SOIL COLOUR

The physical property of 'colour' of the soil, though a minor factor, can tell much about its formation and constituents. Soil horizons are distinguishable by col~ur differences. One sequence of colours ranges from white, through brown to black, as a result of an increasing quantity of humus. In the middle latitudes soils range from black or dark brown in the cool, humid areas to light brown or grey in the semi-arid steppe lands and deserts. Desert soils have little humus.

Red and yellow are common soil colours and are the results of small quantities of iron compounds. Red colour is particularly associated with iron oxide (Fep3)' Red colour indicates that the soil is well-drained, but locally the colour may be derived from a red source rock, like red shale or sandstone. Yellow colour may indicate the presence of the same iron compound combined with water (hydrated iron oxide). Greyish and bluish colour in soils of humid climates often means the presence of reduced iron compounds in the soil and indicates poor drainage or bog conditions. Greyish soils in dry climates mean a meagre amount of humus; a white colour may be the result of salt depositions in soil. Although some recently formed soils retain the colour of the parent material, the colour of a fully developed soil is independent of what lies beneath it.