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forms of bacilli, cocci, sarcinae, clostridia, etc.; in fact, all the forms common in the soil are held to be only stages in the life cycle of a single species. If this should be confirmed by future investigations, the whole basis of the science of bacteriology will be profoundly modified.

Industrial Applications of Microbiology. In the fermentation industries much use has been made of the variations that can be induced in microorganisms by cultural methods. For example, in the alcoholic fermentation by yeast glycerine figures as a by- product to the extent of some two or three per cent of the sugar fermented; by the addition of sodium sulphite to the fermenting complex the process is profoundly altered and the percentage of glycerine is increased to some 33 per cent. Again, dextrose is converted by Citromyces into citric acid, oxalic acid and carbon dioxide; the percentage of citric acid is normally not great, but by high concentration of sugar and low concentration of nitro- genous food it can be raised to 50%. The production of acetone and that of alcohol from maize by biological methods are processes which have been successfully worked during the World War, and encourage one to look forward to considerable developments of microbiology as applied in the factory.

Bacteria of the Soil: Partial Sterilization. Researches at the Rothamsted Experimental Station have proved that soils which have been treated with certain volatile antiseptics or heated to temperatures between 56 and 100 C. show a marked increase in fertility. This results from a parallel increase in the bacterial activity, whereby the rate of the conversion of the organic nitrogenous matter of the soil into nitrogen compounds which are readily available as food for the plant is considerably enhanced. The number of bacteria normally present in soils varies from about 4 to 60 million organisms per gram. Under the above treatment with antiseptics or heat the majority of these are destroyed and the number of active bacteria is reduced to a few hundreds only. By no means all are destroyed, however, since many of the organisms of the soil are of the spore-forming kind and are thus able to withstand the treatment. After the removal of the volatile antiseptics, or after cooling of the soil, the germination of the spores is unhindered and the bacterial population of the soil is quickly reestablished. The treatment renders the soil more suitable as a medium for bacterial growth, so that the number of organisms quickly exceeds by some six or sevenfold the original bacterial content of the soil, or rather that of a control sample of untreated soil kept under the same physical conditions as the treated sample. This remarkable discovery was made in 1909. As a matter of fact it was not an entirely new discovery; reference to the literature showed that the phenom- enon had been observed many years earlier by German scientists, but they had curiously failed to grasp the important significance in its relation to the fertility of the soil. Naturally under such drastic treatment the bacterial flora of the soil does not remain unaltered; many species, in fact practically all those which do not form spores, are entirely annihilated. The very important group of ammonia-producing organisms contains, however, very many of the sporing kind, and the increased fertility of the soil is mainly due to the increased production of ammonia. The nitrifying bacteria on the other hand are destroyed, and on the belief, current at that time, that the nitrogen of ammonia had first to be converted into the form of nitrate before it could be utilized by the plant, it was difficult to explain the increase in fertility. It has been shown, however, that this belief had no real foundation but that, in the absence of nitrates, plants can obtain their necessary nitrogen in the form of ammonia and many other of the simpler nitrogenous compounds.

The enrichment of the soil as a medium for bacteria seems to be the result of the removal of an inhibitory factor which militates against bacterial development. This factor in all probability, al- though the hypothesis is not universally accepted, is the protozoal fauna of the soil. On this view, which is supported by the strongest circumstantial evidence, though at the moment direct proof is lack- ing, the protozoa living mainly upon bacteria keep down the num- bers of the latter within the limits stated above, and the removal or depression of the protozoa by partial sterilization results in a corresponding enhancement of bacterial activity.

Methods have recently been developed at Rothamsted by which the numbers of the different protozoa can be ascertained and the

interesting fact has come to light that encystment of the protozoa takes place with rhythmic periodicity; certain species investigated pass from the trophic to the resting condition simultaneously every forty-eight hours, a phenomenon which has its parallel in the de- velopment of the malarial parasite in the human blood. By counting daily the numbers of protozoa, active and resting, and relating these to the numbers of bacteria in the soil, it has been shown that the bacterial numbers vary inversely with the numbers of the trophic amoebae.

The effect of partial sterilization upon the fertility of the soil is such that it has become a common practice and a paying proposition for the nurserymen in the cucumber- and tomato-forcing industries to sterilize their soils either annually or every second year. The beneficial effect is of rather short duration and in the course of a few years the soil reverts to its former degree of productivity, and in some cases shows, after the initial enhancement, an actual re- duction of fertility. These facts are not easily explained on the cur- rent hypothesis as set out above. Much attention was being fo- cussed upon the subject in 1921, and very interesting results were being obtained by the workers at the Rothamsted station, results which bid fair to revolutionize accepted views, so that the future might well produce a theory more in accordance with the facts.

Nitrogen It has been recognized for some time that the nitrogen- fixing organisms of the soil are physiologically dependent for their energy upon carbohydrates, and that the amount of atmospheric nitrogen they are able to fix bears a close relationship to the amount of carbohydrate material used up. It was demonstrated in 1915 that the amount of fixation of nitrogen was also influenced by the pres- ence of simple soluble nitrogenous compounds in the soil solution; urea, glycocoll, formamide, etc., had a marked effect in depressing the amount of nitrogen assimilated. These results have been fully confirmed, and it is now known that so long as an available supply of soluble nitrogenous matter is present the organisms will make use of this source in preference to that of free nitrogen, for which a greater expenditure of energy on their part is required.

It is only in recent years that the energy relations of soil bacteria have received due consideration; in 1916 it was pointed out that Bacillus mycoides, a typical member of the group of ammonifiers, produces ammonia, not as an essential by-product of its metabolism, but rather in virtue of its power of obtaining energy from the protein molecule. If other sources of energy are available, e.g. carbohydrates, these will be drawn upon in preference to the protein molecule with corresponding diminution of ammonia production ; in fact, in pres- ence of much carbohydrate the proteins will be entirely neglected and the organism will utilize the ammonia present in the soil as its source of nitrogen, thus competing with the growing crop. Probably most of the bacteria and moulds of the soil are capable under suit- able conditions of assimilating ammonia. The process has not been observed in soils poor in organic matter, but in peaty soils it has been demonstrated to the extent of some 30 % of the added ammonia.

Obviously then, in the use of farmyard manure, the proper ratio of carbohydrate to protein material is a matter of considerable im- portance. If the amount of carbohydrate is in large excess, most of the bacterial species will tend to reduce the quantity of nitrates and ammonia already existing in the soil ; at the same time under these circumstances, provided the temperature conditions are satisfactory, the nitrogen-fixing organisms will work energetically. The effect will be a temporary depression of fertility, but eventually the nitrogen fixed will become beneficial to the growing plant.

If the material is particularly rich in protein the organisms will produce considerable quantities of ammonia and the effect will be at once beneficial.

If the air supply is insufficient the organisms will tend to produce denitrification, taking some of their oxygen from the nitrates and liberating nitrogen as gas. It has been shown that dressings of farmyard manure may in exceptional cases do more harm than good.

Symbiotic Nitrogen Fixation. At the commencement of the decade the application to the soil of cultures of Pseudomonas radi- cicola was advocated as a means of improving the crops of legumi- nous plants. As far as the soils of the Old World are concerned hopes of such improvement have been shattered by experience; its soils are already heavily infected with the nodule-producing organism and to inoculate them with any more is merely a case of " bringing coals to Newcastle." In the New World virgin land exists which has never carried leguminous crops; here inoculation with pure cultures of the organism has met with marked success. Although from an economic point of view the study of symbiotic nitrogen fixation has lost much of its interest, in its academic aspects it still retains undiminished fascination. The adaptability of the organism has been further investigated and it has transpired in cross-inoculation experiments that several strains of the organism exist. Based upon trials made by various investigators the nodule organisms are separable into at least nine groups with reference to their power of infection of the various leguminous plants. Thus in one group fall the organisms from all the true clovers, species of Trifolium; in a second those from broad bean, peas, vetches, sweet pea, etc. ; in a third those of species of Phaseolus; while those from soja bean, lupine and locust form each a separate group, no cross inoculations with these having been effected. It is also of great interest to find that on inoculation into animals a reaction occurs, agglutinins being produced which are