Chapter 12: Solonetzic Order

A diagrammatic representation of profiles of some subgroups of the Solonetzic order is shown in Figure 39. Individual subgroups may include soils that have horizon sequences different from those shown. In the description of each subgroup, presented later in this chapter, a common horizon sequence is given; diagnostic horizons are underlined and some other commonly occurring horizons are listed.

Soils of the Solonetzic order have B horizons that are very hard when dry and swell to a sticky mass of very low permeability when wet. Typically the solonetzic B horizon has prismatic or columnar macrostructure that breaks to hard to extremely hard (when dry) blocky peds with dark coatings. They occur on saline parent materials in some areas of the semiarid to subhumid Interior Plains in association with Chernozemic soils and to a lesser extent with Luvisolic and Gleysolic soils. Most Solonetzic soils are associated with a vegetative cover of grasses and forbs. Although some occur under tree cover, it is thought that the trees did not become established until solodization was well under way.

Solonetzic soils are thought to have developed from parent materials that were more or less uniformly salinized with salts high in sodium. Leaching of salts by descending rainwater presumably results in deflocculation of the sodium-saturated colloids. The peptized colloids are apparently carried downward and deposited in the B horizon. Further leaching results in depletion of alkali cations in the A horizon, which becomes acidic and a platy Ae horizon usually develops. The underlying B horizon usually consists of darkly stained, fused, intact columnar peds. Structural breakdown of the upper part of the B horizon apparently occurs at an advanced stage of development as exchangeable sodium is leached downward. At this stage the solonetzic B usually breaks readily to blocky peds coated with white silicious powder. Complete destruction of the solonetzic B horizon is the most advanced stage of solodization. The rate of evolution through the stages of development depends on the salt content and hydraulic conductivity of the parent material and on the climate.

Most Solonetzic soils in Canada have a neutral to acidic A horizon indicating that some solodization has occurred. Soils with strongly alkaline A horizons, an early stage of Solonetzic soil formation, are uncommon in Canada. As solodization proceeds, the horizons of salt and lime accumulation move downward from the B to the C horizon. In most Solonetzic soils the saturation extract of the C horizon has a conductivity of at least 4 mS cm^-1. Solodization is arrested where saline groundwater is within capillary reach of the solum, and resalinization may occur in groundwater discharge areas.

Soils of the Solonetzic order have a solonetzic B horizon (Bn or Bnt). This horizon has columnar or prismatic structure, is hard to extremely hard when dry, and has a ratio of exchangeable Ca to Na of 10 or less. The macrostructural units usually break to form hard to extremely hard blocky peds with dark coatings. Solonetzic soils do not have permafrost within 1 m of the surface, a surface organic layer 60 cm or more in thickness if fibric or 40 cm or more if mesic or humic, a podzolic B horizon, or evidence of gleying strong enough to meet the criteria for Gleysolic soils.