AMELIORATION OF A HARDSETTING ALFISOL THROUGH DEEP MOLDBOARD PLOWING, GYPSUM APPLICATION AND DOUBLE CROPPING .2. SOIL-WATER RELATIONS

Deep mouldboard ploughing to a depth of 0.45 m, gypsum (5 t ha-1) and double cropping wheat (Triticum aestivum) with a summer forage crop were applied individually, and in combination, to modify a hardsetting red-brown earth (Alfisol) used for flood-irrigated wheat production. The effects of these treatments upon water infiltration, storage and intake during irrigations, application efficiency, and depletion between irrigations were measured. Gypsum markedly increased saturated hydraulic conductivity (K(S)) immediately after being applied in June 1984 but not in 1985. Mouldboard ploughing and double cropping had little effect on K(S). All K(S) values were less than 0.5 m day-1. Plant available water content (PAWC) was increased by gypsum application and mouldboard ploughing after the initial irrigation in 1984, and by the three main treatments in 1985. The order in which the main treatments increased PAWC was: gypsum > mouldboard ploughing > double cropping. Water applied and water intake during irrigations were increased by mouldboard ploughing and gypsum during the first irrigation in 1984 by as much as 60 mm. Thereafter mouldboard ploughing, gypsum and double cropping had little effect on water intake, and only gypsum increased the amount of water applied in 1985. The ratio of water intake to water applied during irrigations (application efficiency) was increased by mouldboard ploughing to 59% in 1984; no treatment had values greater than 40% during 1985. Water depletion was significantly increased by mouldboard ploughing in 1984 and by gypsum and double cropping in 1985. Few significant interactions were found between the three main treatments, each of which would improve soil-water relations for crop growth, although the effect of mouldboard ploughing on soil-water relations declined after the first year. Despite substantial improvements after 2 years in some treatments, soil hydraulic properties were inadequate for optimal irrigated cropping.