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Salt-affected soils usually appear as small areas in a field surrounded by relatively normal soils. They are irregular and vary in size from a few square feet to an acre or more. These areas are often low-lying, have poor soil drainage, and dry out slowly.
We need to answer several questions to properly identify and manage salt-affected soils. This information is required to design a feasible, economical management strategy.
The term “salts” refers to the precipitated or crystalized combinations of ions that were dissolved in a water solution. These are typically the chlorides, sulfates, bicarbonates, and carbonates of sodium, calcium, and magnesium found in the soil water (soil solution).
Salt-affected soils are identified by two primary criteria: salinity and sodicity. “Saline” soils have salt concentrations that exceed some threshold and affect plant growth. “Sodic” soils refer to soils with elevated levels of sodium on the cation exchange complex and often contain free sodium carbonate (Na2CO3) which may cause the soil pH to exceed 7.5. The excess sodium causes soil dispersion and deteriorating soil structure. The traditional classifications for salt-affected soils are shown in Table 1.
¶ Table 1. Salt-Affected Soil Classification |
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| -------------------- | Exchangeable sodium percentage | |
| Electrical conductivity of extract, ECe |
% Na < 15% | % Na ≥ 15% |
| < 4 mmho/cm | Non-saline, non-sodic | Non-saline, sodic |
| ≥ 4 mmho/cm | Saline, non-sodic | Saline, sodic |
Strongly saline soils used to be called "white alkali" because of a powder-like layer or crust of salts on the soil surface. Saline areas may have good soil structure and good internal drainage. Sometimes gray to white flecks of salt crystals can be found in the soil itself rather than surface crusts. Slightly or moderately saline soils may not have visible salt crystals. When soils are moist, salts are in solution and do not leave visible deposits. Thin, patchy salt crusts may form on the shady side of clods upon drying.
High water tables can keep saline areas moist for several days after the rest of the field dries out. A moisture probe may detect a dry zone in normal, non-saline areas. Saline areas are likely to be wet the full depth of the probe. Salt tolerant crops growing in saline areas may stay green well into summer because of the subsoil moisture.
Saline soils have soluble salt concentrations high enough to interfere with the plant's ability to extract water from the soil. The first evidence of a salinity problem is likely to be delayed seed germination because the seed has a hard time imbibing the saline soil water. Emergence and stand establishment may not be affected initially, but often decline as the salinity increases. Poor or spotty stands can be caused by problems other than salinity, so consider other possible causes.
Vigorous, full sized plants next to a barren spot may indicate a localized saline area. Stunted areas and irregular sized plants indicate a more general saline condition. Some plants may show a blue-green color when salinity is excessive. This occurs from a waxy coating that forms on the foliage and from a high concentration of chlorophyll per unit of foliage.
Another common symptom is drought stress even when the soil is moist. Slightly or moderately saline soils may support good plant growth when soil moisture content is high and salt concentrations are dilute. As the soil dries, the salts concentrate and plants show water stress even if the soil appears moist.
Strongly saline soils are often devoid of vegetation, except for very salt tolerant weeds. Less salt tolerant plants cannot grow in these areas. Salt tolerant weeds include Russian thistle, kochia, wild barley, greasewood, or saltgrass.
Sodic soils typically have poor soil structure that limits air and water penetration. Common terms for sodic areas are "gumbo spots" or "slick spots". Sodic soils are extremely sticky and plastic. They dry out slowly once they are wetted. They till poorly, make poor seedbeds, and establishing a stand is difficult.
Sodic soils tend to crust and form large cracks as they dry out. When dry, these soils are hard, cloddy, and even brick-like.
Some sodic soils have a thin film of dispersed organic matter on the soil surface which gives the soil a black appearance. This gave rise to the name "black alkali". Color is not a positive indicator because sodic soils may be various shades of brown, gray, or white.
Sodic soils often have a layer of dispersed clay with a low water infiltration rate. Water may be standing on the surface while a few inches down the subsoil is dry. This dispersed layer is often found in the top foot of soil, but may be deeper in the profile. The thickness of the dispersed layer can range from a few inches to as much as two feet thick. Field diagnosis may be difficult because the dispersed layer can be overlaid with a soil layer with has good structure and permeability.
Plant growth in sodic soils is usually poor because of poor air and water infiltration. Plants have a hard time establishing themselves because of poor seedbed conditions. Plants that do establish have difficulty developing good root systems. Water stands in sodic areas, so drowning is a common problem.
Water sources: Salt-affected areas form when salts are transported to a certain location from a source of water containing dissolved salts. Locating the original water source or recharge area is important. Water movement into affected areas must be stopped or arrested before these areas can be reclaimed. If this is not feasible, then other strategies are needed to cope with the problem.
Water sources can be anywhere from a few hundred feet to three or four miles away from the salt-affected area. Water can be recharged from rivers, streams, lakes, or ponds that are at the same level or greater than 2% upslope. Seepage from unlined canals or drainage ditches is a common water source. Fallow areas or windbreaks that trap snow and off season precipitation can recharge shallow water tables.
Internal drainage: Unrestricted downward movement of percolating water is needed to leach salts from salt affected areas. Early spring flooding or subirrigation results in a shallow water table that impedes drainage. It may be feasible to time cropping practices around a seasonal water table to avoid flooding and other adverse effects.
Tillage-induced compaction can often limit downward water movement. In these cases, it is important to locate compacted zone and deal with them. Soil layers that differ by two or three textural classes also limit water percolation. An example is a clay loam soil that underlays a sandy loam surface soil or vice versa.
NRCS soil survey Current soil survey information – whether printed or on-line (http://websoilsurvey.sc.egov.usda.gov/) can be invaluable. It lists important physical properties for the soil series. Soil texture, infiltration rate, and shrink-swell potential are listed by soil depth. Other information is often available, including depth to seasonal water table, interpretations of engineering properties, and soil features affecting agricultural drainage.
Much of the damage has already occurred by the time saltaffected areas can be easily seen. Lab analysis provides an early warning for developing salt problems and provides more accurate diagnosis. Lab analysis should be used to identify the total salt content and the salt composition. (Also, see Crop File
4.03.013, Salt-Affected Soils: Interpreting Laboratory Results)
Total salt content is estimated from the electrical conductivity reading, usually on a saturated soil paste extract or “ECe”. The ECevalue is used to classify soils, to help choose salt tolerant crops, and to establish leaching water requirements.
Soils are formally considered “saline” when the ECe is over 4 mmho/cm, but plants have different tolerance to soil salinity. Salinity should be defined case by case since a salinity level detrimental to sensitive crops may not affect a tolerant crop. Table 2 has general guidelines for crop response to increasing salinity.
¶ Table 2. General Crop Response to Increasing Soil Salinity |
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| ECe mmho/cm | Salinity Category | Crop Response |
| 0 – 1 | Non-saline | Salinity effects mostly negligible |
| 1 – 2 | Very slightly saline | Yields of very sensitive may be restricted |
| 2 – 4 | Slightly saline | Yields of moderately sensitive crops may be restricted |
| 4 – 8 | Moderately saline | Yields of many crops may be restricted |
| 8 – 16 | Strongly saline | Only tolerant crops expected to yield satisfactorily |
| > 16 | Very strongly saline | Only a few tolerant crops expected to yield satisfactorily |
Salt composition can be partially identified with the exchangeable sodium percentage (ESP) and/or the sodium adsorption ratio (SAR). These values help predict and evaluate the effect of sodium on the soil structure. Then, they are used to calculate rates of soil amendments needed for reclamation.
A soil is formally classified as “sodic” when the ESP is 15% or greater. Some use an SAR value of 12 (from a saturated soil paste extract) to identify sodic soils. The soil texture (especially clay content) and the shrink-swell potential (clay mineralogy) will affect the point at which excess sodium has an adverse effect on a particular soil. Table 3 has guidelines for more precisely identify when a soil may have symptoms of excess sodium.
¶ Table 3. Range of Exchangeable Sodium Percentage Used to Identify Sodic Conditions Based on Soil Characteristics |
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| Shrink-swell potential | |||
| Soil Texture | High | Medium | Low |
| Fine | 6 – 8 | 9 – 12 | 11 – 14 |
| Medium | 9 – 12 | 12 – 15 | 15 – 20 |
| Coarse | 12 –15 | 18 –20 | 23 – 26 |
| Fine-textured = silty clay loams, clays; Medium-textured = loams, silt loams, clay loams; Coarse-textured = sands, loamy sands, sandy loams | |||
Other lab analysis that may be useful include pH, chloride (Cl), boron (B), sulfate (SO4), and bicarbonate (HCO3). These tests help identify salt composition and potential nutrient toxicities more precisely. They may also help “fingerprint” and pinpoint the source of dissolved salts.
Only one set of samples is needed if the field is uniformly saline. When salt-affected areas are spotty, take at least one set of samples from the salt-affected area and one set from the nonsaline area for
Soil samples can be taken any time, but are probably best taken after the growing season, before post-harvest precipitation begins. Salt concentrations should be highest at this time.
Salinity can vary with soil depth. For example, some soils have a non-sodic surface, a sodic subsoil, and the deep horizons are saline or calcareous. Thus, separate samples taken from two or more soil depths can be very useful. Patterns of salt distribution at different depths can give clues to the salt source and necessary management strategies. Deep sampling also helps identify subsoil texture and compacted layers.
A good strategy is to collect three sets of samples from 0 - 6", 6 - 12", and 12 - 24" depths. Take samples 3 to 4 inches from the row to include the active root zone. Make sure the soil is compressed to insure uniform core size. About 10 to 15 cores are required per sample. Sometimes it is useful to take a surface sample immediately after planting to determine if salinity is causing germination problems. Take this sample 0 to 4 inches deep, right out of the seed slice.
If the salinity is high in the surface and declines progressively with depth, salts are probably being moved upward by capillarity from a water table. The capillary movement transports salts more rapidly upward than leaching transports them downward.
A wave-like "bulge" or enriched salinity zone in the soil profile is caused by salts being gradually moved upward or downward. This is evidence of a developing salinity problem or shows that water tables fluctuate through the year. Upward capillary movement may move salts upward for part of the year, while leaching moves them downward the rest of the year.
In recharge areas, like fallow fields or wind breaks, the salinity is low at the surface and increases progressively with depth. This occurs because salts are being leached out of the recharge area and being transported to a new area.
Lab analysis is needed to identify water quality in irrigated situations. Water is needed to leach salts below the root zone for reclaiming or maintaining salt-affected areas. This requires some degree of over-irrigating. Irrigation must be applied to exceed the crop demand with the extra water devoted to leaching.
Specific leaching requirements are based on soil salinity, water quality, and crop to be grown. Collect water samples after one irrigation cycle is complete. Water quality in wells drawing from shallow aquifers can change rapidly, so samples may be needed periodically through the irrigation season to adjust water management.
Plant analysis may be useful to help identify nutrient toxicities from constituents, like chloride and boron. Collect plant samples from the prescribed parts and growth stages.
Nutrient ranges are not always well defined for every crop at every growth stage. Comparison samples from "good" and "bad" areas are very useful.
Visual characteristics are useful for providing a general idea of the extent and severity of saline or sodic problems. Lab analysis is necessary to accurately measure the salt content and composition. Developing a management strategy can proceed, once all the information about the soil, crop, and water is in hand
James, D.W., R.J. Hanks, and J.J. Jurinak. 1982. “Salt Affected Soils” in Modern Irrigated Soils. Wiley-Interscience Publications, New York. pg. 136-169
Sumner, M. & R. Naidu. 1998. Sodic Soils - Distribution, Properties, Management, and Environmental Consequences. Oxford Univ. Press, New York. 171 pg.
Tanji, K.K., ed. 1990. “Overview: Diagnosis of Salinity Problems and Selection of Control Practices” in Agricultural Salinity Assessment and Management.
Amer. Society of Civil Engineers, New York. pg. 18-41.
Tanji, K.K., ed. 1990. “Field and Laboratory Measurements” in Agricultural Salinity Assessment and Management. Amer. Society of Civil Engineers, New York. pg. 201-219.