The Part Played by Soil Fertility in Agriculture
The Operations of Nature
The introduction to this book describes an adventure in agricultural research and records the conclusions reached. If the somewhat unorthodox views set out are sound, they will not stand alone but will be supported and confirmed in a number of directions -- by the farming experience of the past and above all by the way Nature, the supreme farmer, manages her kingdom. In this chapter the manner in which she conducts her various agricultural operations will be briefly reviewed. In surveying the significant characteristics of the life -- vegetable and animal -- met with in Nature particular attention will be paid to the importance of fertility in the soil and to the occurrence and elimination of disease in plants and animals.
What is the character of life on this planet? What are its great qualities? The answer is simple. The outstanding characteristics of Nature are variety and stability.
The variety of the natural life around us is such as to strike even the child's imagination, who sees in the fields and copses near his home, in the ponds and streams and seaside pools round which he plays, or, if being city-born he be deprived of these delightful playgrounds, even in his poor back-garden or in the neighbouring park, an infinite choice of different flowers and plants and trees, coupled with an animal world full of rich changes and surprises, in fact, a plenitude of the forms of living things constituting the first and probably the most powerful introduction he will ever receive into the nature of the universe of which he is himself a part.
The infinite variety of forms visible to the naked eye is carried much farther by the microscope. When, for example, the green slime in stagnant water is examined, a new world is disclosed -- a multitude of simple flowerless plants -- the blue-green and the green algae -- always accompanied by the lower forms of animal life. We shall see in a later chapter (Chapter 8, Rice) that on the operations of these green algae the well-being of the rice crop, which nourishes countless millions of the human race, depends. If a fragment of mouldy bread is suitably magnified, members of still another group of flowerless plants, made up of fine, transparent threads entirely devoid of green colouring matter, come into view. These belong to the fungi, a large section of the vegetable kingdom, which are of supreme importance in farming and gardening.
It needs a more refined perception to recognize throughout this stupendous wealth of varying shapes and forms the principle of stability. Yet this principle dominates. It dominates by means of an ever-recurring cycle, a cycle which, repeating itself silently and ceaselessly, ensures the continuation of living matter. This cycle is constituted of the successive and repeated processes of birth, growth, maturity, death, and decay.
An eastern religion calls this cycle the Wheel of Life and no better name could be given to it. The revolutions of this Wheel never falter and are perfect. Death supersedes life and life rises again from what is dead and decayed.
Because we are ourselves alive we are much more conscious of the processes of growth than we are of the processes involved in death and decay. This is perfectly natural and justifiable. Indeed, it is a very powerful instinct in us and a healthy one. Yet, if we are fully grown human beings, our education should have developed in our minds so much of knowledge and reflection as to enable us to grasp intelligently the vast role played in the universe by the processes making up the other or more hidden half of the Wheel. In this respect, however, our general education in the past has been gravely defective partly because science itself has so sadly misled us. Those branches of knowledge dealing with the vegetable and animal kingdoms -- botany and zoology -- have confined themselves almost entirely to a study of living things and have given little or no attention to what happens to these units of the universe when they die and to the way in which their waste products and remains affect the general environment on which both the plant and animal world depend. When science itself is unbalanced, how can we blame education for omitting in her teaching one of the things that really matter
For though the phases which are preparatory to life are, as a rule, less obvious than the phases associated with the moment of birth and the periods of growth, they are not less important. If once we can grasp this and think in terms of ever-repeated advance and recession, recession and advance, we have a truer view of the universe than if we define death merely as an ending of what has been alive.
Nature herself is never satisfied except by an even balancing of her processes -- growth and decay. It is precisely this even balancing which gives her unchallengeable stability. That stability is rock-like. Indeed, this figure of speech is a poor one, for the stability of Nature is far more permanent than anything we can call a rock -- rocks being creations which themselves are subject to the great stream of dissolution and rebirth, seeing that they suffer from weathering and are formed again, that they can be changed into other substances and caught up in the grand process of living: they too, as we shall see (Chapter 7), are part of the Wheel of Life. However, we may at a first glance omit the changes which affect the inert masses of this planet, petrological and mineralogical: though very soon we shall realize how intimate is the connection even between these and what is, in the common parlance, alive. There is a direct bridge between things inorganic and things organic and this too is part of the Wheel.
But before we start on our examination of that part of the great process which now concerns us -- namely, plant and animal life and the use man makes of them -- there is one further idea which we must master. It is this. The stability of Nature is secured not only by means of a very even balancing of her Wheel, by a perfect timing, so to say, of her mechanisms, but also rests on a basis of enormous reserves. Nature is never a hand-to- mouth practitioner. She is often called lavish and wasteful, and at first sight one can be bewildered and astonished at the apparent waste and extravagance which accompany the carrying on of vegetable and animal existence. Yet a more exact examination shows her working with an assured background of accumulated reserves, which are stupendous and also essential. The least depletion in these reserves induces vast changes and not until she has built them up again does she resume the particular process on which she was engaged. A realization of this principle of reserves is thus a further necessary item in a wide view of natural law. Anyone who has recovered from a serious illness, during which the human body lives partly on its own reserves, will realize how Nature afterwards deals with such situations. During the period of convalescence the patient appears to make little progress till suddenly he resumes his old-time activities. During this waiting period the reserves used up during illness are being replenished.
The Life of the Plant
A survey of the Wheel of Nature will best start from that rather rapid series of processes which cause what we commonly call living matter to come into active existence; that is, in fact, from the point where life most obviously, to our eyes, begins. The section of the Wheel embracing these processes is studied in physiology from the Greek root meaning to bring to life, to grow.
But how does life begin on this planet? We can only say this: that the prime agency in carrying it on is sunlight, because it is the source of energy, and that the instrument for intercepting this energy and turning it to account is the green leaf.
This wonderful little example of Nature's invention is a battery of intricate mechanisms. Each cell in the interior of a green leaf contains minute specks of a substance called chlorophyll and it is this chlorophyll which enables the plant to grow. Growth implies a continuous supply of nourishment. Now plants do not merely collect their food: they manufacture it before they can feed. In this they differ from animals and man, who search for what they can pass through their stomachs and alimentary systems, but cannot do more; if they are unable to find what is suitable to their natures and ready for them, they perish. A plant is, in a way, a more wonderful instrument. It is an actual food factory, making what it requires before it begins the processes of feeding and digestion. The chlorophyll in the green leaf, with its capacity for intercepting the energy of the sun, is the power unit that, so to say, runs the machine. The green leaf enables the plant to draw simple raw materials from diverse sources and to work them up into complex combinations.
Thus from the air it absorbs carbon-dioxide (a compound of two parts of oxygen to one of carbon), which is combined with more oxygen from the atmosphere and with other substances, both living and inert, drawn from the soil and from the water which permeates the soil. All these raw materials are then assimilated in the plant and made into food. They become organic compounds, i.e. compounds of carbon, classified conveniently into groups known as carbohydrates, proteins, and fats; together with an enormous volume of water (often over 90 per cent of the whole plant) and interspersed with small quantities of chemical salts which have not yet been converted into the organic phase, they make up the whole structure of the plant -- root, stem, leaf, flower, and seed. This structure includes a big food reserve. The life principle, the nature of which evades us and in all probability always will, resides in the proteins looked at in the mass. These proteins carry on their work in a cellulose framework made up of cells protected by an outer integument and supported by a set of structures known as the vascular bundles, which also conduct the sap from the roots to the leaves and distribute the food manufactured there to the various centres of growth. The whole of the plant structures are kept turgid by means of water.
The green leaf, with its chlorophyll battery, is therefore a perfectly adapted agency for continuing life. It is, speaking plainly, the only agency that can do this and is unique. Its efficiency is of supreme importance. Because animals, including man, feed eventually on green vegetation, either directly or through the bodies of other animals, it is our sole final source of nutriment. There is no alternative supply. Without sunlight and the capacity of the earth's green carpet to intercept its energy for us, our industries, our trade, and our possessions would soon be useless. It follows therefore that everything on this planet must depend on the way mankind makes use of this green carpet, in other words on its efficiency.
The green leaf does not, however, work by itself. It is only a part of the plant. It is curious how easy it is to forget that normally we see only one- half of each flowering plant, shrub, or tree: the rest is buried in the ground. Yet the dying down of the visible growth of many plants in the winter, their quick reappearance in the spring, should teach us how essential and important a portion of all vegetation lives out of our sight; it is evident that the root system, buried in the ground, also holds the life of the plant in its grasp. It is therefore not surprising to find that leaves and roots work together, forming a partnership which must be put into fresh working order each season if the plant is to live and grow,
If the function of the green leaf armed with its chlorophyll is to manufacture the food the plant needs, the purpose of the roots is to obtain the water and most of the raw materials required -- the sap of the plant being the medium by which these raw materials (collected from the soil by the roots) are moved to the leaf. The work of the leaf we found to be intricate: that of the roots is not less so. What is surprising is to come upon two quite distinct ways in which the roots set about collecting the materials which it is their business to supply to the leaf; these two methods are carried on simultaneously. We can make a very shrewd guess at the master principle which has put the second method alongside the first: it is again the principle of providing a reserve -- this time of the vital proteins.
None of the materials that reach the green leaf by whatever method is food: it is only the raw stuff from which food can be manufactured. By the first method, which is the most obvious one, the root hairs search out and pass into the transpiration current of the plant dissolved substances which they find in the thin films of water spread between and around each particle of earth; this film is known as the soil solution. The substances dissolved in it include gases (mainly carbon dioxide and oxygen) and a series of other substances known as chemical salts like nitrates, compounds of potassium and phosphorus, and so forth, all obtained by the breaking down of organic matter or from the destruction of the mineral portions of the soil. In this breaking down of organic matter we see in operation the reverse of the building-up process which takes place in the leaf. Organic matter is continuously reverting to the inorganic state: it becomes mineralized: nitrates are one form of the outcome. It is the business of the root hairs to absorb these substances from the soil solution and to pass them into the sap, so that the new life-building process can start up again. In a soil in good heart the soil solution will be well supplied with these salts. Incidentally we may note that it has been the proved existence of these mineral chemical constituents in the soil which, since the time of Liebig, has focused attention on soil chemistry and has emphasized the passage of chemical food materials from soil to plant to the neglect of other considerations.
But the earth's green carpet is not confined to its remarkable power of transforming the inert nitrates and mineral contents of the soil into an active organic phase: it is utilized by Nature to establish for itself, in addition, a direct connection, a kind of living bridge, between its own life and the living portion of the soil. This is the second method by which plants feed themselves. The importance of this process, physiological in nature and not merely chemical, cannot be over-emphasized and some description of it will now be attempted.
The Living Soil
The soil is, as a matter of fact, full of live organisms. It is essential to conceive of it as something pulsating with life, not as a dead or inert mass. There could be no greater misconception than to regard the earth as dead: a handful of soil is teeming with life. The living fungi, bacteria, and protozoa, invisibly present in the soil complex, are known as the soil population. This population of millions and millions of minute existences, quite invisible to our eyes of course, pursue their own lives. They come into being, grow, work, and die: they sometimes fight each other, win victories, or perish; for they are divided into groups and families fitted to exist under all sorts of conditions. The state of a soil will change with the victories won or the losses sustained, and in one or other soil, or at one or other moment, different groups will predominate.
This lively and exciting life of the soil is the first thing that sets in motion the great Wheel of Life. Not without truth have poets and priests paid worship to "Mother Earth", the source of our being. What poetry or religion have vaguely celebrated, science has minutely examined, and very complete descriptions now exist of the character and nature of the soil population, the various species of which have been classified, labelled, and carefully observed. It is this life which is continually being passed into the plant.
The process can actually be followed under the microscope. Some of the individuals belonging to one of the most important groups in this mixed population -- the soil fungi -- can be seen functioning. If we arrange a vertical darkened glass window on the side of a deep pit in an orchard, it is not difficult to see with the help of a good lens or a low-power horizontal microscope (arranged to travel up and down a vertical fixed rod) some of these soil fungi at work. They are visible in the interstices of the soil as glistening white branching threads, reminiscent of cobwebs. In Dr. Rogers's interesting experiments on the root systems of fruit trees at East Malling Research Station, where this method of observing them was initiated and demonstrated to me, these fungous threads could be seen approaching the young apple roots in the absorbing region (just behind the advancing root tips) on which the root hairs are to be found. Dr. Rogers very kindly presented me with two excellent photographs -- one showing the general arrangement of his observation chamber (Plate I), the other, taken on 6th July 1933, of a root tip (magnified by about twelve) of Lane's Prince Albert (grafted on root stock XVI) at sixteen inches below the surface, showing abundant fungous strands running in the soil and coming into direct contact with the growing root (Plate II).
Plate I. Observation Chamber for Root Studies at East Malling
But this is only the beginning of the story. When a suitable section of one of these young apple roots, growing in fertile soil and bearing active root hairs, is examined, it will be found that these fine fungous threads actually invade the cells of the root, where they can easily be observed passing from one cell to another. But they do not remain there very long. After a time the apple roots absorb these threads. All stages of the actual digestion can be seen.
The significance of this process needs no argument. Here we have a simple arrangement on the part of Nature by which the soil material on which these fungi feed can be joined up, as it were, with the sap of the tree. These fungous threads are very rich in protein and may contain as much as 10 per cent of organic nitrogen; this protein is easily digested by the ferments (enzymes) in the cells of the root; the resulting nitrogen complexes, which are readily soluble, are then passed into the sap current and so into the green leaf. An easy passage, as it were, has been provided for food material to move from soil to plant in the form of proteins and their digestion products, which latter in due course reach the green leaf. The marriage of a fertile soil and the tree it nourishes is thus arranged. Science calls these fungous threads mycelium (again from a Greek word), and as the Greek for root is rhiza, cf. rhizome, the whole process is known as the mycorrhizal association.
The reader who wishes to delve into the technical details relating to the mycorrhizal association and its bearing on forestry and agriculture should consult the following works:
- Rayner, M. C. and Neilson-Jones, W. -- Problems in Tree Nutrition, Faber & Faber, London, 1944.
- Balfour, Lady Eve -- The Living Soil, Faber & Faber, London, 1944.
- Howard, Sir Albert -- An Agricultural Testament, Oxford Press, 1940.
What is urgently needed at the moment is an account in simple, non-technical language, of this remarkable link between a fertile soil and the roots of the vast majority of flowering plants and its significance in nutrition and disease resistance. (See Trees and Toadstools by M.C. Rayner, 1945.)
Plate II. The Beginnings of Mycorrhizal Association in the Apple. Root-tip (x 12) of Lane's Prince Albert on root-stock M XVI at sixteen inches below the surface, showing root-cap (A), young root hairs (C), and older root hairs with drops of exudate (Cl). The cobweb-like mycelial strands are well seen approaching the rootlet in the region marked (C).
This partnership is universal in the forest and is general throughout the vegetable kingdom. A few exceptions, however, exist which will be referred to in the next paragraph.
Among the plants in which this mycorrhizal association has hitherto not been observed are the tomato and certain cultivated members of the cabbage family, many of which possess a very diffuse root system and exceptionally elongated root hairs. Nevertheless, all these examples respond very markedly to the condition of the soil in which they are grown and if fed with dressings of humus will prosper. The question naturally arises: Exactly how does this take place? What is the alternative mechanism that replaces the absent mycorrhizal association?
A simple explanation would appear to be this. Fertile soils invariably contain a greatly enhanced bacterial population whose dead remains must be profusely scattered in the water films which bathe the compound soil particles and the root hairs of the crops themselves; these specks of dead organic matter, rich in protein, are finally mineralized into simple salts like nitrates. We have already mentioned this breaking-down process of the soil population. What is here to be noted is that it is no sudden transformation, but takes place in stages. May not, therefore, some at least of the first-formed nitrogen complexes, which result from this breaking down, be absorbed by the root hairs and so added to the sap current? That is to say that the non-mycorrhiza-forming plants, not drawing on the soil fungi, do compensate themselves by absorbing organic nitrogen in this form -- they catch the bacterial soil population, as it were, before it has been reduced to an entirely inert phase and so have their link also with the biological life of the soil. That there must be some such passage of matter on a biological basis is suggested by the fact that only in fertile soil, i.e. in soils teeming with bacteria, do these non-mycorrhiza formers reveal resistance to disease and high quality in the produce, which means that only in these soils are they really properly fed.
This would be a third method used by plants for feeding themselves, a sort of half-way method between the absorption powers exercised by the root hairs and the direct digestive capacity of the roots: as the mechanism used in this method is presumably the root hairs, the diffuseness of the root system of plants of the cabbage family would be explained. It is possible that even mycorrhiza formers use this alternative passage for organic nitrogen. There seems no reason at all why this should not be so.
But how do the various agencies concerned in these intricate operations manage to carry on their work, buried as they are away from the light and thus unable to derive anything from the source of energy, the sun? How do they do their initial work at all until they can hand over to the green leaf? They derive their energy by oxidising (i.e. burning up) the stores of organic matter in the soil. As in an ordinary fire, this process of oxidation releases energy. The oxygen needed for this slow combustion is drawn from the air, in part washed down by the rain, which dissolves it from the atmosphere in its descent. Incidentally this explains why rain is so superior as a moistening agency for plants to any form of watering from a can: incidentally, again, we can understand the need for cultivating the soil and keeping it open, so that the drawing in of oxygen, or the respiration of the soil, can proceed and the excess carbon dioxide can be expelled into the atmosphere.
Humus is the Latin word for soil or earth. But as used by the husbandman humus nowadays does not mean just earth in general, but indicates that undecayed residue of vegetable and animal waste lying on the surface, combined with the dead bodies of these bacteria and fungi themselves when they have done their work, the whole being a highly complex and somewhat varying substance which is, so to say, the mine or store or bank from which the organisms of the soil and then, in direct succession, the plant, the tree, and thereafter the animal draw what they need for their existence. This store is all important.
The Significance of Humus
Humus is the most significant of all Nature's reserves and as such deserves a detailed examination.
A very perfect example of the methods by which Nature makes humus and thus initiates the turning of her Wheel is afforded by the floor of the forest. Dig down idly with a stick under any forest tree: first there will be a rich, loose, accumulation of litter made up of dead leaves, flowers, twigs, fragments of bark, bits of decaying wood, and so forth, passing gradually as the material becomes more tightly packed into rich, moist, sweet-smelling earth, which continues downwards for some inches and which, when disturbed, reveals many forms of tiny insect and animal life. We have been given here a glimpse of the way Nature makes humus -- the source from which the trunk of the tree has drawn its resisting strength, its leaves their glittering beauty.
Throughout the year, endlessly and continuously, though faster at some seasons than at others, the wastes of the forest thus accumulate and at once undergo transformation. These wastes are of many kinds and mix as they fall; for leaf mingles with twig and stem, flower with moss, and bark with seed-coats. Moreover, vegetable mingles with animal. Let us beware of the false idea that the forest is a part of the vegetable kingdom only. Millions of animal existences are housed in it; mammals and birds are everywhere and can be seen with the naked eye. The lower forms of animal life -- the invertebrates -- are even more numerous. Insects, earthworms, and so forth are obvious: the microscope reveals new worlds of animal life down to simple protozoa. The excrete of these animals while living and their dead bodies constitute an important component of what lies on the forest floor; even the bodies of insects form in the mass a constituent element not without importance, so that in the end the two sources of waste are completely represented and are, above all, completely mingled. But the volume of the vegetable wastes is several times greater than that of the animal residues.
These wastes lie gently, only disturbed by wind or by the foot of a passing animal. The top layer is thus very loose; ample air circulates for several inches downwards: the conditions for the fermentation by the moulds and microbes (which feed on the litter) are, as the scientist would say, aerobic. But partly by pressure from above and partly as the result of fermentation the lower layers are forced to pack more closely and the final manufacture of humus goes on without much air: the conditions are now anaerobic. This is a succession of two modes of manufacture which we shall do well to remember, as in our practical work it has to be imitated (Chapter 13, The New Zealand Compost Box).
This mass of accumulated wastes is acted on by the sunlight and the rain; both are dispersed and fragmented by the leaf canopy of the trees and undergrowth. The sunlight warms the litter; the rain keeps it moist. The rain does not reach the litter as a driving sheet, but is split up into small drops the impetus of whose fall is well broken. Nor does the sunlight burn without shade; it is tempered. Finally, though air circulates freely, there is perfect protection from the cooling and drying effects of strong wind.
With abundant air, warmth, and water at their disposal the fungi and bacteria, with which, as we have already noted, the soil is teeming, do their work. The fallen mixed wastes are broken up; some passes through the bodies of earthworms and insects: all is imperceptibly crumbled and changed until it decomposes into that rich mass of dark colour and earthy smell which is so characteristic of the forest floor and which holds such a wealth of potential plant nourishment.
The process that takes place in a prairie, a meadow, or a steppe is similar; perhaps slower, and the richness of the layer of humus will depend on a good many factors. One, in particular, has an obvious effect, namely, the supply of air. If, for some reason, this is cut off, the formation of humus is greatly impeded. Areas, therefore, that are partly or completely waterlogged will not form humus as the forest does: the upper portion of the soil will not have access to sufficient free oxygen, nor will there be much oxygen in the standing water. In the first case a moor will result; in the second a bog or morass will be formed. In both these the conditions are anaerobic: the organisms derive their oxygen not from the air but from the vegetable and animal residues including the proteins. In this fermentation nitrogen is always lost and the resulting low-quality humus is known as peat.
But the forest, the prairie, the moor, and the bog are not the only areas where humus formation is in progress. It is constantly going on in the most unlikely places -- on exposed rock surfaces, on old walls, on the trunks and branches of trees, and indeed wherever the lower forms of plant life -- algae, lichens, mosses, and liverworts -- can live and then slowly build up a small store of humus.
Nature, in fact, conforming to that principle of reserves, does not attempt to create the higher forms of plant life until she has secured a good store of humus. Watch how the small bits of decayed vegetation fall into some crack in the rock and decompose: here is the little fern, the tiny flower, secure of its supply of food and well able to look after itself, as it thrusts its roots down into the rich pocket of nourishment. Nature adapts her flora very carefully to her varying supplies of humus. The plant above is the indicator of what the soil below is like, and a trained observer, sweeping his eye over the countryside, will be able to read it like the pages of a book and to tell without troubling to cross a valley exactly where the ground is waterlogged, where it is accumulating humus, where it is being eroded. He looks at the kind and type of plant, and infers from their species and condition the nature of the soil which they at once cover and reveal.
But we are not at the end of the mechanisms employed by Nature to get her great Wheel to revolve with smooth efficiency. The humus that lies on the surface must be distributed and made accessible to the roots of plants and especially to the absorbing portions of the roots and their tiny prolongations known as root hairs -- for it is these which do the delicate work of absorption. How can this be done? Nature has, perforce, laid her accumulation on the surface of the soil. But she has no fork or spade: she cannot dig a trench and lay the food materials at the bottom where the plant root can strike down and get them. It seems an impasse, but the solution is again curiously simple and complete. Nature has her own labour force -- ants, termites, and above all earthworms. These carry the humus down to the required deeper levels where the thrusting roots can have access to it. This distribution process goes on continually, varying in intensity with night and day, with wetness or dryness, heat or cold, which alternately brings the worms to the surface for fresh supplies or sends them down many feet. It is interesting to note how a little heap of leaves in the garden disappears in the course of a night or two when the earthworms are actively at work. The mechanism of humus distribution is a give and take, for where a root has died the earthworm or the termite will often follow the minute channel thus created a long way.
Actually the earthworm eats of the humus and of the soil and passes them through its body, leaving behind the casts which are really enriched earth -- perfectly conditioned for the use of plants. Analyses of these casts show that they are some 40 per cent richer in humus than the surface soil, but very much richer in such essential food materials as combined nitrogen, phosphate, and potash. Recent results obtained by Lunt and Jacobson of the Connecticut Experiment Station show that the casts of earthworms are five times richer in combined nitrogen, seven times richer in available phosphate, and eleven times richer in potash than the upper six inches of soil.
It is estimated that on each acre of fertile land no less than twenty-five tons of fresh worm casts are deposited each year. Besides this the dead bodies of the earthworms must make an appreciable contribution to the supply of manure. In these ways Nature in her farming has arranged that the earth itself shall be her manure factory.
As the humus is continually being created, so it is continually being used up. Not more than a certain depth accumulates on the surface, normally anything from a few inches to two or three feet. For after a time the process ceases to be additive and becomes simply continuous: the growing plants use up the product at a rate equalling the rate of manufacture -- the even turning of the Wheel of Life -- the perfect example of balanced manuring. A reserve, however, is at all times present, and on virgin and undisturbed land it may be very great indeed. This is an important asset in man's husbandry; we shall later see how important.
The Importance of Minerals
Is the humus the only source from which the plant draws its nourishment? That is not so. The subsoil, i.e. that part of the soil derived from the decay of rocks, which lies below the layer of humus, also has its part to play. The subsoil is, as it were, a depository of raw material. It may be of many types, clay, sand, etc.; the geological formation will vary widely. It always includes a mineral content -- potash, phosphates, and many rarer elements.
Now these minerals play an important part in the life of living things They have to be conveyed to us in our food in an organic form, and it is from the plant, which transforms them into an organic phase and holds them thus, that we and the other animals derive them for our well-being.
How does the plant obtain them? We have seen that there is a power in the roots of all plants, even the tiniest, of absorbing them from the soil solution. But how is the soil solution itself impregnated with these substances? Mainly through the dissolving power of the soil water, which contains carbon dioxide in solution and so acts as a weak solvent. It would appear that the roots of trees, which thrust down into the subsoil, draw on the dissolved mineral wealth there stored and absorb this wealth into their structure. In tapping the lower levels of water present in the subsoil -- for trees are like great pumps drawing at a deep well -- they also tap the minerals dissolved therein. These minerals are then passed into all parts of the tree, including the foliage. When in the autumn the foliage decays and falls, the stored minerals, now in an organic phase, are dropped too and become available on the top layers of the soil: they become incorporated in the humus. This explains the importance of the leaf-fall in preserving the land in good heart and incidentally is one reason why gardeners love to accumulate leaf-mould. By this means they feed their vegetables, fruit, and flowers with the minerals they need.
The tree has acted as a great circulatory system, and its importance in this direction is to be stressed. The destruction of trees and forests is therefore most injurious to the land, for not only are the physical effects harmful -- the anchoring roots and the sheltering leaf canopy being alike removed -- but the necessary circulation of minerals is put out of action. It is at least possible that the present mineral poverty of certain tracts of the earth's surface, e.g. on the South African veldt, is due to the destruction over wide areas and for long periods of all forest growth, both by the wasteful practices of indigenous tribes and latterly sometimes by exploiting Western interests.
Before we turn to consider the ways in which man has delved and dug into all these riches and disturbed them for his own benefit, let us sum up with one final glance at the operations of Nature. Perhaps one fact will strike us as symptomatic of what we have been reviewing, namely, the enormous care bestowed by Nature on the processes both of destruction and of storage. She is as minute and careful, as generous in her intentions, and as lavish in breaking down what she has created as she was originally in building it up. The subsoil is called upon for some of its water and minerals, the leaf has to decay and fall, the twig is snapped by the wind, the very stem of the tree must break, lie, and gradually be eaten away by minute vegetable or animal agents; these in turn die, their bodies are acted on by quite invisible fungi and bacteria; these also die, they are added to all the other wastes, and the earthworm or ant begins to carry this accumulated reserve of all earthly decay away. This accumulated reserve -- humus -- is the very beginning of vegetable life and therefore of animal life and of our own being. Such care, such intricate arrangements are surely worth studying, as they are the basis of all Nature's farming and can be summed up in a phrase -- the Law of Return.
We have thus seen that one of the outstanding features of Nature's farming is the care devoted to the manufacture of humus and to the building up of a reserve. What does she do to control such things as insect, fungous, and virus diseases in plants and the various afflictions of her animal kingdom? What provision is to be found for plans protection or for checking the diseases of animals? How is the work of mycologists, entomologists, and veterinarians done by Mother Earth? Is there any special method of dealing with diseased material such as destruction by fire? For many years I have diligently searched for some answer to these questions, or for some light on these matters. My quest has produced only negative evidence. There appears to be no special natural provision for controlling pests, for the destruction of diseased material, or for protecting plants and animals against infection. All manner of pests and diseases can be found here and there in any wood or forest; the disease-infected wastes find their way into the litter and are duly converted into humus. Methods designed for the protection of plants and animals against infection do not appear to have been provided. It would seem that the provision of humus is all that Nature needs to protect her vegetation; and, nourished by the food thus grown, in due course the animals look after themselves.
In their survey of world agriculture -- past and present -- the various schools of agricultural science might be expected to include these operations of Nature in their teaching. But when we examine the syllabuses of these schools, we find hardly any references to this subject and nothing whatever about the great Law of Return. The great principle underlying Nature's farming has been ignored. Nay more, it has been flouted and the cheapest method of transferring the reserves of humus (left by the prairie and the forest) to the profit and loss account of homo sapiens has been stressed instead. Surely there must be something wrong somewhere with our agricultural education.
Next: 3. Systems of Agriculture
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