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The Place of Microbiology in Soil Science
The Place of Microbiology in Soil Science Uniuersity
of
A. G. NORMAN Michigan, Ann Arbor, Michigan CONTENTS
I. Introduction . . . . . . . . . . . . . . . . . . . 11. The Study of the Microbial Population of Soils . . . . . . . . 111. The Application of Microbiological Information to the Solution of Problems i n Soil Science . . . . . . . . . . . . . . . . IV. Epilogue . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .
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399 401 405 409 410
I. INTRODUCTION Soil science is an earth science with roots into geology; yet it is also a biological science because it deals with the formation and characteristics of soils developed in part as a result of biological events, and especially because in its applications it is vitally concerned with the growth of vegetation. But essentially and inherently soil science is a microbiological science too, because in the multifarious processes involved in soil genesis, nutrient release, physical structure, organic matter transformation, to name only a few areas, microorganisms are involved directly. If soil science is indeed a microbiological science, then microbiology should be all-pervading in soil science, without which it would be incomplete and indeed largely inexplicable. The current literature of soil science, when critically examined in this respect, is not found to provide support for the view that microbiology is all-pervading. It is infrequently the case that there is any reference to microorganisms or microbial processes in most papers reporting soil researches. There is a small percentage of papers dealing directly with soil microorganisms in these same journals, but the percentage of these seems to be less now than 25 years ago. This is not because this material is being published elsewhere. If the general microbiological literature is scrutinized, there can be found few papers that relate microorganisms to soils or plants. Some will be found describing physiological studies on organisms originally isolated from soil, and in some circles these studies may be called soil microbiology. 399
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It is accordingly necessary to make the distinction that although almost all soil problems have microbiological aspects, not all microbiology, so-called, has direct and obvious relationships with soil science. There is a kind of soil microbiology, which is perhaps really a subfield of general microbiology or microbial physiology, in which the prime object of study is some organism with interesting or unique characteristics, isolated originally from soil. I n the soil it may be a wholly unimportant member of the vast community of soil organisms, or it may be a prima donna, readily picked out from the crowd. What this organism does may seem to bear not at all on the problems of the soil scientist. The researches are usually not even intended to provide an answer as to whether the organism has any role affecting plant growth or soil processes. The information may, however, add to the general store of microbiological knowledge into which the soil microbiologist proper may dip. The point emphasized here is that the study of organisms isolated from soil is not necessarily to be regarded as a subfield of soil science. It is perhaps for this reason that some have maintained that soil microbiology is an independent science-a viewpoint which many find unacceptable, because no area of science is independent, least of all a biological science. I n attempting to re-evaluate the place of microbiology in soil science one might start with the observation, made earlier, that little of the published work in soils reflects the basically microbiological nature of many soil processes, and inquire how such a situation could arise. It could have several causes. There might be among soil scientists in general a lack of appreciation or understanding of the roles played by microorganisms in transformations in the soil which affect fertility. Alternatively, there might be among soil microbiologists a lack of realism and a reluctance to subordinate their primary microbiological interests to the search for solutions to problems of plant nutrition and soil management. The documentation of these charges-lack of appreciation on the one hand, or lack of helpfulness on the other-would be something less than convincing. Both may be contributory up to a point, but is it not possible that the situation arises mainly because of the unique and baffling microbiological system that is involved? The systems with which microbiologists have had real success in control are those in which some clearly determinable end product results from the activity of organisms. There is no parallel to this in soil; no specific organism directly releases plant nutrients, no specific organism determines whether or not a soil is fertile. The plant is an interloper in the soil microbiological world-admittedly an interloper that is not without
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influence-but one that is dependent on the by-products and side reactions of the soil microflora. 11. THE STUDYOF
THE
MICROBIAL POPULATION OF SOILS
There have been changing concepts of the soil population and the procedures that should be followed in its study, It should be remembered that the soil chemists had identified the major plant nutrients before the existence of an active microbial population in the soil was recognized. Next the nitrification process, first in sewage filters and later in soil, was recognized as being microbiological. At this stage, however, bacteriologists were primarily interested in the identification of pathogenic organisms and their recovery from the soil, in connection with public health problems. The device of the enrichment culture, widely employed by Beijerinck and Winogradsky, marked a great advance. By addition to soil of a compound of interest followed by inoculation into a selective medium containing the same compound, organisms were isolated having specialist properties. I n this way many important soil organisms were isolated and described, and the microbiological basis of a number of important chemical transformations was established. Nitrogen fixation, cellulose decomposition, ammonification, ammonia oxidation, nitrate reduction, sulfur oxidation, and so forth were all recognized. The simple concept of cause and effect stemming from the consideration of disease-producing organisms, began to dominate the thinking of those concerned with soil bacteriology. Organisms had to be classified as beneficial or harmful with respect to the growth of plants. With the recognition that many of the activities of organisms in the soil would be reflected in the supply of nitrogen to plants, attempts were made to express the fertility of soils in terms of its ability to carry out particular transformations, such as, for example, ammonification, this being assessed by the rate of liberation of ammonia from a peptone solution inoculated with the particular soil sample. It was soon apparent that this was not particularly helpful because all that was demonstrated was whether or not the soil contained sufficient organisms capable of carrying out the selected transformation so that an enrichment culture resulted. In the next phase the test substance was added to the soil itself, and again the rate of change was ascertained after incubation under optimum conditions. A great deal of effort went into this type of study; yet the correlation with crop yields was so tenuous that such procedures were finally abandoned. This was frustrating to the microbiologist, who was being pressed to explain fertility differences, at least in part, by
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the microbial make-up of the soil, and who was unable to achieve that measure of control t h a t had proved so spectacular and effective in the field of disease and in the fermentation industries. It should be pointed out that the procedures that were tried were based on the assumption that the soil population consisted of a relatively simple collection of bacterial species each with specialist roles, acting more or less independently, and that the summation of their activities would provide an expression of the potentialities of the soil. The next phase and one that lasted about 20 years resulted in a considerable broadening of the understanding of what constitutes the soil population. Fungi, algae, protozoa, nematodes, and actinomycetes were all recognized as being normal soil inhabitants in varying degrees of abundance, and many census-like studies were carried out in which attempts were made to count representatives of each group. All were considered to be capable of participating in some of the transformations of interest, though the specialist bacteria still seemed to occupy the center of the stage. Direct interactions were considered as likely; some protozoa, for example, were pictured as being dependent for food on bacterial cells. Once again, however, there was disillusionment when the attempt was made to correlate the results of such counts with known properties of the soil. That these counts had some meaning was apparent from profile studies in which great differences were obvious between adjacent horizons. Similarly, in some management trials the effects of different rotations showed up clearly in population differences. Yet the proper interpretation of numbers remained in grave doubt; it was not clear whether a high count of a particular group was necessarily more desirable than a modest count. Moreover, grave sampling difficulties are encountered if real statistical significance is sought; the recognition of short-period fluctuations in numbers of bacteria was particularly difficult to overlook. In the next phase many workers adopted the philosophy that the population of the soil is so complex that to endeavor to separate out from it individual organisms to which specialized functions must be ascribed is largely unprofitable. They considered that to ascertain what the population as a whole can accomplish is more important than to attempt to determine what individual does what. Their attitude is summed up in the quotation from Cutler " . . . a population has been developed by evolution which is on the whole so unspecialized that almost any substance that finds its way into the soil, whether naturally, or as a result of agricultural practice, will eventually be incorporated in the general soil economy." Most soil organisms possess a wide range of enzymes or have the
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capability of developing adaptive systems when presented with an unusual, alien, or exotic energy source. The versatility of a population is therefore astonishing, all the more so perhaps because soil in a beneficent environment for inactive forms which maintain viability through long periods of inactivity. The net effect is that a soil can be treated experimentally as if it were a simple organism or a tissue, the capability of which to utilize any compound or group or compounds can be determined. This approach has been widely used in biochemical studies in recent years. A modification involving percolation or perfusion with a solution of the test material, developed by Lees and Quastel, combines this principle with the elective culture principle of Winogradsky. Continued presentation of the test compound results ultimately in the mass development and domination of those organisms which can most efficiently utilize it. Information about possible routes of breakdown may be obtained by supplying suspected intermediates or inhibitors. It should be said that this biochemical approach has not been too acceptable to some microbiologists trained along classical lines of pure cultural bacteriology, and these have been responsible for a dichotomous development. Some of these men have concentrated their attention on the immediate vicinity of plant roots, which they designate as the rhizosphere. They have demonstrated that in this zone and as a direct result of the presence of living roots there develops a population both qualitatively and quantitatively different from that present in the uncropped soil or in the soil at a distance from the roots. They have shown that the types and numbers present are influenced by the species of plant, and that the rhizosphere population is not just a more numerous and more active version of the soil population at large. They have found that the bacteria of the rhizosphere possess more complex nutritive requirements than do many of those in the same soil uncropped, and indeed have attempted to use this in characterizing the rhizosphere flora and in determining whether changes can be effected therein by management practices. The development of the rhizosphere population is, of course, an enrichment phenomenon; the forms that arise and become numerous are stimulated to do so by the nutritional circumstances in the vicinity of the roots. The rhizosphere flora presumably contains no organisms that are not present in the soil at large. However, the question does arise as to which soil population is the one of significance with respect to the nutrition and welfare of the plant. Should there be primary interest in the microflora of the zone in which the roots are present, which in great measure results from the presence of the roots, or should attention be confined to the microflora of the zone unaffected by plant roots? If a
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cropped soil is sampled, does the information obtained relate to a nonexistent population that is just a mixture of the floras of the two zones? If SO, how should any data obtained thereon be interpreted? It is not at all unlikely that the presence of a crop may have a greater influence on the nature of the soil population active in the vicinity of the roots, than differences between the populations of two soils may have upon the growth of a crop. Each of these various phases of study or approaches has ended in a measure of frustration to the investigator because he has found himself unable to disentangle the mass of information which he obtained. The nonmicrobiologist should understand, however, that this is not because of any lack of intelligence o r industry on the part of the investigators but because of the nature of the subject. The soil flora, as a population, forms an extremely complex system, which is further complicated by the physical characteristics of the soil environment. Even if it were possible to distinguish and separate all the individuals, it would still not be possible to put this information together in such a way that the composite activity of the whole community could be seen or understood, any more than the economic life of a city could be developed by knowledge of the number of Jones, Smiths, dogs, and trees to be found therein, or a knowledge of what each might do if placed on an island in isolation. That which is possible for simpler microbiological systems is just not feasible for such a highly complex system. There are other examples in microbiology which illustrate this fact. The bacteriologist has been quite successful in working out and controlling the active population in microbiological systems such as those involved in cheese-making, fiber-retting, or the production of sauerkraut, to name only three in which relatively simple mixed populations may be involved. He has been far less successful with more complex systems such as the intestinal flora of man or of chickens, the flora of the filter beds in sewage purification plants, or the rumen flora of the cow. If it is true therefore that the classical analytical approach to the nature of the soil population is unproductive in the kind of information than can be used to solve problems of all sorts that are presented to the soil scientist, is it correct to assert so boldly that soil microbiology has a place in soil science? The evidence is not sufficiently overwhelming to justify rejection of this assumption. The fact that the microbiologists has not been able to make use of many of his data and observations in the solution of soil problems means merely that he started with an incorrect concept of the type of information that would be useful. There are kinds of soil chemistry and soil physics that have not proved re-
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warding in terms of information enriching soil science, but the value of all soil chemistry or soil physics in application to soil science is not therefore discounted. There is nothing at all original in these comments about the inadequacies of the methods available for studying the soil microflora. They were made most emphatically by Winogradsky more than 20 years ago. Winogradsky was perhaps the greatest of all soil microbiologists, but few of his contributions really dealt with the problems of soil science. He developed the so-called spontaneous culture method for encouraging the increase of any organism bringing about a process of interest in soil, but made relatively little use of this in his later work. His criticisms were valid, but for all his genius he did not really chart a way out of the wilderness. Is there a way out, or are the difficulties insurmountable? In view of the ingenuity and elegance of many of the newer experimental procedures in biology, there is no need to take a deeply pessimistic view, though one must be a realist and recognize the shortcomings of much that has been attempted in the past. The way out probably lies in a combination of the biochemical treatment of the population as a whole, with pure cultural physiological studies of organisms that may be involved in whatever transformation is of interest at the moment. The potentialities and capabilities of the organism are determined; its nutritional requirements, the intermediate steps, and end products are ascertained. Then with this information available the same reaction is studied in the soil, possible intermediates are added, specific or nonspecific inhibitors, if available, are used, and a decision can then be taken as to whether this particular process in the soil proceeds in a manner that is consonant with the view that the pure cultural and the mixed cultural mechanisms are identical. This needs checking, not only when there is present a vast excess of the compound under study, as is the case in enrichment cultures or percolation procedures, but also at levels that more closely reflect the normal condition in soils. The soil microflora is ordinarily on a low-calorie diet. Normal renewal of the food supply through cropping or incorporation of residues of the vegetation, occurs only sporadically. Many of our soil microbiological experiments have been run at unreasonably high energy levels. 111. THE APPLICATION OF MICROBIOLOGICAL INFORMATION TO THE SOLUTION OF PROBLEMS IN SOILSCENCE In the previous section the thesis was developed that misconceptions as to the nature of the soil population and oversimplification in nutritional and environmental studies account in great measure for the
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doubts that have been expressed as to the place of soil microbiology in soil science. Does this mean that microbiology is not going to contribute much to the unraveling of some of the important problems of soil science and agronomic practice that are known to involve microbiological activities? Is there no way of using much of the accumulated information about soil microorganisms in solving some of these problems? Some, indeed, seem to think that a great gulf lies between, a gulf that defies bridging. It is the contemplation of this gulf that has caused doubts as to whether microbiology is capable of giving information that will be helpful in relation to everyday soil problems. It would be difficult to sustain the argument that the only proper occupation of a soil microbiologist is to investigate processes or problems that relate directly to agricultural practices. There might be agreement, however, that such problems have all too infrequently been approached as vigorously and as doggedly as have more abstract, but very basic, studies relating to the physiology of certain soil organisms. Consider the enormous efforts that have gone into the investigation of the mechanism of nitrogen fixation by nonsymbiotic organisms, and compare with this the volume of effort that has been brought to bear on one of the most important questions in soil science, whether in fact nonsymbiotic nitrogen-fixing organisms in soil in the field actually contribute significantly to the nitrogen economy of the soil and, if they do, how much, and under what circumstances. If critically examined the textbook dogma on the subject will be found to be most insecurely based. Because of the truly unique nature of the enzymic processes that must be involved, the nitrogen-fixing mechanism has been most appealing to some of the best'microbiologists, who see it as the really key problem of these soil organisms, whereas the soil scientist, to whom the maintenance of organic nitrogen in soils under various kinds of land use is important, sees as the key problem the contribution which may be made by nitrogen-fixing organisms in soil in the field. Currently the nitrifying organisms are receiving a good deal of attention again. The biochemical steps involved and the energy aspects are likely soon to emerge rather clearly. But although entirely relevant it is uncertain yet whether this information will much clarify some of the unsolved problems relating to nitrification in field soils, such as, for example, the situation in acid soils, in poorly drained soils, and in soil zones heavily charged with ammonia such as occur when fertilization with gaseous ammonia is practiced. Once again, there is needed a really vigorous attack at these field nitrification problems by a microbiological team, using to the full the best techniques and information available. These two examples will serve to lead up to a point which has been
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made before, that, although the establishment of the various steps in the nitrogen cycle was one of the great accomplishments of classical microbiology, the quantitative aspects of the nitrogen transformation in soil need careful re-examination and scrutiny. The conventional version may be an oversimplification. There are varying degrees of specialization in those groups of the soil population capable of carrying out the several steps in the cycle. The steps involving nitrate reduction particularly require review. There is probably less substance in the many statements about denitrification than in any other field of microbiology. This topic is reviewed in detail by Allison elsewhere in this volume. Advantage should be taken of new procedures. For example, it was most interesting to see that someone had thought it worth while to check the composition of soil air by infrared absorption, only to find present a significant amount of nitrous oxide. At high moisture levels the amounts increase rapidly but even at low moisture content very slow evolution of this gas takes place. If this is so, the soils of the earth may well be the source of supply of the nitrous oxide in the atmosphere. There does seem, therefore, to be a little evidence of unwillingness or reluctance among microbiologists to tackle some of the more practical yet vital problems of soil science or to push their findings to those logical conclusions that may influence agronomic practice. Another field, currently of substantial importance, where the microbiologist is not yet playing the part that he should, is in connection with the stability, persistence, accumulation, or disappearance in soil of the many organic compounds developed for use as herbicides, fungicides, insecticides, etc. In some cases, persistence or moderate persistence is desired, and the effectiveness of the treatment may depend on it; in other cases slow or even rapid disappearance may be desirable. The fate of such compounds, though ultimately decided by their microbiological availability, may be further complicated by adsorption by clay or organic colloids, or removal downward by leaching. The exotic character of many of these compounds makes the problem all the more intriguing, because there may have to be a lag phase or adaptive phase of considerable length before decomposition a t a significant rate takes place. Some workers in this field have put a good deal of effort into the isolation of the active organisms, which is the classical bacteriological approach, and one which opens up interesting physiological avenues, but may not provide answers to the specific problem that formed the starting point. The really significant questions which relate to the employment of these compounds may be left to others who cannot do more than approach them empirically.
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It would be inappropriate to conclude on this note of mild criticism without making reference also to the other possibility referred to in the introduction, namely, that there has been some lack of appreciation or understanding of the contribution of microbiology to general soil problems. One of the really significant advances made in the understanding of soil processes in the last 25 years is the recognition of the complete interdependence of the carbon and nitrogen cycles, the carbon and phosphorus cycles, the carbon and sufur cycles, and so on. This can be stated in another way, namely, that there cannot be decomposition without concurrent synthesis, or there cannot be dissimilation without concurrent assimilation. The quantitative aspects of the interdependence are reasonably well worked out for nitrogen, but less completely so for phosphorus, sulfur, or other nutrients. However, it is in effects on supplies of available nitrogen and phosphate that organic matter transformations affecting yield are most influential. Most of the shorter range effects of different rotational systems on yield are reasonably predictable from existing information. Considerable time, effort, and money could probably be saved in this type of work if full advantage were taken of microbiological advice before commencing such experiments. The microbiologist should not be scolded or chided for unhelpfulness if his help is not sought. To attempt to give a forthright answer now to the question ‘‘What is likely to be the place of microbiology in soil science?” would clearly be futile. So much yet remains to be done. There is one area, in particular, developments in which might weigh considerably in the answer. This is an area that has as yet been investigated realistically by very few microbiologists. It involves the signficance of interactions between organisms in the soil, and between soil organisms and crop plants. It is impossible to deny that antibiotic substances are produced by many microorganisms that occur in soils. It is possible to demonstrate the presence of some antibiotics in soil. It is reasonable to assume that the production of such compounds may well affect the nature of the active flora, which otherwise would be determined primarily by the nutritional status, which in turn may be greatly affected by the presence of plant roots. The active microbial complex is a team, fitting together in such a way that the metabolic activities of its members together utilize the energy sources efficiently. Production of antibiotics may disrupt or distort the team structure. Some workers, and seemingly particularly those with the greatest experience in the study of antibiotics, seem inclined to discount their significance in soil processes for a variety of reasons, perhaps the chief of which is that antibiotics are not uni-
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versally effective, so that there will always be present some organisms that are unaffected and which may utilize the antibiotic itself as an energy source. Whatever may be the answer to this, it does seem that insufficient attention has been given to possible direct effects of compounds produced by microorganisms on root elongation, root growth, and root function. There is some risk in leaving out of the calculations just the organism in which there is most interest, namely, the plant. Root elongation, and particularly that of seedling roots, is affected, almost always adversely, by extremely low concentration of many microbial products and some antibiotics. Conversely, there are in the literature many references to favorable effects in plant growth produced by organic manures or humus constituents, that cannot be accounted for solely by the inorganic nutrients supplied. These more subtle relationships between plants and microorganisms deserve the very best study that can be brought to bear upon them. They may well account for some of the hard-to-explain differences between soils and between soils differently cropped.
IV. EPILOGUE This review is unsatisfying in that no straightforward answers are given to the wholly reasonable questions “What place does microbiology have in soil science?” and “What is likely to be the place of microbiology in soil science?” The discussion of these questions has consisted in part of explanations and alibis. There is little doubt that the frustration felt by some soil microbiologists as they try to evaluate the progress made through a microbiological approach to some of the tougher problems of soil science is very real, and has infected some of their colleagues in other branches of soil science, who therefore tend to discount or overlook the microbiological nature of many soil processes. Together this may contribute to the impatience with which the subject is regarded in some administrative circles, which in turn has reflected adversely on the support available for such work in this country. No battles were ever won by armies in a defeatist frame of mind. Nor is there any justification for defeatism; it is no discredit that there may have been some false starts; it is no discredit that some of the viewpoints and hypotheses have been found incorrect. Biology as a whole is advancing at a rapid rate. Tough as are some of the problems in soil science, there is n o good reason for assuming that they are far more intractable or refractory than those in other areas. A science that is now immature may be expected t o mature. In the meantime all who are soil scientists or whose work involves soils should never overlook the fact, abundantly established, that the
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soil is a living system with a dynamic population, nutritionally competitive and highly responsive to changes in its food supply, and that there are few problems in soil science in which the role of the soil inhabitants can be ignored. Microbial participation or involvement, directly or indirectly, should be assumed until proved otherwise. The cropping of soils presents very special microbiological situations that relate in part to the effects of the crop and its residues on the microflora, and in part on the activities of the organisms in influencing the physical and nutritional environment for the plant.
REFERENCES The nature of this paper is such that direct citation to original work is not feasible. Instead there is listed below a number of papers which survey the progress of research in soil microbiology, or in major fields thereof, and in a few of which attempts are made to evaluate the contributions which such work may make to soil science. Blair, I. D. 1951. Lincoln College New Zealand Tech. Publ. 5 , 42. Brian, P. W. 1949. Symposia Soc. Ezptl. Biol. 3 , 357-372. Clark, F. E. 1949. Advances in Agron. 1, 241-288. Coppier, O., and Pochon, J. 1951. Ann. Agron. 2 , 425-428. Crowther, E. M. 1953. Trans. Intern. SOC.Soil Sci. Comm. I 1 d? IV 2, 14-21. Garrett, S. D. 1951. New Phytologist 50, 149-166. Gibson, T. 1950. Agr. Progr. 24, 108-111. Jensen, H. L. 1951. Maataloustieteeblinen Aikakauskirja 23, 127-134. Katznelson, H., Lochhead, A. G., and Timonin, M. I. 1948. Botan. Reu. 14, 543-587. Lipman, J. G., and Starkey, R. L. 1935. New Jersey Agr. Ezpt. Sta. Bull. 595. Lochhead, A. G. 1952. Ann. Reu. Microbiol. 6 , 185-206. Norman, A. G. 1946. Soil Sci. Soc. Amer. Proc. 1 1, 9-15. Russell, E. J. 1928. Proc. 1st Intern. Congr. Soil Sci. 1, 36-52. Smith, N. R. 1948. Ann. Reu. Microbiol. 2 , 453-484. Starkey, R. L. 1955. In “Perspectives and Horizons in Microbiology” (S. A. Waksman, ed.), pp. 179-195. Rutgers Univ. Press, New Brunswick, N. J. Waksman, S. A. 1932. Proc. 2nd Intern. Congr. Soil Sci. 3, 1-12. Waksman, S. A. 1936. Ann. Reu. Biochem. 5, 561-584. Waksman, S. A. 1953. “Soil Microbiology,” Wiley, New York. Winogradsky, S. I. 1949. “Microbiologie du sol: Problemes et m6thodes.” Masson, Paris.
of
A. G. NORMAN Michigan, Ann Arbor, Michigan CONTENTS
I. Introduction . . . . . . . . . . . . . . . . . . . 11. The Study of the Microbial Population of Soils . . . . . . . . 111. The Application of Microbiological Information to the Solution of Problems i n Soil Science . . . . . . . . . . . . . . . . IV. Epilogue . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .
Page
399 401 405 409 410
I. INTRODUCTION Soil science is an earth science with roots into geology; yet it is also a biological science because it deals with the formation and characteristics of soils developed in part as a result of biological events, and especially because in its applications it is vitally concerned with the growth of vegetation. But essentially and inherently soil science is a microbiological science too, because in the multifarious processes involved in soil genesis, nutrient release, physical structure, organic matter transformation, to name only a few areas, microorganisms are involved directly. If soil science is indeed a microbiological science, then microbiology should be all-pervading in soil science, without which it would be incomplete and indeed largely inexplicable. The current literature of soil science, when critically examined in this respect, is not found to provide support for the view that microbiology is all-pervading. It is infrequently the case that there is any reference to microorganisms or microbial processes in most papers reporting soil researches. There is a small percentage of papers dealing directly with soil microorganisms in these same journals, but the percentage of these seems to be less now than 25 years ago. This is not because this material is being published elsewhere. If the general microbiological literature is scrutinized, there can be found few papers that relate microorganisms to soils or plants. Some will be found describing physiological studies on organisms originally isolated from soil, and in some circles these studies may be called soil microbiology. 399
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A. G . N O R M A N
It is accordingly necessary to make the distinction that although almost all soil problems have microbiological aspects, not all microbiology, so-called, has direct and obvious relationships with soil science. There is a kind of soil microbiology, which is perhaps really a subfield of general microbiology or microbial physiology, in which the prime object of study is some organism with interesting or unique characteristics, isolated originally from soil. I n the soil it may be a wholly unimportant member of the vast community of soil organisms, or it may be a prima donna, readily picked out from the crowd. What this organism does may seem to bear not at all on the problems of the soil scientist. The researches are usually not even intended to provide an answer as to whether the organism has any role affecting plant growth or soil processes. The information may, however, add to the general store of microbiological knowledge into which the soil microbiologist proper may dip. The point emphasized here is that the study of organisms isolated from soil is not necessarily to be regarded as a subfield of soil science. It is perhaps for this reason that some have maintained that soil microbiology is an independent science-a viewpoint which many find unacceptable, because no area of science is independent, least of all a biological science. I n attempting to re-evaluate the place of microbiology in soil science one might start with the observation, made earlier, that little of the published work in soils reflects the basically microbiological nature of many soil processes, and inquire how such a situation could arise. It could have several causes. There might be among soil scientists in general a lack of appreciation or understanding of the roles played by microorganisms in transformations in the soil which affect fertility. Alternatively, there might be among soil microbiologists a lack of realism and a reluctance to subordinate their primary microbiological interests to the search for solutions to problems of plant nutrition and soil management. The documentation of these charges-lack of appreciation on the one hand, or lack of helpfulness on the other-would be something less than convincing. Both may be contributory up to a point, but is it not possible that the situation arises mainly because of the unique and baffling microbiological system that is involved? The systems with which microbiologists have had real success in control are those in which some clearly determinable end product results from the activity of organisms. There is no parallel to this in soil; no specific organism directly releases plant nutrients, no specific organism determines whether or not a soil is fertile. The plant is an interloper in the soil microbiological world-admittedly an interloper that is not without
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influence-but one that is dependent on the by-products and side reactions of the soil microflora. 11. THE STUDYOF
THE
MICROBIAL POPULATION OF SOILS
There have been changing concepts of the soil population and the procedures that should be followed in its study, It should be remembered that the soil chemists had identified the major plant nutrients before the existence of an active microbial population in the soil was recognized. Next the nitrification process, first in sewage filters and later in soil, was recognized as being microbiological. At this stage, however, bacteriologists were primarily interested in the identification of pathogenic organisms and their recovery from the soil, in connection with public health problems. The device of the enrichment culture, widely employed by Beijerinck and Winogradsky, marked a great advance. By addition to soil of a compound of interest followed by inoculation into a selective medium containing the same compound, organisms were isolated having specialist properties. I n this way many important soil organisms were isolated and described, and the microbiological basis of a number of important chemical transformations was established. Nitrogen fixation, cellulose decomposition, ammonification, ammonia oxidation, nitrate reduction, sulfur oxidation, and so forth were all recognized. The simple concept of cause and effect stemming from the consideration of disease-producing organisms, began to dominate the thinking of those concerned with soil bacteriology. Organisms had to be classified as beneficial or harmful with respect to the growth of plants. With the recognition that many of the activities of organisms in the soil would be reflected in the supply of nitrogen to plants, attempts were made to express the fertility of soils in terms of its ability to carry out particular transformations, such as, for example, ammonification, this being assessed by the rate of liberation of ammonia from a peptone solution inoculated with the particular soil sample. It was soon apparent that this was not particularly helpful because all that was demonstrated was whether or not the soil contained sufficient organisms capable of carrying out the selected transformation so that an enrichment culture resulted. In the next phase the test substance was added to the soil itself, and again the rate of change was ascertained after incubation under optimum conditions. A great deal of effort went into this type of study; yet the correlation with crop yields was so tenuous that such procedures were finally abandoned. This was frustrating to the microbiologist, who was being pressed to explain fertility differences, at least in part, by
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the microbial make-up of the soil, and who was unable to achieve that measure of control t h a t had proved so spectacular and effective in the field of disease and in the fermentation industries. It should be pointed out that the procedures that were tried were based on the assumption that the soil population consisted of a relatively simple collection of bacterial species each with specialist roles, acting more or less independently, and that the summation of their activities would provide an expression of the potentialities of the soil. The next phase and one that lasted about 20 years resulted in a considerable broadening of the understanding of what constitutes the soil population. Fungi, algae, protozoa, nematodes, and actinomycetes were all recognized as being normal soil inhabitants in varying degrees of abundance, and many census-like studies were carried out in which attempts were made to count representatives of each group. All were considered to be capable of participating in some of the transformations of interest, though the specialist bacteria still seemed to occupy the center of the stage. Direct interactions were considered as likely; some protozoa, for example, were pictured as being dependent for food on bacterial cells. Once again, however, there was disillusionment when the attempt was made to correlate the results of such counts with known properties of the soil. That these counts had some meaning was apparent from profile studies in which great differences were obvious between adjacent horizons. Similarly, in some management trials the effects of different rotations showed up clearly in population differences. Yet the proper interpretation of numbers remained in grave doubt; it was not clear whether a high count of a particular group was necessarily more desirable than a modest count. Moreover, grave sampling difficulties are encountered if real statistical significance is sought; the recognition of short-period fluctuations in numbers of bacteria was particularly difficult to overlook. In the next phase many workers adopted the philosophy that the population of the soil is so complex that to endeavor to separate out from it individual organisms to which specialized functions must be ascribed is largely unprofitable. They considered that to ascertain what the population as a whole can accomplish is more important than to attempt to determine what individual does what. Their attitude is summed up in the quotation from Cutler " . . . a population has been developed by evolution which is on the whole so unspecialized that almost any substance that finds its way into the soil, whether naturally, or as a result of agricultural practice, will eventually be incorporated in the general soil economy." Most soil organisms possess a wide range of enzymes or have the
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capability of developing adaptive systems when presented with an unusual, alien, or exotic energy source. The versatility of a population is therefore astonishing, all the more so perhaps because soil in a beneficent environment for inactive forms which maintain viability through long periods of inactivity. The net effect is that a soil can be treated experimentally as if it were a simple organism or a tissue, the capability of which to utilize any compound or group or compounds can be determined. This approach has been widely used in biochemical studies in recent years. A modification involving percolation or perfusion with a solution of the test material, developed by Lees and Quastel, combines this principle with the elective culture principle of Winogradsky. Continued presentation of the test compound results ultimately in the mass development and domination of those organisms which can most efficiently utilize it. Information about possible routes of breakdown may be obtained by supplying suspected intermediates or inhibitors. It should be said that this biochemical approach has not been too acceptable to some microbiologists trained along classical lines of pure cultural bacteriology, and these have been responsible for a dichotomous development. Some of these men have concentrated their attention on the immediate vicinity of plant roots, which they designate as the rhizosphere. They have demonstrated that in this zone and as a direct result of the presence of living roots there develops a population both qualitatively and quantitatively different from that present in the uncropped soil or in the soil at a distance from the roots. They have shown that the types and numbers present are influenced by the species of plant, and that the rhizosphere population is not just a more numerous and more active version of the soil population at large. They have found that the bacteria of the rhizosphere possess more complex nutritive requirements than do many of those in the same soil uncropped, and indeed have attempted to use this in characterizing the rhizosphere flora and in determining whether changes can be effected therein by management practices. The development of the rhizosphere population is, of course, an enrichment phenomenon; the forms that arise and become numerous are stimulated to do so by the nutritional circumstances in the vicinity of the roots. The rhizosphere flora presumably contains no organisms that are not present in the soil at large. However, the question does arise as to which soil population is the one of significance with respect to the nutrition and welfare of the plant. Should there be primary interest in the microflora of the zone in which the roots are present, which in great measure results from the presence of the roots, or should attention be confined to the microflora of the zone unaffected by plant roots? If a
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cropped soil is sampled, does the information obtained relate to a nonexistent population that is just a mixture of the floras of the two zones? If SO, how should any data obtained thereon be interpreted? It is not at all unlikely that the presence of a crop may have a greater influence on the nature of the soil population active in the vicinity of the roots, than differences between the populations of two soils may have upon the growth of a crop. Each of these various phases of study or approaches has ended in a measure of frustration to the investigator because he has found himself unable to disentangle the mass of information which he obtained. The nonmicrobiologist should understand, however, that this is not because of any lack of intelligence o r industry on the part of the investigators but because of the nature of the subject. The soil flora, as a population, forms an extremely complex system, which is further complicated by the physical characteristics of the soil environment. Even if it were possible to distinguish and separate all the individuals, it would still not be possible to put this information together in such a way that the composite activity of the whole community could be seen or understood, any more than the economic life of a city could be developed by knowledge of the number of Jones, Smiths, dogs, and trees to be found therein, or a knowledge of what each might do if placed on an island in isolation. That which is possible for simpler microbiological systems is just not feasible for such a highly complex system. There are other examples in microbiology which illustrate this fact. The bacteriologist has been quite successful in working out and controlling the active population in microbiological systems such as those involved in cheese-making, fiber-retting, or the production of sauerkraut, to name only three in which relatively simple mixed populations may be involved. He has been far less successful with more complex systems such as the intestinal flora of man or of chickens, the flora of the filter beds in sewage purification plants, or the rumen flora of the cow. If it is true therefore that the classical analytical approach to the nature of the soil population is unproductive in the kind of information than can be used to solve problems of all sorts that are presented to the soil scientist, is it correct to assert so boldly that soil microbiology has a place in soil science? The evidence is not sufficiently overwhelming to justify rejection of this assumption. The fact that the microbiologists has not been able to make use of many of his data and observations in the solution of soil problems means merely that he started with an incorrect concept of the type of information that would be useful. There are kinds of soil chemistry and soil physics that have not proved re-
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warding in terms of information enriching soil science, but the value of all soil chemistry or soil physics in application to soil science is not therefore discounted. There is nothing at all original in these comments about the inadequacies of the methods available for studying the soil microflora. They were made most emphatically by Winogradsky more than 20 years ago. Winogradsky was perhaps the greatest of all soil microbiologists, but few of his contributions really dealt with the problems of soil science. He developed the so-called spontaneous culture method for encouraging the increase of any organism bringing about a process of interest in soil, but made relatively little use of this in his later work. His criticisms were valid, but for all his genius he did not really chart a way out of the wilderness. Is there a way out, or are the difficulties insurmountable? In view of the ingenuity and elegance of many of the newer experimental procedures in biology, there is no need to take a deeply pessimistic view, though one must be a realist and recognize the shortcomings of much that has been attempted in the past. The way out probably lies in a combination of the biochemical treatment of the population as a whole, with pure cultural physiological studies of organisms that may be involved in whatever transformation is of interest at the moment. The potentialities and capabilities of the organism are determined; its nutritional requirements, the intermediate steps, and end products are ascertained. Then with this information available the same reaction is studied in the soil, possible intermediates are added, specific or nonspecific inhibitors, if available, are used, and a decision can then be taken as to whether this particular process in the soil proceeds in a manner that is consonant with the view that the pure cultural and the mixed cultural mechanisms are identical. This needs checking, not only when there is present a vast excess of the compound under study, as is the case in enrichment cultures or percolation procedures, but also at levels that more closely reflect the normal condition in soils. The soil microflora is ordinarily on a low-calorie diet. Normal renewal of the food supply through cropping or incorporation of residues of the vegetation, occurs only sporadically. Many of our soil microbiological experiments have been run at unreasonably high energy levels. 111. THE APPLICATION OF MICROBIOLOGICAL INFORMATION TO THE SOLUTION OF PROBLEMS IN SOILSCENCE In the previous section the thesis was developed that misconceptions as to the nature of the soil population and oversimplification in nutritional and environmental studies account in great measure for the
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doubts that have been expressed as to the place of soil microbiology in soil science. Does this mean that microbiology is not going to contribute much to the unraveling of some of the important problems of soil science and agronomic practice that are known to involve microbiological activities? Is there no way of using much of the accumulated information about soil microorganisms in solving some of these problems? Some, indeed, seem to think that a great gulf lies between, a gulf that defies bridging. It is the contemplation of this gulf that has caused doubts as to whether microbiology is capable of giving information that will be helpful in relation to everyday soil problems. It would be difficult to sustain the argument that the only proper occupation of a soil microbiologist is to investigate processes or problems that relate directly to agricultural practices. There might be agreement, however, that such problems have all too infrequently been approached as vigorously and as doggedly as have more abstract, but very basic, studies relating to the physiology of certain soil organisms. Consider the enormous efforts that have gone into the investigation of the mechanism of nitrogen fixation by nonsymbiotic organisms, and compare with this the volume of effort that has been brought to bear on one of the most important questions in soil science, whether in fact nonsymbiotic nitrogen-fixing organisms in soil in the field actually contribute significantly to the nitrogen economy of the soil and, if they do, how much, and under what circumstances. If critically examined the textbook dogma on the subject will be found to be most insecurely based. Because of the truly unique nature of the enzymic processes that must be involved, the nitrogen-fixing mechanism has been most appealing to some of the best'microbiologists, who see it as the really key problem of these soil organisms, whereas the soil scientist, to whom the maintenance of organic nitrogen in soils under various kinds of land use is important, sees as the key problem the contribution which may be made by nitrogen-fixing organisms in soil in the field. Currently the nitrifying organisms are receiving a good deal of attention again. The biochemical steps involved and the energy aspects are likely soon to emerge rather clearly. But although entirely relevant it is uncertain yet whether this information will much clarify some of the unsolved problems relating to nitrification in field soils, such as, for example, the situation in acid soils, in poorly drained soils, and in soil zones heavily charged with ammonia such as occur when fertilization with gaseous ammonia is practiced. Once again, there is needed a really vigorous attack at these field nitrification problems by a microbiological team, using to the full the best techniques and information available. These two examples will serve to lead up to a point which has been
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made before, that, although the establishment of the various steps in the nitrogen cycle was one of the great accomplishments of classical microbiology, the quantitative aspects of the nitrogen transformation in soil need careful re-examination and scrutiny. The conventional version may be an oversimplification. There are varying degrees of specialization in those groups of the soil population capable of carrying out the several steps in the cycle. The steps involving nitrate reduction particularly require review. There is probably less substance in the many statements about denitrification than in any other field of microbiology. This topic is reviewed in detail by Allison elsewhere in this volume. Advantage should be taken of new procedures. For example, it was most interesting to see that someone had thought it worth while to check the composition of soil air by infrared absorption, only to find present a significant amount of nitrous oxide. At high moisture levels the amounts increase rapidly but even at low moisture content very slow evolution of this gas takes place. If this is so, the soils of the earth may well be the source of supply of the nitrous oxide in the atmosphere. There does seem, therefore, to be a little evidence of unwillingness or reluctance among microbiologists to tackle some of the more practical yet vital problems of soil science or to push their findings to those logical conclusions that may influence agronomic practice. Another field, currently of substantial importance, where the microbiologist is not yet playing the part that he should, is in connection with the stability, persistence, accumulation, or disappearance in soil of the many organic compounds developed for use as herbicides, fungicides, insecticides, etc. In some cases, persistence or moderate persistence is desired, and the effectiveness of the treatment may depend on it; in other cases slow or even rapid disappearance may be desirable. The fate of such compounds, though ultimately decided by their microbiological availability, may be further complicated by adsorption by clay or organic colloids, or removal downward by leaching. The exotic character of many of these compounds makes the problem all the more intriguing, because there may have to be a lag phase or adaptive phase of considerable length before decomposition a t a significant rate takes place. Some workers in this field have put a good deal of effort into the isolation of the active organisms, which is the classical bacteriological approach, and one which opens up interesting physiological avenues, but may not provide answers to the specific problem that formed the starting point. The really significant questions which relate to the employment of these compounds may be left to others who cannot do more than approach them empirically.
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It would be inappropriate to conclude on this note of mild criticism without making reference also to the other possibility referred to in the introduction, namely, that there has been some lack of appreciation or understanding of the contribution of microbiology to general soil problems. One of the really significant advances made in the understanding of soil processes in the last 25 years is the recognition of the complete interdependence of the carbon and nitrogen cycles, the carbon and phosphorus cycles, the carbon and sufur cycles, and so on. This can be stated in another way, namely, that there cannot be decomposition without concurrent synthesis, or there cannot be dissimilation without concurrent assimilation. The quantitative aspects of the interdependence are reasonably well worked out for nitrogen, but less completely so for phosphorus, sulfur, or other nutrients. However, it is in effects on supplies of available nitrogen and phosphate that organic matter transformations affecting yield are most influential. Most of the shorter range effects of different rotational systems on yield are reasonably predictable from existing information. Considerable time, effort, and money could probably be saved in this type of work if full advantage were taken of microbiological advice before commencing such experiments. The microbiologist should not be scolded or chided for unhelpfulness if his help is not sought. To attempt to give a forthright answer now to the question ‘‘What is likely to be the place of microbiology in soil science?” would clearly be futile. So much yet remains to be done. There is one area, in particular, developments in which might weigh considerably in the answer. This is an area that has as yet been investigated realistically by very few microbiologists. It involves the signficance of interactions between organisms in the soil, and between soil organisms and crop plants. It is impossible to deny that antibiotic substances are produced by many microorganisms that occur in soils. It is possible to demonstrate the presence of some antibiotics in soil. It is reasonable to assume that the production of such compounds may well affect the nature of the active flora, which otherwise would be determined primarily by the nutritional status, which in turn may be greatly affected by the presence of plant roots. The active microbial complex is a team, fitting together in such a way that the metabolic activities of its members together utilize the energy sources efficiently. Production of antibiotics may disrupt or distort the team structure. Some workers, and seemingly particularly those with the greatest experience in the study of antibiotics, seem inclined to discount their significance in soil processes for a variety of reasons, perhaps the chief of which is that antibiotics are not uni-
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versally effective, so that there will always be present some organisms that are unaffected and which may utilize the antibiotic itself as an energy source. Whatever may be the answer to this, it does seem that insufficient attention has been given to possible direct effects of compounds produced by microorganisms on root elongation, root growth, and root function. There is some risk in leaving out of the calculations just the organism in which there is most interest, namely, the plant. Root elongation, and particularly that of seedling roots, is affected, almost always adversely, by extremely low concentration of many microbial products and some antibiotics. Conversely, there are in the literature many references to favorable effects in plant growth produced by organic manures or humus constituents, that cannot be accounted for solely by the inorganic nutrients supplied. These more subtle relationships between plants and microorganisms deserve the very best study that can be brought to bear upon them. They may well account for some of the hard-to-explain differences between soils and between soils differently cropped.
IV. EPILOGUE This review is unsatisfying in that no straightforward answers are given to the wholly reasonable questions “What place does microbiology have in soil science?” and “What is likely to be the place of microbiology in soil science?” The discussion of these questions has consisted in part of explanations and alibis. There is little doubt that the frustration felt by some soil microbiologists as they try to evaluate the progress made through a microbiological approach to some of the tougher problems of soil science is very real, and has infected some of their colleagues in other branches of soil science, who therefore tend to discount or overlook the microbiological nature of many soil processes. Together this may contribute to the impatience with which the subject is regarded in some administrative circles, which in turn has reflected adversely on the support available for such work in this country. No battles were ever won by armies in a defeatist frame of mind. Nor is there any justification for defeatism; it is no discredit that there may have been some false starts; it is no discredit that some of the viewpoints and hypotheses have been found incorrect. Biology as a whole is advancing at a rapid rate. Tough as are some of the problems in soil science, there is n o good reason for assuming that they are far more intractable or refractory than those in other areas. A science that is now immature may be expected t o mature. In the meantime all who are soil scientists or whose work involves soils should never overlook the fact, abundantly established, that the
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soil is a living system with a dynamic population, nutritionally competitive and highly responsive to changes in its food supply, and that there are few problems in soil science in which the role of the soil inhabitants can be ignored. Microbial participation or involvement, directly or indirectly, should be assumed until proved otherwise. The cropping of soils presents very special microbiological situations that relate in part to the effects of the crop and its residues on the microflora, and in part on the activities of the organisms in influencing the physical and nutritional environment for the plant.
REFERENCES The nature of this paper is such that direct citation to original work is not feasible. Instead there is listed below a number of papers which survey the progress of research in soil microbiology, or in major fields thereof, and in a few of which attempts are made to evaluate the contributions which such work may make to soil science. Blair, I. D. 1951. Lincoln College New Zealand Tech. Publ. 5 , 42. Brian, P. W. 1949. Symposia Soc. Ezptl. Biol. 3 , 357-372. Clark, F. E. 1949. Advances in Agron. 1, 241-288. Coppier, O., and Pochon, J. 1951. Ann. Agron. 2 , 425-428. Crowther, E. M. 1953. Trans. Intern. SOC.Soil Sci. Comm. I 1 d? IV 2, 14-21. Garrett, S. D. 1951. New Phytologist 50, 149-166. Gibson, T. 1950. Agr. Progr. 24, 108-111. Jensen, H. L. 1951. Maataloustieteeblinen Aikakauskirja 23, 127-134. Katznelson, H., Lochhead, A. G., and Timonin, M. I. 1948. Botan. Reu. 14, 543-587. Lipman, J. G., and Starkey, R. L. 1935. New Jersey Agr. Ezpt. Sta. Bull. 595. Lochhead, A. G. 1952. Ann. Reu. Microbiol. 6 , 185-206. Norman, A. G. 1946. Soil Sci. Soc. Amer. Proc. 1 1, 9-15. Russell, E. J. 1928. Proc. 1st Intern. Congr. Soil Sci. 1, 36-52. Smith, N. R. 1948. Ann. Reu. Microbiol. 2 , 453-484. Starkey, R. L. 1955. In “Perspectives and Horizons in Microbiology” (S. A. Waksman, ed.), pp. 179-195. Rutgers Univ. Press, New Brunswick, N. J. Waksman, S. A. 1932. Proc. 2nd Intern. Congr. Soil Sci. 3, 1-12. Waksman, S. A. 1936. Ann. Reu. Biochem. 5, 561-584. Waksman, S. A. 1953. “Soil Microbiology,” Wiley, New York. Winogradsky, S. I. 1949. “Microbiologie du sol: Problemes et m6thodes.” Masson, Paris.