Chapter 8: Luvisolic Order

A diagrammatic representation of profiles of some subgroups of the Luvisolic order is shown in Figure 33. 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 Luvisolic order generally have light-colored, eluvial horizons and have illuvial B horizons in which silicate clay has accumulated. These soils develop characteristically in well to imperfectly drained sites, in sandy loam to clay, base-saturated parent materials under forest vegetation in subhumid to humid, mild to very cold climates. Depending on the combination of soil environmental factors, some Luvisolic soils occur under conditions outside the range indicated as characteristic. For example, some Luvisolic soils develop in acid parent materials and some occur in forest-grassland transition zones.

Luvisolic soils occur from the southern extremity of Ontario to the zone of permafrost and from Newfoundland to British Columbia. The largest area of these soils occurs in the central to northern Interior Plains under deciduous, mixed, and coniferous forest. The Gray Luvisols of that area usually have well-developed, platy Ae horizons of low chroma, Bt horizons with moderate to strong prismatic or blocky structures, calcareous C horizons, and sola of high base saturation (neutral salt extraction). Gray Luvisols of the Atlantic Provinces commonly have Bt horizons of weak structure and low to moderate base saturation. The Gray Brown Luvisols of southern Ontario and some valleys of British Columbia characteristically have forest-mull Ah horizons, moderate to strong blocky structured Bt horizons, and calcareous C horizons.

Luvisolic soils have an eluvial and Bt horizon as defined. The Bt horizon must have the specified increase in clay over that in the eluvial horizon, clay skins indicative of translocated clay accounting for 1% or more of the area of a section through the horizon, and be at least 5 cm thick. Luvisolic soils may have Ah, Ahe, or dark-colored Ap horizons that satisfy one or more of the following conditions:

1. The dark-colored A horizon does not meet the requirements of a chernozemic A.
2. The dark-colored A horizon is underlain by a thicker, light-colored Ae horizon that extends to a depth of 15 cm from the mineral surface.
3. The dark-colored A horizon shows evidence of eluviation (Ahe or Ap) and is underlain by an Ae horizon at least 5 cm thick.
4. If the soil moisture regime is humid or wetter, the dark-colored A horizon may be of any kind.

Luvisolic soils have none of the following: a solonetzic B horizon, a podzolic B horizon if the upper boundary of the Bt horizon is at a depth of more than 50 cm from the mineral surface, evidence of gleying strong enough to meet the requirements of the Gleysolic order, organic horizons thick enough to meet the requirements of the Organic order, or permafrost within 1 m of the mineral surface and 2 m if the soils are cryoturbated.

The genesis of Luvisolic soils is thought to involve the suspension of clay in the soil solution near the soil surface, downward movement of that clay with the soil solution, and the deposition of the translocated clay at a depth where downward movement of the soil solution ceases or becomes very slow. Commonly, the subhorizon of maximum clay accumulation occurs above a Ck horizon. The eluvial horizon (Ahe, Ae) commonly has platy structure due perhaps to the periodic formation of ice lenses. Any condition that promotes dispersion of clay in the A horizons and deposition of this clay in a discrete subsurface horizon favors the development of Luvisolic soils.