Alfalfa water-production functions under conditions of deficit irrigation with saline water

Abstract

This experiment was designed to determine the shape of the yield response function relating crop yield to total amount of saline irrigation water applied. Such a function contains a built-in leaching fraction that is the inevitable consequence of the inability of the plant to extract 100 % of the water from a saline soil. In order to define the production function and to determine the leaching fractions, alfalfa (Medicago sativa L. cv. 'Mesa Sirsa') was planted in soil columns in a greenhouse. Two experiments were run sequentially. These were irrigated with water of differing salinities. The first with an EC of 4 dS/m (1.4 bars) and the second with an EC of 8 dS/m (2.9 bars). Both solutions were prepared by adding equivalent amounts of sodium chloride and calcium chloride to distilled water. The treatment variables were amounts of irrigation water applied. The amounts in both experiments were 110%, 100%, 75%, 50% and 25% of the measured evapotranspiration (ET). Four crop harvests were made in each experiment. At the end of experiment 1, (approximately 120 days), one column from each treatment was destructively sampled for soil salinity and water content measurements. The remaining columns were similarly sampled at the end of experiment 2 (approximately 120 days). The crop-saline water production functions for both experiments were linear. Leaching fractions in experiment 1 were 9, 9, 6, 5 and 5% for treatments 1-5 respectively. Experiment 2 leaching fractions for treatments 1-5 respectively were 23, 25, 18, 15 and 17%. The lowest rootzone soil water osmotic potentials achieved by the end of experiment 1 for treatments 1-5 were -19, -20, -18, -26 and -24 bars. Corresponding treatment values achieved by the end of experiment 2 were -18, -22, -28, -31 and -45 bars.