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Leaching Requirement Studies: Sensitivity of Alfalfa to Salinity of Irrigation and Drainage Waters¹

ABSTRACT

Alfalfa (Medicago sativa L. cv. Sonora) was grown in 0.6 by 1.5 m greenhouse lysimeters and irrigated with two waters of EC 1 and 2 mmho/cm prepared by adding equivalent amounts of NaCl and CaCl₂ to a 0.4 mmho/cm tap water. Yields showed relatively little effect of leaching fraction (LF) within the limits consistent with steady-state salt balance for suction-drained lysimeters but decreased 26% at the lowest LF for gravity-drained lysimeters. Yields with the 2 mmho/cm irrigation water treatments were consistently about 10% less than those with the 1 mmho/cm water. Cessation of leaching or reduction of LF to levels requiring drainage water salinities for salt balance at steady state to exceed 35 mmho/cm, the maximum salinity achievable by alfalfa roots, eventually reduced yields. Yields were significantly higher (by 9%) when leaching was done every third or sixth irrigation than in every irrigation while maintaining the same average LF of 1/32 and 1/16 for the 1 and 2 mmho/cm irrigation waters, respectively. Since alfalfa yield was affected as much by a 1 mmho/cm difference in salinity of the irrigation water as by a ca. 20 mmho/cm difference in salinity of the drainage water, alfalfa yields obviously were not responding to the soil water salinity averaged by depth. Yield response appears to be related rather to the calculated mean salinity against which water was absorbed, which is influenced more by the salinity of the irrigation water than by the salinity of the drainage water.

Steady-state salinity profiles were achieved after 4 to 8 irrigations when LF was equal to or greater than 1/8, but at lower LF's of 1/16 and 1/32, steady-state profiles were achieved only after 20 and 55 irrigations, or 1 and 2 years, respectively. Increasing the degree of water depletion caused major salt accumulation at lower depths corresponding to the increased depths at which major water depletion occurred. LF in the broad range that permitted nearly maximum growth had essentially no effect on water requirement (grams water evapotranspired per gram dry matter produced) or on Na and Cl contents of the harvested alfalfa. Increasing irrigation water salinity in the range 0.4 to 2 mmho/cm consistently increased plant Na and Cl contents, but had no effect on water requirement.

Implications of the demonstrated lower leaching requirements for irrigation management and drainage and for water quality assessment are discussed.

¹ Contribution from the US Salinity Laboratory, Western Region, ARS, USDA, Riverside, California.

² Plant Physiologist and Agronomist, respectively.

Received for publication March 19, 1973. Accepted for publication August 6, 1973.