Abstract
The variation in soil created by agriculture under irrigation is assessed by comparing the soil on two adjacent 3.75-ha plots on vertisol in the Sudan Gezira. One plot has been cultivated and irrigated by flooding for over 50 years; the other remained in its original natural condition. The topsoil (to 30 cm) was sampled at intervals of 6.25 m on a grid and the pH, electrical conductivity and sodium adsorption ratio measured. The sample variances were up to 3 times larger on the irrigated plot than on the natural one. A spatial analysis showed almost no spatial dependence on the natural plot but a fairly strong dependence after irrigation with marked anisotropy. Most variation occurred perpendicular to the present river channel, but there was also subsidiary short-range variation approximately in the direction of flooding. To illustrate these effects, statistical surfaces were estimated by simple kriging from the data and their variograms and displayed as block diagrams.
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