Rotation of crops is based in most countries upon the accumulated experience of past years; experiments testing the relative merits of various rotations are rarely resorted to unless some serious disturbance arises, such as the inclusion of a new crop. But in the Sudan Gezira (lat. 14–15° N., long. 33–34° E.) the need for rotation experiments was urgent, since growing cotton by artificial irrigation was a recent introduction, and, moreover, the soil was known to be intractable. Any improvement in rotation which would allay seasonal fluctuation in yield and raise the general level would greatly add t o the prosperity of the area.

Thus when the Gezira Research Farm was inaugurated in 1918 some unreplicated plots of various two-course rotations, and of continuous cotton, were laid down. These were supplemented in 1925–6 by a replicated trial comparing various three-course rotations, designed by E. M. Crowther of Rothamsted. Later, in 1931–2, a new ‘Combined Rotation’ experiment, designed by M. A. Bailey and E. M. Crowther, was added, and this comprised comparisons of one-, two-, three- and four-course rotations. These last