Salinity and Sodicity in North Dakota Soils

EB 57, May 2000
B. D. Seelig, Soil Scientist

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

What is Salinity and Sodicity?

Soil salinity and sodicity are related problems that are commom in North Dakota. Both eastern and western North Dakota have their share of saline and sodic soils. For example, 24 and 17 percent of the soils in Grand Forks County and Slope County, respectively, are affected by salinity or sodicity (Soil Survey Staff, 1987). Further, soil survey data from 34 of the 52 counties in North Dakota show approximately 1,900,000 acres affected by sodium and 700,000 acres affected by salinity. These problem soils compose about 9 percent of the 34-county area.

Salinity and sodicity are often seen as bare ground with whitish crust that has a scabby appearance. Scab-land, salt-land, alkali, alkaline, and sour-ground are a few of the local terms that describe these areas. Some of these terms are misleading, because they imply problems that are not necessarily related to salinity or sodicity. For instance, alkaline and sour-ground actually refer to soil acidity. Alkaline soils are basic (pH > 7) and sour-ground is acid (pH < 7). Many soils in North Dakota are basic in pH but are not affected by salinity or sodicity. On the other hand few soils in North Dakota have high enough acidity to reduce crop yields. Acid soils are occasionally found in in in the extreme south-central and southwestern parts of North Dakota. They have formed in acid sediments of the Fox Hills formation and are generally used as rangeland.

We are concerned with salinity and sodicity levels high enough to affect production. Clear definitions
of these two problems are necessary for proper identification and management.

SOLUBLE SALTS AND SALINE SOILS

Soils with high amounts of soluble salts are called saline soils. They often exhibit a whitish surface crust when dry. The solubility of calcium sulfate or gypsum (CaSO₄) is used as the standard for comparing solubilities of salts (Table 1). Salts more soluble than gypsum are considered to be soluble and cause salinity. Examples are sodium sulfate or Glauber's salt (Na₂SO₄) and sodium chloride, or table-salt (NaCl). Salts less soluble than gypsum are considered insoluble and do not cause salinity. Calcium carbonate (CaCO₃) or lime is an example of an insoluble salt commonly found in North Dakota soils.

Salts found in North Dakota soils are of three types: sulfates (SO₄); carbonates (CO₃); and chlorides (Cl). Most saline soils in North Dakota are composed of sulfate salts (Keller et al., 1984). However, the northern Red River Valley has extensive areas of saline soils that have high amounts of chloride salts. Soils with high amounts of carbonates of sodium occur rarely, and are usually associated with coarse textured materials.

Table 1. Composition and solubility of some common evaporite 
minerals (salts).
---------------------------------------------------------------
Mineral         Composition    Solubility   Chemical Name
---------------------------------------------------------------
                             (moles/liter)
Calcite (lime)  CaCO3           0.00014     Calcium Carbonate
Gypsum          CaSO4�2H2O      0.0154      Calcium Sulfate
----            CaCl2�6H2O      7.38        Calcium Chloride
Magnesite       MgCO3           0.001       Magnesium Carbonate
Hexahydrite     MgSO4�6H2O      4.15        Magnesium Sulfate
Epsomite        MgSO4�7H2O      3.07        Magnesium Sulfate
Bischofite      MgCl2�6H2O      5.84        Magnesium Chloride
(Washing soda)  Na2CO3�10H2O    2.77        Sodium Carbonate
(Baking soda)   NaHCO3          1.22        Sodium Bicarbonate
Mirabilite      Na2SO4�10H2O    1.96        Sodium Sulfate
Thenardite      NaSO4           3.45        Sodium Sulfate
Halite          NaCl            6.15        Sodium Chloride
---------------------------------------------------------------

SODIUM AND SODIC SOILS

Soils high in sodium (sodic soils) may present physical restrictions to plant growth. Sodium (Na⁺) is a positively charged component, or cation, of many salts. Sodium problems are due to its behavior when attached to clay particles. If 15 percent or more of the clay adsorption sites are occupied by sodium
(sodium-clay), poor physical condition of the soil often restricts root growth and makes tillage difficult.

The forces that hold clay particles together are greatly weakened when sodium-clay and water come into contact. In this condition clay particles are easily detached from larger aggregates, or dispersed (Figure 1). When dried, however, sodium-clay particles settle out in dense layers that are impenetrable to plant roots and seedling emergence.

(6KB illustration)
Figure 1. When the exchange complex has greater than 15 % sodium (Na) the clay is dispersed (A) resulting in poor soil structure. When calcium (Ca) replaces enough sodium the clay is flocculated (B); stable soil aggregates are formed that create good soil structure (C). (After Rengasamy, P., et al., 1984.)

SODIUM ADSORPTION RATIO AND SODICITY

Sodicity can be defined in terms of the sodium adsorption ratio (SAR). The SAR is a calculation from laboratory measurements made on a water sample or water extracted from a soil. It is based on the concentration of sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) in the sample. There is a relationship between the amount of each ion in solution and its relative amount adsorbed on the clay; however, the relationship is not direct. Na⁺, Ca²⁺, and Mg²⁺ are adsorbed to the clay with unequal strength. Na⁺ is weakly held compared to Ca²⁺ and Mg²⁺. The SAR computation (equ. 1) accomodates the difference in adsorption strengths.

SAR = [Na⁺] / {([Ca²⁺] + [Mg²⁺]) / 2}^1/2 [1]
where: [ ] = concentration in milliequivalents/liter (meq./l)

The U.S. Salinity Laboratory Staff (1954) determined a SAR of 13 from a saturated soil extract is comparable to 15 percent of the adsorption sites being occupied by Na⁺. Because of the difficulty and laboratory expense of analyzing the amount of actual Na⁺, Ca²⁺, and Mg²⁺ adsorbed on the clay, the SAR determination has become the standard measure for sodicity.

To demonstrate the SAR calculation and the importance of the ratio of ions as opposed to the total concentration of sodium, let's look at the following two examples:

Example 1

[Na⁺] = 25 meq/l
[Ca²⁺] = 60 meq/l
[Mg²⁺] = 30 meq/l
SAR = 25 / {(60 + 30) / 2}^1/2 = 25 / (45)^1/2 = 25 / 6.7 = 3.7

Example 2

[Na⁺] = 25 meq/l
[Ca²⁺] = 4 meq/l
[Mg²⁺] = 2 meq/l
SAR = 25 / {(4 + 2) / 2}^1/2 = 25 / (3)^1/2 = 25 / 1.7 = 14.7

The extracts from these two samples contain the same amount of sodium. However, because the amount of calcium and magnesium is different, only example 2 is from a soil that would have a sodicity problem. This points out the fact that some soils may have high amounts of sodium but are not sodic. The high amounts of calcium and magnesium counter-balance the sodium.

U.S. SALINITY LABORATORY CLASSIFICATION

The U.S. Salinity Laboratory Staff (1954) established a system for classifying saline and sodic soils in three broad categories.

Saline Soils
Saline soils are defined as having an electrical conductivity (EC) greater than 4 deciSiemens/meter (dS/m) and sodium adsorption ratio (SAR) of less than 13 in their saturation extract.

Saline-Sodic Soils
Saline-sodic soils have an EC greater than 4 dS/m and a SAR greater than 13 in their saturation extract.

Sodic Soils
Sodic soils have an EC of less than 4 dS/m and a SAR greater than 13 in their saturation extract.

NATURAL RESOURCES CONSERVATION SERVICE, USDA CLASSIFICATION

Saline and sodic soils are recognized and shown on soils maps made by the USDA Natural Resources Conservation Service (NRCS). Soils shown on maps are classified using a system that depends on both observable and laboratory properties. Although the NRCS has incorporated much of the U. S. Salinity Laboratory system into its classification scheme (Table 2), it relies heavily upon soil characteristics that can be observed in the field as opposed to laboratory determinations. Field observations of soil properties such as structure and salt crystals are used regularly, but laboratory data is determined on relatively few samples.

Table 2. NRCS classification of saline and sodic soils. 
(Soils Survey Staff, 1993)
---------------------------------------------------------
Saline Soils
class     Non-saline   Very slightly to    Strongly 
                       moderately saline    saline 

criteria   ^S.E. EC < S.E. EC = S.E. EC > 
             2 dS/m      2-16 dS/m 16 dS/m
---------------------------------------------------------
Natric (Sodic) Soils 
class      glossic   typic or udic    leptic     aquoll 
          subgroups    subgroups    subgroups   suborder 

subsoil 
criteria  *SAR > 13    *SAR > 13    *SAR > 13   *SAR > 13 
            weak        strong       claypan     claypan 
          claypan      claypan        high       poorly 
                                    salinity     drained 
---------------------------------------------------------
*Most natric soils meet this chemical requirement; however, 
some soils are also considered natric with SAR < 13 under 
certain conditions.
^Saturated extract (S.E.) electrical conductivity from the 
topsoil or the average of the soil profile, whichever 
is greater. 

Effects of Salinity and Sodicity

OSMOTIC EFFECT

High salinity (high EC) causes dehydration of plant cells. Reduced plant growth and often death is the result. Dissolved salts cause plant cell dehydration by decreasing the osmotic potential of soil water. Water flows from high potential (low salts) to a low potential (high salt) (Figure 2). When a soil solution has a lower osmotic potential than plant cells, plants cannot extract water from the soil. The effect on a plant is similar to drought stress.

(12KB illustration)
Figure 2. Plants can extract water easily from soils with high total water (A). This is a condition encountered in moist non-saline soils. Dry conditions and dissolved salts decrease the total soil water potential, thus retarding or stopping plant uptake of water (B).

Crop Yield and Soil EC
Yields of many crops are reduced noticeably when the soil extract EC reaches 4 dS/m (U.S. Salinity Laboratory Staff, 1954). Yields will decline proportionately as EC levels (salinity) increase above 4 dS/m. Some crops, such as sugar beets, are quite tolerant to EC between 4 and 8 dS/m. At an EC of 16 dS/m the growth and yields of nearly all crops are severely affected. The effect of salts on plant growth is the basis for the NRCS soil salinity categories (Table 2).

Crop Symptoms of Salinity
Crop symptoms from high salinity are generally the same as symptoms of moisture stress from dry conditions. Plants are stunted with cupped leaves to reduce water loss through the stomata. Initially some plants take on a deep blue-green color from excessive accumulation of wax as an attempt to reduce water loss. Eventually leaves become brown and brittle on the tips and margins as stress continues.

Salinity in the field is usually characterized by broad transitions that run gradually from high to low salinity. The pattern of crop response can also be a gradual change from normal plants to no growth at all. However, in some cultivated species, the plant is most sensitive to damage during germination or early growth stages. This may result in a pattern of sharp change from normal plants to bare ground where seeds failed to germinate. Often plants are able to flourish during later growth stages at salinity levels that would not have allowed germination or growth during earlier stages.

Plant Indicators
Plants can be used as indicators of saline and sodic conditions, because tolerance varies with plant species. All plants attempt to adjust to osmotic moisture stress caused by salinity. The osmotic adjustment requires energy that would otherwise be used for growth and production. As a result, when plants attempt to adjust, growth and yields are reduced.

HALOPHYTIC (SALT LOVING) PLANTS
Plants tolerant to salinity are known as halophytes. Technically, plants that can survive in soils that have greater than 0.5 percent (by weight) soluble salts are considered to be halophytes. Some halophytes have developed sophisticated systems of taking in saline water, extracting the salts, and then excreting the salts. Halophytes common to our area that can be used as indicators of high salinity include salt grass (Distichlis spicata), alkali grass (Puccinellia nuttalliana), cordgrass (Spartina gracilis), spearscale (Atriplex subspicata), salt-wort (Salicornia rubra), and sea blite (Suaeda depressa). Other less reliable indicator plants are wild barley (Hordeum jubatum) and kochia (Kochia scoparia). Although they often occur on saline soils, they are also regularly found in non-saline environments.

VEGETATION PATTERNS AND SALINITY
A study of vegetation and saline seeps in northwestern North Dakota showed a consistent pattern of salinity and vegetation (Figure 3). Wild barley was the dominant plant along seep margins with low salinity. Kochia was the dominant plant on the interior of the seep with high salinity. This is a common pattern seen on salt affected soils in North Dakota. Salt grass was the dominant vegetation on sodic soils with high levels of surface salinity on pastureland in central North Dakota (Seelig et al., 1990).

(4KB graph)
Figure 3. The dominance of plant species in saline seeps is shown by weight % versus EC. (After Worcester and Seelig, 1976.)

SPECIFIC ION EFFECT --
PLANT TOXICITIES AND DEFICIENCIES

Saline soils may also have specific ion effects on plant growth. Higher than normal concentrations of some ions can hinder or block nutrient uptake and certain physiological processes. Few instances of specific ion effects have been reported in North Dakota saline soils. Of the three common cations (Ca²⁺, Mg²⁺, and Na⁺) responsible for saline soils in North Dakota, no toxic ion effects have been reported. Toxic ion effects related to sodium (Na⁺) have been observed in some plants in other parts of the U.S.

Under rare circumstances, boron has been reported to cause toxic ion effects. Boron toxicity is a potential problem when concentrations in the saturation extract are greater than 1 milligram/liter (mg/l). This level of boron may occur in some saline soils in the northern Red River Valley. However, almost without exception, the osmotic effect plays the most im-portant role in limiting plant growth and yield on saline soils in North Dakota.

Neither cloride (Cl^-) nor sulfate (SO₄^2-) have been shown to cause plant toxicities in saline soils. Plant toxicities from excessive bicarbonate (HCO₃) have been reported in other areas; however, few saline soils in North Dakota have high levels of soluble HCO₃^-. Many soils in North Dakota do have large accumulations of calcium carbonate (CaCO₃) that is quite insoluble. Both phosphorus and micronutrient deficiencies are possible in high lime soils. It may be necessary to increase additions of phosphorus and micronutrients to overcome these problems.

HIGH pH AND SODIC SOILS

High pH (7.8 to 8.5) generally occurs in sodic soils. Extremely high pH (greater than 8.5) occurs in sodic soils when soda (NaHCO₃) or sodi-um carbonate (Na₂CO₃) is present. Sodium-clay, bicarbonate (HCO₃^-), and carbonate (CO₃^2-) react with water to form hydroxyl (OH^-) ions that cause high pH [equations 2, 3, and 4]. Soda and sodium carbonate are relatively soluble; therefore, high amounts of soluble carbonate is avail-able to react with water to produce OH^-. For this reason, sodic soils with soda and sodium carbonate are likely to have a pH greater than 8.5.

Na-clay + H₂O –> H-clay + Na⁺ + OH^- [2]

HCO₃^- + H₂O –> H₂CO₃ + OH^- [3]

CO₃^2- + H₂O –> HCO₃^- + OH^- [4]

Extremely high pHs (greater than 8.5) are generally not encountered in sodic soils of North Dakota, because soda and sodium carbonate are rarely present. When soluble carbonate salts are found, the soils generally have coarse textures. When the soil pH is greater than 9.5 live plant roots are vulnerable to deterioration.

Nutrient Availability and High pH
High pH affects plant growth by reducing availability of some plant nutrients. Phosphates are most avail-able to the plant at a pH between 6 and 7. Micronutrients such as iron, manganese, zinc, copper, and cobalt are all much less available at a pH greater than 7.

RESTRICTIVE SUBSOILS AND CRUSTING PROBLEMS

Slowly Permeable Claypans
Soil structure in sodic soils is poor and permeability is low. Permeability is the ease of air and water movement through soils. Good permeability allows water and gasses (oxygen and carbon dioxide) to circulate easily to and from plant roots. This is needed for good plant growth.

A dense layer of clay occurs at or near surface of sodic soils. This natural layer, often called a claypan, is a barrier to roots. Most roots are restricted to the topsoil above the claypan, because movement of water, nutrients, and gases is too slow in the claypan. A dry claypan can be hard enough to physically restrict root penetration. The overall effect on plant growth is one of stress similar to that caused by extremely dry or saline conditions.

UNDULATING OR SPOTTY PLANT GROWTH ON SODIC SOILS
Both claypan depth and root restriction vary considerably within a few feet. Depth to saline material is also highly variable. This variation in soil properties creates extreme differences in plant available water, plant rooting depth, and osmotic potential. Crop response to these extremes is an undulating or spotty pattern of growth that is characteristic of sodic soils.

Soil Crusting
Surface crusting is a common problem with cultivated sodic soils. Plowing mixes sodium-clay from the claypan with topsoil or surface material. Surface material with high amounts of sodium-clay is highly erodible, because it has low aggregate stability and is easily detached by rain drop impact. When dry, the surface forms a hard crust that is a barrier to seedling emergence.

Salinity and Metal Corrosion
Saline soils are corrosive with respect to certain materials commonly used for construction. When metal is placed in the soil, it is ex-posed to electrochemical reactions that change its physical properties; iron rusting is a good example. Soils high in soluble salts enhance electrochemical corrosion. Electrochemical corrosion can be reduced by coating the metal with organic materials, such as tar or pitch. Metal pipelines are often protected by passing an electrical current through them (cathodic protection) to replace the electrons lost to chemical reactions.

Sulfates and Concrete Corrosion
Soil solutions high in sulfate (SO₄^2-) are corrosive to concrete structures. The sulfate solution penetrates concrete and reacts with calcium in the cement to form CaSO₄ that precipitates within the pores. This reaction destroys the integrity of concrete in two ways. The cementing agent is changed to a non-cementing material (CaSO₄) and large crystals of CaSO₄ are formed within voids, causing physical disruption of the concrete.

Much of the damage caused by sulfate solutions can be avoided by using sulfate-resistant concrete. Type I (standard Portland cement) has little resistance to the corrosive action of sulfatic solutions. Type II has medium resistance to sulfates, and Type V has high resistance.

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EB-57, May 2000