The micro-organisms that are found in great variety in soils must have an important influence on the processes of metabolism that are constantly taking place in the soil itself; and the results of their activities, which are not limited to processes of putrefaction and nitrification, can not be measured solely by the amount of nutritive materials appropriated. In my own experiments with soil-microbes they have proved their ability to take their required supplies of lime and potash from solid fragments of gypsum and feldspar, and even from the glass tubes in which cultures were made, which were deeply etched by their action.
The roots of plants undoubtedly aid in determining conditions of the soil that favor the vital activities of certain microbes, and interfere with the well-being of others of different habits; and the plants, in their turn, are presumably benefited by the activities of the microbes best adapted to the prescribed conditions. In the struggle for existence the dominance of these favored forms can not, however, be indefinitely maintained. The roots of one species of plant and their associated microbes, in appropriating their required supplies of nutritive materials, induce a metabolism of the soil that, sooner or later, renders it better fitted for other species of plants and other microbe associates; and these, in their turn, prepare the way for species of still different requirements in their processes of nutrition.
Soil metabolism, and the involved liberation or elaboration of plant food, will thus be promoted by a succession of plants of different habits of growth, each with its associated microbes; and the elements of fertility stored in, or permeating the soil, must, under such conditions, be more completely utilized.
It is practically misleading and inaccurate to say that leguminous plants appropriate the free nitrogen of the atmosphere. The evidence clearly shows that the soil-microbes which find favorable conditions for the exercise of their vital activities in the vicinity of, or in contact with, the roots of leguminous plants, are able to make use of the free nitrogen that permeates the soil, and that it is thus made available as combined nitrogen in the nutrition of the higher chlorophyl-bearing leguminous plants. The latest investigations are, therefore, strictly in accordance with the earlier experiments by Boussingault, and at Rothamsted, in showing that the soil is the source of the nitrogen of plants, and we must look to soil conditions as essential factors in determining the vital activities of the microbes that bring free nitrogen into the combined form that is available for the nutrition of the higher plants.
It must be admitted that red clover appropriates nitrogen that has been prepared for it from the free nitrogen of the soil through the agency of its symbiont microbes, but it is well known that it