Final Preparation of the Soil
Broadly speaking there are four paramount reserves in Nature -- the atmosphere, the waters, the rocks, and the soil. We have just been reasserting yet once again that it is in the soil humus that Nature's reserves begin to pass into the active round; it is here that the chain of living has it start. Here the principle of accumulation gives way to Nature's decision to take her minerals into use; she begins to call on them in a very special way. First comes the microbial and bacteriological life of the soil; from this proceeds the manufacture of the materials needed by the plant; these are conveyed into the plant and then by means of sunlight and the green leaf converted into food; this nourishment passes in due course into the bodies of animals and finally of man. At every point in this upward swing there should be an easy passage, ensuring the smooth transference of new materials and of reserves from one type of organism to the next.
For the initial principle of accumulation is not in any way set aside. Everywhere along the living chain will be found storage of useful elements specially placed at points of danger to guard the organism from shock, strain, or disaster. Every plant, every animal including ourselves carry within their structure or bodies such banks of reserve material. Life is far too great a thing to be carelessly handled, and though we commonly say that Nature is indifferent in the extreme to life and callous in her indifference, she cannot be accused of failing to supply each of her creatures with abundant means of protecting itself, if it is in any way fitted to do so.
To carry out this system Nature has devised most careful methods, nowhere better exemplified than in the way she sets about her first great task of bringing her store of humus into action. For her purpose throughout is not idleness but activity. It is an interesting fact that the humus in the soil does not increase indefinitely: this reserve is not accumulated in order to be hoarded: what is made is also to be used. In undisturbed forest or prairie the depth of fertile soil may be considerable, as we have seen; but this accumulation is no mass of inert matter; it is not a banker's lock-up investment but a true working capital in constant circulation.
Of the four paramount reserves of Nature the soil is the only one over which we have such mastery as may amount to a dominion. Our use of the atmosphere, waters and rocks can only be temporary and incidental. But it is our destiny to interfere in a fundamental way with the surface of the earth, which is our habitat and on which we have been placed. This fate of ours puts into our hands power of a peculiar kind, for as well as utilizing rightly we can also injure. To avoid this disaster we have everything to learn from the closest observation of natural methods. More especially is it advisable to do so at the outset, in our initial treatment of the humus reserve. Provided we get this treatment right, our liberty of action at the later stages of production may be considerable.
How does Nature put her humus into action, how does she cultivate and energize the soil? For cultivate it she does in a very definite and deliberate way; this is her first step in the upward movement. With that cultivation she closely links processes of transmutation of the soil contents; her mechanics, her chemistry, her biology all run together. In choosing some part of these processes to describe, we do not pretend to exhaust the subject. But certain steps in her treatment of the humus reserve have recently been emphasized among us or have for the first time attracted the notice which they merit. These examples of her working methods we may discuss with advantage and from them draw our definite conclusions.
First, her cultivation, drainage and aeration. This work is done by means of agents of many types and kinds. We need not refer again to the piercing and combing action of her vegetable partners, the trees and plants. As we know, their larger roots can shatter rocks, while the pulverization of the topsoil resulting from the spread in all directions of a network of the fine root-systems of some of the lesser plants has to be seen to be believed; it rivals the results of the most delicate machinery. But tree and plant roots are by no means the only instruments employed. The soil is worked and moved and changed about by animals also, especially by those smaller animals whose normal habitat is on the surface or just below the surface of the earth. These are Nature's willing army of diggers and of hoers, her ploughmen and her carters, endowed like ourselves with the ability of motion and in that respect endowed with the greater capacity and power.
These more mobile agents of Nature are indeed very efficient. They are million-handed and myriad-footed and make our own coarser farming instruments look clumsy and ridiculous. They are the ants and termites, the beetles and insects, the grubs and larvae, the worms and slugs and snakes and lizards. They are also such animals as the mice and rats, the rabbits, badgers, moles, in fact, all the burrowing animals, large and small alike. Some of these agents are familiar. Most of us know that the mole in the course of his burrowings throws up mounds of finely pulverized soil which form the materials for a perfect mineral top-dressing. His efforts are rather of the violent kind and may be disturbing to the roots of growing crops, so that he is often pursued and killed by the farmer; but he is a natural soil digger, all the same, and we could ill afford to do without him. The same may be said of the wireworm, on whom so much abuse is lavished. His function is to cut through the matted roots of worn-out sour decaying turf and prepare them for Nature's composting. That is why when such wornout turf is disturbed and a crop like wheat immediately planted, he is -- to our mind -- so destructive; he has been robbed of his proper task and naturally eats what he can, namely, our badly grown wheat. If we are wise we do not let our grassland get into such a state as to invite the wireworm, or, if it has, we allow time for re-adjustment; after an interval we shall find that Nature, not needing the wireworm any longer, has quietly removed him; it is not part of her design to keep workers idle on the dole. These simple examples will introduce our principle, which is that throughout the turning of the Wheel there is placed exactly that kind and type of working agent which is required at each point.
Of Nature's varied army of cultivators and diggers we shall choose the earthworm for our attention. Not only is this creature beyond question one of the most important of Nature's agents, but its work is such that it can easily be followed in our climate. Were we situated in a tropical zone we should choose the termite, whose operations are equally ubiquitous and very similar.
The role of the earthworm is varied. In the first place, the earthworms provide a ventilating system by means of their tunnels; through these the soil draws in oxygen and nitrogen and expels the air it has used up. Surplus rainfall also drains away. The earthworm population truly plough and drain the ground and have been well named the cultivator's unpaid labour force. The work done by the earthworm as digger and cultivator was wonderfully investigated by Charles Darwin, whose book on The Formation of Vegetable Mould through the Action of Worms and other Invertebrates with Observations on their Habits, the last he wrote, was published in 1881. He showed conclusively the colossal power of these lowly creatures, how they are always at work raising the surface of the earth by bringing up the lower layers of the soil and strewing them on top. Though completely deaf, nearly blind and with feeble smell, for their size they have great muscular power. They are indefatigable and so immense is the cumulative effect of their labours that they gradually cover great monuments. It was the first explanation ever given of the well-known phenomenon of the sinking of stones and buildings. (The general character of what the earthworm does, both in digging and aerating powers and its manurial value, was known to Gilbert White, in whose History of Selborne, first appearing in 1789, there is an interesting short passage on this creature. [Letter No. XXXV, 20 May 1777, to the Hon. Daines Barrington.])
An earthworm lives about ten years, not longer. A considerable proportion of the earthworm population therefore dies each year. Their bodies are added to the animal organic matter in the soil. Only rough calculations have as yet been made of the amount and value of this animal element; it must be considerable. From this point of view alone the maintenance of a prolific earthworm population is important; in death as in life the earthworm helps to build the soil. It is therefore no small matter to note that the earthworm, ubiquitous and prolific though it be, cannot survive every condition; like other living things it ceases to exist where its habitat is interfered with or its food supply destroyed. Where there is no humus the earthworm is not found; the natural agent and its natural environment are inextricably conjoined, and a loss of humus means the retreat of the earthworm. That is why the presence of a good lobworm, as thick as his little finger, in every spadeful of earth which he turns up is so pleasing to the gardener; when he sees this creature in abundance he knows that all is well with his soil. No further tests are needed; the earthworm, Nature's broadcaster, will tell him all he wants to know.
But earthworms do a great deal more than aerate and drain the soil or add to its crude organic content. Wonderful as is their mechanical work when living and important as is the continuous addition of animal matter won out of their dead bodies, this does not exhaust their usefulness. They actually eat and digest the soil and in doing so add enormously to its fertility. Their casts, manufactured in their alimentary canals from dead vegetable matter and particles of soil, are thrown up on the surface in hugh quantities; when the land is in very good heart the total of casts, in and above ground, may exceed twenty-five tons to the acre in the course of a year. Having been acted on by the earthworm's three pairs of calciferous glands these casts emerge in the first place very finely ground; having been neutralized by constant additions of carbonate of lime in their passage they have a heavy calcium content; having been saturated with the intestinal and urinary secretions of the earthworms they are exceedingly rich in certain nutrients. American investigators have shown these casts to be five times richer in available nitrogen, seven times richer in available phosphates, and eleven times richer in available potash than the upper six inches of soil. The earthworm is not only the gardener's ploughman, he is the gardener's manure factory.
If the roots of a potato are followed downwards, they will be found to be making full use of the earthworm tunnels. Where a fresh wormcast has been deposited inside the tunnel, a new web-like net of roots may be developed penetrating the deposit in all directions; only when this has been accomplished does the root pass on. Obviously there is something in the deposit which the root desires, which is of great value to it, which it pauses to absorb, something which renders the earthworm's cast more welcome to the plant than untreated soil. Thus these humble creatures are made welcome and encouraged, the plant will have supplies of material from which it can build up the life it needs.
Long before man existed the land was regularly ploughed and manured and still continues to be thus ploughed and manured by the earthworm. "It may be doubted", says Darwin, "whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures." It is a marvellous reflection, he adds, that the whole superficial bed of soil is brought to the surface through their bodies every few years. We may agree that what they do is indeed important and that we have every inducement to provide the conditions which favour their existence.
One of the points of interest in studying the earthworm is the way in which it unites physical with biological and chemical action. If generously handled our earthworm friends will at the same time aerate and drain the soil for us and will also handle our manurial problems, especially the phosphate and potash problems in which their contribution is of special importance. This comprehensive action is in accordance with what happens to be a general rule in Nature's preparation of the soil; it is hardly possible to distinguish very rigidly between physical and biological soil effects, they seem so closely interwoven. Again and again we find the one involves the other; if the soil is injured physically it suffers in its biological content; if it is biologically starved it collapses physically. The soil microbial population, for instance, change the soil content chemically and biologically, but also glue the compound soil particles together.
Nevertheless, there appears to be a stage when Nature, satisfied with her physical treatment of the soil, proceeds to concentrate on her biological operations; she energizes the soil. For soil, to be effective, to be the medium from which the earth's green carpet can arise, must be alive. Nature does not care to work on a sterile soil, not even very greatly on her own deserts. These sands sometimes contain enormous stores of mineral wealth, but Nature does not trouble to draw these into her moving Wheel unless some chance accident starts up the life-round; the occurrence of moisture in the scattered oases shows what fertility lies dormant. Elsewhere the presence of the streams and rivers starts the Wheel into first motion; the soil comes to life, its uncountable populations take up their abode, the humus is accumulated, the green carpet springs.
We admitted at the outset of this book that we do not really understand the origin of life and that we probably never shall. But we do know that protein is the vehicle or the agency by which life is conveyed; we are here on the edge of a mystery which still eludes our knowledge but which may become clearer with time. We know further that protein is various. There are innumerable forms of it. There are minute specks of protein in the soil, there is vegetable protein and animal protein. As if aware of the supreme value of the final stages of the preparation of her soils, Nature has designed a special agency or instrument -- it is difficult to choose the right word -- to insure that these most essential life elements, the proteins, shall be abundantly and above all safely transferred from the soil to the plant; having endowed the soil richly with a supply, she takes measures to secure the smooth transmission of these intensely important substances to the next of her creations, the green plant. One of the means by which this is done is known as the mycorrhizal association.
The mycorrhizal association is an association between the mycelium in the soil and the plant root. It has been described as a living bridge, and it is a fact that it is itself alive and connects two things which are also alive, namely, the plant root and the soil fungus; mycelium is only another word for fungus and soil mycelium means the fungus found within soils. In describing the changes which take place in a compost heap we mentioned thin threads of grey matter clinging to decaying roots and twigs; similar threads can often be recognized attached to the roots of plants which are still growing. But this is not all. The microscope reveals far more; it shows us these thin threads of living matter being drawn in by those roots, and finally their digestion by the plant; every phase of this can be followed and studied under the lens. (The microscopic technique is advanced and has been perfected by Dr. M. C. Rayner and by her assistant, Dr. Ida Levisohn. Dr. Rayner has been a pioneer in the investigation of the mycorrhizal association. See Trees and Toadstools by M.C. Rayner, 1945.)
This remarkable process is a true symbiosis or living together (see Chapter 2) in that it is beneficial; the mycelium which attaches itself to the plant root is no enemy to the plant, no parasite; it is an enricher not a plunderer. Quite a number of fungi have been found thus to be linked with certain trees, especially with conifers; indeed, there are cases in which a plant organism is unable to complete its life cycle unless it has such a fungus as partner attached to it. This was not appreciated until lately. Mycorrhiza were first investigated by Frank in the 1880's, when at the request of the German Government he undertook researches into one such fungus, the truffle, with practical aims in view. From that standpoint the research was rather a failure, and the scientific side of the subject also remained somewhat of a dead letter. It is only recently that attention has been revived, and it was not until the intimate bearing of the mycorrhizal association on soil fertility emerged that the whole subject assumed its proper place and importance.
The first fact which has been established (specimens were collected by Sir Albert Howard from India and Ceylon and at his instance were procured also from South America and many other countries) is that the mycorrhizal association extends far beyond the tree world to which at one time it was assumed to be confined. The following cultivated plants have been proved to be mycorrhiza formers: tea, coffee, rubber, sugar cane, cotton, rice, maize, wheat, banana, cacao, vine, custard apple, mango, lime, guava, peach, san hemp and other tropical leguminous plants, many species of clover, tobacco, and so forth; the association appears to exist throughout the great grass family (this includes the cereals) and -- what is interesting -- also among the nodule-forming plants; in fact about 80 per cent of cultivated plants are mycorrhiza-formers. The mycorrhizal association is therefore no isolated curiosity in Nature, no speciality of conifers or a few forest trees; it is a prevailing and wonderful system.
How has it come about that this has escaped notice? In the first place, mycorrhiza are only present when there is humus in the soil; this has been established beyond dispute. As much of our research has for years past been conducted on plant specimens from soils poor in humus it is not in the least surprising that the presence of mycorrhiza escaped notice; they escaped notice because they had ceased to exist. Something may also be attributed to the delicacy and difficulty of the microscopical technique called for. But the principal reason why so little attention was paid to the mycorrhizal association until lately was no doubt the fact that only a portion of the plant's nutriment is conveyed by this means; much of what the plant needs is sucked up from the soil solution by the ordinary action of the root-hairs. The elaborate and as it were roundabout process which induces first the independent growth of a fungus conjoined to the root, then its absorption, and finally its disappearance, all in stages, appears at first sight to be a subordinate route for the passage of food materials.
It would seem that this inattention on our part is a mistake. The mycorrhizal association is of cardinal significance. The invading mycelium contains 10 per cent of nitrogen -- this is a very high content -- and it seems clear that these fungi have the capacity of handing on their protein to the plant in a very special way and that the subsequent synthesis or building up by the plant of its own protein is done in an ideal manner. Where the mycorrhizal association is absent, this perfect synthesis is not accomplished, and in some way, somehow, the protein which the plant builds up is of inferior quality. In energizing her soils for the use of plants Nature has provided something rather special for the purpose of transmitting the life principle both smoothly and also with an ample margin. For the mycorrhizal association is undoubtedly in the first place a safety mechanism and must be primarily regarded in that light.
If this is so, the question at once arises: what of the 20 per cent of cultivated plants which are not mycorrhiza formers? How do they manage? The question is important. In view of the fact that the nodule-forming plants, which are supposed to be the great suppliers of nitrogen to the soil, nevertheless indulge in this extra means of catching at nitrogen (apparently mycorrhiza and nodules are not always found on the same root), which is certainly rather surprising, the query is one which ought to be answered. The presupposition is that in the absence of the mycorrhizal association there must be some other method by which the plant insures itself an extra intake of protein or of its digestion products. The fact that not even the nodule-forming plants, in spite of their own special capacity for fixing nitrogen, dare forego this method is surely most instructive.
There is no certain knowledge as yet of what these other methods may be, but there are some suggestions. (Howard: The Soil and Health -- Farming and Gardening for Health or Disease, p. 35.) It is possible that the needed intake of protein is accomplished by means of the absorption by the root hairs from the soil solution of the digested products of the dead bodies of the soil microorganisms before these break down completely into the inert form. These specks of so-called dead organic matter are also rich in protein; to that extent they are still alive. What we may call the plant's normal procedure is to wait until these bodies have finally become mineralized into simple nitrates to be held in the soil solution and absorbed therefrom. But is it not possible that the root hairs may also have the capacity of catching these substances at the intermediate stages, before they are completely inert, we may say, before they are completely mineralized? In that case there would be a very definite link with the biological content of the soil while still alive, which could be parallel to and could replace the mycorrhizal association. Indeed, we cannot yet tell how many such other links there may be between the protein of the soil and the protein of the plant. It is certain that there is a more intricate connection than we had realized, and that there are facts of crucial importance still to be discovered. The suggestion that we have here a still uncharted field can be based on the observation, which has been established in a number of cases, that nonmycorrhiza forming plants like the tomato nevertheless reveal exactly the same qualities of stamina and health and taste when grown in a humus-rich soil as do the mycorrhiza-forming plants and also exhibit exactly the same weaknesses when deprived of humus. Incidentally this suggestion gives a very good explanation of why some cultivated members of the cabbage family possess such a very diffuse root system and such elongated root hairs; they would need these, if, not being mycorrhiza-formers, their roots have this extra duty of combing the soil for the specks of disintegrating protein.
This does not exhaust the possibilities. Investigators (the pioneer investigator on these aspects is B.F. Lutman, author of Actinomycetes in Various Parts of the Potato and other Plants, Univ. of Vermont and State Agricultural College Bulletin 522, 1945) are just beginning to discover other very delicate forms of symbiosis of protein-rich intercellular filaments (Actinomycetes) within the plant. Their function has not yet been elucidated with certainty. If however they do contribute to the synthesis of the plant protein which seems likely, they would be a kind of internal mycorrhizal association. The potato is one plant whose life cycle and reproductive powers may be only explicable in the light of further knowledge on this point, the bearing of which may be rather wide. The history of the wild and cultivated potato is indeed a most extraordinary one, which would repay further investigation.
The mycorrhizal association can best be appreciated if it is assumed to be one out of a number of arrangements instituted by Nature, part of a general system. There are evidently what we might almost call secret and concealed mechanisms devoted to preparing and transferring the proteins which have to pass from soil to plant. How far such secret mechanisms occur farther along the great Wheel we have no idea, but the vitamins are in all probability something of the kind linking up the plant with the animal. This synthesis of proteins is the thin red line which runs through the biological life of our planet. In the ordinary way the plant commands more straightforward means of feeding itself; its daily intake of raw materials is given to it from the soil solution by the suction of the root hairs. It can exist on that basis alone. But its existence is then an imperfect and precarious one, and sometimes forbids the consummation of the life cycle, namely, the setting of flowers and the ripening of fruit.
If then the passage of protein is of this supreme importance can we do anything to aid it? Can we assist Nature's alchemy in the same way as our cultivation operations assist her own wonderful methods of aeration and drainage? We do not hesitate to dig and delve and turn and hoe, and though our efforts are clumsy indeed as compared with those of the earthworm and the ant, yet they are powerful aids in evoking richer harvests than we could otherwise obtain. Can we in the same way directly add something to the store of nutritive material on which our crops might draw and thus enhance their amounts and volume? For just as Nature links her physics with her chemistry and operates simultaneously and often by means of identical agents in either field, so might we add our plant-feeding arrangements to our cultivations: the final preparation of the soil is our business no less than hers.
The answer to this pertinent question is both yes and no. We can add to the plant's nutriment with far-reaching results; but we must do so within the limits and by the methods laid down by Nature.
Our ordinary means of adding nitrogen, carbon and the other elements that make up the plant's food materials including its protein have already been discussed; they include animal and green manuring, composting, a supply of water, the use of the algal film and of the Azotobacter group. These contributions of material are in principle straightforward, even if we often contrive curiously to deviate from the path in attempting them. But they are always in the nature of an ultimate return of what already belonged to Nature's living soils; they are wastes which have themselves recently been part of a living system and are now being replaced at the point from which they were lifted up into the active cycle; they are never additions from an alien kingdom. Perhaps the atmospheric gases, oxygen and more especially the inert nitrogen, might seem at first sight a contradiction to this principle, but there appears to be so intimate a circulation of these gaseous elements between soil and air that they can be considered soil constituents no less than the soil elements; the atmosphere actually penetrates into the soil and inhabits it and must be looked on as a soil component, just as water is a soil component.
In attempting to supply extra nutritive elements from outside this already comprehensive section of the Wheel, we are bound to intrude into a different sphere: we cross over, as it were, a significant boundary. This is the boundary between the dynamic and the static, between the living and the non-living. We enter that mysterious zone, that elusive no-man's-land of which we know so little though our eyes always scan it with so much anxiety. In this land we shall find that our authority has ceased to run. It is impossible for us to add directly from remoter sources to the intensity of existence on this planet; all we can do is to create conditions favourable to, Nature's own methods of intensification and thus indirectly to stimulate and enrich her results.
This important principle needs to be stated because it has been contravened. There is a great temptation on us to add to the resources of material existence. We live under the shadow of fear -- the fear of scarcity. This fear has troubled us not only during the present period of upheaval, famine, and disturbance, but for many years past. It has particularly beset our minds since one great scientist (Sir William Crookes at the British Association for the Advancement of Science at the end of the nineteenth century) pointed to the apparently growing disparity between the numbers of human beings destined to be born into the world and the amount of food which he believed it would be possible to grow for them. It was therefore with relief that men heard the advice of agricultural science not slowly and carefully to intensify Nature's production but quickly and rapidly to force up her activity: to make weighed and measured additions to her current stores of raw materials for the plant and thereby to increase above all the volumes of crops.
We were not to wait until the roots of trees fetched up for us the mineral wealth lying in the subsoil and rocks; we were to seek the numerous outcrops of such substances as phosphates and potash deposits and ourselves to collect these and give them to the soil. When it was discovered during the first World War that we could also by means of a new invention catch the atmospheric nitrogen and transform it into a portable and purchasable material, the scheme seemed complete; the three great raw materials for plant growth -- nitrogen, phosphorus and potash -- would be available in unmeasured quantity to relieve our anxiety and if carefully and justly allocated would immensely increase the food supplies of humanity.
The plant food materials thus to be procured by mining or manufacture were candidly described as artificial, a name which has since been deprecated, but which nevertheless conveys the truth; for they are all alien substances from the point of view of the growth of the plant. The theory which underlies their application goes back roughly a hundred years, but their extended use only dates from after the turn of this century. Their inventors and advocates have until recently referred to them with pride, and have been not a little surprised to encounter the keen and biting criticism which is now being directed against them.
The starting point was Liebig's chemical researches in the 1840's. This great scientist put order and sense into what had until then been a subject for the most erratic speculation; he helped to make organic chemistry what it is. On the question of plant nutrition he carried out a series of experiments which swept away many current delusions and put certain facts on a firm basis. These facts were in his own domain, that of chemistry. By burning plants and analyzing the ash, he was able to establish beyond controversy what mineral elements went into the structure of plants. He made a further deduction, and here he was less well-inspired. He argued that these minerals could be given to plants from an extraneous source in whatever quantities were desired and that the plants, making use of them, would show a corresponding increase of growth.
Thus artificial manures were born out of what we may fairly describe as the NPK mentality (NPK from the chemical formulae N for nitrogen, P for phosphorus and K for potash [Kalium]). Though their widespread popularity is, as has already been stated, a comparatively recent phenomenon, their use was continuously kept before the public mind by certain famous experiments on the Broadbalk field at Rothamsted; these experiments date back to Liebig's own day, for it was one of his own friends who instituted them and left money for their continuation. They were designed to show the effects of artificial manures on cereals and were still claimed only the other day as "the greatest contribution ever made to the art of food production". (C. S. Orwin in The Sunday Times, 14 October 1945.) The very searching criticism now being directed against them throws great doubt on this claim. (The criticisms attack the trials at their core in maintaining that the methods employed have been inherently unsound and the results therefore unreliable. Two principal defects stand out: the small size of the plots, rendering them liable to earthworm invasion from outside: the use each year of fresh seed procured from elsewhere; the latter defect invalidates the evidence of a hundred years. These criticisms, which yet await any answer, will be found set forth in Howard: The Soil and Health -- Farming and Gardening for Health or Disease, pp. 79-80; this author's description of the appalling state of the plots when visited by him about 1919 should be read.)
Let us recall briefly the processes of nutrition. The organic matter in the soil, i.e. the principal part of the humus, is constantly combining with the oxygen of the atmosphere and of the air in the soil spaces. This slow oxidation releases energy enabling the living soil organisms, existing in incredible numbers, to do their work, which is to attack the oxidizing matter and break it down into simple salts, nitrates, etc. These salts in turn, dissolved in the soil solution, constitute the raw materials for the food manufacture of the plant, which sucks them in through its root hairs. In these processes, which follow on each other, there is a balance. The work of the soil organisms is adjusted to the amount of the organic matter in process of oxidation ready and available for them; as they eat it up more is being manufactured; the consumption processes at the latter end are always being made good at the starting point. The result is a layer of humus which is remarkably stable both in amount, i.e. depth, and in quality.
When a dressing of artificial manure is given it might be imagined that all that was being done was to introduce quickly and easily at a comparatively late stage in the chain of processes an additional supply of those salts which are being laboriously produced by the soil organisms; the plant should proceed to feed abundantly on this enhanced food store; the whole thing should go forward more rapidly. But this is not what happens. The chain of processes instead of being sped forward is diverted or may even be described as reversed. The additional salts do not simply lie there while the plant gradually eats them up; on the contrary, they start another cycle. They stimulate the life of the soil organisms, and do this to such an extent that these organisms begin to comb and devour the soil humus for every thing they can get. They go much further than they have ever gone before, for in their frantic search for food they sometimes attack the nitrates which they themselves have produced, entering into rivalry with the needs of the plant to the eventual detriment of the latter, and finally invade the colloidal glue or paste which holds the compound soil particles together, glue which is itself an organic substance. This glue is not ordinarily subject to their attacks. The glue is composed of the bodies of dead bacteria; it is therefore rich in nitrogen; the living bacteria, in fact, are feeding on their dead brothers. Here is a perfect example of cannibalism, for it arises, just as cannibalism in the human race arises, out of an unsatisfied craving for protein on the part of a protein-dependent organism.
The result is a strange degree of superactivity. The slow crumbling down process, the making and remaking of humus, yields to an intense glow, a sort of fire; the soil is correctly described as burnt up. This fire, like all fires, is destructive, for the glue once consumed is not restored. The compound soil particle falls apart; a dusty condition sets in; the land "blows".
The soil population, in fact, has been so stimulated as grossly to have outstripped its food supply; it has eaten like a starving creature, which in these conditions it is. Indeed, it is not difficult to realize what happens. The soil humus is being used up. Nature's working capital is being transferred to current account; and there is no replacement.
This wasteful and extravagant practice has all the results we might foresee. At first there is considerable stimulation of plant growth: large dark green foliage is formed: fruits and flowers increase in size: there is an apparent initial success. This may last some time, and it is obvious that the period depends on how great is the original store of humus which can be burnt up. But when this precious capital is exhausted, the true effects are to be observed.
(Almost all comparative trials of artificials and other manures break down in accuracy on this point; the humus present everywhere bolsters up the action of the artificial fertilizer for a period. The only true comparison would be in experiments done on the bare subsoil.)
The first effect is physical. The compound soil particles fall apart into dust, which either sets itself into a dense mass preventing the pentration of air and needing constant reworking (an effect very noticeable to the cultivator) or disintergrating into a lifeless powder which can be carried away by wind in one of those destructive dust storms which we have already noted. It scarcely matters which disastrous condition confronts us. We have wasted our store of humus: we have destroyed the food of our faithful allies, the soil bacteria: they have now perished: the soil is dead.
So notorious have these effects become that a general retreat all along the line has had to be made. It is agreed that the physical condition of the soil must at all costs be maintained. No one is now so enthusiastic an advocate of the presence of humus in the soil as he who wishes to use artificial manures. Humus, it is argued, must be insured, but surely it is possible to use these extra amounts of plant raw materials as well? Are they not, after all, themselves natural, part of Nature's wealth? Where can be the harm in conducting to the root of the plant an additional supply of the mineral wealth it needs just as we conduct an additional supply of air, water or organic manure?
It is not altogether an unreasonable thesis; indeed, it is the old Liebig thesis restated. The temptation not entirely to abandon the use of artificials after years of their use and in face of the constant pressure from authority is very great. What harm can we apprehend from their introduction provided only that we manage to guard against their abuse in bringing destruction of the physical soil texture?
What we have been learning in this chapter of Nature's own methods of dealing with her soils will direct us to an answer. Artificial manures kill the earthworms outright; they ruin mycorrhizal association; they destroy quality and lead to the loss of reproductive power. This is Nature's first categoric NO to the use of such unsuitable means. Her further unmistakable answer is conveyed in the form of warnings which by now ought surely to be sufficient, namely, in the modern spread of all types of plant and animal disease. To this, Nature's own final condemnation of the use of these manures, we shall have to devote a lengthy discussion which had best be reserved for our following and last chapter; for the present, therefore, we shall confine ourselves to the first effects mentioned.
On the earthworm population the effect of artificial manures, especially of the acid-forming manures such as sulphate of ammonia and superphosphate is of the nature of a cataclysm: a single heavy dressing overnight of sulphate of ammonia will strew them on the surface so that their dead bodies can be ,wept up by the shovelful in the morning. (Sulphate of ammonia is advocated as the most effective means of clearing tennis lawns of earthworms. U. S. A. Dept. of Agriculture; Farmers' Bulletin 1569, 1935.) Poison sprays -- those offshoots of the artificial manure idea -- will do the same; after one application of tar oil or lime sulphur in the orchards of Kent the ground is often a carpet of dead worms. Owing to the earthworm's habit of reinvading a territory the effect is best seen where artificials have been used for a number of years over very large areas. This has taken place in the great potato-growing areas of Lincolnshire, where now there are few worms to be found.
There can be no doubt that other life perishes with the worm, the grubs, the beetles, the insects generally, probably the whole microfauna. What is the result? Just what we should expect; the thousand aerating tunnels fall in, the infinitely delicate drainage and aeration provided by these creatures ceases, the soil degenerates -- the ploughman is gone. Is it surprising that in these potato fields of Lincolnshire we see in our own country the first signs of the dust bowl, that final result of the wind and sheet erosion which has wrought such terrible destruction in other countries?
But there is more to face than this. There are complicated organic changes to account for. On every two acres of land in good heart the earthworms are so numerous that their bodies would aggregate a full-grown bullock -- on every hundred acres of good land we have the equivalent of fifty underground cattle. The sudden addition of this quantity of animal organic matter to the soil content must have a marked effect. It is the possible explanation in part of the initially stimulating effect of a dressing of such manures. Curious though it may be, this subject has never been worked out, but it is not to be disregarded. It is consonant with the general extravagance which attends the use of artificials. Nature, as we know, allows about one-tenth of her earthworm population to die annually and adds their bodies to her soil organic matter; in our insane haste we slay this whole population outright, not even troubled to know that we are doing so. How long this addition lasts as fertilizing material it would be difficult to say; general experience would suggest that it can operate for a season or two. Then come the unmistakable permanent effects. The earthworm is no longer there as alchemist, let alone as ploughman. Those casts of his, rich in phosphates and potash, have disappeared-the whole biological condition of the soil becomes impoverished.
Is it surprising in these circumstances that the mycorrhizal association also ceases to function or at least to function properly? We called it a symbiosis, a living together, and the implication is that it operates between substances and agencies which are themselves alive. If the soil population is dead it is impossible to suppose that it can continue; nor in fact does it. Where it can still be found it is never carried to its proper fulfilment; investigation has shown that the breakdown comes in the processes of digestion of the fungus by the root; this digestion, as the microscope shows, falls to be consummated when the soil is sterile. In truth the soil population including the earthworm, the insects and the invisible microfauna, the humus in the soil, and the mycelium which is the bridge leading towards the plant root and ultimately therefore towards the green leaf, are so intimately bound up together that it is hard to describe one phase of this existence without implying all the others. In fact, humus, fungi, and bacteria, earthworms and other soil fauna, are one world; they exist together or not at all. We must therefore reluctantly come to the conclusion that any attempt on our part to short-cut these well-established connections and put a supply of minerals directly at the disposal of the plant roots will never be satisfactory. The attempt to do more for ourselves than Nature is willing to do for us is bound to end in disaster. The temptation is born partly of greed but more largely of fear, motives which dominate us to our detriment. The true cultivator should not yield to these emotions -- they are out of place in his great occupation. He should trust to Nature: not necessarily with the worship appropriate to a religion, but with a sound respect based on observation, understanding, and proved experience. She is the greatest cultivator, and the greatest alchemist whom he is ever likely to encounter: he will do well to follow all her indications and to obey all her laws.
There is almost no end which we can foresee to our use of the green carpet. It is true that there are no longer many unknown surface areas of this planet to be opened up: it is forgotten that we can equally well delve downwards and by proper working and treatment of our soils increase our available acreage many times; the increased surface that comes from increased pore space in a well-cultivated soil adds thousands of square yards to our fields. Nor need we stop here. We can maintain Nature's alchemy in such a form as will best assist her own operations, insure to her the return of all she needs, see that this material is constituted as she wishes, and so speed up her fertility cycle to an unbelievable extent. The idea that the earth can now do no more for us, that we have reached the saturation point of human possession of this planet, is nonsense: there are vast stores of wealth in the shape of future food awaiting us: we have only to seek these stores in the right way.
The fear of scarcity -- this great fear which is still doing so much harm to our thought and so distorting our emotions -- need not pursue us. It has been far too dominating among us, and a much more hopeful, more cheerful outlook would do us good. The world needs hope, and we should fix our minds on the unquestionable bounty of Nature, which is really there, which truly does exist, of which we ourselves are a part, and to make use of which we are endowed with so extraordinary a capacity. It is not necessary to assume our existence as the central point of the universe or to suppose that Nature leads up to ourselves as a final end: on such topics we may think as our individual beliefs direct. What we all must agree in stating, what we must acknowledge to be a fact established by scientific veracity, is that in the world as we know it, it is possible to pursue human existence, not on a precarious basis, but in satisfying, safe and universal plenty for all the peoples of the earth.
Next: 9. Disease as Censor
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