THE SECRET OF THE SOIL.
NVHEN the farmer in Aesop's fable told his sons that he was leaving them treasure buried in his fields, which they would find if they dug for it, be gave them nearly the sum of the knowledge which the modern agriculturist possesses of the soil be ploughs. With all the added learning of a thousand experiments in manuring, in irrigating, and in applying the science of bacteriology to the cultivation of crops, the conclusion is still the same. Tilth is the essential, imperative need. Farmers have learnt a great deal about the values of different manures for different crops, and a great deal, too, as to the necessities and possibilities of rotation of one crop after another, though the Roman farmer had dis- covered that principle two thousand years ago. Virgil's first Georgic is full of advice as to alternating crop and crop—lupins before oats, for example—which is really the practical teaching of our modern experiments for extracting nitrogen from the air. Varro, before Virgil, even came nearer modern practice, for he advised the sowing of certain crops, not with the immediate hope of harvest, but in the knowledge that such crops ploughed in would increase the fertility of the soil. That is the practical experience of high farming of to-day. Crops are sown to catch and detain chemical elements necessary for crops that are to succeed them, and then are ploughed in. But the great thing is the mixing and breaking up and ventilating the soil, ploughing it to enable the frost to do its proper work, pulverising it so that the water which is to hold the food for the plants' roots can cling round every tiny particle of soil. Then the roots can push free and far and find food and drink wherever they push. That is the substance of human know- ledge of, the oldest of man's industries. The soil holds all that a plant needs if it can be broken up sufficiently small for the plant to get at its food. To increase tilth and to decrease manure is the main tendency of modern scientific farming.
An interesting little book, summing up in a condensed form the conclusions of many experiments in soils and farming generally, has just been published by Mr. Primrose McConnell, a practical farmer of Southminster, Essex, under the title Soils : their Nature and Treatment " (Cassell and Co., ls. net). How much is there still waiting for the farmer to discover, of the soil which Varro wrote two thousand years ago, and man had been learning for uncounted centuries before him P If the need for good tilth still remains the greatest need of all, what have we learnt that the Roman farmer did not know ? Of the actual practical work of ploughing and sowing probably very little. Of the composition of soils and subsoils perhaps a little more. We know, for instance, what he had no oppor- tunity of knowing,—that the rule of cultivable soils is pretty nearly the same the whole world over. First the top layer, some three inches of turf ; under that a layer, which may be shallow or deep, but is seldom much more than a foot deep, of soil; under that, a subsoil; under that, a layer of " brash " or rubble ; and, last of all, the bedrock itself, from whose surface all the rest have been rubbed and washed and broken by frost and rain and sun and clinging plants. The Roman farmer could get a practical knowledge of the actual earth which be himself worked, but he could not, of course, compare the geology of five continents. What has been reserved for a generation living two thousand years after him to discover is that the soil is something very different from what English farmers believed it to be forty or fifty years ago. Perhaps the Roman, or the Greek before him, knew it instinctively, for ib was not an Englishman who first called the earth Mother. But it was not until our day that science established the fact that the soil is not a mass of dead, inert matter, to be shovelled here and shifted there, merely a fortuitous collection of powdered rocks which can be " manured," or worked by the hand, into different places and various uses ; but is a teeming world of living creatures, an aggregate of millions of tons of so-called " soil" which can bring forth life because it is itself alive ; which bolds in its open, arable sur- face secrets of living organism and growth and fertility which men who have ploughed it since the beginning of earning bread only yesterday began to suspect, only to-day recognise faintly, and perhaps only in the end of things, perhaps never, will fathom. The more man learns of other life, the further the horizon of his own life retreats from him.
The principles of the plainer methods of improving soils are simple enough. Wet soil can be drained, and so made more capable of holding water. It sounds contradictory, but an ill-drained soil does not hold water as plants like it held,— that is, collected by capillary attraction to tiny particles, with room for air next to the water. Rather it contains stagnant settlements of water at which roots will not drink, and which need to be drained away. Some soils must be drained of water, others cleared of stones. Others, again, need to be limed, or supplied with a body which acts in various ways, nearly invariably beneficially, on various soils. Lime stiffens sand, makes clay friable, and helps plants whose roots take up nitrogen from the air in the soil in retaining surpluses of the gas which otherwise might be too much for them. Plants which take up nitrogen sometimes succeed in choking themselves, and lime, as it were, helps them with what they cannot manage of their plateful. But the most interesting, and, regarded from different points of view, at once the darkest and best lighted of the problems of soil and cultivation, is the question of what perhaps may be called bacterial aid to plant-growth. Soil, separated into its constituents, consists of some twelve or thirteen chemical substances, existing in various compounds, of which the most important are seven: nitrogen, phosphoric acid, potash, lime, ferric oxide, magnesia, and sulphuric acid. Roughly and vaguely speaking, for many of the chemical processes which go on in the soil are still entirely obscure, of three or four of these there is such an abundance that no imaginable process of crop-growing could exhaust them. Three, or perhaps four—nitrogen, phosphoric acid, potash, and lime—might conceivably be removed by plant-growth to an extent harmful to the soil, and might need to be replaced. Of these, lime may be necessary to assist the presence of nitrogen, but is very little required as a direct manure to the plant itself. Potash, again, need be added in comparatively few cases. But nitrogen and phosphoric acid are abundantly and perpetually wanted, and one of them, nitrogen, has been the subject of some extraordinarily interesting discoveries. Two German chemists, Hellriegel and Willfarth, first established in 1883 that on the roots of certain leguminous plants, vetches, beans, lupins, and so on, there grow little nodules, which are the result of the growth of microbes within them. The microbes extract nitrogen from the air in the soil round them, and, as it were, build it up into the substance of the root of the leguminous plant which is their " host." That discovery was the beginning of a series, of which we can only guess the possibilities. To it has already been added the discovery that seeds can be dressed with cultures of particular microbes which feed, and help the plant to feed, on nitrogen, and so help it to a vigorous growth, and
to vigorous growths in other plants that are to follow it in the soil. The almost certain induction seems to be that all plants have particular microbes which, in feeding themselves, help the plant to feed.
If that is so, the science of " manuring " a soil will resolve itself, first, into perfect tilth, so that every particle of the soil can be surrounded with its coat of water containing soluble food ; and second, into the provision of the right kind of microbe in the neighbourhood of the plant that needs it. Of the four substances known to be absolutely necessary to plant life, and also liable to exhaustion, we still have no better means of supplying phosphoric acid than in the form of manure. But is it to be supposed that Science has yet made all her discoveries ? Mr. McConnell states, surely a little dogmatically, that phosphoric acid "is the one ingredient that is universally deficient in the soil, that is not added by the ordinary farmyard dung of the farm in sufficient quantity, and that cannot be developed or extracted from anywhere by any system of cropping or treatment, and must be returned by extraneous manuring." Is not that, considering that we knew nothing of nitrogen-extracting nodules thirty years ago, rather too sweeping ? We have not yet heard all the secrets even whispered. The secret of the life of the soil is not to be dragged from her in thirty years, possibly not in three thousand. Our posterity, perhaps, will decide that we were little further on the road to discovery than was Varro.
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