CHANGES IN THE PHYSICAL-PROPERTIES OF A VERTISOL FOLLOWING AN IRRIGATION OF COTTON AS INFLUENCED BY THE PREVIOUS CROP

Structural degradation of Vertisols depresses cotton lint yield by extending the period of waterlogging following irrigation or heavy rainfall. Break crops such as wheat and safflower are often grown without irrigation to improve the macroporosity of degraded Vertisols, by encouraging deep cracking, after several years of cotton production. Investigations were made into the effects of cultivated fallow, wheat and safflower on soil structure, and the growth of a subsequent cotton crop. The tests reported in this paper, done over a 12-day period following irrigation, were: soil water content and potential, core and clod bulk density, air-filled porosity, an index of macropore continuity using Rhodamine dye, and penetration resistance. The results, when compared with established limits of aeration and penetration resistance to cotton root growth, indicate that soil physical conditions at 0.25 m should have impeded root growth regardless of water content. However, cotton roots continued to extract water in this hostile soil environment; this favourable performance was attributed to the better conditions for root growth in the interaggregate fissures than those within aggregates. Apart from water content, all indicators of soil structure were able to differentiate consistently between the three crop treatments to a depth of 0.25 m. Below 0.8 m, the cropped plots, particularly safflower, had lower water contents than the fallow plots.