This knowledge of the ripening process is further teaching the farmer how to prevent waste.  In the ordinary decomposition of the compost heap not an inconsiderable portion of the nitrogen is lost in the air by dissipation as ammonia or free nitrogen.  Even his nitrates may be thus lost by bacterial action.  This portion is lost to the farmer completely, and he can only hope to replace it either by purchasing nitrates in the form of commercial fertilizers, or by reclaiming it from the air by the use of the bacterial agencies already noticed.  With the knowledge now at his command he is learning to prevent this waste.  In the decomposition one large factor of loss is the ammonia, which, being a gas, is readily dissipated into the air.  Knowing this common result of bacterial action, the scientist has told the farmer that, by adding certain common chemicals to his decomposing manure heap, chemicals which will readily unite with ammonia, he may retain most of the nitrogen in this heap in the form of ammonia salts, which, once formed, no longer show a tendency to dissipate into the air.  Ordinary gypsum, or superphosphates, or plaster will readily unite with ammonia, and these added to the manure heap largely counteract the tendency of the nitrogen to waste, thus enabling the farmer to put back into his soil most of the nitrogen which was extracted from it by his crops and then used by his stock.  His vegetable crops raise the nitrates into proteids.  His animals feed upon the proteids, and perform his work or furnish him with milk.  Then his bacteria stock take the excreted or refuse nitrogen, and in his manure heap turn it back again into nitrates ready to begin the circle once more.  This might go on almost indefinitely were it not for two facts, the farmer sends nitrogenous material off his farm in the milk or grains or other nitrogenous products, which he sells, and the decomposition processes, as we have seen, dissipate some of the nitrogen into the air as free nitrogen.

To meet this emergency and loss the farmer has another method of enriching the soil, again depending upon bacteria.  This is the so-called green manuring.  Here certain plants which seize nitrogen from the air are cultivated upon the field to be fertilized, and, instead of harvesting a crop, it is ploughed into the soil.  Or perhaps the tops may be harvested, the rest being ploughed into the soil.  The vegetable material thus ploughed in lies over a season and enriches the soil.  Here the bacteria of the soil come into play in several directions.  First, if the crop sowed be a legume, the soil bacteria assist it to seize the nitrogen from the air.  The only plants which are of use in this green manuring are those which can, through the agency of bacteria, obtain nitrogen from the air and store it in their roots.  Second, after the crop is ploughed into the soil various decomposing bacteria seize upon it, pulling the compounds to pieces.  The carbon is largely dissipated into