Chapter 7: Gleysolic Order

A diagrammatic representation of profiles of some subgroups of the Gleysolic order is shown in Figure 32. 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.

Gleysolic soils are defined on the basis of color and mottling, which are considered to indicate the influence of periodic or sustained reducing conditions during their genesis. The criteria that follow apply to all horizons except Ah, Ap, and Ae. However, if the Ae horizon is thicker than 20 cm and its lower boundary is more than 60 cm below the mineral soil surface, the criteria do apply to the Ae. Also, if the Ah or Ap horizon is thicker than 50 cm, the color criteria apply to the mineral horizon immediately below. Apart from these exceptions the criteria are as follows: Gleysolic soils have a horizon or subhorizon, at least 10 cm thick (the upper boundary of which occurs within 50 cm of the mineral surface), with moist colors, as follows:

1. For all but red soil materials (hues of 5YR or redder where the soil color fades slowly upon treatment of the soil with dithionite).
   1. Dominant chromas of 1 or less or hues bluer than 10Y with or without mottles; or
   2. Dominant chromas of 2 or less in hues of 10YR and 7.5YR accompanied by prominent mottles 1 mm or larger in cross section and occupying at least 2% of the exposed, unsmeared 10 cm layer; or
   3. Dominant chromas of 3 or less in hues yellower than 10YR accompanied by prominent mottles 1 mm or larger in cross section and occupying at least 2% of the exposed, unsmeared 10 cm layer.
2. For red soil materials (hues of 5YR or redder where the soil color fades slowly upon treatment of the soil with dithionite).
   1. Distinct or prominent mottles at least 1 mm in diameter occupy at least 2% of the exposed, unsmeared 10 cm layer.

Soils of the Gleysolic order have properties that indicate prolonged periods of intermittent or continuous saturation with water and reducing conditions during their genesis. Saturation with water may result from either a high groundwater table or temporary accumulation of water above a relatively impermeable layer, or both. In contrast, soils saturated periodically with aerated water or saturated for prolonged cold periods, which restricts biological activity without developing properties associated with reducing conditions, are not classified as Gleysols.

Gleysolic soils are associated with a number of different moisture regimes that may change during the genesis of the soil. They commonly have peraquic or aquic regimes, but some have aqueous regimes and others are now rarely, if ever, saturated with water. Those that are rarely saturated now presumably had aquic moisture regimes in the past and were once under reducing conditions. Drainage, isostatic uplift, or other factors have resulted in a changed moisture regime in these soils.

Gleysolic soils occur in association with other soils in the landscape, in some cases as the dominant soils, in others as a minor component. In areas of subhumid climate, Gleysolic soils occur commonly in shallow depressions and on level lowlands that are saturated with water every spring. In more humid areas, they may also occur on slopes and on undulating terrain. The native vegetation associated with Gleysolic soils commonly differs from that of associated soils of other orders.

Some notes on the basis of the color criteria follow. The criteria are based on color because it is the most easily observable and most useful indicator of the oxidation-reduction status prevailing during soil genesis. Redox potential (Eh) measured at several depths within pedons throughout the period when the soil is not frozen provides useful information on current redox conditions. Values of Eh of 100 mV or less are associated with reduced forms of Mn and Fe. Such values, however, indicate only present redox conditions, not those that existed over long periods during which the soil developed. Similarly, monitoring of moisture regime properties, such as depth to water table, provides valuable information on the present state of the soil, but it does not necessarily indicate the prevailing moisture regime during soil genesis.

Color criteria used for red soil materials are different from those used for material of other colors because even prolonged saturation and, presumably, reducing conditions have not been found to result in the development of drab gray colors in such materials. Usually, however, such soils are mottled in horizons near the surface. In some cases gray mottles occur in a reddish matrix, in others strong brown or yellowish-red mottles occur in a matrix of lower chroma. The dominant color is considered to be the matrix color.

Exceptions had to be made in applying the criteria to soils with Ah, Ap, or Ae horizons because chromas of 1 occur in some horizons of oxidized soils. Furthermore, prominent mottling may occur in Ae horizons overlying relatively impermeable horizons of generally oxidized soils. In the case of thick Ae horizons, however, prominent mottling in the upper part of the horizon is thought to indicate periodic reducing conditions near the surface. These exceptions have not been tested and will probably require adjustment.

The color criteria specify a minimum size and abundance of mottles in a subhorizon 10 cm thick or thicker, because it seems unreasonable to base classification at the order level on the occurrence of few or fine mottles in a thin layer. Care is required in estimating the abundance of mottles; smearing of ocherous material on the profile can result in overestimates, and failure to look for both inped and exped mottles can result in underestimates. The use of mottle charts facilitates estimates of abundance.