INOCULATING' THE SOIL.*
SUCH a series of discoveries as have revolutionized medical science during the past twenty years is in process of being made in relation to the cultivation of the soil. Bacteriology is yielding wonderful secrets for the horticulturist and agri- culturist. Friendly bacteria can be made to fight the battles of the gardener and the husbandman. Probably, as in the treatment of human bodies, bacteria often conquer the diseases and defects of the soil by introducing into it what is in itself a minor disease. It is known that this has been the method of Nature herself, though for centuries the meaning of certain familiar phenomena was not understood. For example, nodules on the roots of leguminous plants are, in one sense, a disease. The plants resisted the formation of them, and very strong plants would have prevented them from forming altogether. But the nodules nevertheless contain the very bacteria which are necessary for bringing nitrogen to the plants and turning a weak growth into a flourishing one. In bacterial methods— introducing the right sorb of bacteria to aid natural processes —lies the future of the fertilization of the soil. Experiments have already gone far enough to leave us in very little doubt on this point. Inoculation with selected bacteria is, after all, a natural method ; it aids Nature ; it does not defy or reverse her ways.
The use of bacteria as manure in place of mineral fertilizers or electrical stimulation—though these methods depend, no doubt, upon their effect on bacteria—opens up amazing prospects. The rapid growth of bacteria enables them to extend their influence at lightning speed. The perfect ferti- lizer of the future, let us boldly guess, will not only contain * The Spirit of the Soil; or, An Account of Nitrogen Fixation in the Soil by Bacteria and of the Production of Auxinionce tin Bacterized Peat. By Gordon D. Knox. With a Foreword by Professor W. 13. Bottomley. London: Constable and Co, Lb. ad, net.]
infinitely more bacteria than exist in stable manure, but will be able to spread far and wide beyond the spot where it is placed. The members of each bacterial colony will form an empire and invade and dominate the surrounding country, carrying plant food wherever it is needed. One thinks of an acre being fertilized from an inch-patch of bacteria. But such things are dreams at present. The use of bacterized peat, which has given much the most encouraging results so far, thanks to Professor Bottomley's brilliant experiments, has not yet been applied to agriculture on a large scale. It had been hoped that the effects of " humogen," as the bacterized peat is called, would have been traceable in the last harvest, but the long drought prevented the bacteria from flourishing. Moisture, within limits, is as necessary to the work of the bacteria as is the absence of acidity in the soil. No doubt the disadvantages of drought, so far as they act in checking the development of bacteria, will be overcome in due course. To some extent, wo suppose, they can be overcome now by the use of farmyard manure, which is an excellent retainer of moisture. But of course we must remember that the fertilizers of the future ought to be independent of the manures and fertilizers we have hitherto used. The resources of the world in nitrates and other minerals are exhaustible, and the supply of stable and farmyard manure becomes continually smaller in these days of motor traction.
Professor Bottomley's experiments, described in the book before us by Mr. Gordon D. Knox with a singularly happy combination of learning and power of popular exposition, are the culmination of much research on similar lines. For several years attempts have been made to produce a bacterial culture that would ensure a steady supply of nitrogen to plants. There have been partial successes, buiriothing like the startling results which have attended Professor Bottomley's efforts.
Professor Bottomley himself has been compelled by the un- expected character of his results to believe in the existence of certain mysterious accessory food substances produced by the action of the bacteria under his methods. It should be under- stood that peat is not in itself a manure or anything like one; it has only been found by Professor Bottomley to be the best medium for a plant-feeding badterial culture.
Mr. Knox, describing the new conception of the soil as a bacterial battlefield, says :—
"It was a revelation to the world when Darwin, lifting a corner of the veil, told of the stupendously great part played by the earthworm that had been carrying on' unheeded or resented for countless ages of time. But to-day we are beginning to know that the soil which the gardener turns with the spade is the site of countless vast empires of bacteria. They are empires that rise and fall in the short space of weeks, that have great tasks to per. form, and devote themselves wholeheartedly to carrying them out empires liable to countless vicissitudes. Now they are over- whelmed by the vast immigrations that come to them suddenly as gardener or husbandman pours into their borders countless myriads of individuals with each spadeful of his manure. Periods of drought wreak havoc on their colonies, the lives of whole empires being dependent on the chances of the climate. They are preyed on by monstrous protozoa that may exact a greater toll than even they with their astounding fertility can cope with. Or they may perish through their own activity, their life clogged by the products they have themselves formed. Widely they differ among themselves. Some of them build up, others of them destroy ; even in the same species there aro vast differences. Like communities of men and like single individuals, there are some that are energetic, others that are lazy, others that are tired ; as food fails, or is abundant, they are poorly fed or well nourished. They are healthy or sick. To a bad environment or a good one they respond as readily and as notably as the people in our great cities, and with them, too, their heredity has a dominating influence. Only within recent years have we realized how intimately our prosperity is dependent on the bacterial population of the soil. Without bacterial activity it would be of no avail to the farmer to dung his crops ; it would be useless for him to attempt to enrich his soil by ploughing in green stuff. The plants and trees that have lived and died wresting Carbon from the air would only cumber the land. All vegetation would bo choked, and the earth would bocomo a vast wilderness, unbeautiful and silent, save for the winds and seas and other manifestations of lifeless forces."
That is a romantic image which is intensified by thoughts of the stupendous speed with which bacterial empires are built up. Mr. Knox says :- "Picture an observer suspended above the equator of the earth, unaffected by the swirl due to the earth's turning on her axis, but following closely her movements through space. Assume that at an instant in time he drops a bacterium on the hurrying surface beneath his feet, and that the bacterium, like the seed of the sower.
that fell on fruitful ground, falls into a medium ideal for its growth and finds nothing to chock its power of reproduction. In twenty-four hours, when the same spot of the earth's surface was again beneath his feet, he would find, instead of the single bacterium he had dropped, a bacterial empire one hundred and seventy thousand times as numerous as the present human popu- lation of the world."
A bacterium reproduces itself by division, and this occurs about every half-hour. If the rate of progress were not checked, a single bacterium could become in twenty-four hours the ancestor of 280,000,000,000,000 bacteria.
It was in 1888 that Beyerwick, following up the famous discoveries of Hellriegel and Wilfarth two years earlier as to the nature of the soil, isolated the bacterium which forms the nodules on leguminous plants and called it Bacillus radicicola. Professor Nobbe tried to make this organism a marketable commodity, but as he sold it in gelatine, a substance which contains nitrogen, the bacteria after a time became overfed and lazy and refused to perform their required function of fixing nitrogen from the air. The United States Department of Agriculture went a step further in 1901. It distributed the bacteria on cotton- wool. The result of inoculating the soil with these bacteria was good in many cases. But the causes of many failures had still to be discovered. The failures, as we know now, were due to the fact that the bacteria retained their vitality for only about six weeks on the cotton-wool. What Professor Bottomley has done incidentally is to find or confirm all the reasons of failure with the culture usually called nitro-bacterin —lack of moisture, acidity of soil, excess of nitrates in soil, and so on. And as a result he has made the enormously important constructive discovery that peat is the best medium for the development of the plant-feeding bacteria, Peat is raw humus—the bacteria require humus for their food—and he calls the baoterized peat " hurnogen." Moreover, he recog- nized that the Bacillus radieicola was only one of several organisms which help to change the nitrogen of the air into nitrogenous plant food. He treated peat to produce his humogen with a mixed culture of Bacillus raelicicola and Azotobacter chroococcum,.
There can be no mistake as to the effects of humogen, as it has been used not only by scientific agriculturists who care- fully recorded the results, but by the highly trained staff at Kew Gardens. It does not merely force leaf growth, like some well-known stimulants, but causes a proportional luxuriance of fruit or flower. Other remarkable effects are the rich variegation in leaves and the symmetry of general form. The photographs of plants treated with humogen side by side with plants not so treated are absolute proof of the efficacy of ihe stimulant. We cannot quote even the typical figures which have been recorded by experimenters with humogen, but we must give one example—the cultivation of potatoes with humogen "Compared with no manure, artificial manures and dung, peat has given an increase in potatoes over no manure of 123 per cent., over artificial manures of 75 per cent., and over dung of 41 per cont. These results were obtained in a light sandy loam in 1913, the land not having been previously cultivated for nine years. In 1914 the same ground treated with humogen gave an increase of 50'5 per cent. over land not manured. A part of each of the plots manured in 1913 was left unmanured in 1914. The land which had received artificial manures showed an increase of 27 per cont. over the unmanured land, the peat-treated land showed an increase of 83.3 per cont., while the land that had received dung showed an increase of 37.7 per cent. The treated peat appeared to leave the land as fertile the second year as in the season of application."
There are still occasional disappointments alongside amazing achievements, In one case humogen appeared actually to have retarded growth. But there is every reason to suppose that the ways of the bacteria in soil will be much further elucidated, and that failure will be provided against by ordinary care. As we said at the beginning, the future of soil-fertilization lies with bacteriology. Questions about price will of course be asked, so we must end by quoting what Mr. Knox says on this subject :— " While the process was in the experimental stage the pries, of the peat was provisionally fixed at 158. for 3 bushels. This price was fixed arbitrarily to meet the convenience of those who wished to conduct experiments, but there is reason to believe that when the substance is dealt with commercially a price will bo prac- ticable at considerably less than £10 per ton. For purposes of comparison with ordinary fertilizers it may be assumed, for the moment therefore, that the cost of it is about £10 per ton. In
view of the fact that the available plant food in peat, as compared with that in rotted stable manure, is as between fifty and eighty to one, the superiority of the peat from the standpoint of its food value alone, when considered in relation to its cost, is strikingly apparent."
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