— 77 — The first important point brought out by the above figures is the efficacy of drainage in preventing the waterlogging of the soil by heavy irrigation.

The decrease in the water content in March was due to the fact that irrigation ceased in February.

The second point is that the amount of soluble salts has not appreciably increased during the experiment. A marked accumulation of gypsum has occurred, but this is of minor importance from an agricultural point of view.

Lastly, if the figures given for the salt content of the same samples in 1912 are compared with the above, it will be seen that irrigation with drainage has actually lessened the amount of soluble salts present in the soil.

SECTION III.

Soil Bacteriology and the Nitrogen Problem.

Green Manures and their Effect on the Soil.—The frequent introduction of organic matter into the arid soils of the Sudan is a vital necessity, for the amounts of humus and nitrogenous matter existing naturally are extremely small. Analyses of twelve samples of soils taken from the farm in March, 1912, showed the percentage composition to be as follows : Depth. Nitrogen.