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.
(10KB
illustration)
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).
(30KB map)
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:
- saline seeps;
- salinization along road ditches;
- salinization along lagoon margins;
- salinization from irrigation;
- 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).
(10KB
illustration)
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.
(26KB
series of maps)
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.
(4KB
illustration)
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).
(15KB
illustration)
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.
(9KB
illustration)
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.
(6KB graph)
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).
(15KB
illustration)
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
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EB-57, May 2000
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