Soil color

A soil's color doesn't necessarily provide useful information on its characteristics and yield potential. For example, it's commonly believed that dark colors (especially black) indicate high organic matter content and, therefore, high natural fertility. This is often true in temperate regions like the prairie grasslands of the Great Plains (USA) where there is a direct relationship between soil color and humus content - the blacker the soil, the more humus it contains and the more fertile the soil. However, this correlation isn't universally valid, because soil humus in warmer regions has a more brownish coloration. Also, parent rock itself can make a soil black. In fact, many black soils in the tropics and elsewhere owe their color not to high humus content but to a reaction of the calcium in their limestone parent material with only a small amount of humus.

Distinct red and yellow colors usually indicate very old and weathered soils likely to be acidic and low in natural fertility; their clay portion usually contains a high amount of ''tropical"-type clays (hydrous oxides of iron and aluminum, and 1:1 clays like kaolinite) that are lower in negative charge but less sticky when wet than soils high in "temperate" type clays (see Chapter 1).

Subsoil color is also a valuable indicator of how well drained a soil is as will be explained in the section on drainage in this chapter. Now, on to the 6 mayor "vital signs" of soil physical health.