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Show PART II PRESENT STATUS OF IRRIGATION Water Quality General effects on irrigation Three principal characteristics contribute to the success of irri- gated agriculture in the region from the standpoint of water quality. (1) The region is normally characterized by high flow, good qual- ity runoff in spring and early summer and low flow, poorer quality run- off the remainder of the year. This has been altered in many cases by construction of reservoirs that store the better quality high runoff for use later in the year. Good quality water is beneficial in early spring when the young plants are more susceptible to injury. (2) There are only a few acres of salt-sensitive crops irrigated in the region, thus the effects of increased salinity of the irrigation water are not great. (3) Calcium and magnesium in the proper proportions to sodium main- tain soil in good conditions of tilth and structure. In most soils of the Upper Colorado Region, calcium and magnesium are the principal cations held by the soil in replaceable or exchangeable form with sodium consist- ing of a smaller amount. Only in a few localities is the ratio of these constituents reversed, with the excess sodium in the soil causing struc- ture to break down, sealing pores, and decreasing permeability. Waters containing predominantly calcium when applied to such soils decrease the sodium adsorption ratio and potential sodium hazard. In discussing quality of water for irrigation it is necessary to consider the effects of the water's constituents on both the plant and the soil. The deleterious effects of salts on plant growth can result from: (l) direct physical effects of salts in preventing water uptake by plants (osmotic effects); (2) direct chemical effects upon metabolic reactions of plants (toxic effects); and (3) indirect effects through changes in soil structure, permeability, and aeration.^' Absolute limits of the permissible concentration of salts in irriga- tion water are difficult to assign for several reasons listed below. (1) It is possible that the soil solution may exceed eight times the concentration of the water that replenishes it because of evapora- tion from the soil surface, transpiration by plants, and selective adsorption of salts by the plants. (2) There is apparently no definite relationship between the con- centration and composition of the irrigation water and soil solution. (3) Plants vary widely in their tolerance of salinity, as well as specific salt constituents. 31 |