A list of the soil-bacteria which have been isolated and more or less carefully cultivated and examined would comprise about fifty species; but it is certain that, as at present classified and named, many more species are to be discovered in any ordinary soil.
The fungi are apparently even more numerous than the bacteria, and we may rest satisfied for the present with the general statement that the life-actions of the myriads of individuals of these organisms in the soil completely alter the question of soil-water as understood by the last generation of agriculturalists.
But there is another aspect of this question of soil-organisms which has grown in importance of late to such an extent that we are more than ever justified in regarding the biology of soil as far more vital to the interests of the plant than its physical or chemical properties. With many of the fungi in the soil the roots of plants have to compete - just as plant competes with plant - for water, salts, and other food-materials. The toadstools which are so conspicuous in fields and forests spring from mycelia which ramify in the ground, and are busily breaking down the remains of other organisms, and just such fungi are known to store up relatively large quantities of salts of potassium and phosphorus - the very salts which are so valuable to crops and occur so sparingly in most soils, but which the extensively spread fungus mycelia can gradually accumulate. Some of these fungi, moreover, are more active in their antagonism, and actually attack and pierce the roots as destructive parasites, but I pass these by for the present, as they form the subject for further consideration when we come to the diseases of plants.
It is obvious that the competition of fungi with root-hairs for mineral salts, oxygen, etc., may be at times acute, and it is extremely probable that cases of so-called sterility of soil, where a particular soil is found unsuitable for a crop, may sometimes be due to this over-competition.
The researches of recent years, however, and especially those of Frank, Winogradsky, Hellriegel, and Stahl, have brought to light a series of relationships between certain of these soil-organisms and the higher plants which place the matter of soil-biology in quite new lights.
On the one hand it has been discovered that groups of bacteria are the active agents in bringing about the destruction of organic nitrogenous matter with the formation of ammonia, in oxidising this ammonia to nitrous and to nitric acids, which combine with bases in the soil to form the corresponding salts; while, on the other hand, other forms can decompose the nitrates and reduce them to nitrites, or set free ammonia or even nitrogen from them. Moreover, there are certain species which can fix the free nitrogen of the atmosphere, and start the cycle of up-building of this inert element into the complex higher compounds we term organic. It is impossible to over-estimate the importance of these processes of nitrification and denitrification going on in the soil about the root-hairs of the higher plants.
But, in addition to this circulation of nitrogen in the soil, it turns out that the life-actions of bacteria, and not mere chemical decompositions, are largely responsible for the circulation of carbon, of iron, of sulphur and other elements formed from the decomposition - also by bacterial and fungal agency - of animal and vegetable remains in the soil.
Even more startling are the biological relations in the soil between the absorbing roots of the higher plants and some of these bacteria and fungi, for it has now been established beyond all doubt that certain fungi enter the living roots and there flourish not as mere destructive parasites, but as messmates not only tolerated by the plant, but even indispensable to its welfare. It is probable that nearly half the plants of our fields, moors, and forests entertain such fungi in their root-tissues. The curious, and long-known nodules on the roots of leguminous plants - peas, beans, clover, etc. - are filled with bacteria which enable these plants to avail themselves of the free nitrogen of the air, and so enrich the soil with nitrogenous substances.
The roots of most forest trees, orchids, and plants of the moorlands, meadows and marshes are similarly occupied by fungi, which in some way convey salts - probably especially phosphates and potassium compounds - to the plant in return for the small tax of organic carbon-compounds it exacts from the latter. In some cases at any rate, as Bernard has lately shown, the very existence of the plant depends on its seedling roots obtaining this advantageous attachment and co-operation (symbiosis) of the fungus immediately on germination.
These remarks must suffice to illustrate this part of my subject, and to emphasise the statement that the question whether a given plant can be grown in a given soil, is by no means one of simply the physical and chemical constitution of the latter. The plant will have to run the gauntlet of a long series of vicissitudes brought about by the presence or absence, relative proportions and vigour, and specific nature of the organisms in the soil at its roots, and it is easy to see that many cases of disease may be due to the absence of advantageous bacteria or fungi, or to circumstances which disfavour their life, as well as to the predominance of competing organisms.
It will now be evident that the old points of view must be abandoned, and with them, especially, the widely prevalent notion that chemical analyses of the plant and soil can explain the real problems of agriculture.
It was of course an enormous advance in the science when, thanks to the splendid labours of the chemists, at the end of the last century and the beginning of this, we obtained that preliminary knowledge of the constitution of the air, and of the composition of the water, acids and salts, etc., which plants require for their food-materials and life-processes. Much was gained by De Saussure's establishment of the fact of oxygen respiration, though we now understand by the term something very different from, and much more complex than, what he understood by it, as, also, much had been gained by the previously acquired knowledge of the gas-exchanges in carbon-assimilation: nor must we forget the services of those who proved, by laborious analyses, continued for long periods, what chemical compounds are found in the tissues of plants, and in the soils at their roots and the atmosphere which surrounded them. We must also remember many other con tributions which have been furnished, and are still being furnished by the chemist; and I for one hope that his labours will continue to go hand in hand with those of the physiologist.