“All vegetation requires nitrogen. Wheat, for example, must have it to develop the grain in the ear; peas, beans, lentils demand it in order to fill out their pods; the pasture and the hay-field need it if they are to furnish the nutriment that the sheep and the cow will transform into milk. But plants cannot take this nitrogen from the air, where it is so abundant; it must be served up to them after a certain necessary preparation. We ourselves need phosphorus, since it enters into the composition of our bones; we need carbon still more, the principal fuel used in maintaining the heat of the body. But are we to eat the charcoal that the charcoal-burner manufactures in his furnace, and the phosphorus used in the making of matches? Certainly not. The first would be a frightful mouthful, the second an atrocious poison. We must have them prepared in a suitable way, such as they are found in bread, milk, meat, fruits, vegetables. In the same manner plant-life requires nitrogen, not as it occurs in the atmosphere, but as it exists in certain combinations, of which the most notable are the compounds of ammonia. This explains to us the highly beneficial effect of manure on our crops. Manure is composed of the bedding used in stables and the animal excrement with which it has become mixed and impregnated. Now this excrementitious matter, especially urine, yields ammonia in decomposing, as is proved by the odor arising from latrines in hot weather and so powerfully affecting the eyes and nose. Thus manure may be said to hold ammonia compounds in storage, and from them plants derive their nitrogen, as also many other ingredients. “Let us summarize these details. In the nutrition of plants four substances are of prime importance. First, carbonic acid gas, which yields carbon, the most widely diffused of all the elements (but which we need not dwell upon here), since plants take it chiefly from the atmosphere, to which it is supplied unceasingly. After carbonic acid come potash, phosphoric acid, and nitrogen, all of which the roots extract from the soil, where it occurs in some compound or other. These are the ingredients that the soil, if it is to remain fertile, must have given back to it as fast as they are exhausted by the crops. Such is the part played by fertilizers, without which the soil becomes exhausted and ceases to produce.” CHAPTER VIII VEGETATION AND THE ATMOSPHERE