Managing Saline Soils
in North Dakota

SF-1087 (revised), March 2007
David Franzen, Soil Science Specialist

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• Salt Accumulation Processes
• The Nature of North Dakota Salts
• Where Do Salts Accumulate?
• Saline Soil Management
  • Tile drainage
  • Tillage and seedbed preparation
  • Soil testing for salinity
  • Crop tolerance
  • Lower the water table and lower salinity risks
• Managing Sodic Soils
  • Use of gypsum as a sodium remediation amendment
• Summary of Saline Soil Management Tools
• Summary of Sodic Soil Management Tools
• References

Salt Accumulation Processes

Saline soils have salt levels high enough that either crop yields begin to suffer or cropping is impractical. Excessive salts injure plants by disrupting the uptake of water into roots and interfering with the uptake of competitive nutrients. Several factors contribute to the development of saline soils in North Dakota, but a high water table is a prime requirement. Recognizing how and why salts accumulate is the first step in farming profitably on land interspersed with saline soils. Preventing further encroachment of salinity and addressing remediation strategies are other steps.

The weathering of geologic materials has given rise to our present soils, but also produces salts that impact crop growth and yield. Lack of leaching in certain landscapes has kept the salts from leaving. The pattern of saline soils across the state results from years of natural salt redistribution. However, land use practices and rainfall patterns can influence the spread and severity of saline soil. A survey of growers from Hettinger County in 1968 showed that 51 percent of the reported saline soils had appeared within the eight years prior to the survey.

Leaching of salts over time has created shallow saline groundwater in wide areas of the state. Water flows down grade within the soil due to gravity. Where shallow saline groundwater occurs, salts often concentrate at or near the soil surface through capillary rise. In capillary rise, water moves from where the soil is saturated, or nearly so, to drier soil against the force of gravity, much like water moving into a dry sponge from a puddle of water on a floor. Evaporation then dries the soil and "pulls" water by capillary flow from the wet soil zone. Because only pure water evaporates, salts are left behind.

In silt loam soils, this rise can reach eight to nine feet above the water table. Theoretically, a rise of up to 15 feet is possible in a loam or silty clay loam soil (Knuteson et al, 1989). In sandy soils, which have larger pore sizes between soil particles, the pull is less, perhaps reaching 1.5 to 2 feet above the water table (Figure 1). Water movement toward the surface due to capillary rise provides a continuous supply of salts which accumulate in the root zone or at the soil surface when the capillary water evaporates. However, in a clay soil, salinity accumulation is rare, due to the development of strong aggregates which do not allow significant vertical capillary water movement.

Figure 1. Capillary rise from a water table depends on soil texture. Capillary rise will extend higher in a silt loam, silty clay loam and a very fine sandy loam than in a clay, clay loam or sand.

Groundwater produces a crop production paradox. Crops can utilize groundwater to supplement precipitation received during the growing season and achieve higher yields. However, groundwater too close to the surface can carry salts as well as water into the crop root zone, causing yield reductions and crop failures. Management of these soils must somehow balance seasonal water needs with salt reduction.

The Nature of North Dakota Salts

The salts most commonly found in concentrations that affect crop growth are sodium sulfate (Na₂SO₄), calcium sulfate (gypsum, CaSO₄) , magnesium sulfate (epsom salts, MgSO₄), sodium chloride (NaCl), calcium and magnesium chloride. North Dakota's saline soils are usually a mixture of salts, with sulfates being the most dominant form.

Most North Dakota salinity is due to calcium sulfate, magnesium sulfate and sodium sulfate. Chloride salts are also found in groundwater which has passed through or resides in geological materials with significant chloride in their solid matrix. Sodium chloride is the dominant salt in the saline soils of eastern Grand Forks County. Artesian flow from geologic deposits with significant sodium and chloride sources has added sodium chloride to shallow groundwater in that area. Saline soils develop where the evaporation exceeds the growing season rainfall, and local landscape features accumulate seasonal runoff to form a water table which at some point rises to less than six feet below the soil surface. The Northern Great Plains of the United States and Canada have vast areas that meet these criteria and where saline soils are common.

Where Do Salts Accumulate?

Figures 2a, 3 and 4 provide examples of where salts are commonly found in North Dakota landscapes due to shallow saline groundwater. It is common for potholes and slow-moving natural drains to have an accumulation, as shown in Figure 2a, a short distance from the water's edge. In this example, water can move significant distances laterally over a long period of time, flushing the soil of salts as it moves and concentrating these salts at the maximum depth above the water table where the capillary water rises and then evaporates. This condition is also common along road ditches, field ditches and next to sewage lagoons.

Figure 2a. Saline soil development near shallow streams, road ditches and sewage lagoons.

Figure 2b. Use of a 30-foot alfalfa strip along borders of shallow stream, road ditch or sewage lagoon prevents fringe salt deposition.

Figure 3 shows surface salt accumulation due to seasonally wet soils. A feature found in seasonally wet saline soils is a relatively low area with white, crusty, salty material, surrounded with sparse crop growth and a sharp boundary where crops grow reasonably well. It is common when examining soil in these low areas to see small pockets of crystalline salts in the plow layer. A subsoil sample beneath the fringe crop plants surrounding the bare area often reveals salt crystals there, also. However, crops in the depression edge usually grow normally. In this example, the crops rooting into the capillary fringe have enough water, but, through drying of the soil around the roots, salts accumulate in the soil at the top of the capillary fringe, somewhere below the surface.

Figure 3. Saline development in a nearly level landscape with a shallow, saline water table. Continuous cropping will help decrease development.

Figure 4 shows a condition in a subtly undulating landscape with a silty clay loam or silty clay texture. This landscape usually would have an elevation difference of only six to eight inches from top to bottom. Rainfall runs off the slowly permeable clay into the microrelief depressions in-between the higher elevations. Water then leaches out the salts in the depressions. Groundwater containing salt rises through capillary flow to the highest soil surface.

Figure 4. Saline development on a high clay content, subtly undulating landscape. Salt accumulates on high clay content ridges, while the low spots are leached of salts. Continuous cropping will help lower water table and stop salinity development.

In addition to these conditions, North Dakota also has large areas where a shallow water table lies under a relatively flat soil surface. Subsoil salt accumulation in these areas is widespread. High rainfall years raise water table levels, which bring salts to or near the surface, adversely affecting crop growth. Following drought and a lower water table, rains leach the salts to a lower depth. As the salts are washed lower, the salt concentration in the rooting zone is decreased and crop growth improves.

Another serious saline soil problem, especially in hillier regions of North Dakota, is saline seeps. Saline seeps form in the landscape when water percolates from higher elevations, reaches a zone of vertical discontinuity, usually a relatively impermeable layer such as loam material over clay, or a coal seam (Figure 5). Water will then move laterally, exiting at the side or bottom of the hill. In dry years, these areas are usually not saline. In wet years, they appear as salty areas in the side slopes and bottoms of hills.

Figure 5. Saline seep development.

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SF-1087 (revised), March 2007