Subsequent researches by Pasteur and others have conclusively demonstrated that the micro-organic life instrumental in effecting the putrefaction or decay of organic matter of any kind, may be divided into two great classes:—
1. Those which require a plentiful supply of oxygen for their development, and which, when bereft of oxygen, die—known as aerobies.
2. Those which, on the contrary, develop in the complete absence of oxygen, and which, when exposed to oxygen, die—known as anaerobies.
In the fermentation of the manure-heap, therefore, we must conceive of the two classes of organisms as the active agents. In the interior portion of the manure-heap, where the supply of oxygen is necessarily limited, the fermentation going on there is effected by means of the anaerobic organism—i.e., the organism which does not require oxygen; while on the surface portion, which is exposed to the air, the aerobic (or oxygen-requiring) organism is similarly active. Gradually, as decay progresses, the aerobic organisms increase in number. It is through their instrumentality that the final products of decomposition are largely produced. The functions of the anaerobic organisms may be, on the contrary, regarded as largely preparatory in their nature. By breaking up the complex organic substances in the manure into new and simpler forms, they advance the process of putrefaction through the initial stages; and when this is accomplished, they die and give place to the aerobic, which, as we have just seen, effect the final transformation of the organic matter into such simple substances as water and carbonic acid gas.
The conditions influencing the fermentation of farmyard manure may be summed up as follows:[163]—
1. Temperature.—The higher the temperature the more rapidly will the manure decay.
2. Openness to the Air.—Of course it will be seen that the effect of exposing the manure to the action of the air is to induce the development of the aerobic type of organism, and thus to promote more rapid fermentation. If, on the other hand, the manure be impacted, the slower but more regular fermentation, due to the anaerobic type of organism, will be chiefly promoted. It must be remembered that in the proper rotting of farmyard manure both kinds of fermentation should be fostered. It is, in fact, on the careful regulation of the two classes of fermentation that the successful rotting of the manure depends. It must further be remembered that, even with a certain amount of openness in a manure-heap, anaerobic fermentation may take place. This is due to the fact that the evolution of carbonic acid gas, in such a case, is so great as to exclude the access of the atmospheric oxygen into the pores of the heap.
3. The dampness of the manure-heap is another important influence. This, of course, will act in two ways. First, by lowering the temperature. Where the manure-heap is found to be suffering from "fire-fang," the common method in practice is to lower the temperature by moistening the heap with water. Secondly, it acts as a retarder of fermentation by limiting the supply of atmospheric oxygen, and thus preventing, as we have just seen, aerobic fermentation.
4. The fourth chief influence in regulating fermentation of the manure-heap is its composition, and more especially the amount of nitrogen it contains in a soluble form. The rate at which fermentation takes place in any organic substance may be said chiefly to depend on the percentage of soluble nitrogenous matter it contains: the greater this is in amount, the more quickly does fermentation go on. There are always a number of soluble nitrogenous bodies in farmyard manure. These are chiefly found in the urine, such as urea, uric and hippuric acids, and ammonia salts.
Products of Decomposition of Farmyard Manure.