We may now turn to consider the species of bacteria found in the interstices of soil. They may be classified in five main groups. The division is somewhat artificial, but convenient: 1. _The Denitrifying Bacteria._ A group whose function has been elucidated in recent years (largely by the investigations of Professor Warington) are held responsible for the breaking down of nitrates. With these may be associated the _Decomposition or Putrefactive Bacteria_, which break down complex organic products other than nitrates into simpler bodies. [Illustration: A METHOD OF GROWING CULTIVATIONS IN A VACUUM OVER PYROGALLIC SOLUTION] 2. _The Organisms of Nitrification._ To this group belong the two chief types of nitrifying bacteria, viz., those which oxidise ammonia into nitrites, and those which change nitrites into nitrates. 3. _The Nitrogen-fixing Bacteria_, found mainly in the nodules on the rootlets of certain plants. 4. _The Common Saprophytic Bacteria_, whose function is at present but imperfectly known. Many are putrefactive germs. 5. _The Pathogenic Bacteria._ This division includes the three types, tetanus, malignant œdema, and quarter evil. Under this heading we shall also have to consider in some detail the intimate relation between the soil and such important bacterial diseases as tubercle and typhoid. To enable us to appreciate the work which the "economic bacteria" perform, it will be necessary to consider shortly the place they occupy in the economy of nature. This may be perhaps most readily accomplished by studying the accompanying table (p. 145). A SCHEME SHOWING THE PLACE AND FUNCTION OF THE ECONOMIC MICRO-ORGANISMS FOUND IN SOIL _Water_ _Chemical Substances_ _Gases_ \ [Nitrates, etc.] [CO_{2}, H, N, O] \ | / \ | / _| \|/ |_ PLANT LIFE | +------------+----------+---+--------+-------------+----------+ | | | | | | Carbohydrates Fats Proteids Vegetable Mineral Water [albumoses, [bodies Acids Salts sugar, containing starch, Nitrogen] etc.] | | ANIMAL LIFE | +-----------------+------------+---+-----------------------+ | | | | Gases Water Urea, Albuminoids, Nitrogen in many [CO_{2}, etc.] Ammonia compounds, etc. forms locked up in the body