— lo-

in nature, but is apparently always very rich in nutriment, as the salts that

arise by weathering of the soil are only very incompletely washed out. Yet

dryness of the soil checks absorption of nutriment by the roots, and this

necessarily reacts upon the dimensions and numbers of the plants present. The abundance of nutriment may be such that the soil is supersaturated with

salt, and the amount present increases with increasing aridity of the climate. The various kinds of soil behave differently in this connection. Salts are

readily washed out of sandy soil, so that, ceteris paribus, sand is poorer in salts

than is clay, which is relatively impervious to water. Finally the soil is utterly

devoid of humus, for sun and drought burn it up completely.

The Desert Climate.—The air is very dry, and the variations in its

temperature are very great. Nowhere else has the atmosphere been observed

in day time to be drier than in North Africa, where the relative humidity may

be 10 to 25 per cent. During the summer the temperature of the air may exceed

50deg. C, and at daytime the soil is considerably hotter than the atmosphere.*

In the Sudan evaporation from water-surfaces and irrigated soils reaches extra-

ordinary figures, the absolute maximum reading for twenty-four hours, during

the three years under review, in the desert screen adjoining the farm amounting

to 41*2 mm. In this connection specially constructed evaporimeters have been

found necessary, those in use in Egypt having proved of inadequate capacity

for the climate of the Northern Sudan.

Where practically the whole of the water required by the crops for their

growth and development has to be raised from the Nile by pumps or water-

wheels, the question of unnecessary losses and wasteful use of this supply has an

enhanced importance. Evaporation from water surfaces can hardly be

prevented by human foresight or energy; seepage losses from canals and the

evaporation of water from the soil on the other hand are, in a large measure,

under the control of human agencies. Such operations as the frequent surface

cultivation of the land, closer planting of crops to ensure early shading of the

surface, the estabhshment of hedges and brakes to limit the desiccating effects

of high winds and blown sands, the introduction of humus in the form of green manures to act as soil sponges, coupled with efficient construction, grading and hning of canals, merit the most careful examination as means of minimising

preventable waste and increasing the duty of our costly water supply.

Irrigation and Drainage.—The absolute necessity for the natural or artificial

drainage of an arid soil, and the constant or periodic application of water in

excess of the actual requirements of the crop, is not immediately obvious even

to many experienced cultivators, and it is a noteworthy fact that the excessive

* Warming : " (Ecology of Plants."