Soil genesis involves evaluating the roles of: 1) parent materials, 2) climate, 3) organisms, 4) topographic position, and 5) time in the development of soils. Aridisols are soils developed in regions where the evapotranspiration exceeds the precipitation in seven out of ten years. These climatic conditions limit the percolation of water, resulting in the accumulation of soluble and difficulty soluble salts. In humid regions, water percolation through the solum typically removes soluble salts; however, some soils in humid climates do exhibit sufficient concentrations of soluble salt that alter the soil profile and its properties.

Saline, sodic and saline-sodic soils are typically located in semi-arid and arid regions; however, these soils are also less frequently located in humid regions [1]. The exact definitions of saline, sodic, and saline-sodic soils are based largely on experimental protocols that reveal the intensity of soil properties associated with salt concentrations [2] [3] [4]. The exchangeable sodium percentage (ESP) is a measure of the sodium concentration associated with a soil horizon’s cation exchange capacity. ESP is expressed as [Na] × 100/CEC, where all concentrations have units of centimole proton charge per kg (cmol_p(+)/kg), or in the older literature as milliequivalents/100g-soil (meq/100g). The sodium adsorption ratio (SAR) is generally a measure of the sodium concentration in water or saturated paste extracts. The SAR is expressed as [ Na ] / { [ Ca ] + [ Mg ] } / 2 , where all concentrations are mole charge/liter. When the ESP is less than 50%, the ESP and SAR frequently exhibit a linear relationship; that is, ESP = K_G × SAR, where K_G is the Gapon constant [2] [3] [4].

Typically soils not considered sodic, saline or saline-sodic have water saturated electrical conductivity extracts (ECe) less than 4 dS∙m⁻¹ at 25˚C and saturated sodium adsorption extract ratios (SARe) less than 13. Saline soils have an ECe greater than 4 dS∙m⁻¹ and a SARe of less than 13. Sodic soils are soils exhibiting an ECe less than 4 dS∙m⁻¹ and a SARe of greater than 13. Typically, saline soils have an ESP of less than 15 and a pH less than 8.5, whereas sodic soils have an ESP of greater than 15 and a pH greater than 8.5. Saline-sodic soils generally exhibit an ESP of greater than 15 and a pH less than 8.5 [2] [3] [4].

The natric horizon, as defined in the “Keys of Soil Taxonomy” [5], requires “1) a thickness requirement based on soil texture and the presence or absence of lamellae, 2) evidence of clay illuviation (examples include clay bridging of sand grains, clay films lining pores, clay films on both vertical and horizontal surfaces of peds), 3) a greater clay content requirement than an overlying eluvial horizon (argillic horizons), 4) typically having columnar or prismatic structures, and 5) typically having an exchangeable sodium percentage (ESP) of 15 percent or more exchangeable magnesium plus sodium than calcium plus extractable acidity”. The reader is encouraged to observe the “Keys of Soil Taxonomy” for a more rigorous and precise listing of the criteria for a natric horizon. The absence of a natric horizon should not infer that plant productivity or the soil’s physical properties are not influenced by the presence of sodium, as sodium intolerant plants may be negatively impacted at an ESP of less than 15%. Therefore, Australia proposes an ESP greater than 6% as the significant onset for soil structure dispersion [6].

Calcite [CaCO₃], dolomite [CaCO₃MgCO₃], gypsum [CaSO₄∙2H₂O], and magnesite [MgCO₃] are abundant minerals in the soil environment and parent materials. However, based on the water status of the soils and the weathering of primary minerals, the following minerals may also be present: nahcolite [NaHCO₃], trona [Na₃H(CO₃)₂∙2H₂O], sylvite [KCl], halite [NaCl], kalicinite [KHCO₃], thermonatrite [Na₂CO₃∙H₂O], pirssonite [CaCO₃∙Na₂CO₃∙2H₂O], gaylussite [CaCO₃∙Na₂CO₃∙5H₂O]. Albite, a sodium bearing silicate (NaAlSi₃O₈), is also present in many soils and parent materials.

Sodium may negatively influence plant growth and development in several distinct domains: 1) degradation of the soil’s physical properties, 2) supporting alkaline to extremely alkaline pH values, and 3) increasing chemical activities involving sodium, molybdenum, and boron to toxic levels [2] [3] [4] [6]. Generally, if as little as 10 percent of the cation exchange complex becomes composed of sodium, then the soil’s physical properties are deleteriously affected.

Soil structures are complex flocculated and aggregated entities primarily composed of sand, silt, clay, and soil organic matter. An important soil physical property that is adversely affected by sodium is the wet aggregate stability. Divalent calcium exhibits stronger phyllosilicate adsorption, subsequently reducing the diffuse layer thickness and permitting greater soil structure strength. Conversely sodium exhibits weaker phyllosilicate adsorption, subsequently increasing the diffuse layer thickness and permitting substantially weaker soil structure strength. The resulting dispersion of the soil structure permits the textural separates to migrate and reduce the soil’s pore space, resulting in reduced: 1) water infiltration and percolation, 2) root penetration, and 3) aeration and gaseous exchange. The diffuse double layer (DDL) theory can predict swelling pressures in smectite systems, particularly with montmorillonite [7] [8]. Swelling pressures are effectively repulsion forces, with Ca-Na systems showing greater macroaggregate inhibition with increasing Na saturation of the exchange complex.

A frequently disturbed chemical property in sodic soils is an excessively alkaline pH. Soil pH increases with increased sodium activity, presumably because exchangeable Na acts as a Lewis acid, fostering hydrolysis and ion exchange, resulting in the generation of exchangeable H⁺ and increased solution activities of Na⁺ and OH⁻ [2] [3] [6]. The hydrolytic behavior of Na rests with its small ionic radius and a large effective ionic radius, permitting Na⁺ to be easily displaced from the diffuse layer by other common ionic species. Monovalent alkali cations display ionic radii patterns that support increased hydration spheres upon progression with atomic number. The hydration radius of ammonium is similar to potassium: Li < Na < K < NH₄ < Rb < Cs. Compared to calcium, magnesium has a greater hydration energy, supporting a larger hydration radius. The larger magnesium hydration shell supports soil structure dispersion, whereas calcium strengthens soil structure stability [6] [7].

The development of soil alkalinity reduces the plant availability of phosphorus, iron, manganese, copper, and zinc, potentially inducing plant nutrient deficiencies. Elemental toxicity may arise from pH-dependent plant availability of sodium, molybdenum, and boron. Boron toxicity arises from the development of soluble sodium metaborate, rather than the less soluble calcium metaborate. Molybdenum typically has greater plant availability in alkaline soils.

Rice (Oryza sativa) and burmuda grass (Cynodon dactylon) are considered examples of plants that are tolerant of sodic soils. Semi-tolerant plants include wheat (Triticum aestivum), barley (Hordeum vulgare) and oat (Avena sativa), whereas cowpea (Vigna unguiculata), lentil (Lens esculenta) and Maize/Corn (Zea mays) are susceptible. Sensitivity varies because of growth stage, environment (rainfall, evapotranspiration, soil drainage, and irrigation water quality), varieties, and cultural practices such as tillage and mulching [6].

Carbon dioxide will interact with water to form carbonic acid (H₂CO₃). The equilibrium expression is:

CO 2 ( g ) + H 2 O = H 2 CO 3         log K = − 1.46

or

[ H 2 CO 3 ] / [ P CO 2 ] = 10 − 1.46 ,

where P CO 2 is the partial pressure of carbon dioxide, having a value of 413 ppm. The acid protonation reactions for carbonic acid are:

H 2 CO 3 = H + + HCO 3 −         log K 1 = − 6.36

HCO 3 = H + + CO 3 2 −         log K 2 = − 10.33

Substitution of the H₂CO₃ protonation reaction will provide a pH of 5.6. Rainfall, which has not been influenced by oxides of nitrogen and sulfur and other anthropogenic species, is acidic.

Weathering reactions are thermodynamic predictable reactions involving earth minerals. Congruent weathering involves the dissolution of solid minerals, an example being the dissolution of calcite to Ca²⁺ and bicarbonate, which is expressed as:

CaCO 3 + H 2 O + CO 2 = Ca 2 + + 2HCO 3 − .

Incongruent weathering involves the conversion of a mineral to another mineral. Important to this study is the incongruent weathering of albite [2NaAlSi₃O₈(s)]. Three examples of the incongruent weathering reactions involving albite include: 1) hydrolysis, 2) carbonation, and 3) oxalic acid, which is an example of an organic acid emanating by plant roots.

1) Hydrolysis

2NaAlSi 3 O 8 ( s ) + 11H 2 O = 2Na + + 2OH − + 4H 4 SiO 4 [ silicicacid ] + Al 2 Si 2 O 5 ( OH ) 4 ( s ) [ kaolinite ]

2) Carbonation

2NaAlSi 3 O 8 ( s ) + 2CO 2 + 11H 2 O = 2Na + + 2HCO 3 − + 4H 4 SiO 4 + Al 2 Si 2 O 5 ( OH ) 4 ( s )

3) Albite weathering by oxalic acid (dicarboxylic acid):

2NaAlSi 3 O 8 ( s ) + H 2 C 2 O 4 + 9H 2 O = 2Na + + C 2 O 4 2 − + 4H 4 SiO 4 + Al 2 Si 2 O 5 ( OH ) 4 ( s ) .

In soil environments that experience leaching, soluble silicic acid may be removed from the system, supporting the weathering of albite to kaolinite. The influence of rainfall over protracted time periods supports congruent and incongruent weathering. Plant acids, particularly present in the root rhizosphere, similarly support weathering [6] [8].

First proposed by Brinkman [9], ferrolysis is mostly observed when seasonal wetting and drying cycles create alternating anoxic and oxic soil environments, which in turn, create sufficient acidity to degrade primary and secondary (phyllosilicate) minerals. In anoxic soil environments, carbon sources (CH₂O) and microbial populations respond with Fe-oxyhydroxides to produce ferrous iron (Fe²⁺):

CH 2 O + 4Fe ( OH ) 3 + 7H + = 4Fe 2 + + HCO 3 − + 10 H 2 O .

Carbon sources include particulate soil organic matter, mineralized humus, microbial or plant exuded organic acids, and other compounds. Ferrous iron (Fe²⁺) will adsorb onto the soil’s cation exchange complex, displacing Ca²⁺, Mg²⁺, and K⁺. Upon the establishment of an oxic soil environment, Fe²⁺ will oxidize to form Fe-oxyhydroxides, resulting in distinct zones of iron accumulation and depletion and the creation of soil acidity. The oxidation of Fe and creation of acidity may be represented as:

4Fe 2 + + O 2 + 10 H 2 O = 4Fe ( OH ) 3 + 8H + .

The establishment of active and reserve acidity should accentuate incongruent weathering; however, incongruent weathering of one phyllosilicate (montmorillonite) to another (kaolinite) may not readily occur [10].

The purpose of this manuscript is: 1) to present an in-depth review of sodium bearing soils in a humid continental climate (southeastern Missouri) and 2) to provide an assessment of the soil processes involved in sodic soil genesis.

The study area comprises the southeastern portion of Missouri and the northeastern portion of Arkansas within the Mississippi Embayment (Middle Mississippi River Alluvial Valley). The geologic history of the study area is complex, with multiple episodes of Pleistocene-Holocene valley entrenchment and deposition [11]. The Advance lowlands occupy areas between the Ozark Border region and Crowley’s Ridge in southeastern Missouri and northeastern Arkansas.

Within the Advance Lowlands, Roxanna loess (Altonian at 55,000 yr BP) and Peoria loess (Woodfordian at 20,000 yr BP) have been identified [12] [13] [14] [15] [16]. In Arkansas, West et al. [12] documented that Crowley’s Ridge loess was a succession of Peoria loess, Roxana loess and Loveland loess. The younger Peoria and Roxana loess sheets had an eastern source area, whereas the older Loveland loess had its source area in the Advance Lowlands. Rutledge et al. [13] investigated silty braided stream terraces in the Advance Lowlands, documenting that Peoria loess was superimposed on Loveland loess. West and Rutledge [14] investigated several silty sites in the Advance Lowlands and determined the source materials to be alluvium.

In Arkansas, Saucier [11] proposed that the Advance Lowlands have Peoria loess superimposed on Roxana loess, presumably on early or middle Wisconsin valley train. Subsequently, Blum et al. [15] using similar transects reinterpreted the valley train ages from middle Wisconsin to late Wisconsin. Royall et al. [16] suggested that extensive river meandering during or around 12,000 yr BP produced extensive areas of backswamp. By 9000 yr BP, the Mississippi River occupied a position along Sikeston Ridge. Approximately 9000 yr BP, the Mississippi River diverted through Thebes Gap and flowed east of Sikeston Ridge, occupying its modern location [16] [17].

Royall et al. [16] documented pollen assemblages in the Missouri Advance Lowlands. Between 14,500 to 18,000 yr BP, pollen assemblages indicate a boreal forest consisting of spruce (Picea) and northern pine (Dploxylon Pinus), fir (Abies) and minor stands of Tamarack (Larix laricina). Broadleaf representation consisted of oak (Quercus) and willow (Salix). With warming conditions (9500 to 14,500 yr BP), the pollen assemblages indicate a large oak, hickory (Carya) and hornbeam (Carpinus) population increase, suggesting the conversion of a boreal forest to a mixed conifer-northern hardwood forest. With continued warming (4500 to 9500 yr BP), oak and ash (Fraxinus) populations increased and the first recognized presence of bald cyprus (Taxodium distichum). The fine-grained sediments also indicated that silt and clays were being deposited from the Salem Plateau by flooding rivers. Willow, elm (Ulmus), sweetgum (Liquidambar), red maple (Acer rubrum) and locust (Gleditsia) dominated the landscape between 4500 yr BP to the present and oak declined in importance.

Currently, the mean annual temperature is 13˚C (56˚F), and mean annual precipitation is 1118 mm (44 inches). Approximately 53% of the rainfall occurs from April to September. The average winter temperature is 4.4˚C (40˚F) and the average summer temperature is 26˚C (79˚F). The average daily summer maximum is 33˚C (91˚F) [18].

Of the many soil series within the study area, the emphasis will be placed on three aqualfs. The Overcup series (fine, smectitic, thermic Vertic Albaqualfs) consists of very deep, poorly drained, very slowly permeable soils that formed in fine-textured alluvium. These soils are on broad low-lying terraces in the Advance Lowlands. The horizon sequence is Ap (ochric) - Eg (albic) - Btg (argillic) - Cg. The Foley series (fine-silty, mixed, active, thermic Albic Glossic Natraqualfs) consists of very deep, poorly drained, very slowly permeable soils that formed in silty terraces. The horizon sequence is Ap (ochric) - Eg (Albic) - B/E (B is argillic) - Btng (natric) - BCng (natric). The Lafe series (fine-silty, mixed, active, thermic Glossaquic Natrudalfs) consists of very deep, somewhat poorly drained, very slowly permeable soils that formed in silty Pleistocene terraces. The horizon sequence is Ap (ochric) - Eg-BE (Albic) - Btng (natric) - BCng (natric) [19]. The important phyllosilicates in the clay separate are smectite (montmorillonite), followed by hydroxy Al-interlayered vermiculite, hydrous mica (Illite), and minor quantities of kaolinite [1]. The Foley series spans 133,950 ha across the states of Arkansas, Missouri, and Louisiana, with Arkansas having the largest areal extent. In western Dunklin County, Missouri, the Foley series spans 982 ha and the Lafe series spans 170 ha. Correspondingly, the Overcup series spans 6615 ha.

Soil pedons were sampled in excavated pits and the soil profile descriptions were according to US Department of Agriculture criteria [20]. Soil pH in water, NH₄-Acetate (pH 7) exchangeable cations (Ca, Mg, K and Na), and BaCl₂-triethanolamine (pH 8.0) acidity are routine soil characterization procedures (Carter, 1993). Soil organic matter by loss on ignition, Bray-1 phosphorus, and 2M KCl extractable SO₄-S were also determined by the University of Missouri Soil Testing Laboratory. The clay, silt, and sand fractions were fractionated by Na-saturation of the exchange complex, washed with water-methanol mixtures, dispersed in Na₂CO₃ (pH 9.2), followed by centrifuge fractionation and wet sieving [21].

An aqua regia digestion was employed to obtain a near total estimation of elemental abundance associated with all but the most recalcitrant soil chemical environments. Aqua-regia does not appreciably degrade quartz, albite, orthoclase and other minerals; however, anorthite and phyllosilicates are partially digested. Homogenized samples (0.75 g) were equilibrated with 0.01 liter of aqua-regia (3 mole nitric acid: 1 mole hydrochloric acid) in a 35˚C incubator for 24 hours. Samples were shaken, centrifuged and filtered (0.45 µm), with a known aliquot volume analyzed using inductively coupled plasma emission - mass spectrometry. Selected samples were duplicated, and known reference materials were employed to guarantee analytical accuracy.