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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. Nitrogen-

recuperation 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 dis-

tinguished 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