Rothamsted Experimental Station

“ Nature seems to be in an unexpectedly economical mood in the soil, and all compounds which can be oxidised with liberation of energy seem to have corresponding organisms capable of utilising them. Even phenol, benzene, hydrogen, and marsh gas can all be oxidised and utilised as energy sources by some of the soil population.
Even with this remarkable power the soil population has insufficient energy to satisfy all its possibilities; our present knowledge indicates that energy supply is, in this country at any rate, the factor limiting the numbers of the population. ”

“ The circumstance that certain soil organisms—bacteria, algæ, and fungi—themselves assimilate ammonia and nitrate may account for the remarkable slowness of nitrate accumulation, to which reference has already been made. The protein formed from the assimilated nitrogen remains in the bodies of the organisms, living or dead, till decomposition sets in. It is not difficult to picture a cycle of events in which much of the nitrate formed is at once reabsorbed by other organisms, and only little is actually thrown off into the soil. ”

“ If, on the other hand, it absorbs more ammonia than it produces, as happens when it derives its energy from non-nitrogenous substances, the plant suffers. Thus, addition of peptone to the soil or an increase in bacterial numbers effected without addition of external energy (e.g. by partial sterilisation) leads to increased ammonia supply, and, therefore, to increased fertility. But addition of sugar to the soil causes so great an increase of numbers of bacteria and other organisms that considerable absorption [177] of ammonia and nitrate occurs, and fertility is for a time depressed. ”

“ The great practical importance of leguminous crops in agriculture has led to numerous attempts being made to increase their growth, and the fixation of nitrogen in them, by inoculating the seed or the soil with suitable nodule-bacteria. This inoculation can be effected either with soil in which the host-plant has been successfully grown, and which should consequently contain the organism in fair numbers, or else pure cultures of the organisms isolated from nodules may be used. ”

“ The fixation of nitrogen in soil conditions was beyond the power of chemists to achieve, and here it was universally agreed that bacteria were the active agents. And finally, chemists were themselves bringing into prominence a set of bodies—the colloids—whose remarkable properties seemed indefinitely expansible, and were in addition sufficiently incomprehensible to the ordinary student to attain much of the magnificence of the unknown. ”

“ A soil population of this magnitude can not be without effect on the fertility of the soil. When growing on the surface of the ground exposed to sunlight the algæ must, by photosynthesis, add considerably to the organic matter of the soil, but when they live within the soil itself their nutrition must be wholly saprophytic, and they can be adding nothing either to the energy or to the food-content of the soil. How these organisms fit into the general scheme of life in the soil is at present undetermined, and there is a wide field for research in this direction. ”

“ The life of fungi in the soil may, for our purposes, be considered from three points of view—their part in the decomposition of carbon compounds, their nitrogen relationships, and their work in the mineral transformations of the soil. ”

“ The first requirement of the soil population is a supply of energy, without which it cannot live at all. All our evidence shows that the magnitude of the population is limited by the quantity of energy available. The percentage by weight of the organic matter is about two to four or five, and the percentage by volume runs about four to twelve. Not all of this, however, is of equal value as source of energy. About one-half is fairly easily soluble in alkalis, and may or may not be of special value, but about one-quarter is probably too stable to be of use to soil organisms. ”

“ The various activities of the soil bacteria have a vital importance to the growth of higher plants, which are dependent for their existence on certain of these processes. In the first place, as we have seen, bacteria decompose the tissues of higher plants and produce humus materials, which are essential to the maintenance of good physical properties in the soil. Then the nitrate supply on which most higher plants depend is produced by the decomposition of organic nitrogen compounds by bacteria in their search for energy. ”

“ The soil is inoculated into a culture medium that will especially favour the group of bacteria to be isolated, to the exclusion of others. For example, if it is desired to isolate the organisms that attack cellulose, a medium is made up containing no other organic carbon compounds except cellulose. Such a selective medium encourages the growth of the group of organisms to be investigated, so that after several transfers to fresh medium a culture is obtained containing only two or three different types of organisms. ”

“ The morphology of bacteria is not the constant thing that is seen in the more stable higher organisms. In many cases the appearance of a single strain is entirely different on different media, and may be quite altered by such conditions as changes in acidity of the medium or temperature of incubation. Even on a single medium remarkable changes in morphology occur, at any rate, in some bacteria. This is well seen in a cresol-decomposing organism under investigation at Rothamsted. In cultures a few days old this organism develops as bent and branching rods ”

“ If, however, the mingling agents are absent, the dead vegetation lies like a mat on the surface of the soil, only partially decomposing, unsuitable for the growth of most seedlings, and effectually preventing most of the vegetation below from pushing a way through: thus there comes to be no vegetation at all, or only a very restricted and special flora. The soil population becomes also specialised. Peats and acid grassland afford examples. ”

“ Now, it is one of the problems of present-day farming that soil is becoming depleted of its humus. This is due to the increasing scarcity of farmyard manure in many districts, and the consequent use of mineral fertilisers to supply nitrogen, potash, and phosphate to the crop. A need has therefore arisen for a substitute for farmyard manure, by means of which the humus content of soils may be kept up in districts where natural manure is scarce. ”

“ The toxic conditions which cause trouble in working with pure cultures of organisms in specific cultures media do not, so far as is known, arise in the soil. All attempts to find bacterio-toxins or plant toxins in normal soils have failed. The product toxic to one organism seems to be a useful nutrient to another, and so the mixed population keeps the soil healthy for all its members. ”

“ When a sufficient amount of easily decomposable organic nitrogen is present in the soil, the ammonifying bacteria use such compounds as sources of energy, and in this case have a nitrogen supply exceeding their requirements.
But where there is an excess of carbohydrate or other non-nitrogenous source of energy available in the soil, the case is different. Here the organisms have a supply of energy which enables them to multiply rapidly until the organic nitrogen is insufficient for their needs. ”

“ From a general survey of the researches that have been carried out on soil fungi during the past two decades certain issues emerge. It would seem clear that fungi occupy, perhaps, a primary place as factors in the decomposition of celluloses, and thus may be the chief agents in the transformation of plant remains to humus and to soluble compounds which can be used as food by the nitrogen-fixing bacteria. ”

“ The relationships between the soil population and soil fertility are readily stated in general outline, but they are by no means clear cut when one comes to details; [176] fertility is a complex property, and some of its factors are independent of soil micro-organisms.
The general relationship between plants and soil organisms is one of complete mutual interdependence. The growing plant fixes the sun’s energy and converts it into a form utilisable by the soil organisms; without the plant they could not exist. ”

“ It is to be hoped that future research will indicate what are the conditions that produce the greatest economy of energy material in nitrogen fixation.
The fixation of nitrogen in soil is depressed by the presence of considerable amounts of nitrates. This is, in all probability, due to the fact that nitrogen-fixing organisms are able to utilise compounds of nitrogen where these are available. The energy needed to build up amino-acids and proteins from nitrate or ammonia is, of course, far less than that required to build up these substances from elemental nitrogen. ”

“ We shall confine ourselves to the normal case where earthworms bring the source of energy into the soil.
Directly the energy is available, it begins to be utilised. Two laws govern the change. The first is well-known to biologists: it states that the total energy of the system remains constant and can neither be increased nor diminished except from outside; in other words, that energy can be neither created nor destroyed. The second law is less familiar: it is [166] that energy once transformed to heat by one organism cannot be used again by another. ”

“ We are at present ignorant as to which organisms are most effective in decomposing cellulose in the soil under field conditions, or what are the conditions best suited to their activity. It is possible that fungi also help in the decomposition of cellulose to a great extent. This subject of the decomposition of cellulose offers one of the most promising fields of research in soil bacteriology. The difficulty of the subject is further increased by our present ignorance of the chemical aspect of cellulose decomposition. ”

“ It is difficult to understand how accurate results could be expected when, to an already little understood complex substance, such as soil, is added a series of substances whose effects are practically unknown.
Free nitrogen-fixation in soils is an important process, more especially in soils of a light sandy nature, from which crops are taken year after year without any application of manure. The effect of protozoa on the organisms causing this process has in the past received little attention. ”

“ The more difficult case, however, is that in which the legume crop has been grown for a long time in the locality, and where the soil is already infected with right organisms. This, the more fundamental problem, applies especially to this country. Here it would seem that inoculation with a culture of the organism will benefit the plant only (1) if the naturally occurring organisms are present in very small numbers; or (2) if the organisms in the culture added are more virulent than those already in the soil. ”

“ That fungi take a very important place as ammonifying agents in the soil can no longer be doubted, but the question yet remains to be considered of the balance of profit or loss resulting from their activities. It has usually been considered that a part of the ammonia freed is used by the fungi themselves, but that the greater part is liberated, and so rendered available to nitrifying organisms. ”

“ With protozoa and bacteria the relation of soil individual to plate colony is direct; with fungi we do not know what is the soil “individual” nor whether it is the same for different fungi; nor can we yet profitably discuss any significant numerical relationship of plate colonies to soil organisms. Thus Conn [4] has pointed out that the plate count of a fungus indicates only the ability to produce reproductive bodies and found that the spores of one colony of Aspergillus, if distributed evenly through a kilogram of soil, could produce the average plate counts obtained by Waksman. ”

“ Chlorella vulgaris, an alga frequently found in soil, has been shown to be extremely plastic in its relations to food substances. Given only a dilute mineral-salts solution as food source, it absorbs CO2 from the air, and grows in sunlight with moderate rapidity. The addition of glucose to the medium in the light greatly increases the rate and amount of growth and the size of the cells, while in the dark the colonies not only remain green but have been shown to develop more vigorously than in full daylight. The organism is also able to use peptone as a source of nitrogen in place of nitrates. ”

“ Some of them were almost certainly in the resting condition as spores, and the new generation of workers had an uneasy feeling that the case for the overwhelming importance of bacteria in the economy of the soil was not too well founded. It was shown that the decomposition of nitrogen compounds to form ammonia would take place without micro-organisms if, as presumably would happen, the plant enzymes continued to act after they got into the soil. Even the oxidation of ammonia to nitrate—the great stronghold of the biological school—was accomplished by chemical agents. ”

“ We have dealt in the first chapter with the supplies of water, air, and heat, without which life is clearly impossible. Equally necessary is the source of energy, for the organism requires energy material as surely as the motor engine requires petrol, and it ceases to function unless an adequate supply is forthcoming. ”