The great world's farm; some account of nature's crops and how they are grown

Front matter

LIBRARY

OF THE

THE GREAT WORLD'S FARM

Contents

PREFACE ••«••• V

I. INTRODUCTORY . . t • • • I

II. PIONEER-LABOURERS - • c • • 1$

II

No one needs to be told that all living things require food of one sort or another to keep them alive; but some people have fancied, even within the last hundred years, that vegetables had such delicate appetites as to need nothing but air and pure water for their sustenance.

If this were so, then, of course, one sort of soil would agree with them as well as another; and, in fact, it would not be wanted at all except as a protection to their roots, and as a means of fixing them to one spot and enabling them to stand against the wind.

As a matter of fact, however, no vegetables live upon a diet of mere air and water.

But then, what of the seaweeds which float about in the ocean ? Are there not vast meadows of weed far away from soil or even rocks of any kind ? Does not the ocean, moreover, swarm everywhere, from the Polar regions to the Equator, with microscopic vegetables ? and is it not a fact that no seaweeds, not even those which cling to the rocks, receive any of their nourishment through their roots, and therefore must live upon water ?

Quite true that they do not feed by means of their roots — indeed, no seaweeds possess true roots ; and it is quite true, also, that they live upon what they obtain from the water, but surely the taste of it is sufficient to prove that it is not mere water.

There is no such thing as pure, absolutely pure, water in nature ; and sea-water, which is much heavier than fresh water, contains thirty-five parts of solid matter in every thousand. The rivers are constantly pouring into it small quantities of every sort of mineral substance that can be dissolved, while the sun draws up from it almost pure water, leaving the salts behind to accumulate and help to feed the crops of seaweed, besides providing material for the skeletons of corals and sponges and the shells and bones of other seacreatures.

All plants, then — whether they tower aloft like the giants of the tropical forest, or send out streamers two or three hundred feet in length like the Giant Kelp of the Atlantic Ocean ; whether they are so minute as to be altogether invisible to the naked eye, and whether they live in earth or in water — are alike in this, that they live upon other food besides mere air and water.

We may easily satisfy ourselves of this by burning a bit of wood, a few grains of corn, or any other vegetable matter. When it has burnt as long as there is anything to burn, and ail the gases and water it contained are driven away, a small quantity of ash will remain, consisting of salts, or compounds of various metals. The whole amount is usually very small — so small that we might perhaps be disposed to think it could not be of any very great consequence.

If, for instance, we were to burn a hundred grains of wheat so thoroughly that nothing but ash remained, we should find the whole amount of this to be equal to about two of the grains, or less ; while if we were to take peas or beans, the amount in a hundred of each would be equal to less than three peas, and less than four beans.

There would be more in the straw or stems of all three kinds of plants, but still the quantity appears so small that one might doubt its being absolutely essential if one did not know that it was so.

However, ' many a mickle makes a muckle,' and when we consider, not single plants or a handful of grain, but a whole crop, the amount of mineral matter becomes large enough to look important. Thus, while a pinch or two of dust might represent the entire amount of ash of all sorts in a single turnip or carrot, there are, on an average, about 40 pounds of lime alone in 22 tons of turnips, more in proportion in the carrots, and very much more in an equal weight of clover.

The mineral substances chiefly taken up by plants are sulphur, phosphorus, silica, potash, soda, lime, iron, magnesia, manganese, together with mineral compounds of the two gases chlorine and fluorine. All these are contained in the rocks ; but the question is, How are they to be made available — how is the plant to get hold of them ? There is abundance of food, but as long as it is stored in the rocks it might as well be locked up so far as most of them are concerned, for they cannot get at it or make use of it. Of course, we all know that seeds cannot grow, though they may sprout, upon a slab of bare stone, even though it may be rich in the very food they want. The stone must be converted into soil before they can turn it to account ; and how is this to be accomplished ?

If man had to make his soil from the rocks before he could grow his crops, he would have to begin with crowbars and pickaxes, if he did not first resort to blasting with gunpowder or dynamite, and even then his progress would be slow and laborious.

Nature usually works in a much more quiet and unobtrusive fashion, but there are times when she, too, has recourse to blasting as a preliminary measure. She mines the rocks and shatters them by means of the earthquake, compared with which the power even of dynamite is insignificant ; and if these rough measures do but little towards preparing the soil, they unquestionably make things easier for the army of labourers who follow.

But it is the noiseless and often invisible workers who accomplish most, for they are at work, some or other of them, incessantly during every moment of every hour, day and night, summer and winter, throughout the whole year.

Usually the first of the silent labourers to begin work upon the rocks are also the invisible ones — the gases of air and water, which wear away the very hardest rocks by degrees. They are not able to work equally fast upon all rocks, but where they can be employed, there the work goes on most rapidly. Those rocks which are best able to resist them decay more slowly, even though they be actually softer and yield more readily to such labourers as wind and rain. In other words, the unseen chemical workers produce more effect on the rocks than do the seen, mechanical workers.

The two gases which do the chief part of the chemical work are oxygen and carbon dioxide, formerly called carbonic acid. Rocks containing much iron are especially open to the attacks of the one, and those containing lime, potash, soda, to the attacks of the other. We are all familiar with the fact that iron and steel become covered with rust if left exposed to the air. Keys rust if left in their locks, and even polished fireirons often rust in the summer, unless they are oiled or greased and so protected from the air. What happens in these cases is that the oxygen, always present both in the air itself and in the watery vapour floating in the air, lays hold of the metal and combines with it to form a compound substance — an oxide — which is looser and softer, and takes up more room than the metal alone. At first the rust is a mere reddish-brown stain ; but as the oxygen eats deeper and deeper, and more and more oxide is formed, it swells up unevenly abovf the surrounding surface, and feels rough to the touch. It is so soft that it may be partly rubbed off by the finger, and when the rust is cleaned away there will be scars and indentations left, showing how much of the metal has been removed.

Now it would take a much harder blow to break a piece of iron than it would to shatter a flint ; but if the two were left exposed to the air, the metal would waste away long before any impression was made on the stone.

Very many rocks contain iron, as, for instance, the slates, sandstones, granites, and basalts, some more, some less, but hardly ever in a pure state. The basalt of the Giant's Causeway contains so much iron that, on those sides which are most exposed to the weather, it not only looks rusty, but is also softer on the surface and less compact within, for nature's labourers do not so Pioneer Labourers

generally work singly and alone, but in union one with the other, and the great ally of oxygen is moisture.

Let us take basalt as an example, and see how this rock is crumbled into soil. In perfectly dry air, at the ordinary temperature, oxygen is powerless to do even so much as tarnish iron in the mass, though it would have no difficulty in reducing it all to oxide — that is, rust — if the same mass of iron were exposed to its action in the form of powder. Fireirons do not rust in winter, or when in constant use, because the fire keeps them dry ; they do rust when unused in summer, because natural air is never perfectly dry, even on the driest summer day, not even in the midst of the parching desert.

But, if iron quickly rusts when exposed to the damp air of such a climate as ours, we all know how much faster it does so when actually wetted ; and therefore it is not surprising to find that basaltic and other rocks containing much iron decay more rapidly on the side which faces the rainiest quarter. Not that the force of the rain makes so much impression on them as on softer rocks, but that the wet enables the oxygen to work faster. The decay is not confined to the surface, moreover, for all rocks, even those which are most close and compact and are called impervious, absorb some amount of moisture, and this also finds entrance through the cracks and joints, from which no large mass of rock is ever entirely free. These joints are especially well developed in the basalt — an ancient lava — which, in cooling down from the molten stnfe, has shrunk and contracted into columns having from tnree to nine faces, and measuring from a few inches to several feet across, The rain, of course, easily finds its way in between these columns ; but patches of wet and brown stains are also found actually inside the columns themselves, when these are broken open, showing that moisture has been sucked up by the rock.

Now, water in the natural state always contains some amount of air dissolved in it, and, wherever the water penetrates, there the oxygen of the air penetrates also, and lays hold of any iron that comes in its way, as we see by the stains that it has done in this instance.

The iron of the basalt is not, indeed, pure iron, being already combined with some amount of oxygen, but it does not acquire the reddish-brown colour of what we familiarly call ' rust ' until it has absorbed as much oxygen as it can hold. In this condition it is, of course, heavier, and, as we have seen, softer than before, and is therefore more easily washed or blown away from the surface. But it is also more bulky, and takes up more space than it did before, so that if it be formed inside the rock where it has not room to expand, the rock is cracked by it. This, of course, opens the way for more water and more oxygen to enter, and so the work proceeds, and the decay goes deeper and deeper.

We have chosen iron-rust as a sample of the way in which oxygen works because it is one of which we all know something, but it must not be forgotten that this is only one of many oxides formed in the rocks ; and that whenever oxygen combines with any other substance in a rock to form an oxide, it makes that substance take up more room than before, and so the rock is cracked and crumbled. The other gas, carbon dioxide, \vorks in a different way, though it also helps the oxygen to rust iron faster than it could do alone. But when it works on its own account it is by combining with such substances as lime, potash, soda and magnesia, which it makes much more soluble than they were before.

Some rocks are said to be impervious, or ' waterproof,' but this only means that they allow water to enter so very slowly that, unless they are actually soaking in it for some time, hardly any is taken up. And so, in a similar way, some minerals are called insoluble, which again means only that pure water has very little effect upon them, dissolves them so very slowly that it hardly seems to do so at all. For some effect it has upon every known mineral, unless it be perhaps upon gold and platinum. But water in nature is never perfectly pure; how can it be, since it dissolves some, though it may be only a very minute quantity, of everything through, or over, which it passes ? Its dissolving powers are greatly increased, too, by the addition of carbon dioxide — the gas we are now speaking of — which is being constantly produced both in earth and air, by the decay of vegetable matter in the one, and by the lungs ot animals, fires and furnaces, in the other.

The rain, as it descends from the clouds, washes down with it some of this gas, and if it comes in contact with such a rock as limestone, soon makes an impression upon it Chalk, limestone and marble, are all composed of carbonate of lime, softer or harder, the lime being already united with a certain quantity of carbon dioxide. But in this condition it dissolves so very slowly as to be called insoluble in pure water. When it comes in contact with the gas, however, whether in air or water, it takes up double the quantity it had before, and is converted into a double, or bi-carbonate, which is easily dissolved and washed away. Marble slabs which are exposed to the weather in such a climate as that of Edinburgh are completely destroyed in this way in less than a hundred years.

Even rocks which consist only in part of carbonate of lime are open to the attacks of carbon dioxide. For instance, there are the sandstones. The grains are hard enough, being composed of silica, and if they are cemented together with silica, too, the stone is one of the most durable that can be found, neither water nor gases, together or separately, being able to make much impression upon it. But if the grains are cemented together by iron oxide, or by carbonate of lime, it is quite another matter. Oxygen or carbon dioxide may get to work on the cement, and as that is removed the grains fall apart and become sand.

Then, again, there are the granites, composed of three minerals, quartz, mica, and felspar ; the first is silica, like flint, and equally hard, and if the rock consist mainly of quartz, as some granites do, it is of course extremely durable, for, besides resisting the action of the gases, it also admits very little water. If, on the other hand, the felspar predominates, the rock is softer, and water finds an easier entrance ; but, besides this, the felspar — which is clay without the water, and in a crystallized form— contains either potash or soda, and often lime as well, and any one of these is easily attacked and made soluble by union with carbon dioxide. Once converted into carbonates, or bicarbonates, as the case may be, these are easily dissolved and washed away by the next shower. As for the single carbonate of potash, it does not need even a shower to help it, but melts away in the moisture of the air, as anyone may know who has ever tried to keep it wrapped in paper.

The carbonates being thus removed, and the clayey part of the felspar softened and washed away to form beds of clay elsewhere, the crystals of quartz and mica must needs fall apart, and so the work of breaking down the rock goes on.

But nature's labourers proceed upon the principle that ' union is strength,' and they so constantly work in company that it is a difficult matter to apportion the result of their labours exactly each to each. We have already seen how water dissolves ; we must now look at it in another capacity, and see how it acts the part of crowbar and pickaxe, and even at times of dynamite. A cubic inch of water, when converted into steam, occupies just 1,728 times as much space as it did before, and it expands with such violent force as to shatter the rocks beneath which it is confined. Such explosions as this sometimes occur during volcanic eruptions, water having found its way down through the earth till it has come into contact with some mass of molten lava, which has converted it into steam, and made it a powerful engine of destruction.

But water expands also, though in a less degree, when it is converted into ice, and it is under this aspect that we are most familiar with its doings. Ice occupies only one-fifteenth more space than the water from which it is formed, and, compared with the expansion which takes place in the formation of steam, this increase may sound insignificant. But its effects are very far indeed from being insignificant the force which it exerts being simply irresistible.

Water, as we have said, finds entrance everywhere, more or less, in one way or another, and wherever it is sufficiently near the surface to freeze, there it has the effect of a multitude of crowbars and chisels of all sorts and sizes wielded by an invisible army of workmen. It widens every crack in which it is formed, prizing up large masses of rock many tons in weight, loosening and eventually forcing them off, and also doing finer work, such as chiselling off splinters and particles of all sizes, large and small. The immense piles of rubbish which strew the surface of the glaciers, and consist of sand, grit, and fragments of all dimensions, are due mainly to the action of the frost, which in mountain regions recurs not merely every winter, but every night throughout the year.

Even in England, where frosts are less frequent, as well as less severe, and therefore penetrate to a less depth, the tops of the higher hills are often buried some feet deep with wreckage of the frost's making ; only very often we fail to realize what is going on because the loosened fragments are being constantly washed away by the rain. In more severe climates the waste is immense, the hill-tops and mountains being crowded with great blocks and slabs wedged off by the frost ; and this is the first step towards their being crumbled into soil. The more water a rock absorbs, the more easily, of course, it is cracked and splintered by the frost ; and though we may not, many of us, have much opportunity of observing the work done in this way upon the rocks, we all know something of glasses and pipes cracked by the freezing of the water in them*

We may have seen, too, how fence-posts are sometimes lifted out of their places, simply by the heaving of the soil beneath them ; the water in the pores of the soil having frozen and swelled, and forced them up.

But even where there is no rain, and no ice can therefore be formed in their cracks and crevices, the rocks themselves feel changes of temperature ; and where these changes are sudden, severe, and often repeated, no rock is strong enough to stand against them. The rocks of the Sahara and other similar regions are crumbled into sand simply by the intense heat of the day and the sharp frost at night. The one causes them to expand and the other makes them contract ; and between the two the outer grains are being constantly loosened and forced out of their places

When the 'Glass Road' was being made in the famous Yellowstone Park (Wyoming, U.S.A.), some huge blocks of obsidian, or volcanic glass, were found to come in the way ; and as they were too hard to be either hewn or drilled, and could therefore not be blasted, the engineer in charge had large fires lighted on the top. When the rocks were scorching hot, a sudden deluge of cold water was poured upon them from the neighbouring lake, and by these means they were thoroughly shattered. This is of course a very extreme instance of the effect produced by changes of temperature, and such as would seldom, if ever, occur in nature; but it may serve to show how very real these effects are.

Of the other ways in which the rocks are broken up, it will not be needful to say much. We must pass over with brief mention the work done by sand, set in motion by wind or water., which cuts and polishes the

Pioneer Labourer $ 27

very hardest rocks when driven against them by the former, and, when driven by water, has produced the great caftons, or narrow gorges some thousands of feet in depth, with which we are familiar in California. Nor will it be necessary to dwell upon the hammering, battering effects of the rains, which are sufficiently obvious even in our climate, and are of course greatly intensified in tropical regions.

But a few words must be said about the glaciers, those frozen rivers, which are among the mightiest of nature's grinders. Looking down upon a glacier, and seeing it strewn with the blocks of stone and vast heaps of rubbish which have fallen upon it from the cliffs above, dislodged by the frost, we should be disposed to think it a very rough labourer indeed, merely engaged in carting away the wreckage made by others. But this would be a great mistake. A little brawling stream makes, it is true, far more noise and fuss, and even more show of work, as it rolls the pebbles over in the bed which it is perpetually deepening and widening; but the glacier is a giant mill-stone, pressing upon the rocks beneath with a power which is simply irresistible.

Glaciers move on in solemn silence, it may be at the rate of perhaps only an inch or two in the twenty-four hours, but they go on steadily and noiselessly, and as they go, they grind the rocks beneath to a powder so fine, that when at last it escapes from the glacier-mill in the stream, which flows out from beneath, it has been reduced to nothing but mud.

One other grinder, equally mighty and thorough, but by no means silent, must be mentioned in conclusion. This is the volcano, which, besides pouring forth streams of lava, often buries the surrounding country many feet deep in the finest dust and ashes, or in mud, if the eruption be accompanied, as it often is, by rain.

Such, then, are the principal pioneer -labourers employed in breaking down the rocks. By the combined efforts of the gases and moisture of the atmosphere, by heat and frost, wind and rain, by rivers and streams, by glacier and volcano, the rocks are gradually split up, worn away, and reduced on the surface to a condition of softness.

In tropical regions, where the air is always moist, and the rainfall large and violent, some rocks are decayed and softened to a depth of a hundred or two hundred feet. In drier climates, the work proceeds more slowly and to a less depth, the rock beneath being to some extent protected from the weather by the looser material above, when this is left to accumulate.

But the labourers which we have been thus briefly considering are only pioneers. They accomplish only the rougher work of preparation, and very much remains to be done before anything that can properly be called ' soil ' is ready for the crops.

III

STANDING before some bare expanse of hard rock, we might well wonder, if we knew nothing of the subject, how it should ever be converted into a surface fit for the support of vegetation. There may be vineyards close by showing that it has been done in other instances ; but what is to be the first step ?

Ploughs, even steam ploughs, are quite useless here, and man must wait until the work is done for him, as it will be, and very thoroughly, too, in the end ; but it will be done deliberately, without hurry or bustle, and it may take years or centuries. Meanwhile, it is of no use for him to look with longing eyes upon the rock ; he can't plough it, and he would be mad to sow it, for any seed he could sow would be washed or blown away.

And yet if he were to look closely at the seemingly bare surface, he might, and in most cases would, find that it was not altogether bare and barren. He might need a microscope to show him the truth, but if he understood what he saw, he would discover that the rock had been sown.

The pioneer labourers, far from finishing, have hardly begun their work here, but seed fcas been scattered in this unlikely place ; and if we look at what has been done in other similar places, we shall see that it has not been wasted.

Floating about in the air, invisible, but in countless multitudes, are — what answer to the seeds of other plants — the spores of those strange forms of vegetation called lichens, which, except in towns, are to be seen beautifying every old wall, roof, and tombstone. They are so light that they cannot settle at all, except when the air is still, and even then the least breath would disturb them.

But they are sticky, and this stickiness enables them to cling fast even to the bare rock. Once settled, they begin to grow, and are the first traces of vegetable life to make their appearance upon recent streams of lava. They may truly be called 'traces/ for the first-comers are nothing more than helplesslooking stains, or dust, hardly noticeable except by those on the look-out for them ; and one would have said anything but dangerous to the rock, for they look not only perfectly inactive, but entirely lifeless.

Just so a stranger to the tropics, newly-arrived, would see with perfect unconcern a single small ant making its way across the floor of his room, a helpless insignificant insect, which he could crush with a finger. But the native knows better. He could crush the one, but behind it is a mighty column of ants two or three hundred yards long coming to take possession of the house, and there is nothing to be done, as he knows, but to give it up to them and retreat for the time. The ants come and go again when their work is done ; but the lichens come to stay until displaced by other and more important members of the family, for they are the advance guard of the vegetable host, and their appearance signifies that the fate of the lava surface upon which they have settled is sealed. Nature has marked it for her own. She is going to break down the hard, barren surface sooner or latter, and convert it into a fertile, productive soil, fit for field or vineyard.

The lava has resisted for some time. For years it did not even cool, and it has scorched innumerable lichen spores to death in their attempt to settle upon it. Even when the surface had cooled there was for a long time heat enough within to dry all the life out of them ; while multitudes have found the glossy surface too glossy even for their powers of clinging, and have been blown away as fast as they came. There are some streams of lava which are as glossy now as when they were first poured forth three or four hundred years ago, and no lichens have as yet managed to gain a footing there. But they are not generally kept so long at bay. They return to the charge again and again, helped by the pioneers who have also been at work meantime, and have gradually roughened the surface a little, or at least have taken off some of the glossiness ; and at last the spores manage to settle and fix themselves so firmly that neither wind nor rain can dislodge them, and they begin to grow and spread at their ease.

Then, in spite of what was said in the previous chapter, these vegetables, at all events, must live on air and water ?

Not at all ! Lichens are very substantial feeders indeed, and consume more mineral matter in proportion to their size than any other plants.