the fungi and bacteria which break down these wastes. This humus also helps to provide the cement which enables the minute mineral soil particles to aggregate into larger compound particles and so maintain the pore space. If the soil is deficient in humus, the volume of the pore space is reduced; the aeration of the soil is impeded; there is insufficient organic matter for the soil population; the machinery of the soil runs down; the supply of oxygen, water, and dissolved salts needed by the root hairs is reduced; the synthesis of carbohydrates and proteins in the green leaf proceeds at a lower tempo; growth is affected. Humus is therefore an essential material for the soil if the first phase of the life cycle is to function.
There is another reason why humus is important. Its presence in the soil is an essential condition for the proper functioning of the second contact between soil and plant -- the mycorrhizal relationship. By means of this connexion certain soil fungi, which live on humus, are able to invade the living cells of the young roots and establish an intimate relation with the plant, the details of which symbiosis are still being investigated and discussed. Soil fungus and plant cells live together in closer partnership than the algal and fungous constituents of the lichen do. How the fungus benefits has yet to be determined. How the plant profits is easier to understand. If a suitable preparation of such roots is examined under the microscope, all stages in the digestion of the fungous mycelium can be seen. At the end of the partnership the root consumes the fungus and in this manner is able to absorb the carbohydrates and proteins which the fungus obtains partly from the humus in the soil. The mycorrhizal association therefore is the living bridge by which a fertile soil (one rich in humus) and the crop are directly connected and by which food materials ready for immediate use can be transferred from soil to plant. How this association influences the work of the green leaf is one of the most interesting problems science has now to investigate. Is the effective synthesis of carbohydrates and proteins in the green leaf dependent on the digestion products of these soil fungi? It is more than probable that this must prove to be the case. Are these digestion products at the root of disease resistance and quality? It would appear so. If this is the case it would follow that on the efficiency of this mycorrhizal association the health and well-being of mankind must depend.
In a fertile soil the soil and the plant come into gear in two ways simultaneously. In establishing and maintaining these contacts humus is essential. It is therefore a key material in the life cycle. Without this substance the wheel of life cannot function effectively.
The processes of decay which round off and complete the wheel of life can be seen in operation on the floor of any woodland. This has already been discussed. It has been shown how the mixed animal and vegetable wastes are converted into humus and how the forest manures itself.
Such are the essential facts in the wheel of life. Growth on the one side: decay on the other. In Nature's farming a balance is struck and maintained between these two complementary processes. The only man-made systems of agriculture -- those to be found in the East -- which have stood the test of time have faithfully copied this rule in Nature. It follows therefore that the correct relation between the processes of growth and the processes of decay is the first principle of successful farming. Agriculture must always be balanced. If we speed up growth we must accelerate decay. If, on the other hand, the soil's reserves are squandered, crop production ceases to be good farming: it becomes something very different. The farmer is transformed into a bandit.
It is now possible to define more clearly the meaning of soil fertility. It is the condition of a soil rich in humus in which the growth processes proceed rapidly, smoothly, and efficiently. The term therefore connotes such things as abundance, high quality, and resistance to disease. A soil which grows to perfection a wheat crop -- the food of man -- is described fertile. A pasture on which meat and milk of the first class are produced falls into the same category. An area under market-garden crops on which vegetables of the highest quality are raised has reached the peak as regards fertility.
Why does soil fertility so markedly influence the soil, the plant, and the animal? By virtue of the humus it contains. The nature and properties of this substance as well as the products of its decomposition are therefore important. These matters must now be considered.
What is humus? A reply to this question has been rendered easier by the appearance in 1938 of the second edition of Waksman's admirable monograph on humus in which the results of no less than 1311 original papers have been reduced to order. Waksman defines humus as
'a complex aggregate of brown to dark-coloured amorphous substances which have originated during the decomposition of plant and animal residues by micro-organisms, under aerobic and anaerobic conditions, usually in soils, composts, peat bogs, and water basins. Chemically, humus consists of various constituents of the original plant material resistant to further decomposition; of substances undergoing decomposition; of complexes resulting from decomposition either by processes of hydrolysis or by oxidation and reduction; and of various compounds synthesized by micro-organisms. Humus is a natural body; it is a composite entity, just as are plant, animal, and microbial substances; it is even much more complex chemically, since all these materials contribute to its formation. Humus possesses certain specific physical, chemical, and biological properties which make it distinct from other natural organic bodies. Humus, in itself or by interaction with certain inorganic constituents of the soil, forms a complex colloidal system, the different constituents of which are held together by surface forces; this system is adaptable to changing conditions of reaction, moisture, and action by electrolytes. The numerous activities of the soil micro-organisms take place in this system to a large extent.'
Viewed from the standpoint of chemistry and physics humus is therefore not a simple substance: it is made up from a group of very complex organic compounds depending on the nature of the residues from which it is formed, on the conditions under which decomposition takes place, and on the extent to which the processes of decay have proceeded. Humus, therefore, cannot be exactly the same thing everywhere. It is bound to be a creature of circumstance. Moreover it is alive and teems with a vast range of micro-organisms which derive most of their nutriment from this substratum. Humus in the natural state is dynamic, not static. From the point of view of agriculture, therefore, we are dealing not with simple dead matter like a sack of sulphate of ammonia, which can be analysed and valued according to its chemical composition, but with a vast organic complex in which an important section of the farmer's invisible labour force -- the organisms which carry on the work of the soil -- is temporarily housed. Humus, therefore, involves the element of labour; in this respect also it is one of the most important factors on the farm.
It is essential at this point to pay some attention to the many-sided properties of humus and to realize how profoundly it differs from a chemical manure. At the moment all over the world field trials -- based on mere nitrogen content -- are in progress for comparing, on the current crop, dressings of humus and various artificial manures. A mere glance at the properties of humus will show that such field trials are based on a fundamental misconception of what soil fertility implies and are misleading and therefore useless.
The properties of humus have been summed up by Waksman as follows:
- Humus possesses a dark brown to black colour.
- Humus is practically insoluble in water, although a part of it may go into colloidal solution in pure water. Humus dissolves to a large extent in dilute alkali solutions, especially on boiling, giving a dark coloured extract; a large part of this extract precipitates when the alkali solution is neutralized by mineral acids.
- Humus contains a somewhat larger amount of carbon than do plant, animal, and microbial bodies; the carbon content of humus is usually about 55 to 56 per cent., and frequently reaches 58 per cent.
- Humus contains considerable nitrogen, usually about 3 to 6 per cent. The nitrogen concentration may be frequently less than this figure; in the case of certain high-moor peats, for example, it may be only 0.5-0.8 per cent. It may also be higher, especially in sub-soils, frequently reaching 10 to 12 per cent.
- Humus contains the elements carbon and nitrogen in proportions which are close to 10:1; this is true of many soils and of humus in sea bottoms. This ratio varies considerably with the nature of the humus, the stage of its decomposition, the nature and depth of soil from which it is obtained, the climatic and other environmental conditions under which it is formed.
- Humus is not in a static, but rather in a dynamic, condition, since it is constantly formed from plant and animal residues and is continuously decomposed further by micro-organisms.
- Humus serves as a source of energy for the development of various groups of micro-organisms, and during decomposition gives off a continuous stream of carbon dioxide and ammonia.
- Humus is characterized by a high capacity of base- exchange, of combining with various other soil constituents, of absorbing water, and of swelling, and by other physical and physico-chemical properties which make it a highly valuable constituent of substrates which support plant and animal life.'
To this list of properties must be added the role of humus as a cement in creating and maintaining the compound soil particles so important in the maintenance of tilth.
The effect of humus on the crop is nothing short of profound. The farmers and peasants who live in close touch with Nature can tell by a glance at the crop whether or not the soil is rich in humus. The habit of the plant then develops something approaching personality; the foliage assumes a characteristic set; the leaves acquire the glow of health; the flowers develop depth of colour; the minute morphological characters of the whole of the plant organs become clearer and sharper. Root development is profuse: the active roots exhibit not only turgidity but bloom.
The influence of humus on the plant is not confined to the outward appearance of the various organs. The quality of the produce is also affected. Seeds are better developed, and so yield better crops and also provide live stock with a satisfaction not conferred by the produce of worn-out land. The animals need less food if it comes from fertile soil. Vegetables and fruit grown on land rich in humus are always superior in quality, taste, and keeping power to those raised by other means. The quality of wines, other things being equal, follows the same rule. Almost every villager in countries like France appreciates these points and will talk of them freely without the slightest prompting.
In the case of fodder a very interesting example of the relation between soil fertility and quality has recently been investigated. This was noticed in the meadows of La Crau between Salon and Aries in Provence. Here the fields are irrigated with muddy water, containing finely divided limestone drawn from the Durance, and manured mostly with farm-yard manure. The soils are open and permeable, the land is well drained naturally. All the factors on which soil fertility depends are present together -- an open soil with ample organic matter, ample moisture, and the ideal climate for growth. Any grazier who saw these meadows for the first time would at once be impressed by them: a walk through the fields at hay-making would prepare him for the news that it pays the owners of high-quality animals to obtain their roughage from this distant source. Several cuts of hay are produced every year, which enjoy such a reputation for quality that the bales are sent long distances by motor lorry to the various racing stables of France and are even exported to Newmarket. The small stomach of the racehorse needs the very best food possible. This the meadows of La Crau help to produce.
The origin of these irrigated meadows would provide an interesting story. Did they arise as the result of a set of permanent manurial experiments on the Broadbalk model or through the work of some observant local pioneer? I suspect the second alternative will be found to be nearer the truth. A definite answer to this question is desirable because in a recent discussion at Rothamsted, on the relation between a fertile soil and high-quality produce, it was stated that no evidence of such a connexion could be discovered in the literature. The farmers of Provence, however, have supplied it and also a measure of quality in the shape of a satisfactory price. For the present the only way of measuring quality seems to be by selling it. It cannot be weighed and measured by the methods of the laboratory. Nevertheless it exists: moreover it constitutes a very important factor in agriculture. Apparently some of the experiment stations have not yet come to grips with this factor: the farmers have. The sooner therefore that effective liaison is established between these two agencies the better.
The effect of soil fertility on live stock can be observed in the field. As animals live on crops we should naturally expect the character of the plant as regards nutrition to be passed on to stock. This is so. The effect of a fertile soil can at once be seen in the condition of the animals. This is perhaps most easily observed in the bullocks fattened on some of the notable pastures in Great Britain. The animals show a well-developed bloom, the coat and skin look and feel right, the eyes are clear, bright, and lively. The posture of the animal betokens health and well-being. It is not necessary to weigh or measure them. A glance on the part of a successful grazier, or of a butcher accustomed to deal with high- class animals, is sufficient to tell them whether all is well or whether there is something wrong with the soil or the management of the animals or both. The results of a fertile soil and proper methods of management are measured by the prices these animals fetch in the market and the standing of the farmer in these markets. It should be a compulsory item in the training of agricultural investigators to accompany some of the best of our English cattle from the pasture to the market and watch what happens there. They would at once discover that the most fertile pastures produce the best animals, that auctioneers and buyers detect quality instantly, and that such animals find a ready sale and command the best prices. The reputation of the pastures is finally passed on to the butcher and to his clients.
Resistance to insect and fungous disease is also conferred by humus. Perhaps the best examples of this are to be seen in the East. In India, the crops grown on the highly fertile soils round the 500,000 villages suffer remarkably little from pests. This subject is developed at length in a future chapter when the retreat of the crop and of the animal before the parasite is discussed.
Soil fertility not only influences crops and live stock but also the fauna of the locality. This is perhaps most easily seen in the fish of streams which flow through areas of widely differing degrees of fertility. An example of such difference is referred to at the end of Chapter 5 of Isaak Walton's Compleat Angler the following words:
'And so I shall proceed next to tell you, it is certain, that certain fields near Leominster, a town in Herefordshire, are observed to make sheep that graze upon them more fat than the next, and also to bear finer wool; that is to say, that in that year in which they feed in such a particular pasture, they shall yield finer wool than they did that year before they came to feed in it, and coarser again if they shall return to their former pasture; and again return to a finer wool, being fed in the fine wool ground. Which I tell you, that you may the better believe that I am certain, if I catch a trout in one meadow he shall be white and faint, and very likely to be lousy; and as certainly if I catch a trout in the next meadow, he shall be strong and red and lusty and much better meat: trust me, scholar. I have caught many a trout in a particular meadow, that the very shape and enamelled colour of him hath been such as hath joyed me to look on him: and I have then with much pleasure concluded with Solomon, "Everything is beautiful in his season".'
Soil fertility is the condition which results from the operation of Nature's round, from the orderly revolution of the wheel of life, from the adoption and faithful execution of the first principle of agriculture -- there must always be a perfect balance between the processes of growth and the processes of decay. The consequences of this condition are a living soil, abundant crops of good quality, and live stock which possess the bloom of health. The key to a fertile soil and a prosperous agriculture is humus.
Rayner, M. C. Mycorrhiza: an Account of Non-pathogenic Infection by Fungi in Vascular Plants and Bryophytes, London, 1927.
-- Mycorrhiza in Relation to Forestry', Forestry, viii, 1934, p. 96; x, 1936, p. 1; and xiii, 1939, p. 19.
Waksman, S. A. Humus: Origin, Chemical Composition, and Importance in Nature, London, 1938.
Next: 3. The Restoration of Fertility
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