Of all the soil-related problems for crop production in Kansas, one of the most potentially damaging for crops is high salinity. Fortunately, salinity problems affect a relatively small percentage of the total acres. The Arkansas River floodplain has the greatest concentration of salt-affected soils in Kansas.
In some cases, salinity problems occur because of a shallow water table and poor quality of the river water, as is the case with the Arkansas River floodplain. Other causes include soil formation from parent material high in soluble salts, poor quality irrigation water, excessive application rates of manure or other waste products, and spillage of brine water associated with oil production.
Some degree of salinity in the soil is normal, and even necessary because essential nutrients exist in the soil as part of the soluble salts. If soluble salt levels are too high, however, salt can reduce seed germination and plant growth. At this point, the soil is termed a “saline soil.” This is sometimes confused with “sodic” soils. Sodic soils are those with excessive levels of exchangeable sodium, but low levels of total salts. Saline-sodic soils have both high salt levels and high exchangeable sodium. The key factors used to diagnose whether a soil is saline, sodic, or saline-sodic are: electrical conductivity of the soil; soil pH; and percent sodium saturation of the cation exchange capacity. The following table summarizes the important differences between saline, sodic and saline-sodic soils.
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|
Soil Conductivity Mmhos/cm |
Soil pH |
Exchangeable sodium content |
Physical conditions |
|
Saline Soil |
Greater than 4.0 |
Less than 8.5 |
Less than 15% |
Normal |
|
Sodic soil |
Less than 4.0 |
Greater than 8.5 |
Greater than 15% |
Poor |
|
Saline-sodic soil |
Greater than 4.0 |
Less than 8.5 |
Greater than 15% |
Normal |
Saline and saline-sodic soils often have a white crust on the soil surface. Sodic soils usually have a brownish-black crust from the dispersion of organic matter.
When salt levels in the soil become too high, the osmotic pressure within the soil is increased to the point that soil water is held too tightly for plant roots to be able to absorb it. Most plants become stressed or die from lack of water uptake in a saline soil. Plant species vary markedly in their tolerance to salinity levels. Some species are quite tolerant, such as salt marsh grasses. Unfortunately, the major agronomic crops grown in Kansas are only moderately tolerant to salinity. Soybeans are slightly more sensitive than sorghum, corn, and wheat to salinity.
Measurement of soluble salt concentrations is normally made on a saturation paste by mixing just enough distilled water with the soil to totally saturate it. The specific conductance is then measured on the mixture directly or on a vacuum extraction of the mixture. A specific conductance of 4 millimhos per centimeter (mmhos/cm) or greater is defined as saline or saline-sodic. You also may see results reported as deciseimens per meter (ds/m). One mmhos/cm is equal to one ds/m, so these numeric values are equivalent.
Soil testing labs typically evaluate EC (electrical conductivity) as part of a routine analysis. Because saturation pastes are labor intensive to prepare and hard to duplicate among technicians without considerable experience, many soil test laboratories use an alternative method of sample preparation, which yields excellent results. An equal amount of distilled water and soil are mixed as a slurry and the specific conductance is determined on the mix. This is the same slurry mix used by the lab for determination of soil water pH. Results obtained by this alternative method cannot be interpreted using published salinity levels in most soils textbooks or handbooks which reference the saturated paste method. Conductivity values for this mix will be roughly half those found for a saturation paste on a medium-textured soil.
Reclamation of salt-affected soils is possible. The first step is to assess the situation through a salt-alkali soil test to verify that a salinity problem exists. Find out whether it is only a salinity problem, or if excess exchangeable sodium also is present.
The second step is to identify the source of the excess soluble salts and, if possible, eliminate the source. This may be a simple as stopping the manure application, or correcting drainage problems. In some cases, such as where the water table is high, it may not be possible to eliminate the source of the problem.
Reclamation of sodic soils requires the addition of a soluble calcium source, such as gypsum (calcium sulfate). The calcium in the gypsum will displace the exchangeable sodium from soil clay particles. Gypsum improves soil structure, allowing better water penetration and movement of the excess sodium into the subsoil. A salt-alkali soil test is needed to confirm the exchangeable sodium level is high, and to calculate the rate of gypsum required. Gypsum rates needed will vary from less than one ton per acre to more than 10 tons per acre. The actual rate is normally 1.7 tons gypsum per millequivalent of exchangeable sodium.
The final step is to leach out the excess salt from the root zone. Under natural rainfall conditions, leaching may be relatively slow. Practices that improve water retention and movement into the soil, such as enhanced residue cover, will be beneficial.
It’s important in a saline-sodic soil to treat with gypsum prior to starting an aggressive leaching program, since the physical properties of a saline-sodic soil are generally good. If one cuts off the source of the salts and leaches the free salts from the soil first, the sodium in the soil will destroy the physical properties, making leaching extremely slow and difficult. So make sure to follow the steps outlined above in sequence.
Abbreviated step-by-step procedure for reclamation:
Step 1. Take a salt-alkali soil test to determine specific problem.
Step 2. Identify source/cause of problem.
Step 3. Eliminate source of salt contamination if possible and establish drainage if necessary.
Step 4. Add chemical amendment (gypsum) to sodic or saline-sodic soils.
Step 5. Incorporate residue to improve water intake.
Step 6. Apply irrigation water (if available).
Step 7. Allow time for leaching and consider planting tolerant crops.
Crops differ in the ability to tolerate salt accumulation in soils, but if levels are high enough (more than 16 mmhos/cm) only tolerant plants will survive. As salts accumulate in soil, the soil solution
osmotic pressure increases. When this happens, the amount of water available for plant uptake decreases and plants exhibit poor growth and wilting even though the soil isn’t dry.
Crop selection can be a good management tool for moderately saline soils. The table below serves as a general guide of salt tolerance ratings for crops, realizing that management practices, irrigation water quality, environment, and crop variety also affect tolerance.
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Salt Tolerance Ratings for Various Field and Forage Crops |
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Sensitive (0-4 mmhos/cm) |
Moderately Tolerant (4-6 mmhos/cm) |
Tolerant (6-8 mmhos/cm) |
Highly Tolerant (8-12 mmhos/cm) |
|
Field beans (Dry) |
Corn |
Wheat |
Barley |
|
Red Clover |
Grain Sorghum |
Oats |
Rye |
|
Ladino Clover |
Soybeans |
Triticale |
Bermudagrass |
|
Alsike Clover |
Bromegrass |
Sunflowers |
Crested Wheatgrass |
|
|
Sudangrass |
Alfalfa |
|
|
|
Sorghum-Sudans |
Tall Fescue |
|
|
|
|
Sweet Clovers |
|
Just as crops differ in tolerance to high salt concentrations, they also differ in their ability to withstand high sodium concentrations. Crop growth and development problems on sodic soils can be nutritional (sodium accumulation by plants), associated with poor soil physical conditions, or both. Plants on sodic soils usually show a burning or drying of tissue at leaf edges, progressing inward between veins. General stunting is also common. If sodium levels are high enough, all crops can be affected.
For more detailed information on saline and sodic soils, see K-State Extension publication MF-1022 at: http://www.ksre.ksu.edu/bookstore/pubs/MF1022.pdf
Dave Mengel, Soil Fertility Specialist
dmengel@ksu.edu
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu
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