the air as carbonic dioxide, where the next generation of plants can get hold of it.  The minerals and the nitrogen remain in the soil.  The nitrogenous portions go through the same series of decomposition and synthetical changes already described, and thus eventually the nitrogen seized from the air by the combined action of the legumes and the bacteria is converted into nitrates, and will serve for food for the next set of plants grown on the same soil.  Here is thus a practical method of using the nitrogen assimilation powers of bacteria, and reclaiming nitrogen from the air to replace that which has been lost.  Thus it is that the farmer’s nitrogen problem of the fertile soil appears to resolve itself into a proper handling of bacteria.  These organisms have stocked his soil in the first place.  They convert all of his compost heap wastes into simple bodies, some of which are changed into plant foods, while others are at the same time lost.  Lastly, they may be made to reclaim this lost nitrogen, and the fanner, so soon as he has requisite knowledge of these facts, will be able to keep within his control the supply of this important element.  The continued fertility of the soil is thus a gift from the bacteria.

Bacteria as sources of trouble to the farmer.

While the topics already considered comprise the most important factors in agricultural bacteriology, the farmer’s relations to bacteria do not end here.  These organisms come incidentally into his life in many ways.  They are not always his aids as they are in most of the instances thus far cited.  They produce disease in his cattle, as will be noticed in the next chapter.  Bacteria are agents of decomposition, and they are just as likely to decompose material which the farmer wishes to preserve as they are to decompose material which the farmer desires to undergo the process of decay.  They are as ready to attack his fruits and vegetables as to ripen his cream.  The skin of fruits and vegetables is a moderately good protection of the interior from the attack of bacteria; but if the skin be broken in any place, bacteria get in and cause decay, and to prevent it the farmer uses a cold cellar.  The bacteria prevent the farmer from preserving meats for any length of time unless he checks their growth in some way.  They get into the eggs of his fowls and ruin them.  Their troublesome nature in the dairy in preventing the keeping of milk has already been noticed.  If he plants his seeds in very moist, damp weather, the soil bacteria cause too rapid a decomposition of the seeds and they rot in the ground instead of sprouting.  They produce disagreeable odours, and are the cause of most of the peculiar smells, good and bad, around the barn.  They attack the organic matter which gets into his well or brook or pond, decomposing it, filling the water with disagreeable and perhaps poisonous products which render it unfit to drink.  They not only aid in the decay of the