“Wood, leaves, herbage, left a long time in contact with air and moisture, undergo a slow combustion; in other words, they rot. The result of this decomposition is a brown substance called humus or vegetable mold. The heart of old hollow willows is converted into humus; it is the same with leaves that fall from the trees and rot on the ground. Humus from the remains of earlier generations of plant-life nourishes the plant-life of to-day, and this in turn will become mold from which future plants will spring. It is in this way that vegetation is maintained in places not cultivated by man. Humus, then, is nature’s manure. Where it is allowed to form freely, vegetation never loses its vigor, using over and over again the same material, which takes alternately the two forms of plant and humus. But hay from the field is stored in the hay-loft, and the annual harvest of wheat is taken to the granary. Thus the land is robbed of the mold that would be formed naturally by the rotting of this hay and wheat; therefore we must give back to it, under some form or other, this mold that has been taken away, since otherwise the soil will become less and less productive until finally it is quite sterile. This restitution is made in the form of animal manure, which is a sort of humus produced by digestive processes instead of by natural decay. “Humus plays a twofold part in the soil. First, it mellows the land, or in other words makes it more easily permeable by air and water. Secondly, by the slow combustion taking place in the humus there is constantly being liberated a small quantity of carbonic acid gas, which is taken up by the adjacent roots. Agriculture can succeed only in so far as the soil contains humus. Wheat requires nearly eight per cent, oats and rye only two per cent. In poor, sandy soils, to increase the amount of vegetable mold, it is customary to plow certain green crops under, as the farmers express it; that is, the surface soil is turned over and the growing crop intended for manuring purposes is buried and left to decay in the ground. That is what is done when the plowman turns under a field of growing grass or a stretch of clover. When it is proposed to improve a piece of land by this process, it is the practice to begin by raising a crop (which will later be turned under) that derives the greater part of its nourishment from the air, since the soil in this instance cannot of itself furnish this nourishment. Among the plants satisfying these conditions are buckwheat, clover, lupine, beans, vetches, lucerne, and sainfoin.