The Application of General Principles
The Retreat of the Forest
Man's disturbance of the natural cycle in the course of his cultivation is more definite that he realizes, but he continues in confidence to repeat his operations season after season because experience has taught him that his work is a success. Yet in the background of his mind lingers a question -- will the never-ceasing miracle really come about, will the seed germinate, will life be born of the dam and the sire, will there be corn, milk and meat, will the earth in truth renew her green carpet?
These doubts have led throughout the ages to innumerable rites and beliefs, particularly prominent when men are in the primitive stage of existence and still widespread and influential, even if maintained in a clandestine way, when far more advanced religions and philosophies have been evolved. Such rites and beliefs are described as fertility rites and beliefs and the concept on which they are based as the fertility cycle concept. Much investigation has been devoted to them over the last forty or fifty years and it has been proved by a mass of evidence that the central idea is that of the expected renewal of birth in plant and animal season by season; to bring this about many magical ceremonies have been and still are instituted all over the world.
That this combined hope and doubt should have been so plainly avowed among men is in itself interesting. Does it point to the fact that the yearly rebirth of the earth's green carpet is indeed an astonishing event, is, as we have already said, a miracle? To modern man the miracle is no longer a manifestation of magic; it is an established certainty, and we can up to a point explain it; we know, more exactly than primitive man could, that the first factor which brings this miracle into existence is in truth fertility, that fertility which rests in the soil.
What is a fertile soil? As we have already had occasion to state, it is nowadays the fashion for the scientist to declare that soil fertility is something which cannot be defined. The farmer would not agree with him. Perhaps it is difficult to give a precise definition in scientific language of what constitutes soil fertility, but that need not deter us from agreeing that fertility is that quality in the soil which stimulates the rebirth of life in constant and regular repetition; it is the prime rendering possible the renewal of natural things.
It is this factor of renewal which is the cardinal one.
We have already in Chapter 1 laid down the tenet that renewal is only made possible because everything that Nature creates she also absorbs again; nothing is cast aside; nothing is lost. The so-called wastes of Nature are dropped on the earth's surface and transformed into fresh forms of life; there is, as we might say, a constancy of substance. We called this the Law of Return. We also noted that this is no hand-to-mouth process, that the accumulation of material undergoing transformation after its return to the soil is immense, forming huge reserves, and that the annual or seasonal miracle springs out of these colossal margins.
Moreover, on this reserve of fertility all created things continue to draw during the course of their existence. We pictured them as jostling each other for their share of food, light, moisture and warmth. Together they form one mass of intertangled lives and functions, displaying intricacies of arrangement which constitute a dramatic illustration of natural law. The principle of mixed existences rules throughout the physical world and cannot with safety be disregarded; this, too, we are bound to bear in mind during our inquiry into the renewal of the earth's green carpet.
This principle of mixed existences embraces a great deal; it implies much of what we shall need to know. It presupposes the use of one form of created life to assist, but also to remove, another form. We may roughly call the act of assistance a living together; the scientific term for certain special manifestations of this is symbiosis, from the Greek, but this expressive word might be given a wider meaning. We may call the act of removal decay or fermentation or, as we shall see, we may classify it as parasitism or disease, and finally we may call it death. In this way or that, beneficently or destructively, one living thing helps, or preys on, another living thing; nothing can exist in isolation, nothing can continue separated whether from its allies or from its foes.
Of all these laws the forest is a perfect example. It exhibits continuous and permanent fertility; it is the pre-eminent portion of the green carpet to which are assigned the living reserves of Nature; it is a marvelous example of mixed existences. The forest is composed, in the first place, of one of the most lovely creations we know, of trees. But it is not composed of trees alone, though we are apt to think of it in these terms. First, it includes a remarkable variety of vegetable growths other than trees, which in a wild forest are always of different kinds. There are to be found in the forest abundant mosses and lichens, annual and perennial plants and flowers, shrubs and bushes, and usually creepers and lianas; in tropical forests lianas with orchids occur in profusion and form a special cover or network, so thick as to constitute a well-defined layer at the top of the tree stems near the light, inhabited by a specialized fauna living in a high world of their own -- a very remarkable fragment of the green carpet. Further, there exists in all forests, temperate or tropical, a mass of fungous growth, which seems essential to the continuance of the trees; indeed, an intimate and necessary connection has lately been proved between the growth of certain tree species and their associate root fungi.
These varied forms of vegetable life are not the end of the matter. The forest is also the home of a vast animal population, far larger than is generally estimated. There are the birds. There are a number of larger mammals, and the fact that our convenience and safety has eliminated these in civilized countries must not tempt us to forget them. There is always a very large population of smaller animals, rodents, snakes, lizards, etc. There are worms and invertebrates generally; finally, there are masses of insects, both those which can reach the air by flight and those found in the soil. Finally, there are microscopic types of animal existence in myriads.
Thus the forest is a combination of forms of life whose character it is to be interlocked and mutually dependent; the finest piece of invented machinery could never rival the delicate and vital adjustments which constitute a forest system.
It is the special nature of trees to form a truly magnificent protective cover to this complex creation. Under their canopy this mixed world goes on undisturbed and richly. The result is that accumulation of reserves, that stored fertility which is the very sign manual of a forest.
Generally speaking we may say that the solid portion of the earth's surface is divided between forest and grass; we may omit rocks and desert. These two forms of the green carpet are alternate and their respective advance and retreat depend on circumstances, climatic and geological. In moist climates on the whole the trees win, especially in the temperate zones; they are what is known as the ultimate succession; in other words, were Nature left to herself she would at once proceed to cover most of these zones, first with scrub, and then with a thick forest -- the process may be watched at its start in any neglected field when the hedges begin to grow out into uncared-for pastures. Yet we know that within the period of our existence the reverse process has taken place -- the grass has won on the trees. The retreat of the forest has been the result of man's wish to cultivate, and has been a major event in the history of the human race.
It has come about gradually. In some parts of the world it is still going on, in others it took place centuries ago. In England, as long as population was scanty, the cultivation of the higher and drier downlands sufficed, but with the increase of its population consequent on the Anglo-Saxon settlement the valley forests were destroyed to make room for fields. The continent of Europe had been subjected to the same process some time previously; the Mediterranean forests perished some considerable period before the birth of Christ, the Northern European forests lasted until the Roman Empire or later. But Europe has not been the sole example of these acts of deforestation. Both the Americas, much of Africa, parts of Central Asia, parts of Australia, almost the whole of New Zealand used to exhibit thick forests, all now destroyed or being destroyed by man.
What have been the consequences? Much of this destruction including a great deal of what is happening at the present day, has been wanton; it is inflicting great injury; it should be stopped. It is not, however, our purpose to dwell on this aspect. Let us rather look at the question in a larger way.
Man is bound to eliminate the forest. He must have fields for he must grow food. But in laying out the earth into tilled and cultivated areas he should grasp what he is doing and set himself with deliberation and with firmness to make good his interference in the natural order. If he is obliged to disturb the earth's green carpet, he should assist Nature to reconsiitute in other forms what he disturbs. He cuts down a tree, a group of trees: well and good; but let him learn of the tree, let him study all the tree can do.
The role of trees in Nature is a very important one. We will take what they do point by point, and base the remainder of this chapter on a consideration of three separate functions which they fulfill. These three functions are as follows:
In the first place, they anchor the soil. This mechanism -- for so it may be called -- is particularly observable wherever there is a slope, for it is here, of course, that the soil tends to slip out of place. But it is effective everywhere. Moreover, the tree stems, together with the leaf canopy, simultaneously protect the top layers of the soil from the effects both of sun and of wind -- the latter far more deadly than might be realizedand also break up the rain into a fine and gentle spray. In the forest the rain drops very quietly on the soil and the wind is broken. The result is that the top layers of forest soil remain not merely anchored in their place but absolutely undisturbed. That is why they are able to remain extremely loose and that again is why they are so beautifully full of air.
Nothing like this is to be found elsewhere. The carpet of grasses and other plants which is Nature's alternative to the forest is much closer and more likely in course of time to cut off the air from the soil; as the roots decay they may form an almost impenetrable mat. It is true that these roots hold the soil securely in place -- it cannot be blown or washed away or burnt by the sun. But the forest alone combines the two functions of affording a completely adequate protection and of yet allowing entire accessibility to the air. The floor of the forest remains therefore as the most perfect illustration of natural soil build-up; the constructional method employed is, so to say, exemplary.
The topsoil of a forest -- the product of tree growth -- is a wonderful thing to examine and will repay almost any amount of investigation; its delicious smell is most attractive to the senses, its lessons are infinite to the mind. But the beneficent action of trees does not stop with the topsoil. The roots of almost all trees go a considerable way down and those of some trees penetrate to a surprising depth. Here they meet the subsoil and often touch the parent rock.
What are their functions in relation to the subsoil? Before answering this question we need to know a little about this subsoil. There are two main points to realize. First, the subsoil, like the topsoil, has a life of its own; second, the life of the topsoil is in intimate relation with this life of the subsoil. Just as the plant is geared to the topsoil, so is the topsoil itself geared to the subsoil. The process goes further. The subsoil itself is geared to the parent rock on which it rests. (Clay and chalk are included in the expression "parent rock".) In fact, each layer is derived from the preceding-the topsoil from the subsoil, the subsoil from the parent rock. There are continuous processes going on by weathering, pressure, upheaval, leaching and chemical action which bring about a constant formation of fresh soil upwards and downwards. The earth's crust is a piled series of such layers (technically known as horizons; the whole set of horizons looked at in vertical sections is called a soil profile), often broken and bent into confusion or otherwise mingled by alluvial deposit, gravity, friction, wind, etc. Nor is this a finished process. It is still going on. There is a geological life proceeding with vigour right under our feet though we do not often become conscious of it.
It is the roots of trees which provide the mechanism required to bring the various soil strata into relation with each other. They shatter the soil in all directions. They provide innumerable channels down which the rain can creep. As they decay they leave positive vents which draw in the atmosphere; aeration follows drainage. It is the combined effect of the passage of water and air which starts up the life of the soil. Without these two elements that life would soon die down; the weathering and leaching processes would cease, the subsoil would be unable to form, as it now does, ever-fresh layers of topsoil; it would not itself be constituted from the parent rock. The effects would be disastrous. There would be completely static conditions. Drainage and aeration are necessary to the life of the soil, which is as sensitive to their cessation as any living being.
But trees have yet a third function besides those, first of anchoring and then of draining and aerating the soil. This third function is a fertilizing one. It is the business of trees to pull the minerals out of the lower layers of the earth. For the store of minerals needed for proper vegetable growth, and eventually for proper animal growth, are banked in the subsoil into which they have passed by slow disintegration from the parent rock. Banked at this level they would, and do in fact, stay there unless sucked up by the pumping action of tree roots. The roots of trees penetrating many feet downwards are the most effective agencies which can perform this necessary task. If the pumping action is unhindered these minerals pass into the true structure and foliage. In due course they leave the tree in the form of the autumn leaf-fall, and in this way they are finally distributed over the surface of the earth. If the pumping action fails, plants and animals have to go short of minerals. The roots of trees are therefore a vital connecting link between the members of the natural round; they are Nature's great circulatory system, and to eliminate them is like removing the veins from a man's body.
For the forest manures itself; it is there that the fertility cycle is most intense. Nothing quite equals the lay-out of the forest. Whether we look at the perfection of its mechanical action in securing the soil in place or at the completeness of the drainage and aeration systems which it provides or at the effectiveness of its manurial renewals, we see something which surpasses any other manifestation of the green carpet. Time and again we can go back to the forest and study it afresh.
We are now in a position to consider what consequences are likely to arise when in our constant search for the areas in which to grow food we cause the forest to retreat. We may sum up our conclusions under the following heads:
There is, first, the obvious effect of loosening the soil. When no longer held by the roots of trees the top layers of earth are easily removed by wind or water through the natural process known as denudation. This continues until the actual configuration of the landscape is altered. In its acute forms denudation becomes erosion, when deep rifts and gullies appear which widen, or the soil of a whole hillside is gradually shifted and carried away. In the second place, there is the interference with the natural drainage and ventilation of the soil which follows on the large-scale removal of trees. The farmer has always sought to make good any harm he has done in this direction. Up to a point his efforts have been successful, but there are dangers here also, as we shall see. In the third place, in replacing forests by fields we have not merely interfered with, but have definitely put out of action the natural circulation of minerals. On this point traditional agriculture has had no certain knowledge, and the farmer has had little to guide him beyond his instinct and feeling for the land.
Let us take these three points one by one. So much has been written about the incidence and effects of erosion that it is not necessary to repeat what is well known. These effects are worst in countries of climatic extremes. Here the damage is rapid and apparently irreparable. The fertile landscape vanishes; the country takes on a bare and hopeless look; stretches of sand lie everywhere and rise in clouds of dust with the pitiless wind or are washed down into dangerous sandbanks by illregulated and torrential streams; the rich vegetation fails to reappear, a few hardy species struggle on; gradually even these may vanish. Invasion by the desert has begun.
Elsewhere the effects may be less pronounced. We may notice here or there a gully or a field of which the topsoil has been washed down to the lower levels with detriment to the crop-growing capacity of the higher land; or after a strong wind in open country we may see topsoil blown away together with the sprouting crop; this now takes place in a few districts over our own country and has caused much alarm. But even if erosion so far plays a very small part at home, we must confess in looking over the world as a whole that it has a long and sinister history. No part of our earth is unvisited by this evil. In Europe any traveller in northern Italy will mark the deep, scooped-out depressions interrupting the contours of this most highly cultivated land. The south of France, Greece, Palestine have not suffered less -- all these countries share the evil, while on the other side of the Mediterranean North Africa has partly collapsed in desert, a desert which advances at the rate of two-thirds of a mile a year; indeed the Mediterranean basin is the classic example of the erosion following prolonged human settlement.
In other countries there are plenty of examples. In the Orange Free States useless gullies replace what in 1879 was an area of rich grass interspersed with reedy pools. In Ceylon, where the tea gardens have been formed on bare slopes stripped of the original jungle, erosion has carried away millions of tons of fruitful soil. In China, where thousands of miles of the higher stretches of the great rivers have been completely denuded of trees in the persistent search for fuel on the part of the peoples living at those levels, the soil, deprived of its natural anchorage, collects as a vast mass of suspended matter in the river streams, gets deposited along the lower courses of these rivers until the river bed is so raised that the waters spill over on to the flat fields on either side in the form of devastating floods; it is estimated that every year two thousand million tons of soil, sufficient to raise an area of four hundred square miles by five feet, are carried down by the Yellow River alone. Both in this country of China -- otherwise so far-seeing in its agricultural technique -- and in the United States of America along the course of the huge Mississippi River and its tributaries, the short-sighted remedy is adopted of trying to keep the river in its course by building mud banks -- levees -- on each side. The river merely rises higher and higher and always ends as the victor, for the human effort fails before the immense force of the waters with their burden of suspended matter. Terrible as is the effect of this water-borne erosion, it is equalled by the wind-borne destruction of the prairie plains of North America, which has carried away in dust storms thousands of dried-out acres of farming land; whole areas have been reduced to the condition known as dust-bowls. These are only a few of the best-known instances of a destructive process which seems to be increasing in its extent and its intensity.
Nevertheless we should not lose sight of the fact that denudation in the form of interchange of soil is normal in Nature. Our lands have been formed by the weathering of rocks and the washing-down of the results; it is one of the positive duties of streams and rivers to transport particles of soil. It is when the process of denudation is exaggerated that we begin to be alarmed. We despair of what we may have done in our centuries of human history; we wonder whether the human race is destined to work nothing but injury on the earth which it inhabits.
We need not be alarmed. Nature does not forbid us to open up the earth. She does not insist on the forest everywhere and at all times. If that were so, we should have to grow our crops as best we could under a forest canopy. It is true that some agricultures have gone much too far in the elimination of trees; but nothing absolute is imposed on us, nor indeed has Nature imposed it on herself. As we have already remarked, the prairie and the steppe exist side by side with the forest and also hold stores of fertility and great marginal reserves.
This may provide us with the secret of what we have to do. If the renewal of life is to take place, if the green carpet is not to be rolled up forever, then there must be no lengthy interruption in verdure -- nor does Nature admit of it herself. Bogs, marshes may form; growth and change may decline to a minimum; but some sort of green cover is always there, if not the forest then the savannah, the prairie or steppe, if not the steppe, the marsh or bog. Once all these are removed we have a desert, and though a desert is not usually entirely without life, it still seems to. remain as an almost ineradicable scar on the face of our earth. If Nature is able to restore deserts, she does so but slowly; most deserts seem permanent, terrible proofs of the need for continuity in the green carpet and of the void which follows on its lapse. In truth deserts are not a normal illustration of Nature's working methods. She prefers her green carpet and exerts all her power towards its maintenance.
The best check on erosion, therefore, is the growing crop. This is the real preventative and also the remedy. One nation, the Japanese, has found it so. Japan is a country where the steep configuration of mountains and valleys, combined with the climatic condition of heavy seasonal rainfall, lends itself peculiarly to the evil of erosion; yet erosion is successfully combated by the Japanese agricultural engineers. Each river system is studied as a whole, and following on that study, dams are gradually built in the upper valleys, with careful observation of results. Sedimentation, and above all, natural revegetation are allowed to run their course. In fact, the Japanese engineer applies to Nature to help him; he confines himself to encouraging the re-establishment of the wild forest; the re-established forest then absorbs the rainfall, the streams cease to be agents of destruction, the precious paddy fields in the lower valleys remain open and unharmed. Thus cultivation areas are secured on condition of leaving the upper slopes to Nature's persistent habit of replacing the tree where it is needed. The areas secured for human use are small -- the areas left to Nature's domination very large; but the Japanese engineer is wise; he knows the folly of attempting more. His reward is that what he has he holds.
If Nature is properly approached she can do even more for us; she will be quite willing to put back what she has taken away even of cultivable areas. This was proved in Gwalior in India, where the simple construction of embankments with spillways in a very short space of time resulted in the formation of fine wheat-bearing areas where there had only been useless eroded land. Erosion, therefore, is by no means the hopeless problem which it is so often supposed to be. The very power of the forces which originate it can also be used to make it good.
Yet it scarcely admits of argument that prevention is the aim. Terracing on erodible lands should never be omitted. Primitive agricultures are exemplary on this point; for it is life and death to the people living by them. Modern plantation agriculture has been terribly neglectful; the classic examples are the plantation areas of Ceylon, which contrast most unfavourably with the island of Java where the Dutch have taken the severest measures to insist on ample terracing. In general, the great rice areas of the East are most marvelously terraced by peoples who have had only their hands and a few tools and animals to help them.
Slowly the Western people are also beginning to correct their past mistakes. A new device is contour-ploughing, i.e. ploughing across a slope and not down it. This is like a miniature form of terracing, especially when each fifth or tenth furrow is ploughed deeper. The furrows act as small channels to collect the rain water, which soaks in gently instead of washing down the slope carrying the mud with it. Or again there is strip cultivation, which means the sowing of two different crops in alternate strips. These two crops grow and are reaped at different times, so that the crop left acts as a protection to the exposed strips; the force of the sun, the rain and especially of the wind is broken; this is a partial but insufficient application of the principle that the earth should not be completely bared of her green carpet. Contour-ploughing and strip cultivation have been found useful in the United States of America and in South Africa and have done a good deal to begin the check on erosion.
The retreat of the forest then can so far be made good; where necessary the forest itself can be re-established over a part of the catchment area or' such configuration on a large or small scale can be put in hand as checks soil loss. Thereafter all is well provided, that the green carpet is maintained, and that nothing else is done to destroy the nature of the soil.
For not even the best contoured and protected field will stand up against the eroding force of sun, wind and rain -- those fierce agents to whom is assigned the great task of reshaping the earth's form and surface -- if its most intimate inner structure is irretrievably damaged. The soil itself must be respected; the soil structure must be retained in its integrity. As well expect our bodies to prosper after all our corpuscles had been blown apart as expect the earth to remain kindly after her beautifully composed structure has been broken down into and dust. .
This truth is appreciated by all farmers. They are experts, trained by centuries of experience, in the feel of the land. They know by the tread, by a rub between the hand, by the resistance of fork or spade, by the draw of the plough or harrow what their soil is like. Is it crumbly? Is it sticky? Is it powdery and poor? The exact condition is apparent to all who have mastered the art of cultivation. They know that a good tilth is everything.
This good tilth depends on retaining the compound soil particle, which, comparatively large, provides shapes of irregular contour and plenty of air space constituting substance and resistance. The effect is, in the first place, physical and depends on laws of mechanics; fine particles would pack close and be impervious; big particles lie loose and cohere; they, so to say, hook into each other. But if the effect appears to be mechanical the cause is something different. The compound soil particle, the particle which, once for all, prevents a complete process of erosion which holds its own against the action of water, sun and wind, which makes possible crop cultivation on the open level and even on the more dangerous slope, is the same in the field, steppe and prairie as on the floor of the forest. It is glued together out of minute fragments of mineral matter by means of a pastry cement supplied mostly by the dead bodies of the soil bacteria. It is when this compound soil particle disintegrates that the soil becomes dust and as such forms the prey of water, gravity and wind. The maintenance of the compound soil particle is therefore what matters, and if this depends on an adequate supply of the pasty glue or jelly referred to, then there must be plenty of the specks of dead microbial matter to make it; in other words, there must be a rich microbial population in any soil which is to hold together. Now the microbial population lives and dies -- we are in the midst of a biological phase. In truth, we have left any mechanical safety devices far behind and have entered into the life of the soil.
This is the gist of the matter. What we know as soil is a changing dynamic complex of living and dying organisms mixed with inert mineral matter and with decaying vegetable or animal wastes, the different constituents being brought into relation with each other in a very remarkable and varied way. But none of this is possible unless the teeming organisms which conduct the life of the soil have plenty of the organic wastes on which to feed. These wastes are their food supply; cut off this food supply and they cease to exist.
Thus we come back to the Law of Return, the essential pre-condition of soil fertility. It is on fertility that the preservation of all soils depends; the problem of erosion is a problem of fertility. This is why in considering the retreat of the forest it is a mistake to lay too much stress on the anchoring action of the tree roots; this anchoring action can be to a large extent forgone -- as we see in our fields -- provided that the fertility natural to the forest floor is not also sacrificed.
We now come to the second of the three consequences which we noticed as arising out of man's interference w ith the growth of trees; we stated that in eliminating trees he destroyed the natural drainage and ventilation which they secured for the soil. As in the matter of the anchorage of the soil in place, this is so obvious that the cultivator has from time immemorial kept the problem in mind. Our ordinary agricultural operations, the antiquity of which has often been remarked upon, rest upon the necessity of doing all that is possible to assist the entry and dispersion both of moisture and air into the soil. Ploughing, harrowing, digging and hoeing are all conducive to the great task of making water and air available where they are required, i.e. where the roots of the plants can use them. To these annual or seasonal operations are added from time to time important efforts designed to secure the same conditions. The lay-out and shaping of fields, but above all their drainage by whatever system, whether by ditches or pipes or sloping, is considered an essential part of the art of cultivation; or there is irrigation to be undertaken, which is an operation requiring the utmost skill and understanding. All these efforts are really one great effort; they all aim at the introduction of air and water at those levels where the roots of our food-bearing crops are sure to look for these two agents to their growth.
In these operations we greatly miss the help of the tree roots. As we have already noted, these roots are unrivalled conductors of air and water. Our habit of substituting something else for their action carries us on for prolonged periods, but ultimate conditions sometimes accumulate which are elusive and baffling and which call for a certain reconsideration of agricultural practice.
One common condition is the formation of what is known as a pan just above the subsoil. A pan is a stratum or layer of closely packed soil which has caked and hardened; even though originally only sand, it may become as impervious as a shelf of stone. (Anyone can observe the process by watching what happens in a jar of sugar left for some time. The fine grains conglomerate into a mass, which if left for long can become quite hard. Sand packs even closer than this and therefore becomes impermeable.) A pan may be close to the surface or some way down. Wherever it is, it is very injurious. The passage both of air and water downwards is impeded, if not entirely cut off. This spells death to the crop. A plant must breathe downwards as well as upwards. It must grow downwards as well as upwards, but only a few hardy plants have the force to shatter the stony opposition of the pan. Most plants find their roots checked and mutilated; is it surprising that they suffer? A peculiar stunted growth above ground is the surest sign of this mutilation below. Wherever the farmer or gardener finds his crops in this state he should at once probe for a pan. And it should be noted that pans may occur almost anywhere, in a suburban garden, for instance, where the builders' gear in constructing the house has pressed heavily or where the lorries have drawn up, even if only for a few weeks.
There is every reason to suppose that the soils of our own country are suffering from the formation of pans. Centuries of the passage of men, animals and machines have had the result of packing and pressing the soil. The invention of large and heavy machines like tractors must have hastened the process. There has been prolonged washing-down of finer particles to lower levels, where they accumulate and choke the soil air spaces. Cultivation has kept the topsoils open, but this has only helped to conceal what has gone on below these. An examination of our subsoils would probably reveal a surprising state of affairs.
A pan is not difficult to deal with. There is a new practice, known as subsoiling, which consists in drawing, by means of a tractor, a sharp thin wedge through the subsoil, which is thereby shattered. Aeration and drainage are at once restored. The effects are quite remarkable. They surpass what has hitherto been accompanied by dressings of basic slag, dressings which are really no longer necessary in view of the new technique of subsoiling.
(Subsoiling was experimentally carried out by the late Sir Bernard Greenwell at Marden Park, Surrey, in 1938 with highly successful results; for details see Howard: The Soil and Health -- Farming and Gardening for Health or Disease, pp. 199 et sqq. Dressings of basic slag [which means the used-up lining of the Bessemer converter] were first introduced about fifty years ago. The usual explanation has been that the phosphorus content, removed from the iron ore, acts as a chemical fertilizer on pastures; the more likely explanation is that the effect has been a purely physical one, of aeration; if this is so, subsoiling is a better and cheaper method. For the superior effects of subsoiling over basic slag dressings see communication by Mr. Delgado of Little Oreham, based on observations, on two treated fields in 1943-4, in the reference given above. Basic slag permanently kills the edible mushroom, but apparently not on all soils.)
But an even better remedy presents itself. Once more we can ask Nature to help us. We need not be fearful of causing the forest to retreat provided that we follow rather more closely than we sometimes do, what she institutes when she herself chooses to alternate her forest with grass. Her arrangements in these areas are not dissimilar from her arrangements in the forest. With marked differentiation of growth the various grasses and meadow plants lift themselves so as to use every inch of the light -- some are tall-stemmed, some short-stemmed, some creeping. Exactly the same rule is followed below the surface, some plant roots thrust deep, some are shallow-rooted, some creep just below or even above ground; thus, down to a depth of three, four, five or six feet every inch of the soil is combed and divided. Here we have the same pattern as in the forest, where both above and below the soil every possibility is sought out. Thus what the forest does on a grand scale is also done by the prairie and meadow in a smaller but no less effective way.
How very thorough, how very useful and indispensable this combing and shattering action of the smaller plants can be is what we ought not to forget. There has been a certain tendency to do so in our own country -- the improvement of our grass mixtures has run in the direction of concentrating on the rich shallow-rooted grasses. The Eastern cultivator has been cleverer; he consistently uses deep-rooting plants like the pigeon pea in his rotations; these he considers indispensible. What a weapon we also might have, what a sure preventive of evils like the soil pan, in our own burnet, chicory and cocksfoot! Fifty years ago a far-seeing reformer described these plants as "the cheapest, deepest and best tillers, drainers and warmers of the soil". From his own experience he gave a truly remarkable picture of what they can do. About fourteen inches below the surface of one of his fields he found a pan ten inches to a foot in thickness so hard that a powerful man with a sharp spade had to use great force to break it open; this pan had been formed by the washing-down from the topsoil of very hard minute particles which by reason of their smallness had arrested capillary attraction; in fact, it was a compacted stony mass without compound particles or the air spaces which these create. Yet close set as was this pan -- the expression he uses is "apparently impenetrable" -- the roots of chicory had passed straight through it without deflections and had succeeded in disintegrating it; their roots had pierced to a depth of thirty inches, the roots of burnet and kidney vetch to twenty inches, and of lucerne eight to ten inches. In general, the plant roots "seemed to have a profound contempt" for the pan. (R.H. Elliot: The Clifton Park System of Farming, ed. Faber & Faber, p. 98.)
This is the help we should use if we wish to make good our interference with natural forest growth. If for valid reasons we feel ourselves compelled largely to eliminate one of the constituents of the green carpet -- the tree -- we have only to turn to another type of constituent, and our difficulties are solved.
Let us first understand Nature, and then be as bold in our practice as we please: understanding is the first and only step to right action.
But is this all we have to apprehend? Do the ultimate consequences of the retreat of the forest stop at the two effects of loosening the soil from its natural anchorage and allowing it to pack? These two effects, though at first sight a little contradictory, clearly arise out of the same cause, namely, the removal of the tree roots. They are mechanical effects having biological consequences. Do the results of the retreat of the forest end here?
The essential role of the tree in Nature is not only to be a mechanical agent whether for anchoring or for shattering the soil, it is also destined to act as the perfect chemist, analyst and distributor of mineral riches. It is not only the roots which perform this function, but the whole tree system contributes, for the minerals absorbed by the roots from great depths pass up the trunk into branches and leaves and drop as the leaf-fall; in this way they reach the surface of the ground and enrich the topsoil, where they are available for plant growth. This we have already stated. The tree therefore serves not only itself but the whole vegetable kingdom in thus pulling up the mineral wealth of the subsoil.
(There is another agency which also distributes the necessary minerals: water. Glacier water is especially rich. Very notable examples of magnificent cultivations fed by glacier water are to be found in places as far apart as the Hunza valley in the Gilgit district of the Himalayas and the La Crau meadows in Southern France; the hay from these meadows is eagerly sought by distant trainers of racehorses; it gives great stamina, a result doubtless of its perfectly mineralized condition; the stamina of the tribesmen of the Hunza valley is by now proverbial. Other examples of cultivation enriched by glacier water could be found, especially in Switzerland. Mud-laden waters also are full of minerals. This mud carried from the higher reaches of the river is soil which has at one time or another been well supplied with minerals. Spread on the fields, it keeps the mineral content at a good or even a notable level. That is why the Chinese practice of spreading canal and river mud is so important. In Egypt the mud is spread by the river itself; and the effects have been adequate for many thousands of years.)
It may be well imagined that this last function of the tree in Nature cannot be disregarded without dire peril. A reasonable number of trees are a necessity in any permanent system of farming. Such, for instance, are the shade trees commonly found in the best tea gardens: these trees do a great deal more than merely provide shade; there are always palm trees in an Eastern rice field. In general, there is a great distinction to be drawn between agricultures which are treeless, such as those of the South African veldt, and agricultures which maintain a real residuum of tree growth, such as the European and the Asiatic.
In nothing has British farming shown itself so sensible as in its retention of the hedgerow. A hedgerow is really a small strip of surface-drained forest left in position. It has been noticed that the grazing animals are very fond of the herbage next to the hedges round their fields. The explanation given is that they here find the wild herbs which may have been omitted from the new pure seed mixtures with which their improved pastures have been sown, herbs necessary to their diet, grown, moreover , in the rich humus-filled and drained soil which collects under an English hedgerow. But perhaps this is not the whole explanation. Do they also find in the vegetation near the trees, of which there are usually a few in our hedges, and in that under the larger bushes supplies of minerals and does instinct cause them to crop these plants by preference? If this is so, we have quite a good contribution to the science of animal husbandry and quite a reasonable additional explanation of why animal breeding so excels in England, where for many years past the tree growth has been generously replanted along our roads, avenues, hedgerows and in our parks, fields and gardens.
Here is the explanation of, and the remedy for, those deficiencies in trace minerals of which so much has lately been written. Trace minerals -- manganese, boron, cobalt, etc. -- are so called because only the veriest traces usually exist in the soil; nevertheless, the absence of these minute quantities is very injurious to the health of all animals, including ourselves. The somewhat limited investigations in this field have left the subject there; the advice given to the farmer not going beyond the idea of establishing a deficiency and re-adding each mineral separately to the soil as indicated. But none of these laborious investigations and re-additions recognizes the fundamental fact that where trees have been too definitely eliminated, the soil will sooner or later deteriorate, and that the only way to deal with the situation is to restore tree growth in whole or in part.
Assuming then that we shall maintain at any rate some trees and if possible some hedgerows, can we now dismiss this subject of minerals? Not entirely. There is one element, the loss of which is almost sure to cause us anxiety; this is calcium. (Calcium is denoted by the formula Ca. Limestone = calcium carbonate, CaC03; quicklime, which is obtained by burning limestone = calcium oxide, CaO; when slaked, this becomes calcium hydrate, Ca(OH)2.)
Calcium, the element to which the farmer is referring when he talks of the necessity of lime for his land, exists in various forms. These range from limestone rock to layers of chalk. Although inorganic it is perhaps worth noting that they have had an organic origin, for they are the residue of millions of shells and skeletal structures of minute marine animals once deposited on the floor of the ocean. In the course of ages these deposits have emerged to form part of the earth's dry crust. This limestone is therefore a natural soil constituent. It is invariably found in an impure state and is present in unequal quantities in different soils; some parent rocks contain little limestone, which means that the subsoil and topsoil will also be deficient. It is, moreover, exceedingly soluble. Dissolved in the slightly acid soil water which percolates downwards in our climates, it quickly leaches away out of reach of the plant roots.
The limestone content of a soil and its accessibility are of great importance in the cultivation of crops. The crops we grow need the calcium carbonate of which the limestone is composed. Not quite all of them-tea is an exception; still, the majority of cultivated crops must have limestone. Not only is this substance a raw material needed by the plant, but it has the invaluable property of making active other soil materials. Without limestone these materials become, so to say, locked up. They are there, but the plant roots cannot use them; crushed limestone sets them free for the plant. Further, certain soil bacteria must have this substance; they are unable to operate in a sour soil. In a limestone-starved sour soil neither bacteria nor plants can use the food materials which are there; we might call this almost a breakdown in the manurial cycle or at least a very great slowing down of the machinery.
This does not suit us. We want to intensify, not to slow down. We are really facing a double or treble difficulty. Our cultivation practices, especially our practice of increasing drainage, greatly add to the tendency of the soil to lose its calcium carbonate by leaching; meanwhile our intensification of growth is making extra demands on this reserve: but simultaneously, by cutting down the forest, we are removing the one agency which can pull it up again. We may add that by depriving the soil of the foliage canopy of the forest we permit the rain to fall with much greater force on to the surface of the earth; this undoubtedly increases its leaching action; a great deal of the leaching effect is to be attributed to this cause. In truth, the retreat of the forest is at the bottom of the whole business; the calcium deficiency in the soils of Europe has arisen because we have interfered with the natural circulation of this element via the tree roots and tree foliage; according to Darwin the dying leaf continues to collect calcium almost to the end, so that the leaf-fall is particularly rich in this substance. It was part of the wisdom of the European peasant to have realized the situation and to have sought a remedy. The practice of liming the land is very old, how old we are not sure, but it is mentioned in Pliny. As it was originally impossible to crush the natural limestone by human labour the easiest practice was to burn it, then slake the quicklime so produced and thus make a product easy to scatter, the remains of old kilns may be seen almost everywhere in our country. It is now, of course, possible to crush the natural limestone or chalk -- a signal instance where the invention of machinery has been of great use to agriculture; the process of burning and slaking are no longer required. Modern transport puts such material at the disposal of every farmer. Nevertheless the universal use of this substance is in itself a confession that our soils have forfeited some of their original quality, that they have lost something, that they are not the same as when Nature first put them into our hands.
The Eastern races do not have recourse to liming. The Indian peasant does not dream of putting slaked lime on his land. The Chinese peasant, though he knows of lime for building purposes, does not use it in his agriculture. It is true that some of the alluvial soils of the tropics are not so subject to leaching as our own. The effect of six or eight weeks of heavy precipitation is nothing like so intense in washing out lime as the continual moisture that falls in temperate zones. But the real reason is not this. The Eastern peasant has learnt the secret of keeping his soils absolutely fertile. He guards himself against any soil. exhaustion with the greatest care: he always uses a deep-rooting crop or trees in his rotation. A really fertile well-managed soil will be sure to contain lime -- the difficulty does not then arise. Shortage of lime comes from short-term policies in farming, in fact, from bad farming. It will be interesting to see how far the lime shortage of Western agriculture can be cured if all natural wastes without exception, including above all such wastes as leaf-fall and twig-fall, are restored to the land. It will be a very long process, for the loss of lime is a very old injury inflicted on the soil. (As calcium carbonate is a natural mineral constituent of the soil it cannot be classed as an artificial manure; the addition, or rather the restitution of lime to a soil is something quite different from the use of artificials; we are simply trying to restore the soil to a certain natural state. Dressings are best added to the compost heap, where they help the work of the bacteria.)
We must at length sum up our analysis. The retreat of the forest is inevitable; it is the condition for human advance. But there is a wide difference between the blind destruction of what is the richest manifestation of the earth's green carpet and a wise and patient cultivation which slowly lays out the fields as they should be laid down, leaves a sufficient growth of trees and so far follows natural principles as to insure that the anchoring, draining, ventilating, and fertilizing action of the trees is continued and maintained by appropriate means. That these means must vary from place to place and even over different periods in our history may be granted. But two devices may recommend themselves to all.
Wherever we go we should leave or should reconstitute a few acres left wholly to Nature, such as portions of an original jungle or national park. Here Nature will maintain or will replace what is most suitable to the climate and the soil; she will arrange her successions as she pleases. From her action we have everything to learn. Time and again we shall be able to study what she does afresh. For Nature, undirected, can direct us. Every extension of the areas of human cultivation should therefore pay attention to this precaution. More especially should the great plantation industries, in opening up virgin regions, be most careful to leave undisturbed some tracts, not too small, of original jungle or forest; these will then serve as control areas to which reference can at all times be made. Such a practice will stand the farmer in good stead and will be of the greatest assistance to the scientist.
In the second place, we should not hesitate to make a larger sacrifice. It is probable that in the long run nothing can completely restore soil fertility except the replacement of forest growth. The forest is such a unique piece of natural machinery for starting up the manurial round that nothing can ever be entirely substituted for it. It would be the highest policy at definite intervals to replant portions of our food-growing areas with trees until they shall have regained that natural status which is the beginning of all wealth. After a sufficient period we could take them into use again. As they are felled, important additions to our timber supply would accrue. The forest, in fact, would be a long-term rotation crop.
This would entail some sacrifice. Only a public authority could embark on a programme which must look forward to the benefit of future generations at cost to the present. It is therefore not easy to introduce or advocate this far-reaching principle. Yet it would be the wisest of all plans, because it would reconstitute the natural sequence which cultivation by man, of its very nature, cannot help injuring. Unless we are prepared to embark on some such fundamental corrective of the errors which have crept into our soil management in the course of centuries, we shall always be plagued with the evils which are bound to follow the slow dwindling of soil fertility consequent on the retreat of the forest.
Man, as we have stated more than once, can and must interfere with Nature if he is to exist to any purpose: but how infinitely small is the degree of his interference when contrasted with the vastness of the obedience he has to render! To lay stress on the interference and to lose sight of the obedience is folly indeed and very bitter are the fruits thereof.
Next: 5. The Animal as our Farming Partner
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