— G8 — The influence of organic matter in improving the texture is again well marked. On the whole, however, the capillary power of these soils is feeble, and explains the rapid desiccation of the surface layers which ensues when irrigation ceases.

Hygroscopic Moisture.—Although subjected to extremely high temperatures and drying winds, the soil always contains a small quantity of water, which can only be driven off by the application of temperatures over 100 deg. C. for a prolonged period of time.

This hygroscopic water, as it is termed, varies in amount according to the size of the component particles, and is naturally highest in clay and fine alluvial soils.

A series of determinations of the moisture content of air-dried soils from the experimental area of the farm gave the following results : Moisture content of first 6 inches of soil another sample rather sandier stifEer clay ...

sandy loam ...

clay /o 3-9 4-2 2-9 5-0 3-5 5-3 Wilting Point.—Hygroscopic moisture is valueless to plant life, and indeed wilting begins when the water content of the soil is far in excess of these figures.

Although no estimations of the amount of water vapour transpired have yet been made, it is obvious that in such arid conditions very great demands are made on the root system of all plants, and since the movement of water in these clay soils is slow, it is not surprising that plants begin to wilt when from all appearances the soil still contains a fair quantity of moisture.

The development of the root system of plants is extraordinary in this climate. The tap root of seedlings of such common weeds as Calotropis procera, Senna obovata and Crotalaria senegalensis, has often been traced down to nearly a metre in depth, whilst the root of full-grown plants often penetrates 3 metres and probably more.

It has been found impossible to grow cotton plants in small pots suitable for laboratory experiments on accoimt of the limited space afforded to root development. In pots containing 500 grams of dry soil, cotton seedlings never grew more than 15 cm. high.

Such seedlings, -however, grown in sealed wire baskets, under the shade of a verandah, wilted when the moisture content of the soil was between 14 per cent, and 15-5 per cent. Death occurred when moisture content fell to 8 per cent.