AN EVALUATION OF CONTROLLED TRAFFIC WITH REDUCED TILLAGE FOR IRRIGATED COTTON ON A VERTISOL

Where Vertisols are used for the mechanised production of irrigated co tton, the main pre-planting tillage options are 'direct listing' (a 'c ontrolled traffic-reduced tillage' treatment with retained ridges), de ep ripping (0.45 m deep) and chisel ploughing (0.25 m deep). An experi ment was established, on a commercial scale under furrow irrigation, t o compare the effects of these treatments on the physical properties ( aeration, strength and water content) of a frequently irrigated Vertis ol over 3 years, The growth and profitability of three cotton crops an d one wheat crop were monitored during this period, The soil was well structured when the experiment commenced. Soil measurements showed tha t whilst the deep ripped treatment had lower resistance to penetration than the direct listed treatment, it provided a less favourable envir onment for root growth than the direct listed soil due to prolonged wa terlogging after irrigation. An excess of water entered the ripped pro file under the prevailing irrigation regime. All treatments had a sodi c subsoil that was poorly drained, The chisel ploughed treatment gener ally behaved in a fashion that was intermediate between the direct lis ted and deep ripped treatments. The direct listed treatment resulted i n higher cotton lint yields and lower land preparation costs, in compa rison with deep ripping. Profitability increases were of the order of 11% when the lint value was 1.78 Australian dollars kg-l. The deep rip ped soil had more stored water than the other treatments throughout ea ch irrigation cycle, but this potential advantage could not be express ed in terms of better crop yield and improved water use efficiency. A supplementary experiment is needed to evaluate the three tillage optio ns where irrigation water is applied less frequently.