In the making of soils, a process that is constantly going on, the most important agents are water, air, frost, sunshine, and the action of living organisms. By this combined action, the mountain, with its rich store of mineral matter, is disintegrated, its constituents are partly dissolved in the water and partly carried mechanically to the plains below; the air is distributed through the soil; seeds are dropped; the living animal forms begin to multiply; the soil is enriched, and gradually it begins to be in a condition suitable to bear the simpler forms of vegetable life, which in turn decaying, add to the richness of the soil. Furthermore, the mechanical condition of the soil has much to do with the successful growth of the plant. If the soil is extremely fine, it is liable to become so compact that the rootlets cannot easily penetrate it, when it is of such a composition as to bake readily in the sun; if very coarse, like gravel, there is not a sufficient capacity to retain moisture. It should, however, be porous enough to allow the air to penetrate it, for upon the aeration of the soil depends much of its fertility. We loosen the soil about the roots of plants to allow the air to penetrate and give an opportunity for the chemical changes constantly undergoing in the soil. Then, too, the work of the earthworms in loosening the soil, and thus adding to its porosity, should not be overlooked. In this soil workshop, too, live and labor certain minute organisms that make it their business to enrich the soil by helping the rootlets to assimilate the nitrogen of the air. Since the soil is composed mostly of ingredients that come from the decomposition of rocks, it follows that is must be of very complex composition. Fortunately, however, there are only a few of the ingredients of the soil that are of interest to the agriculturist, as only a few of the elements, as they are called, go to make up the plant structure, or at least only a few are essential ingredients of the plant. Nitrogen, though very abundant in the air, is not abundant in the soil. In fact, the soil has to depend largely on the nitrogen compounds that are washed out of the atmosphere in small quantities by the rain. Another source of nitrogen is the action of certain bacteria, that make little sacs on the rootlets and, living on the juices of the plants, fix the nitrogen of the air, and thus fertilize the soil; especially on plants of the leguminous family, as peas, beans, and clover.