garden soil different from sand, or the rich soil different from the sterile soil.  If the soil is cultivated year after year, its food ingredients are slowly but surely exhausted.  Something is taken from the humus each year, and unless this be replaced the soil ceases to be able to support life.  To keep up a constant yield from the soil the farmer understands that he must apply fertilizers more or less constantly.

This application of fertilizers is simply feeding the crops.  Some of these fertilizers the farmer purchases, and knows little or nothing as to their origin.  The most common method of feeding the crops is, however, by the use of ordinary barnyard manure.  The reason why this material contains plant food we can understand, since it is made of the undigested part of food, together with all the urea and other excretions of animals, and contains, therefore, besides various minerals, all of the nitrogenous waste of animal life.  These secretions are not at first fit for plant food.  The farmer has learned by experience that such excretions, before they are of any use on his fields, must undergo a process of slow change, which is sometimes called ripening.  Fresh manure is sometimes used on the fields, but it is only made use of by the plants after the ripening process has occurred.  Fresh animal excretions are of little or no value as a fertilizer.  The farmer, therefore, commonly allows it to remain in heaps for some time, and it undergoes a slow change, which gradually converts it into a condition in which it can be used by plants.  This ripening is readily explained by the facts already considered The fresh animal secretions consist of various highly complex compounds of nitrogen, and the ripening is a process of their decomposition.  The proteids are broken to pieces, and their nitrogen elements reduced to the form of nitrates, leucin, etc, or even to ammonia or free nitrogen.  Further, a second process occurs, the process of oxidation of these nitrogen compounds already noticed, and the ammonia and nitrites resulting from the decomposition are built into nitrates.  In short, in this ripening manure the processes noticed in the first part of this chapter are taking place, by which the complex nitrogenous bodies are first reduced and then oxidized to form plant food.  The ripening of manure is both an analytical and a synthetical process.  By the analysis, proteids and other bodies are broken into very simple compounds, some of them, indeed, being dissipated into the air, but other portions are retained and then oxidized, and these latter become the real fertilizing materials.  Through the agency of bacteria the compost heap thus becomes the great source of plant food to the farmer.  Into this compost heap he throws garbage, straw, vegetable and animal substances in general, or any organic refuse which may be at hand.  The various bacteria seize it all, and cause the decomposition which converts it into plant food again.  The rotting of the compost heap is thus a gigantic cultivation of bacteria.