k horizon. The accumulation of pedogenic carbonates in a soil is indicated by the suffix "k". It can occur within the three principal horizons: Ak, Bk or Ck. Ck is the most frequent situation, but also in the lower parts of the B carbonate accumulations are normal (Bk). In other terminologies the suffix "ca" is used instead of "k".
km horizon. Accumulation of calcium carbonate strongly cemented, consolidated, indurated (for example, Cmk marking a petrocalcic horizon within a C horizon). In some cases it appear with laminar habit (crusted horizon).
Soil Taxonomy (Soil Survey Staff, 1975) defines two types of diagnostic horizons for soils with carbonate acumulations:
Calcic horizon. The calcic horizon is enriched with secondary carbonates over a thickness of 15 cm or more, has a calcium carbonate equivalent content of 15 percent or more and at least 5 percent greater than that of a deeper horizon. The latter requirement is expressed by volume if the secondary carbonates in the calcic horizon occur as pendants on pebbles, or as concretions or soft powdery forms. If such a calcic horizon rests on very calcareous materials (40 percent or more calcium carbonate equivalent), the percentage of carbonates need not decrease with depth.
Petrocalcic horizon. A petrocalcic horizon is a continuous cemented or indurated calcic horizon and in places by calcium and some magnesium carbonate. Accesory silica may be present. The petrocalcic horizon is continuously cemented to the extent that dry fragments do not slake in water and roots cannot enter. It is massive or platy, extremely hard when dry so that it cannot be penetrated by spade or auger, and very firm to extremely firm when moist. Noncapillary pores are filled; hydraulic conductivity is moderately slow to very slow. It is usually thickner than 10 cm. A laminar capping is commonly present but is not required. Carbonates constitute half or more of the weight of the laminar horizon if this is present.
Calcic horizons evolution
The formation of a calcic horizon in soils has been studied by Gile and collaborators (1966) who have defined four phases during its formation , distinguishing two different situations depending whether the soils contained originally carbonates or not.
Evolution of non-gravelly calcic horizon, after Gile et al. (1966)
Stage I. The carbonate accumulation consists of a horizon with a few filamentary deposits or thin ped coatings of carbonates.
Stage II. Few to common carbonate nodules with K-fabric are formed; they may be soft or indurated.
Stage III. Is characterized by numerous, commonly cemented or indurated carbonate nodules with diffuse internodular carbonate impregnation and cementation. The pores in nodules are plugged with carbonate, and at the end of stage III, the internodular matrix also is impregnated and plugged with carbonates; the horizon has then a continuous K-fabric and is cemented.
Stage IV. Is achieved by deposition
of one or more almost pure carbonate laminae on top.
Evolution of gravelly calcic horizons
Stage I of carbonate accumulation in gravelly materials consists of a horizon in which pebble bottoms have thin, discontinuous authigenic carbonate coatings.
Stage II, the pebbles are continuously coated with carbonate,and some pebble interstices are filled with carbonate; the horizon has a continuous K-fabric and may be discontinuously cemented.
Stage III is completed when all, except the finest pores between pebbles, are plugged with authigenic carbonate, and the horizon commonly is cemented.
Stage IV consists of the formation of one or more thin carbonate laminae on top.
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