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Fluid Journal : Summer 2014
5 The Fluid Journal Summer 2014 leaching fraction of 15 percent. More complete crop salinity tolerance data can be found in the Western Fertilizer Handbook produced by the Western Plant Health Association, Sacramento, CA available at http:// www.waveland.com/browse.php?t =17&r=a|728 Under normal irrigation practices, the ECe should be about 1.5 times that of the ECw. When the ECe exceeds about 3 times that of ECw, then insufficient water is passing through the root zone to leach salts. This may be due to either sub-adequate amounts of irrigation water and rainfall or a subsoil condition such as a hardpan or another restrictive layer that is preventing deeper percolation of water. Periodic assessment of both water and soil salinity provides very useful information to growers and crop consultants and should be standard procedure, particularly for new wells or where salts are known to be a concern. There are two basic processes by which salts harm plants: • Osmotic potential or stress • Specific ion toxicity. Osmotic stress is likely the dominant cause of salt-induced yield and growth reductions in most situations. In a non- saline soil, the concentration of ions internal to the plant is higher than the one external to the plant. This allows for osmosis--the passive diffusion of water from an area of lower salt content to an area of higher salt content. As soils become saltier, the difference in salt content between the plants and the soil narrows, which slows the rate of osmosis. This is called osmotic stress, which makes water less available to plants. Plants respond by either taking in more ions (salts) to raise the salt content in the roots or by synthesizing more sugars or organic acids, which has the same effect as absorbing more salt ions. Both processes require energy that would otherwise be used for growth and yield. Eventually, when the concentrations of ions in the soil exceed the concentration of ions in the plant, osmosis stops and plants wilt, collapse, and die. In essence, the soil is physically wet but physiologically dry. Osmotic stress can greatly stunt plants without plants developing visual necrosis symptoms common to specific ion toxicity as can be seen in Figure 2. Specific ion toxicity pertai ns to the accumulation of sodium, chloride, or boron to levels that cause cellular death. Whereas sodium and chloride also can be dominant ions contributing to total salinity, boron concentrations are typically in the single digit ppm range and therefore have no effect on soil salinity. Specific ion toxicities cause leaf chlorosis and necrosis and leaf drop (in milder cases), and collapse Figure 1. Visible salt accumulation will become a more common site in California as grower’s reliance on salty ground water increases. Table 1: Drought, Salinity and Crop Nutrition: Interactions and Management. 0% 10% 25% 50% 100% Crop ECe ECw ECe ECw ECe ECw ECe ECw ECe Alfalfa 2 1.3 3.4 2.2 5.4 3.6 8.8 5.9 16 Almond 1.5 1 2 1.4 2.8 1.9 4.1 2.7 6.8 Brocolli 2.8 1.9 3.9 2.6 5.5 3.7 8.2 5.5 14 Corn 1.7 1.7 2.5 1.7 3.8 2.5 5.9 3.9 10 Cotton 7.7 5.1 9.6 6.4 13 8.4 17 12 27 Grape 1.5 1 2.5 1.7 4.1 2.7 6.7 4.5 12 Lettuce 1.3 0.9 2.1 1.4 3.23 2.1 5.1 3.4 9 Rice 3 2 3.8 2.6 5.1 3.4 7.2 4.8 11 Tomato 2.5 1.7 3.5 2.3 5 3.4 7.6 5 13 Adapted from Quality of Water for Irrigation. R . S. Ayers, Journ. Of the Irrig. And Drain.Div. ASCE. Vol. 103, No. IR2, June 1977, R140, and Salt Tolerance of Plants, E. V. Maas. Appl. Agri. Res.Vol. 1, No. 1, 1986 Note: On gypsiferous soils ECe readings may be as much as 2 dS/m higher on the laboratory report than under actual field conditions.