Solonetzic soils occur in the same ecozones (Prairies and Boreal Plains) as Chernozemic soils. The distinction between the two orders arises from the effect of a particular positively charged ion (or base cation) on soil formation and properties – sodium (Na+).
Soils of both orders form primarily on parent materials derived from glacial re-working of sedimentary rocks. In soils of the Chernozemic order, the main base cations released from weathering of the glacial sediments are calcium (Ca2+) and, to a lesser extent, magnesium (Mg2+); Na+ is much less common. In Chernozemic soils the amount of Ca2+ may be 20 to 40X greater than Na+. In some landscapes the amount of sodium in the soil is higher, and the ratio of Ca2+ to Na+ decreases; in Solonetzic soils the ratio is 10 or less (formally, the ratio of exchangeable Ca2+ to exchangeable Na+is 10 or less). This low Ca2+ to Na+ ratio is the key diagnostic criterion of the Solonetzic order, and horizons that meet this criterion are denoted with the n suffix (Bn).
The higher amounts of Na+ can arise from two main causes. First, the parent material itself may be high in sodium. This occurs mainly where shales that formed originally in marine environments (which are high in salts such as sodium chloride) have been incorporated into parent material through glacial re-working. In the second situation, the sodium has been increased through deposition of salts from groundwater in the soil profile (or salinization). In this process the salts are present in a dissolved form in the groundwater and transported through the sediments. When the groundwater is found close to the surface, water is lost by uptake by plants or by evaporation. The salts re-form (or precipitate), and the relative percentage of ions common in salts, such as Na+, increase in the soil.
If the groundwater table falls below the depth where it can interact with soil forming processes (beyond 1.5 to 2 m in typical soils), the negatively charged component of the salts (such as chloride (Cl-) or bicarbonate (HCO3-)) are transferred by vertically draining water into the C horizon (Cs, Csa, Csak) or out of the soil profile (called desalinization). This leaves Na+ behind in the upper part of the soil, where it is bound to negatively charged clay minerals (if clay is not present or is only present in low amounts, the Na+ will also be transferred out of the soil profile and Solonetzic soils will not develop). The high amount of sodium is recognized by the n suffix (Bn, Bnsa, Bnsak).
Sodium attracts a large sheath of water around it, and the individual clay particles are pushed apart and are readily moved with the vertically draining water to lower depths in the soil. This leads to a horizon which is depleted in clay (Ae), which is underlain by a horizon where the clay movement ceases and which therefore becomes enriched in clay and sodium (Bnt).
Eventually most of the clay is lost from the Ae, and the transfer of clay ceases (called a self-terminating process). The deposited clay clogs the pores in the Bnt horizon, which progressively reduces its ability to transmit water. The deposition of clay and shrinking and swelling of the clay causes a very strong prismatic structure to develop in this horizon. The prismatic structures break into well developed angular or sub-angular peds, often with dark staining on the surfaces of the peds.
Water cannot readily move into the Bnt horizon and accumulates at the base of the Ae, which accelerates chemical weathering of the Ae and top of the Bnt. The tops of the prismatic structural units become rounded and grey in colour (called roundtops), and a transitional horizon with characteristics of both the A and B forms (AB or BA horizon) from the weathering of the Bnt. The process of destruction of the Bnt by weathering is termed solodization.
In cultivated fields the upper soil horizons (Ap and Ae) are often lost by erosion, and the dense Bnt is found at the soil surface. When this layer dries out it is very difficult to penetrate with tillage equipment and is an inhospitable layer for root development. This layer is often referred to as hardpan. These eroded soils can be difficult to classify due to the absence of Ae horizon.
Solonetzic Great Groups
There are four great groups of the Solonetzic order. Three of them represent an idealized evolutionary sequence, which begins with soil with a Bn horizon (Solonetz great group). Sodium-enhanced clay translocation begins, creating an Ae and a Bnt horizon (Solodized Solonetz great group). When the upper Bnt is altered by the weathering processes outlined above, the soils are placed into the Solod great group. In many landscapes all three of these great groups can be found, often occupying distinct topographical positions in the catena.
In soils formed in parent material high in clay (typically > 60% clay) the Ae/Bnt sequence may be weakly expressed. These soils are classified into the Vertic Solonetz great group.
Solonetz soils
These soils have a solonetzic B horizon and either no Ae horizon or one less than or equal to 2 cm thick.
Solodized Solonetz soils
Soils of this great group have an Ae thicker than 2 cm and an intact Bnt or Bn.
Solod soils
These soils also have an Ae thicker than 2 cm as well as a distinct AB or BA horizon (formed ion the disintegrating Bnt).
Vertic Solonetz soils
These soils have any of the features listed above plus a slickenside horizon within 1 m of the soil surface.
Solonetzic Subgroups
For the most part the subgroups used in the Solonetzic order are based on the A horizon colour criteria that define the great groups of the Chernozemic order (view map of prairie soil zones). The A horizon colours in turn are regulated by the below-ground organic matter inputs into the soil and the decomposition rates of this organic matter, which are governed by vegetation and climate.
| Subgroup |
Great Groups
|
|||
|
Solnetz
|
Solodized Solonetz |
Solod
|
Vertic Solnetz
|
|
| Brown |
X
|
X
|
X
|
X
|
| Dark Brown |
X
|
X
|
X
|
X
|
| Black |
X
|
X
|
X
|
X
|
| Dark Gray |
Not applicable
|
X
|
X
|
Not applicable
|
| Gray |
Not applicable
|
X
|
X
|
Not applicable
|
| Alkaline |
X
|
Not applicable
|
Not applicable
|
Not applicable
|
: If faint to distinct mottles are found with 50 cm of the surface the term Gleyed is added before the subgroup (e.g. Gleyed Brown Solonetz)
Brown
These soils have an A horizon with colour values higher than 4.5 dry and usually have chromas higher than 1.5 dry.
Dark Brown
These soils have an A horizon with colour values between 3.5 and 4.5 dry and usually have chromas higher than 1.5 dry.
Black
These soils have an A horizon with colour values less than 3.5 dry and usually have chromas less than 1.5 dry.
Dark Gray
These soils have an A horizon with colour values between 3.5 and 4.5 dry and chromas less than 1.5 dry.
Gray
These soils have an A horizon with colour values greater than 4.5 dry and chromas usually lower than 2.
Alkaline
These soils have a Bn horizon and highly saline parent materials. The alkaline A horizon (ph of 8.5 or more) may be Brown, Dark Brown, or Black (according to the criteria outlined above). Soils of this subgroup are commonly associated with areas of active saline groundwater discharge.