96 — Duty of Water.—Of the first importance is an adequate and exact knowledge of the duty of water for every crop. This varies with the locality and the texture of the soil. Porous, sandy soils require more water than heavy, clay land, but retain their moisture longer. It is therefore necessary to determine the duty of water and method of application best suited to each district. It would be impossible and absurd to expect figures approved for one centre to apply to the entire cotton belt of the Sudan.

As stated previously, the cotton plant is affected by atmospheric changes. When deleterious, such changes may, in a measure, be counteracted by a judicious application of water at the right moment. To be really effective, however, such control should be exercised before the change occurs, and though it may be possible to forecast the weather for twenty-four or even forty-eight hours, it would rarely be practicable to complete the watering of five or six hundred feddans from a single source of supply in so short a time It is, therefore, the object of all cotton growers to determine the amount of water and the intervals between waterings which will best meet the vicissitudes of the average season. Even when a formula which is likely to suit most seasons is devised, experience, or " instinct," must play a great part in the decision as to how far the system should be rigidly adhered to, and what departures should be made to meet unexpected or extraordinary changes.

The construction of a formula suited to the average season is, however, the first step to be taken, and to this end experiments have been conducted for the last three seasons over areas of 9 feddans.

The following diagram illustrates the arrangement, amounts of water and intervals between waterings :