— 11 — development of alkalies in Egypt, India and California is the result of irrigation practices modern in their origin and methods, and instituted by people lacking in the traditions of the ancient irrigators, who have worked these same lands for thousands of years. Alkali, or " salt " as it is locally known, is produced in situ from the decomposition of the soil and subsoil under the action of water and air. Where conditions are persistently maintained which allow the formation of salts without permitting them to be removed by any cause whatever, there must come a time, sooner or later, when the amount produced and accumulated in the soil reaches a degree of concentration which is intolerable to cultivated crops. In the natural conditions of rainy countries there is usually a sufficient amount of leaching to permit of the alkalies being borne away in the country drainage with sufficient completeness to prevent their effects attracting general attention. In arid countries some method must be devised by which periodically, at least, sufficient water is applied to the cultivated lands to bring into solution and carry out of the country the soluble alkali salts which are fatal to cultivated crops.* Among other advantages attaching to an adequate system of drainage is the reduction of the water-table to a minimum depth, and the consequent avoidance of root destruction during the period of high Nile which has been so conclusively demonstrated by recent experiments in Egypt. The elimination of malarial infection which is unfortunately rife on all undrained irrigated areas in the Northern Sudan, is in itself ample justification for the expenditure involved, while numerous agricultural considerations, such as the better aeration of our stiff clay soils, a more equable soil temperature, and the removal of the finest silt which is largely responsible for " pan " formation, would lend additional weight, if necessary, to the essential argument for the removal of salt.

The Nitrogen Problem.—Stress has been laid on the total absence of " humus " in desert soils. We are thus faced with an almost universal excess of the mineral constituents and an abnormal deficiency of organic matter as agents of soil fertiUty. Moisture, the humus-body and the mineral particles of the soil, however, affect fertiHty principally, if not entirely, through the agency of vitally active organisms, which have been provisionally grouped as soil bacteria, though including forms which must be relegated to such other groups as the fungi and protozoa. That the vital activity of bacteria is essential to sustained fertility may be readily proved by sterihsing a soil at a temperature sufficient to destroy organic life, by treating it with an antiseptic such as bichloride of mercury, or by applying an anaesthetic such as chloroform. In all cases the soil will be left virtually unaltered in composition, but sterility early ensues as a result of the destruction of hving soil organisms ; the decomposition * Wilcox : " Irrigation Farming.''