Reduction of nitrates to nitrites or to ammonia or even to free nitrogen is brought about by a great many different kinds of bacteria. In many instances this phenomenon is due to a lack of free oxygen, which is obtained by the bacteria from these easily reducible salts. In other cases a portion of the nitrogen is removed to be used as food material in the building up of new protein in the bacterial cell. This latter use of the nitrogen of nitrates by bacteria might theoretically result in considerable loss of “available nitrogen” in the soil as has actually been shown in a few experiments. The reduction of nitrates as above mentioned would also diminish this supply, but probably neither of these results has any very great practical effect on soil fertility. The building up of protein from these mineral salts by bacteria in the intestines of herbivorous animals has been suggested by Armsby as a considerable source of nitrogenous food, and this suggestion appears possible. The liberation of nitrogen from nitrates or nitrites, either as free nitrogen or as ammonia, is spoken of as “dentrification,” though this term was formerly applied to such liberations, from compounds of nitrogen generally even from proteins. Certain bacteria may also reduce sulphates and other sulphur compounds to H₂S, a phenomenon frequently observed in sewage and likewise of importance in the soil. It is possible that phosphates may be similarly reduced.[14] Further and more careful study of the reducing actions of bacteria is needed. OXIDATION. As has been stated in discussing the respiration of bacteria (Chapter VIII) most of these organisms gain their energy through the oxidation of carbon in various forms, chiefly organic, so that CO₂ is a product of the activity of nearly all bacteria. Some few oxidize CO to CO₂, others CH₄ and other paraffins to CO₂ for this purpose. One class of bacteria even oxidizes H in small amounts for its energy and uses the carbon dioxide of the air or traces of organic carbon in the air as a source of carbon for “building” purposes. One of the familiar oxidations of organic carbon is that of the acetic acid bacteria in the making of vinegar. These oxidize the alcohol which results from the action of yeast to acetic acid according to the formula CH₃CH₂OH + O₂ = CH₃COOH + H₂O (see Fig. 67).