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General description. The term secondary carbonates refers to translocated lime, soft enough to be cut readily with a finger nail, precipitated in place from the soil solution rather than inherited from a soil parent material. As a diagnostic property it should be present in significant quantities.
Field identification. Secondary carbonates must have some relation to the soil structure or fabric. Secondary carbonate accumulations may disrupt the fabric to form spheroidal aggregates or 'white eyes', that are soft and powdery when dry, or lime may be present as soft coatings in pores or on structural faces. If present as coatings, secondary carbonates cover 50 percent or more of the structural faces and are thick enough to be visible when moist. If present as soft nodules, they occupy 5 percent or more of the soil volume. Filaments (pseudomycelia), which come and go with changing moisture conditions, are not included in the definition of secondary carbonates.
General description. Soil material has stagnic properties (from L. stagnare, to flood) if it is, at least temporarily, completely saturated with surface water, unless drained, for a period long enough to allow reducing conditions to occur (this may range from a few days in the tropics to a few weeks in other areas), and show a stagnic colour pattern27.
27 A stagnic colour pattern shows mottling in such a way that the surfaces of the peas (or part of the soil matrix) are lighter (one Munsell value unit or more) and paler (one chrome unit or less) coloured, and the interior of the peas (or parts of the soil matrix) are more reddish (one hue unit or more) and brighter (one chrome unit or more) coloured than the non-redoximorphic parts of the layer, or of its mixed average. This mottling pattern may occur directly below the surface horizon or plough layer, or below an albic horizon.
Diagnostic criteria. Reducing conditions are evident by:
1. a value of rH in the soil solution of 19 or less; or
2. the presence of free Fe2+ as shown by the appearance of either:
a. a solid dark blue colour on a freshly broken surface of a field-wet soil sample, after spraying it with a 1% potassium ferric cyanide (K3Fe(III)(CN)6) solution; or
b. a strong red colour on a freshly broken surface of a field-wet soil sample after spraying it with a 0.2% a,a, dipyridyl solution in 10% acetic acid; and
3. an albic horizon or a stagnic colour pattern either:
a. in more than 50 percent of the soil volume if the soil is undisturbed; or
b. in 100 percent of the soil volume if the surface horizon is disturbed by ploughing.
Field identification. The distribution pattern of the redoximorphic features, with iron and manganese oxides concentrated in the inside of peas (or in the matrix if peas are absent) gives a good indication of stagnic properties.
General description. Strongly humic properties refer to soils which have a high organic carbon content in the upper metre of the soil.
Diagnostic criteria. To be strongly humic, soil material must have more than 1.4 percent organic carbon as weighted average over a depth of 100 cm from the soil surface (the same weighted average over 100 cm applies if the soil is 50-100 cm deep; soils less than 50 cm deep cannot be strongly humic). The calculation assumes a bulk density of 1.5 g cm-3.
Anthropogeomorphic soil material
Calcaric soil material
Fluvic soil material
Gypsiric soil material
Organic soil material
Sulfidic soil material
Tephric soil material
It appeared appropriate to define diagnostic soil materials. These diagnostic soil materials are intended to reflect the original parent materials, in which pedogenetic processes have not yet been so active that they have left a significant mark. They comprise anthropogenic, calcaric, fluvic, gypsiric, organic, sulfidic and tephric soil material. The fluvic, calcareous and calcaric, and gypsiferous properties of the Revised Legend (FAO, 1988) are redefined under fluvic, calcaric and gypsiric soil material.
Anthropogeomorphic soil material
General description. Anthropogeomorphic soil material (from Gr. anthropos, human) refers to unconsolidated mineral or organic material resulting largely from land fills, mine spoil, urban fill, garbage dumps, dredgings, etc., produced by human activities. It has, however, not been subject to a sufficiently long period of time to find significant expression of pedogenetic processes.
Descriptions of some anthropogeomorphic soil materials are given in Table 2.
TABLE 2 Some anthropogeomorphic soil materials
Aria |
Mineral soil material which has,
in one or more layers between 25 and 100 cm from the soil
surface, 3 percent or more (by volume) fragments of
diagnostic horizons which are not arranged in any
discernible order. |
Garbic |
Organic waste material; land fill
containing dominantly organic waste products. |
Reductic |
Waste products producing gaseous
emissions (e.g. methane, carbon dioxide) resulting in
anaerobic conditions in the material. |
Spolic |
Earthy material resulting from
industrial activities (mine spoil, river dredgings,
highway constructions, etc.). |
Urbic |
Earthy material containing
building rubble and artifacts (cultural debris > 35
percent by volume). |
Definition. Calcaric soil material (from En. calcareous) shows strong effervescence with 10 percent HCl in most of the fine earth. It applies to material which contains more than 2 percent calcium carbonate equivalent.
General description. Fluvic soil material (from L. fluvius, river) refers to fluviatile, marine and lacustrine sediments, which receive fresh material at regular intervals, or have received it in the recent past28.
28 Recent past covers the period during which the soil has been protected from flooding, e.g. by empoldering, embanking, canalization or artificial drainage, and during which time soil formation has not resulted in the development of any diagnostic subsurface horizon apart from a salic or sulfuric horizon.
Diagnostic criteria. Fluvic soil material is soil material which shows stratification in at least 25 percent of the soil volume over a specified depth; stratification may also be evident from an organic carbon content decreasing irregularly with depth, or remaining above 0.2 percent to a depth of 100 cm. Thin strata of sand may have less organic carbon if the finer sediments below, exclusive of buried A horizons, meet the latter requirement.
Field identification. Fluvic soil material shows stratification. Alternating darker coloured soil layers may reflect an irregular decrease in organic carbon content with depth.
Definition. Gypsiric soil material (from L. gypsum) is mineral soil material which contains 5 percent or more gypsum (by volume).
General description. Organic soil material consists of organic debris which accumulates at the surface under either wet or dry conditions and in which the mineral component does not significantly influence the soil properties.
Diagnostic criteria. Organic soil material must have one of the two following:
1. if saturated with water for long periods (unless artificially drained), and excluding live roots, either:
a. 18 percent organic carbon (30 percent organic matter) or more if the mineral fraction comprises 60 percent or more clay; or
b. 12 percent organic carbon (20 percent organic matter) or more if the mineral fraction has no clay; or
c. a proportional lower limit of organic carbon content between 12 and 18 percent if the clay content of the mineral fraction is between 0 and 60 percent; or
2. if never saturated with water for more than a few days, 20 percent or more organic carbon.
General description. Sulfidic soil material (from E. sulphide) is waterlogged deposit containing sulphur, mostly in the form of sulphides, and only moderate amounts of calcium carbonate.
Diagnostic criteria. Sulfidic soil material must have:
1. 0.75 percent or more sulphur (dry weight) and less than three times as much calcium carbonate equivalent as sulphur; and
2. pH (H2O) of more than 3.5.
Field identification. Deposits containing sulphides often show in moist or wet condition a golden shine, the colour of pyrite. Forced oxidation with a 30 percent hydrogen peroxide solution lowers the pH by 0.5 unit or more. Oxidation also gives rise to the smell of rotten eggs.
Tephric soil material29
29 Description and diagnostic criteria are adapted from Hewitt (1992).
General description. Tephric soil material (from Gr. tephra, pile ash) consists either of tephra, i.e. unconsolidated, non or only slightly weathered primary pyroclastic products of volcanic eruptions (including ash, cinders, lapilli, pumice, pumice-like vesicular pyroclastics, blocks or volcanic bombs), or of tephric deposits, i.e. tephra which has been reworked and mixed with material from other sources. This includes tephric loess, tephric blown sand and volcanogenic alluvium.
Diagnostic criteria. Tephric soil material must have:
1. 60 percent or more tephra; and
2. less than 0.4 percent Al + 1/2Fe, both extractable in acid oxalate (pH 3).
Relationships with some diagnostic horizons. The low amount of acid oxalate extractable aluminium and iron sets tephric soil material apart from vitric horizons.