acid-producing bacteria after a little begin to grow slowly, and in time the silage is rendered somewhat sour by the production of acetic acid.  But the exclusion of air, the close packing, and the small amount of moisture appear to prevent the growth of the common putrefactive bacteria, and the silage remains good for a long time.  In other methods of filling the silo, the food is very quickly packed and densely crowded together so as to exclude as much air as possible from the beginning.  Under these conditions the lack of moisture and air prevents fermentative action very largely.  Only certain acid-producing organisms grow, and these very slowly.  The essential result in either case is that the common putrefactive bacteria are prevented from growing, probably by lack of sufficient oxygen and moisture, and thus the decay is prevented.  The closely packed food offers just the same unfavourable condition for the growth of common putrefactive bacteria that we have already seen offered by the hard-pressed cheese, and the bacteria growth is in the same way held in check.  Our knowledge of the matter is as yet very slight, but we do know enough to understand that the successful management of a silo is dependent upon the manipulation of bacteria.

The fertility of the soil.

The farmer’s sole duty is to extract food from the soil.  This he does either directly by raising crops, or indirectly by raising animals which feed upon the products of the soil.  In either case the fertility of the soil is the fundamental factor in his success.  This fertility is a gift to him from the bacteria.

Even in the first formation of soil he is in a measure dependent upon bacteria.  Soil, as is well known, is produced in large part by the crumbling of the rocks into powder.  This crumbling we generally call weathering, and regard it as due to the effect of moisture and cold upon the rocks, together with the oxidizing action of the air.  Doubtless this is true, and the weathering action is largely a physical and chemical one.  Nevertheless, in this fundamental process of rock disintegration bacterial action plays a part, though perhaps a small one.  Some species of bacteria, as we have seen, can live upon very simple foods, finding in free nitrogen and carbonates sufficiently highly complex material for their life.  These organisms appear to grow on the bare surface of rocks, assimilating nitrogen from the air, and carbon from some widely diffused carbonates or from the Co2 in the air.  Their secreted products of an acid nature help to soften the rocks, and thus aid in performing the first step in weathering.