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Growth problems in salt affected soils are generally due to high soil osmotic tension caused by excess soluble salts or poor soil structure caused by excess sodium. An excess of certain ions in soil solution may cause toxicity symptoms or nutritional disorders in susceptible plants. Three ions of major concern are sodium (Na+), chloride (Cl-), or boron (B+).
These ions can be translocated from the roots to the leaves, so toxicity symptoms show in the foliage rather than the roots. When foliage is wetted by sprinkler irrigation these ions can be absorbed directly through the leaf cuticle (surface tissue) Sprinkler irrigated crops may accumulate toxic ions faster than surface irrigated crops because the ions are absorbed by the roots plus being deposited directly on the leaves.
Sodium toxicity is not easily diagnosed. Poor soil structure caused by excess sodium usually causes more growth problems than ion toxicity. Sensitive crops can show toxicity symptoms at exchangeable sodium percentages (ESP or % Na) that do not affect soil structure. At high sodium percentages, plants show the combined effects of both sodium toxicity and poor soil structure.
Typical toxicity symptoms are leaf burn, scorch, and dead tissue along the outside edges of leaves. It normally takes time (days or weeks) before sodium accumulates to toxic levels. Symptoms appear first on outer edges of older leaves. As the severity increases, symptoms move progressively inward between the veins and toward the leaf center. Sodium and chloride toxicity may both occur at the same time, but chloride toxicity symptoms normally begin at the extreme leaf tip.
Sodium toxicity symptoms are usually noted when sensitive plants, like tree crops, accumulate more than 0.25% to 0.50% Na in the leaves. High sodium affects the balance of calcium, magnesium, and potassium in leaf tissue. This may interfere with plant transpiration control mechanisms and can affect water relations in the leaves. Table 1 contains the relative tolerance of various crops to different soil sodium percentages.
Chloride can be considered an essential plant nutrient, but toxicity occurs when plants accumulate excessive amounts in leaf tissue. Chloride moves readily with soil water. It is taken up by the roots, translocated to the shoot, and accumulates in the leaves. Individual crops and varieties differ in the speed at which they accumulate chloride. Chloride tolerance is apparently controlled genetically. Plants tend to exclude chloride or block movement through the plant rather than to accumulate it.
Both chloride and sodium can be absorbed directly into the plant when applied as droplets onto the leaves during sprinkler irrigation. The chloride absorbed into the leaf speeds the rate of ion accumulation, resulting in toxicity symptoms even if soil levels are low. Accumulations occur during periods of high temperature and low humidity which result in rapid evaporation of water from the droplets, leaving the chloride behind.
Fine water droplets have more surface area per droplet than coarse droplets and evaporate faster. Some producers have noticed toxicity symptoms under the inside spans of center pivot sprinkler systems with no symptoms under the outside spans. This is due to the difference in water application rate due to the distance traveled by the respective spans. For example, water is applied six to seven times faster under the outside span of a center pivot system than under the inside span, assuming a 1300-foot radius.
Trees and woody perennials are generally more sensitive to chloride than are non-woody plants. Necrotic (dead) spots or burning of leaf tips and margins may show up when the leaves of woody plants accumulate more than 0.5% Cl by weight. Excess chloride may cause also cause bronzing, burning, or early leaf drop. Most agronomic crops can tolerate 5% to 10% Cl in the leaf tissue. Table 2 gives the maximum chloride levels for different crops as measured in the saturated paste soil extract.
Boron is an essential plant nutrient, but is generally required in small amounts. Many irrigation water sources can supply adequate boron for normal crop production and prevent nutrient deficiency, but may be present or can accumulate to toxic concentrations.
Boron moves with soil water, but can accumulate more rapidly than other salts because it is more tightly adsorbed on soil particles. For example, 0.2 mg/L boron in water may prevent deficiency in some crops, but 1.0 to 2.0 mg/L B may be toxic.
Boron is considered immobile within the plant. Toxicity research showed that foliar applied boron increased foliar damage and reduced yields, but did not increase leaf tissue concentrations. The relative toxicity of boron that enters through leaf absorption is greater than boron entering via the roots.
Boron toxicity appears as leaf burning, chlorosis, or necrosis. Symptoms usually show first on older leaves as yellowing, spotting, or drying of leaf tips and edges. Toxicity symptoms often appear when the dry weight leaf tissue concentrations exceed 250 to 300 ppm B.
Toxicity may occur in some plants, like stone fruits, even with low leaf tissue boron concentration. Soil, plant tissue, and irrigation water analyses are needed to properly diagnose boron toxicity. Table 3 compares relative crop tolerance to soil boron levels.
¶ Table 1. Tolerance of Various Crops to Exchangeable Sodium Percentage (% Na) Under Non-Saline Conditions |
||
| Relative tolerance | Crop | Growth response under field conditions |
| Extremely sensitive (2% to 10% Na) |
Avocado | Sodium toxicity symptoms even at low sodium percentages |
| Citrus | ||
| Cotton (at germination) | ||
| Deciduous fruits | ||
| Nut crops | ||
| Sensitive (10% to 20% Na) |
Beans | Stunted growth at low sodium percentages even though the physical condition may be good. |
| Corn | ||
| Cowpea | ||
| Peanut | ||
| Moderately tolerant (20% to 40% Na) |
Dallisgrass | Stunted growth due to both nutritional factors and adverse soil conditions. |
| Carrot | ||
| Clover | ||
| Lettuce | ||
| Oats | ||
| Onion | ||
| Radish | ||
| Rice | ||
| Rye | ||
| Tall fescue | ||
| Tolerant (40% to 60% Na) |
Alfalfa | Stunted growth usually due to adverse physical condition of soil. |
| Barley | ||
| Beets, garden | ||
| Cotton | ||
| Sorghum | ||
| Spinach | ||
| Sugarbeet | ||
| Tomatoes | ||
| Wheat | ||
| Most tolerant (> 60% Na) |
Bermudagrass | Stunted growth usually due to adverse physical condition of soil. |
| Rhodes grass | ||
| Wheatgrass | ||
¶ Table 2. Maximum Chloride Levels in the Saturated Paste Soil Extract Without Leaf Injury.* |
|
| Crop | Chloride, mg/L Cle |
| Alfalfa | 800 |
| Avocado | 175 - 300 |
| Barley | 3000 |
| Beans, kidney | 850 |
| Beans, navy | 640 |
| Berries | 350 |
| Beets | 2800 |
| Citrus | 3005 - 900 |
| Corn (young) | 2500 |
| Cotton | 1800 |
| Dallisgrass | 675 |
| Flax | 1800 |
| Grapes | 800 |
| Rice | 800 |
| Sorghum | 800 |
| Stone fruits | 250 - 900 |
| Strawberry | 175 - 275 |
| Tomato | 1600 |
| Wheat (young) | 900 |
| *The maximum Cle values apply to crops under surface irrigation. Toxicity symptoms may appear at lower soil levels if crops are irrigated with overhead sprinklers that apply chloride directly to the leaf surface. | |
¶ Table 3. Relative Crop Tolerance to Soil Boron, as Measured in the Saturated Paste Extract |
||
| Name | Relative Tolerance | mg/L Be |
| Alfalfa | T | 4.0 - 6.0 |
| Apricot | S | 0.5 - 0.75 |
| Artichoke | MT | 2.0 - 4.0 |
| Asparagus | VT | 6.0 - 15.0 |
| Avocado | S | 0.5 - 0.75 |
| Barley | S | 0.75 - 1.0 |
| Bean, kidney | S | 0.75 - 1.0 |
| Bean, lima | S | 0.75 - 1.0 |
| Bean, mung | S | 0.75 - 1.0 |
| Beet, red | T | 4.0 - 6.0 |
| Blackberry | VS | < 0.5 |
| Bluegrass | MT | 2.0 - 4.0 |
| Cabbage | MT | 2.0 - 4.0 |
| Carrot | MS | 1.0 - 2.0 |
| Celery | MT | 2.0 - 4.0 |
| Cherry | S | 0.5 - 0.75 |
| Corn | MT | 2.0 - 4.0 |
| Cotton | VT | 6.0 - 15.0 |
| Cucumber | MS | 1.0 - 2.0 |
| Fig, kadota | S | 0.5 - 0.75 |
| Garlic | S | 0.75 - 1.0 |
| Grape | S | 0.5 - 0.75 |
| Grapefruit | S | 0.5 - 0.75 |
| Lemon | VS | < 0.5 |
| Lettuce | MT | 2.0 - 4.0 |
| Lupine | S | 0.75 - 1.0 |
| Muskmelon | T | 2.0 - 4.0 |
| Mustard | MT | 2.0 - 4.0 |
| Oats | MT | 2.0 - 4.0 |
| Onion | S | 0.5 - 0.75 |
| Orange | S | 0.5 - 0.75 |
| Parley | T | 4.0 - 6.0 |
| Pea | MS | 1.0 - 2.0 |
| Peach | S | 0.5 - 0.75 |
| Peanut | S | 0.75 - 1.0 |
| Pecan | S | 0.5 - 0.75 |
| Persimmon | S | 0.5 - 0.75 |
| Plum | S | 0.5 - 0.75 |
| Potato | MS | 1.0 - 2.0 |
| Radish | MS | 1.0 - 2.0 |
| Sesame | S | 0.75 - 1.0 |
| Sorghum | T | 4.0 - 6.0 |
| Squash | MT | 2.0 - 4.0 |
| Strawberry | S | 0.75 - 1.0 |
| Sugarbeet | T | 4.0 - 6.0 |
| Sunflower | S | 0.75 - 1.0 |
| Sweet potato | S | 0.75 - 1.0 |
| Sweet clover | MT | 2.0 - 4.0 |
| Tobacco | MT | 2.0 - 4.0 |
| Tomato | T | 4.0 - 6.0 |
| Turnip | MT | 2.0 - 4.0 |
| Vetch, purple | T | 4.0 - 6.0 |
| Walnut | S | 0.5 - 0.75 |
| Wheat | S | 0.75 - 1.0 |
Bresler, E., B.L. McNeal, and D.L. Carter. 1982. Saline and sodic soils. Springer-Verlag, New York. pg. 180.
UN-FAO. 1985. Water Quality for Agriculture, Irrigation and Drainage Paper No. 29. United Nations Food & Agriculture Org., Rome. pg. 77-83.
Ben-Gal, A. 2007. The Contribution of Foliar Exposure to Boron Toxicity. Journ. of Plant Nutrition. 30:1705-1716.
Hanson, B., S.R. Grattan, and A. Fulton. 1993. Agricultural Salinity and Drainage. Univ. of California, Davis. pg. 11-18.