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1 - Frequently Asked Questions About Salinity
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Web adaptation from Handbook 60 (p. 65-67) originally published in 1954 |
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Because of
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| As has already been pointed out, the availability of water to plants is always a factor under saline conditions. For example, suppose alfalfa is being grown on a loam having a salt content of 0.2 percent sodium chloride and a wilting percentage of 6 when the latter is determined on a nonsaline sample. Under such conditions, because the osmotic effect is additive with soilmoisture tension, alfalfa will stop growing when the soil dries to a moisture content of only 13 percent. In other words, if the soil contains 0.2 percent salt, the alfalfa plant cannot use a large part of the soil moisture that is normally available under nonsaline conditions. The presence of even smaller quantities of salt in this soil would cause a fraction of the soil moisture above the wilting percentage to be unavailable to the plant. More frequent irrigation would be required to decrease the inhibitory effect of the salt on the growth of alfalfa. | |
| Although it has been shown that crop growth on saline soils is definitely benefited by more frequent irrigation, the need for this irrigation may not be indicated by the appearance of the crop (Richards and Wadleigh, 1952). In nonsaline soils, there is usually a relatively abrupt transition from low moisture stress to high moisture stress conditions, and the wilting of the plant indicates the need for irrigation. In saline soils, changes in moisture stress are more gradual and, although the plants may be subjected to high stress, there is no abrupt transition in the turgor condition of the plant and, hence, no sign of the need for irrigation. Nevertheless, experiments have shown that crop growth is greatly improved by more frequent irrigation under such conditions. Careful leveling of the fields to insure more uniform moisture distribution during irrigation will also improve chances for successful crops on saline soils. | |
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Germination |
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| In selecting crops for saline soils, particular attention should be given to the salt tolerance of the crop during germination because poor crops frequently result from a failure to obtain a satisfactory stand. This problem is complicated by the fact that some crop species which are very salt tolerant during later stages of growth may be quite sensitive to salinity during germination (fig. 19). Sugar beets, for example, which are very salt tolerant during later stages of growth, are extremely sensitive during germination. On the other hand, barley has very good salt tolerance during all stages of growth, although it is more sensitive during germination than at later stages (Ayers and others, 1952). Under field conditions, it is possible by modification of planting practices to minimize the tendency for salt to accumulate around the seed and to improve the stand of crops that are sensitive to salt during germination (Heald and coworkers, 1950). | |
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Figure 19 |
Relative Salt Tolerance of Crop Plants |
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| The salt tolerance of many species and varieties of crop plants has been investigated at the Laboratory. Previously published lists (Magistad and Christiansen, 1944, and Hayward and Magistad, 1946) have been modified on the basis of recent findings and are presented in Table 8. | |
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The salt tolerance of a crop may be appraised according to three criteria:
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| Many previous observations on salt tolerance have been based mainly on the first criterion, ability to survive; but this method of appraisal has very limited practical significance in irrigation agriculture. Although it is recognized that the second criterion is perhaps of greater agronomic importance, the third criterion was used in compiling the present salt-tolerance lists because it provides a better basis of comparison among diverse crops. | |
| The salt-tolerance lists are arranged according to major crop divisions; and, in each division, crops are listed in three groups. Within each group, the crops are listed in the order of decreasing salt tolerance, but a difference of 2 or 3 places in a column may not be significant. EC, values given at the top of a column represent the salinity level at which a 50 percent decrease in yield may be expected as compared to yields on nonsaline soil under comparable growing conditions. For example, for crops with high salt tolerance in the division of field crops, EC, values of 16 mmhos/cm occur at the top of the column and 10 mmhos/cm at the bottom. This indicates that crops near the top of this column will produce about 50 percent as well on a soil having an EC, of 16 mmhos/cm. as on a nonsaline soil under similar conditions, and crops near the bottom of this column will produce about 50 percent as well on soils having an EC, of 10 mmhos/cm. as on a nonsaline soil. EC, values having similar significance have been shown for each group of plants for which such data are available. | |
| In most instances, these data are based on a field-plot technique in which crops are grown on soils that are artificially adjusted to various salinity levels after the seedlings are established. By this method, crop yields were related to EC, values for comparable saline and nonsaline soils, and the salinity level associated with a 50 percent decrement of yield was determined graphically. In many of these studies, a number of varieties of a given crop were compared. Significant varietal differences were found for cotton, barley, and smooth brome, while for truck crops such as green beans, lettuce, onions, and carrots varietal differences were not of practical significance. | |
| In applying the information in the following table, it is important to remember that climatic conditions may influence profoundly the reaction of plants to salinity. The choice of suitable salt-tolerant varieties and strains will depend on local climatic factors; and, consequently, information on salt-tolerant varieties should be evaluated with reference to the conditions under which the crops are to be grown. The position of each crop in this table reflects its relative salt tolerance under management practices that are customarily employed when this crop is grown under irrigation agriculture and not the inherent physiological ability of the crop to withstand salinity under some given set of conditions that is uniform for all crops. | |
| A salt-tolerance list for some important crops of Holland has recently been prepared by Van den Berg (1950) . Based on field-plot studies in areas which had been inundated by salt or brackish water in 194.4.-45, the salinity values ("salt index," expressed as grams NaCl per liter of soil water) associated with 75 percent of normal yields for 14 crops were determined. Despite obvious differences in climate and cultural practices, Van den Berg's results for relative salt tolerance are in good agreement with those in Table 8. | |
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Relative Boron Tolerance of Crop Plants |
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| Plant species differ markedly in their tolerance to excessive concentrations of boron. In sections where boron tends to occur in excess in the soil or irrigation water, the boron-tolerant crops may grow satisfactorily, whereas sensitive crops may fail. The relative boron tolerance of a number of crops was determined by Eaton (1935), and his results are reported in Table 9 with minor modifications based on field observations. The boron tolerance lists are analogous to the salt-tolerance lists and subject to much the same limitations in interpretation. Differences in position of a few places may or may not be significant, and there is no sharp division between successive classes. Climate and variety may also be factors in altering the indicated tolerance of a given species under specific conditions. | |
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| Available information on boron tolerance does not permit the establishment of definite permissible limits of boron concentration in the soil solution. Irrigation waters are classified on the basis of boron content in table 14, chapter 5, with reference to sensitive, semitolerant, and tolerant crops. The effect of a given concentration of boron in the irrigation water on the boron content of the soil solution will be conditioned by soil characteristics and management practices that influence the degree of boron accumulation in the soil. In the discussion of saturation extracts of soils (ch. 2), 0.7 ppm. boron in the saturation extract was indicated as the approximate safe limit for sensitive crops. | |
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