Sulphidic material (WRB)

Sulphidic material is one of the diagnostic materials used, in the WRB system, to discriminate some soils from others.

Description

Sulphidic material (from English sulphide) is a waterlogged deposit containing S, mostly in the form of sulphides, and only moderate amounts of calcium carbonate.

Diagnostic criteria

Sulphidic material has :

• a pH (1:1 in water) of 4.0 or more and 0.75 percent or more S (dry mass) and less than three times as much calcium carbonate equivalent as S

or :

• a pH (1:1 in water) of 4.0 or more that, if the material is incubated as a layer 1 cm thick, at field capacity at room temperature, drops 0.5 or more units to a pH of 4.0 or less (1:1 in water) within 8 weeks.

Field identification

In moist or wet conditions, deposits containing sulphides often show a golden shine, the colour of pyrite. Forced oxidation with a 30-percent hydrogen peroxide solution lowers the pH to 2.5 or less, the reaction may be vigorous in sunlight or on heating. Munsell colours range: hues of N, 5 Y, 5 GY, 5 BG, or 5 G; values of 2, 3 or 4; chroma always 1. The colour is usually unstable, and blackens upon exposure. Sulphidic clay is usually practically unripe. If the soil is disturbed, a whiff of rotten eggs may be noticed. This is accentuated by application of 1 M HCl.

See also

• The FAO reference text, (2007 version)