From Soil Taxonomy:
Lower-case letters are used as suffixes to designate specific kinds of master horizons and layers. The term accumulation is used in many of the definitions of such horizons to indicate that these horizons must contain more of the material in question than is presumed to have been present in the parent material. The suffix symbols and their meanings are as follows:
a -Highly decomposed organic material -This symbol is used with O to indicate the most highly decomposed organic materials, which have a rubbed fiber content of less than 17 percent of the volume.
b -Buried genetic horizon -This symbol is used in mineral soils to indicate identifiable buried horizons with major genetic features that were developed before burial. Genetic horizons may or may not have formed in the overlying material, which may be either like or unlike the assumed parent material of the buried soil. This symbol is not used in organic soils or to separate an organic from a mineral layer.c -Concretions or nodules -This symbol indicates a significant accumulation of concretions or nodules. Cementation is required, but the cementing agent is not specified, except that it cannot be silica. This symbol is not used if the concretions or nodules consist of dolomite or calcite or more soluble salts, but it is used if the nodules or concretions are enriched with minerals that contain iron, aluminum, manganese, or titanium.
d -Physical root restriction -This symbol indicates root-restricting layers in naturally occurring or man-made unconsolidated sediments or materials, such as dense basal till, plow pans, and other mechanically compacted zones.e -Organic material of intermediate decomposition -This symbol is used with O to indicate organic materials of intermediate decomposition. Their rubbed fiber content is 17 to 40 percent (by volume).
f -Frozen soil -This symbol indicates that a horizon or layer contains permanent ice. The symbol is not used for seasonally frozen layers or for so-called dry permafrost (material that is colder than O°C but does not contain ice).g -Strong gleying -This symbol indicates either that iron has been reduced and removed during soil formation, or that saturation with stagnant water has preserved it in a reduced state. Most of the affected layers have a chroma of 2 or less, and many have redox concentrations. The low chroma can represent either the color of reduced iron or the color of uncoated sand and silt particles from which iron has been removed. The symbol g is not used for materials of low chroma that have no history of wetness, such as some shales or E horizons. If g is used with B, pedogenic change in addition to gleying is implied. If no other pedogenic change besides gleying has taken place, the horizon is designated Cg.
h -Illuvial accumulation of organic matter -This symbol is used with B to indicate the accumulation of illuvial, amorphous, dispersible organic-matter-sesquioxide complexes if the sesquioxide component is dominated by aluminum but is present only in very small quantities. The organo-sesquioxide material coats sand and silt particles. In some horizons, these coatings have coalesced, filled pores, and cemented the horizon. The symbol h is also used in combination with s as "Bhs" if the amount of sesquioxide component is significant but the color value and chroma, moist, of the horizon is 3 or less.i -Slightly decomposed organic material -This symbol is used with O to indicate the least decomposed of the organic materials. Its rubbed fiber content is 40 percent or more (by volume).
k -Accumulation of carbonates -This symbol indicates an accumulation of alkaline-earth carbonates, commonly calcium carbonate.m -Cementation or induration -This symbol indicates continuous or nearly continuous cementation. It is used only for horizons that are more than 90 percent cemented, although they may be fractured. The cemented layer is physically root-restrictive. The predominant cementing agent (or the two dominant cementing agents) may be indicated by using defined letter suffixes, singly or in pairs. The horizon suffix km indicates cementation by carbonates; qm: cementation by silica; sm: cementation by iron; ym: cementation by gypsum; kqm: cementation by lime and silica; and zm: cementation by salts more soluble than gypsum.
n -Accumulation of sodium -This symbol indicates an accumulation of exchangeable sodium.o -Residual accumulation of sesquioxides -This symbol indicates a residual accumulation of sesquioxides.
p -Tillage or other disturbance -This symbol indicates a disturbance of the surface layer by mechanical means, pasturing, or similar uses. A disturbed organic horizon is designated Op. A disturbed mineral horizon is designated Ap even though it is clearly a former E, B, or C horizon.q -Accumulation of silica -This symbol indicates an accumulation of secondary silica.
r -Weathered or soft bedrock -This symbol is used with C to indicate root-restrictive layers of saprolite such as weathered igneous rock, or of soft bedrock such as partly consolidated sandstone, siltstone, and shale. Excavation difficulty is low to high.s -Illuvial accumulation of sesquioxides and organic matter -This symbol is used with B to indicate an accumulation of illuvial, amorphous, dispersible organic-matter-sesquioxide complexes if both the organic-matter and sesquioxide components are significant, and if the color value and chroma, moist, of the horizon is 4 or more. The symbol is also used in combination with h as "Bhs" if both the organic-matter and sesquioxide components are significant, and if the color value and chroma, moist, is 3 or less.
ss -Presence of slickensides -This symbol indicates the presence of slickensides. Slickensides result directly from the swelling of clay minerals and shear failure, commonly at angles of 20 to 60 degrees above horizontal. They are indicators that other vertic characteristics, such as wedge-shaped peds and surface cracks, may be present.t -Accumulation of silicate clay -This symbol indicates an accumulation of silicate clay that has either formed and subsequently been translocated within the horizon or has been moved into the horizon by illuviation, or both. At least some part of the horizon should show evidence of clay accumulation either as coatings on surfaces of peds or in pores, or as lamellae or as bridges between mineral grains.
v -Plinthite -This symbol indicates the presence of iron-rich, humus-poor reddish material that is firm or very firm when moist and hardens irreversibly when exposed to the atmosphere and to repeated wetting and drying.w -Development of color or structure -This symbol is used with B to indicate the development of color or structure, or both, with little or no apparent illuvial accumulation of material. It should not be used to indicate a transitional horizon.
x -Fragipan character -This symbol indicates a genetically developed layer that has a combination of firmness, brittleness, very coarse prisms with few to many bleached vertical faces, and commonly a higher bulk density than adjacent layers. Some part of the layer is physically root-restrictive.y -Accumulation of gypsum -This symbol indicates a gypsum accumulation.
z -Accumulation of salts more soluble than gypsum -This symbol indicates an accumulation of salts that are more soluble than gypsum.