Salinity and Sodicity in North Dakota Soils (continued)

EB 57, May 2000

What is Salinity and Sodicity?
Effects of Salinity and Sodicity
Location and Occurrence of Saline and Sodic Soils
Saline and Sodic Soil Management

Location and Occurrence of Saline and Sodic Soils

NATURALLY OCCURING SALINE SOILS

Saline and sodic soils generally occur on landscape positions where groundwater discharges by evapotranspiration from a shallow water table. In these soils, the water table will flucuate seasonally, but generally will be within 6 feet of the surface for a large part of the growing season. Because soil texture affects the flow of water through soil, it also affects the depth of the water table necessary to cause salinization. Coarse textured soils allow upward capillary movement of water to occur only over a short distance. Therefore a higher water table is required for salinization of coarse textured soils, such as sands or sandy loams, compared to finer textured soils, such as loams or clays. However, because the large pores in coarse textured soils offer much less resistance to water flow compared to the smaller pores in fine textured soils, salinization can occur much quicker in coarse textured soils.

As groundwater is discharged through the soil surface, dissolved salts precipitate and accumulate. Accumulations generally occur near or at the surface of the soil; however, they may occur anywhere in the soil where water is extracted by plant roots.

Discharge as evapotranspiration is only one part of the water balance at a given landscape position. Seasonal rain causes downward percolation of water that leaches salts from the upper parts of the soil.

Salinization occurs in those soils that favor discharge (evapotranspiration loss) over leaching. Not all soils with a water balance favoring discharge are saline. Salinization also depends on groundwater quality. If the groundwater has a only a small amount of soluble salts, salinization is very slow or may not occur at all.

Many combinations of ground-water discharge, water quality, and landscape position exist in North Dakota. As a result, a variety of different types of both saline and sodic soils have developed. Saline and sodic soils with similar properties can often be located by geographical area.

NORTHERN RED RIVER VALLEY
Saline soils in the Grand Forks County area (Figure 4) are the result of regional discharge of artesian groundwater from the Dakota Sandstone formation (Benz et al., 1961). The chemistry of these saline soils is uncommon for the northern Great Plains because of the accumulation of chloride salts. Sulfate salts generally dominate saline soils in North Dakota.

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Figure 4. The geologic cross section of Grand Forks County shows the relationship between the Dakota sandstone and the overlying glacial deposits. (After Benz et al., 1961.)

GLACIATED AREAS NORTH AND EAST OF THE MISSOURI RIVER
In glaciated areas, saline soils are associated with the edges of closed depressions or broad swales where discharge of groundwater can occur. Saline soils with most salts at the surface are associated with very shallow water tables. Saline soils with lower water tables may actually have the highest levels of salinity deeper in the soil (Seelig et al., 1987). Leached soils with little salt are often found in depressions within areas of saline soils.

WEST AND SOUTH OF THE MISSOURI RIVER
In nonglaciated areas of North Dakota the water table generally approaches the soil surface on low slopes that gently grade to natural drainageways (streams or rivers). Surface water flow and storage is controlled by the system of interconnected drainageways. This is different than the hydrology in glaciated areas that is controlled by a system of closed depressions (potholes).

Some saline and sodic soils west of the Missouri River are not necessarily related to groundwater discharge. These soils inherited salts from sedimentary material (marine shales, sandstones, etc.) in which they formed. They may be found on landscape positions that have very deep water tables.

Naturally Occurring Sodic Soils
Groundwater discharge may lead to the development of sodic soils as sodium salts are concentrated in the soil. During evaporation, sodium salts separate from calcium salts because of different solubilities. The soluble sodium salts concentrate high in the soil profile. As the soil solution becomes more concentrated with sodium, the percentage of sodium on the exchange complex also increases. Eventually there is enough exchangeable sodium to cause dispersion of clay and organic matter.

In the unglaciated parts of western North Dakota, sodium affected soils are quite common and the saline soils are generally also affected by sodium (Figure 5). Sodic soils are also found in the glaciated parts of North Dakota; however, saline soils unaffected by sodium are far more prevalent (Figure 5).

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Figure 5. The distribution of saline and sodic soils in North Dakota. (Adapted by D. D. Patterson, C. Fanning, and B. D. Seelig from the General Soil Map of North Dakota, Soil Survey Staff, 1961).

SECONDARY SALINIZATION

Secondary salinization is the term used to describe soils salinized as a consequence of human activities. In North Dakota there are five types of secondary salinization:

1. saline seeps;
2. salinization along road ditches;
3. salinization along lagoon margins;
4. salinization from irrigation;
5. salinization from wetland drainage.

Saline Seep Formation
Most saline seeps have developed recently (post settlement) in the northern Great Plains. In the last 20 years, investigators have concluded that saline seep formation is closely related to the practice of summer fallow for moisture conservation. Researchers have determined that the soil in the rooting zone under summer fallow reaches its storage capacity long before the end of the fallow period. Deep percolation of additional moisture beyond the rooting zone is likely to occur in this situation.

Rate and amount of downward percolation is controlled by soil texture. Average pore diameter in coarse textured soils is larger than in fine textured soils. Faster rates of water flow and less water storage are directly related to larger pore diameters. As a result, deep percolation is more likely to occur in coarse textured soils. In some places relatively coarse textured soils overlay impermeable material of finer texture. Under these circumstances, deep percolation leads to lateral water flow along the surface of the impermeable material (Deutsch, 1977; Seelig, 1978). If the contact between the two different materials approaches the soil surface along a hill slope, as often happens, the laterally moving water will create a wet spot that eventually becomes saline as the water is evaporated (Figure 6).

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Figure 6. A generalized diagram of a saline seep.

LOCATION AND TYPES OF SALINE SEEPS
Saline seeps are most common south and west of the Missouri River and on the Missouri Coteau. Areas most prone to saline seeps have sloping stratified geologic materials. They have been classified into six general groups according to the underlying materials (Figure 7). Doering and Sandoval (1976) estimated that 100,000 acres of western North Dakota cropland are affected by saline seeps. Although this is a small percentage of the total acreage of saline and sodic soils, saline seeps are a serious problem. They are responsible for deterioration of land that was once productive. Efforts to avoid wet saline spots on tracts of cultivated land increases costs of production, especially with today's farm machinery.

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Figure 7. Six classes of saline seeps found in North Dakota. (After Worcester et al., 1975.)

Road Ditch and Lagoon Margin Salinity
Saline soils that have developed around lagoons and drainage ditches are also controlled by the water source. Salinity along drainage ditches is due to lateral movement of water from the ditches to adjacent fields (Figure 8). Extremely flat areas, such as the Red River Valley, are particularly susceptible to this type of secondary salinization. The low grade on most drainage ditches allows water to stand for long periods of time.

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Figure 8. The location of maximum salinity and crop damage along a drainage ditch in the Red River Valley. (After Skarie et al., 1986.)

Salinity and Irrigation
Under irrigated agriculture, secondary salinization may occur if water is not properly managed. All water from sources other than precipitation contains some dissolved salt. As irrigation water is used by crops, salts are precipitated in the soil. This process may eventually lead to saline conditions that plants cannot tolerate (Figure 9).

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Figure 9. Secondary salinization by irrigation (A) and (B) may be prevented, if the field is adequately drained (C).

The rate at which salinization proceeds depends on the amount and quality of water added. Water with high amounts of dissolved salts will cause rapid salinization. Poorly designed and improperly managed irrigation systems have been responsible for salinization of thousands of acres in the U.S. Before an irrigation system is developed, soil and water compatibility should always be determined by a qualified soil scientist.

Wetland Drainage and Salinity
Drainage of certain types of wetlands in the northern Great Plains may also lead to soil salinization in the wetland interior. Class 3 and 4 wetlands are noted for their non-saline soils but are often surrounded by saline soils on the wetland edge. Hydrologically, many of these wetlands are called flowthrough wetlands, because water flows laterally through the wetland soils and does not allow accumulation of salts (Figure 10). When the hydrology of a flowthrough wetland is disrupted by drainage, evaporative discharge can occur through the drained wetland soil. Under the undrained condition, evaporative discharge is confined to the wetland edge; soils may be extremely saline at this location. When the area of evaporative discharge is expanded to the wetland interior, saline conditions may be expanded to the entire wetland (Figure 10). The final result of flowthrough wetland drainage is often no gain in crop production, but a loss of flood protection, groundwater recharge, and wildlife habitat.

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Figure 10. Salinity is confined to the edges of flowthrough wetlands (A), unless the wetlands are drained, and the area of evaporative discharge is expanded to the wetland basin (B).

Management stategies for saline and sodic soils must be designed with processes of salt accumulation in mind. Different management techniques may be necessary for soils with different salt compositions and water regimes.

Most saline and sodic soils are the result of evaporation from a shallow water table. Before effective methods of management can be designed for a specific parcel of land, the water table level must be determined. As long as a shallow water table exists, potential for salinization and sodification exists.

Saline and Sodic Soil Management

SALINE SOIL MANAGEMENT

The type of ions in the soil solution does not influence the osmotic potential to any large degree; it is the total of all ionic species that controls the osmotic potential. From a practical point of view, management of saline soils with different salt compositions is essentially the same.

Plant Tolerance to Salinity
Salt tolerant plant species should be selected on the basis of salt concentrations found in saline areas. Drought resistant plants such as grasses and small grains are generally more tolerant to salinity than row crops, trees, shrubs, and vegetables (U.S. Salinity Laboratory Staff, 1954). If an accurate estimate of field salinity is known, crop tolerance information can be used to determine the economic feasibility of growing certain crops.

Crop tolerance to salinity has been tested for some of the major crops in North Dakota (Figure 11). It is noteworthy that sunflower is more resistant to salinity than experience from other production areas would indicate.

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Figure 11. Thresholds of yield reduction due to salinity for some of the major crops in North Dakota. Salinity was expressed as the electrical conductivity (EC) of the saturated soil extract from the plow layer (0 to 6 inch depth). (After Maianu, 1983; 1984; and unpublished data; Maianu and Lukach, 1985; Nelson, unpublished data.)

Many cultivated crops are most susceptible to salinity during germination and early growth stages. If planting coincides with periods shortly after salts have been flushed from the surface, plant germination and seedling survival is more likely to be successful.

Reduction of Evaporative Discharge
Control of salinity must include management practices that reduce evaporative discharge. Subsurface drainage is an effective means of lowering the water table; however, it may not be economically feasible. Evaporative discharge is also affected by the condition of the soil surface and the plants growing on it. Surface mulches can be used to reduce evapotranspiration and salt accumulation.

SODIC SOIL MANAGEMENT

Soils affected by sodium must be managed similarly to saline soils with respect to drainage. Management for these soils includes drought tolerant plant species. Chemical amendments and physical disruption of the claypan may help to reduce the restrictive nature of these soils.

Calcium Amendments for Sodic Soils
Sodic soils may be improved by replacing adsorbed sodium with calcium. A number of amendments have been used with limited success over the years (U.S. Salinity Laboratory Staff, 1954). Amendments that release high amounts of calcium to the soil solution are the most effective. Unfortunately very soluble amendments, such as calcium chloride (CaCl₂), are cost prohibitive. Gypsum (CaSO₄) is an amendment less effective than calcium chloride but popular in many areas because of cost. Gypsum, however, is often ineffective on sodic soils in North Dakota, because they already have high amounts of gypsum in them. For those sodic soils in North Dakota that do not have gypsum in the upper part of the claypan, CaSO₄ may be an effective amendment. Soil inspection and analysis by a qualified soil specialist before calcium amendments are applied to fields with sodic soils is recommended.

Deep Plowing and Sodic Soil Improvement
Deep plowing has improved some sodic soils in western North Dakota (Sandoval, 1978). Deep plowing not only disrupts the restrictive claypan, but may also mix CaSO₄ from deeper soil layers into the claypan.

Caution should be taken not to plow too deep or too shallow. Plowing too deep may bring excessive amounts of soluble salts to the surface, creating a salinity problem. Plowing too shallow will not disrupt the lower part of the claypan and will not reach gypsum accumulations below it. The effectiveness of the deep plowing technique depends on the location of the water table and the presence of native gypsum in the soil.

Soil improvement through deep plowing is not expected to be sustained on a sodic soil that has a shal-low water table, because sodium salts are continually introduced to the soil through evaporative discharge. If the claypan already has high amounts of gypsum, mixing of gypsum from deeper substrata will have no effect on replacement of adsorbed sodium. Consultation with a qualified soil specialist is recommended before deep-plowing is attempted to improve fields with sodic soils.

SALINE-SODIC SOIL MANAGEMENT

Saline-sodic soils have unique problems with respect to management, because they have both high salinity and sodicity. Some saline-sodic soils have well aggregated structure, although high amounts of sodium are present. The SAR ranged from 18 to 30 and EC ranged from 10 to 20 dS/m in a typical saline-sodic soil from northeastern North Dakota. High salinity counteracts the dispersive effect of sodium. Attempts to leach the salts will likely result in a puddled soil, because the counteractive effect against dispersion is lost as the salts are removed.

MANAGEMENT OF SOILS AFFECTED BY SECONDARY SALINIZATION

Saline Seeps

RECHARGE AREA MANAGEMENT AND SALINE SEEP CONTROL
Saline soil improvement often includes managing the water table. In the case of saline seeps, the water table may be controlled by reducing local groundwater recharge. Eliminating summer fallow in upland recharge positions is generally the most efficient method of controlling the water table in a saline seep. Compared to native vegetation, cropping systems may not use moisture efficiently, particularly when summer fallow is included in the rotation. The result is high water tables that contribute to evaporative discharge and salinization of farmland. Where feasible, reduced summer fallow, continuous cropping, and inclusion of hay or pasture in long term rotations should reduce groundwater recharge in areas prone to saline seeps. Deep-rooted crops such as alfalfa have been shown to effectively withdraw moisture from recharge areas and reduce discharge from saline seeps downslope (Brun and Worcester, 1974; 1975).

INTERCEPTION MANAGEMENT AND SALINE SEEP CONTROL
Other management practices focus on interception of lateral flow with tile drains or bands of deep rooted crops. These methods have been successful; however, they have serious disadvantages. Interception of saline water with tile drains can be costly, and the problem of saline water disposal is created. Interception of lateral water with deep rooted crops may be short lived if salts build up in the root zone. Interception methods fail to deal with the source of the problem, recharge from locations farther upslope.

Road Ditch and Lagoon Margin Management for Salinity
Lateral movement of water to soils adjacent to road ditches can be reduced by preventing water from standing in the drainage ditches. Ditches should be designed and maintained to move water rapidly and minimize standing water. Lagoons should be designed to prevent leakage that causes salinization of adjacent soils.

Irrigation Management to Prevent Secondary Salinization

LEACHING AND DRAINAGE
When subsurface drainage is adequate, salt accumulation in irrigated soils can be avoided by applying more water than is needed by the crop. Excess water dissolves salts and percolates beyond the root zone (Figure 9).

The amount of water needed to adequately leach the salts (leaching requirement) is determined by the quality of the irrigation water. Poor quality water (high salinity) dictates a larger leaching requirement. Proper subsurface drainage is absolutely necessary to prevent shallow water tables from developing below irrigated soils. A shallow water table under an irrigated soil defeats the application of leaching water, because leached salts are moved back into the rooting zone by capillary action (Figure 9).

FIELD LEVELLING
Secondary salinization can occur from uneven distribution of irrigation water due to irregular topography. Microdepressions act as points of focused recharge; salts are leached from the recharge locations (Figure 12). Adjacent microknolls, however, act as points of focused evaporative discharge; salts accumulate and may cause salinity problems at the discharge locations (Figure 12). Levelling irrigated fields allows more even water distribution and avoids concentration of water and salts at specific places in the field (Figure 12).

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Figure 12. Secondary salinization in irrigated fields at points of evaporative discharge (A) may be eliminated by even distribution of irrigation water after field levelling (B).

BACK | Contents

REFERENCES

Benz, L. C., R. H. Mickelson, F. M. Sandoval, and C. W. Carlson. 1961. Ground-water investigations in a saline area of the Red River Valley, North Dakota. J. Geo. Res. 66:2435-2443.

Brun, L. J. and B. K. Worcester. 1974. The role of alfalfa in saline seep prevention. ND. Farm Res. 31:5:9-14.

Brun, L. J. and B. K. Worcester. 1975. Soil water extraction by alfalfa. Agron. J. 67:586-588.

Deutsch, R. L. 1977. Detection and characterization of saline seeps in western North Dakota. MS. Thesis, Soil Sci. Dep., North Dakota State University, Fargo.

Doering, E. J. and F. M. Sandoval. 1976. Hydrology of saline seeps in the northern Great Plains. Trans. Am. Soc. Agric. Eng. 19:856-865.

Keller, L. P., G. J. McCarthy, and J. L. Richardson. 1986. Mineralogy and stability of soil evaporites in North Dakota. Soil Sci. Soc. Am. J. 50:1069-1071.

Maianu, A. 1983. Salt tolerance of hard red spring wheat. In proceedings annual Agricultural Short Course and Trade Show, North Dakota Agric. Assoc. 311-312.

Maianu, A. 1984. Salt tolerance of soybeans in the Red River Valley. In proceedings annual Agricultural Short Course and Trade Show, North Dakota Agric. Assoc. 170-171.

Maianu, A., J. R. Lukach. 1985. Salt tolerance of barley. In proceedings annual Agricultural Short Course and Trade Show, North Dakota Agric. Assoc. 102-103.

Rengasamy, P., R. S. B. Greene, and G. W. Ford. 1984. The role of clay fraction in the particle arrangement and stability of soil aggregates - a review. Clay Res. 3:2:53-67.

Sandoval, F. M. 1978. Deep plowing improves sodic claypan soils. ND. Farm Res. 35:4:15-18.

Seelig, B. D. 1978. Hydrology and stratigraphy of saline seeps. MS. Thesis, Soil Sci. Dep., North Dakota State University, Fargo.

Seelig, B. D., J. L. Richardson, and W. T. Barker. 1990. Charactersitics and taxonomy of sodic soils as a function of landform position. Soil Sci. Soc. Am. J. 54:1690-1697.

Seelig, B. D., R. Carcoana, and A. Maianu. 1987. Identification of critical depth and critical salinity of the groundwater on North Dakota high water table cropland: Final project report to the North Dakota Water Resources Research Institute, Fargo.

Skarie, R. L., J. L. Richardson, A. Maianu, and G. K. Clambey. 1986. Soil and groundwater salinity along drainage ditches in eastern North Dakota. J. Env. Quality 15:335-340.

Soil Survey Staff, North Dakota State University and Soil Conservation Service. 1963. General soil map North Dakota. Agric. Exp. Station, North Dakota State University, Fargo.

Soil Survey Staff. 1975. Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. Agric. Handb. No. 436, SCS, USDA. U.S. Gov. Print. Off., Washington, D. C.

Soil Survey Staff. 1987. Sodic, sodic-saline, and saline soils of North Dakota. Misc. Publ., Soil Conservation Service, USDA, Bismarck, ND.

Soil Survey Staff. 1993. Soil Survey Manual, Hndbk 18, USDA, NRCS. U.S. Gov. Print. Off., Washington, D.C.

U.S. Salinity Laboratory Staff, 1954. Diagnosis and improvement of saline and alkali soils. Agric. Hanb. No. 60, USDA. U.S. Gov. Print. Off., Washington, D. C.

Worcester, B. K. and B. D. Seelig. 1976. Plant indicators of saline seep. ND. Farm Res. 34:1:18-20.

Worcester, B. K., L. J. Brun, and E. J. Doering. 1975. Classification and management of saline seeps in western North Dakota. ND. Farm Res. 33:1:3-7.

EB-57, May 2000