— 12 -of organic matter is suspended, and the mineral food supplies are curtailed with the exhaustion of such quantities as are present in immediately available form.

The fixation of atmospheric nitrogen by bacteria living in nodular stinictures in the roots of leguminous plants is already a matter of common knowledge.

Apart from this symbiotic relationship with bacteria, plants are unable to assimilate nitrogen gas, and there is no natural supply of nitrogen salts from the underlying rocks. Another source of organic nitrogen is, however, found in the direct fixation of nitrogen by bacteria existing independently of other plants in the soil, of which Azotobacter may be taken as a typical form. Nitrogenrecuperation may thus ensue even when no legruninous crop has been introduced into the rotation, but such activity is directly dependent upon the presence of organic matter in the soil which furnishes the bacteria with a food supply in the form of carbonaceous matter. Arid soils lacking in humus consequently display a remarkable preponderance of leguminous forms in their flora, of which the Acacia is an outstanding type, as such are rendered independent of external supplies of organic nitrogen by the activity of nitrogen-fixing nodules on their roots.

The successful cultivation of non-leguminous crops, such as cotton, in desert soils will consequently be determined either by the regular introduction of legumes as rotation staples, the constant application of organic matter to sustain the activity of such free soil forms as Azotobacter, or the use of such artificial fertilisers as nitrate of soda.

Nitrification.—The nitrogen, as fixed by soil or nodule-inhabiting bacteria, as well as that found in the humus of the soil, takes the form of complex organic compounds which are no more available to the higher plants than is the nitrogen gas. The intervention of other micro-organisms is consequently necessary to the process of nitrification or decomposition, which should be carefully distinguished from nitrogen-fixation. To be capable of yielding good crops the land must contain a supply of nitrogen which can be dissolved in soil water. A first stage in decomposition is due to organic ferments, known as peptonisers, similar in action to the pepsins of the digestive juice, which reduce proteid substances to simpler forms. A group of bacteria, known as ammonifiers, then split up the still complex nitrogenous substances and derive therefrom ammonia. A second group change ammonia into nitrous acid, and a third oxidises the latter substance into nitric acid or nitrate, in which form it is readily soluble in water and most available for the use of plants. It should be carefully noted that this beneficial function of the nitrifying bacteria is essentially a process of oxidation or combustion, and is dependent upon the presence of free oxygen. The organic matter rapidly changes in a well-aerated soil, darkening in colour, losing all structure, and being finally resolved into ammonia, carbonic acid gas and