The nitrate produced in this way is the main source of nitrogen supply to plants under normal conditions. Experiments have shown that a number of plants are capable of utilising ammonia as a source of nitrogen, and Hesselmann[34] has found forest soils in Sweden where no nitrification was proceeding, and where, therefore, plants would presumably obtain their nitrogen in this way, but such cases must be regarded as exceptional. Another group of bacteria capable of deriving their energy from an inorganic source exists in the soil. This comprises the sulphur bacteria, which are able to derive energy by the oxidation of sulphur, sulphides, or thiosulphates to sulphuric acid:-- S + 3O + H₂O = H₂SO₄ + 141 Cals. One organism studied by Waksman and Joffe[63] is able to live in inorganic solution, deriving its carbon from carbon dioxide. The sulphur bacteria have recently come into prominence in America owing to their faculty for producing acid. Thus Thiospirillum will increase the acidity of its medium to a reaction of P_{H} 1·0 before growth ceases. The potato scab disease in America is now treated by composting with sulphur. This treatment depends on the production of sulphuric acid by the sulphur oxidising bacteria, which renders the soil too acid for the parasite. There is some evidence also that acid thus produced can be used to render insoluble phosphatic manures more available in the soil. Analogous to the sulphur organisms are certain bacteria isolated from sheep dig tanks in South Africa by Green,[28_b_] which can derive energy by the oxidation of sodium arsenite to arsenate. (4) _Anaerobic Respiration._ As is seen in the examples mentioned, energy is commonly obtained by bacteria through an oxidation process in which free oxygen is utilised. In water-logged soil, however, or in soil overloaded with organic matter, anaerobic bacteria may develop, which obtain their oxygen from oxidised compounds. Thus there are soil organisms described by Beijerinck[2] and others which can obtain oxygen by reducing sulphates to sulphides. A more important source of oxygen under these conditions is nitrate, which can supply oxygen to a larger number of bacteria. The stage to which the reduction can be carried varies according to the organism. A very large number of bacteria are capable of reducing nitrates to nitrites. Many can reduce nitrate to ammonia, and some can produce an evolution of nitrogen gas from nitrate. The effects of nitrate reduction, therefore, appear under water-logged conditions in soils. For example, in swamp soils in which rice is grown, it has been found by Nagaoka,[53] in Japan, that treatment with nitrate of soda depresses the yield, probably owing to the formation of poisonous nitrites by reduction.