We are accustomed to think of bacteria as living only on the richest food and being the cause solely of putrefaction. This is not true. There are bacteria that live on the bare rocks and get their sustenance from those rocks and the surrounding air. Under these conditions they actually store up food material. They are really miracle workers. They render the soil fertile and the farmer is largely dependent upon them for the growth of his crops. They reduce mineral substance to the powdered form, assist in storing up organic matter for the soil and thus render it suitable for the growth of higher plants. Under the influence of sunlight the green portions of plants, by absorbing carbon dioxide from the air and water from the soil, produce starch, which is one of the important foods for animals. This is not the work of bacteria, yet some of these organisms are capable of accomplishing similar work in the dark. It has been suggested that they derive their energy directly from the oxidizing processes that they set up. This is one of the problems of the soil bacteriologist. Many micro-organisms act upon nitrogenous matters and destroy them, in that the substances are rendered less available for higher plants. These are denitrifying agents. There is another class of these micro-organisms, however, that takes nitrogenous food, which is only a little or not at all utilized by higher vegetables, and makes it available plant food. These micro-organisms thus contribute to the fertility of the soil. They oxidize ammonium compounds, from whatever sources they may be had, and form nitrites first and then nitrates. The nitrates are used by common plants. The nitrifying bacteria are composed of two groups of active agents: one group serves to make the first change, namely from ammonium compounds to nitrites; and the other, to change the nitrites to nitrates. The conditions most favorable for their activity are just the opposite for most other micro-organisms. In a soil having much organic matter already decomposed, these bacteria may be practically inactive until the decomposition has passed certain stages. During this transition, the soil is barely suited for crops. Nitrification requires the oxygen of the air; denitrification can go on in a compact soil with the least portion of the atmosphere. So really the soil is cultivated largely to favor the growth and propagation of the nitrifying bacteria. The result of experimentation has shown that this may be facilitated in some cases by denitrification first, and in other cases by stirring the soil for the penetration of the air, and again by applying lime to the soil to overcome the acidity. Professor Burrill recently said, “Some day farmers will come to understand that specialists working in the laboratory, and for years gazing through microscopes, are gaining knowledge for them.”