Salinity and Sodicity Changes under Irrigated Alfalfa in the Northern Great Plains

Insufficient water is the greatest limitation to crop production and choice of crops grown in the Northern Great Plains. Supplemental irrigation can overcome this limitation. Uncertainties about the drainage capacity of flne-textured subsoils and the effect of irrigation on soil properties has impeded irrigation development. In this study we quantified salinity changes in soils with fine-textured subsoils receiving a range of irrigation treatments. Alfalfa (Medicago saliva L.) was planted in 18 nonweighing ly si meters at two sites having fine-textured subsoils. Irrigation was applied at three levels so that irrigation plus precipitation equaled either one, two, or three times the calculated evapotranspiration rate using two water qualities (electrical conductivity of irrigation water [EG] 0.1 S m~' , sodium adsorption ratio of irrigation water [SAR,] 4; or Ed 0.34 S m', SARi 16). Changes in the electrical conductivity of saturated soil extracts (ECe) and the sodium adsorption ratio of saturated soil extracts (SAR«) were determined from soil cores collected to a depth of 1.5 m nine times between the years of 1984 and 1993. Averaged across irrigation levels, the profile-averaged ECe increased from 0.03 to 0.12 S m" and the SAR, increased from 1 to 6 in lysimeters receiving the 0.1 S m" water. In lysimeters receiving the 0.34 S m"' water, the profile-averaged ECe increased from 0.03 to 0.23 S m -' and the SAR, increased from 1 to 11. Salinity exhibited seasonal fluctuations. Changes in sodicity were persistent, exhibiting little seasonal variation. Supplemental irrigation of alfalfa is a viable management option in the Northern Great Plains when irrigation water quality is not a problem. S IRRIGATION is a management strategy that can be used by producers in the Northern Great Plains to meet crop moisture needs during periods of below-normal precipitation or to grow crops that require more water than is normally available. Supplemental irrigation is economical in certain areas of North Dakota (Leitch et al., 1991) but remains underutilized by producers. Uncertainty about the drainage capacity of many fine-textured or stratified soils and the effects irrigation will have on physical and chemical properties of these soils are reasons producers may be unwilling to invest in an irrigation system. Soils are classified as nonirrigable when the subsoil has a hydraulic conductivity <20% of the weighted hydraulic conductivity of the overlying soil layers and the slowly permeable layer is within 1.8 m of the surface (U.S. Department of Interior, Bureau of Reclamation, 1993). Adequate drainage capacity has been demonstrated in situ, using large (2.5 m square by 2.3 m deep) nonweighing lysimeters, for two sites having fine-textured subsoils classified as nonirrigable using U.S. Bureau of Reclamation criteria (Doering et al., 1986; Trooien and Reichman, 1990). The drainage capacity of these soils was sufficient to prevent development of a perched water table above the fine-textured subsoil even when irrigation USDA-ARS, Northern Great Plains Research Lab., P.O. Box 459, Mandan, ND 58554-0459. Received 28 Nov. 1994. ""Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 59:1709-1714 (1995). water was applied at rates three times the ET (Trooien and Reichman, 1993). In addition, 280000 ha of land with similar soils in southern Alberta have been irrigated for >50 yr with few drainage problems (Chang et al., 1985). Adequate drainage allows application of irrigation water in excess of that needed by the crop so that salts are leached out of the root zone and salinization of the soil does not occur. Most studies dealing with salinization associated with irrigation have been conducted in Cl~dominated systems. Most soil and groundwater systems in the Northern Great Plains are not Cl~ dominated but rather contain proportionately large amounts of SC>4~, HCO3-, and COi" (Prunty et al., 1991). A greenhouse lysimeter study using four commonly irrigated North Dakota soils and six waters (distilled water and five with a range of chemical compositions similar to those of groundwater in North Dakota) measured the yield response of alfalfa (Prunty et al., 1991) and quantified changes in soil chemical and physical properties (Costa et al., 1991). Little drainage was allowed as water inputs were such that drainage from the lysimeters was <2% of the applied water. The total amount of water applied was similar to what would be applied during 17 yr under field conditions. A seventh treatment alternated irrigation with saline water and distilled water to simulate the effect of precipitation during the growing season. Alfalfa yields decreased 30 to 60% in all treatments by the end of the experiment (Prunty et al., 1991). Substantial amounts of salts were deposited in the soil profile in these treatments. Precipitation of Ca and Mg salts resulted in an increase in SARe (Costa et al., 1991). The effects of irrigation on soil chemical properties in the field have not been reported for the Northern Great Plains. The objectives of this study were to quantify soil profile salinity changes at two sites with fine-textured subsoils when these soils were irrigated with two qualities of irrigation water applied at three rates. MATERIALS AND METHODS Two Northern Great Plains sites were used; the Menoken township site (T139N, R78W, Sec. 19, SE 1/4) (Doering et al., 1986) and the Naughton township site (T140N, R79W, Sec. 35, SW 1/4) (Trooien and Reichman, 1990). These sites are located in central North Dakota, 50 km east of Bismarck in Burleigh County. Soils at the Menoken site are Lihen sandy loam (sandy, mixed Entic Haploboroll), Roseglen loam (fineloamy, mixed Pachic Haploboroll), and Parshall (coarseloamy, mixed Pachic Haploboroll) (Table 1). These soils developed on aeolian-lacustrine sediments deposited over fine lacustine sediments. Soil structure at the Menoken site is massive or single grained and is similar throughout the sampled profile (M.D. Sweeney, 1982, unpublished data). Soils at the Naughton site are Falkirk loam (fine-loamy, mixed Pachic Abbreviations: EC, electrical conductivity; SAR, sodium adsorption ratio; ET, evapotranspiration.