Although a minor factor in itself, soil colour is perhaps the characteristic that is first noticed about a soil. Colour can tell much about how a soil is formed and what it is made of. Soil horizons are usually distinguishable by colour differences. One sequence of colours ranges from white, through brown, to black as a result of an increasing content of humus which is finely divided, partially decomposed organic matter. Abundance of humus depends in a general way on luxuriance of vegetation and upon intensity of microbial activity, which in turn depend on climate. Thus we find that in middle latitudes, soils range from black or dark brown in the cool, humid areas to light brown or gray in the semi-arid steppe lands and deserts. Desert soils have little humus.
Reds and yellows are common colours in soils and are the result of small quantities of iron compounds. The red colour is particularly associated with sesquioxide of iron (Fe2O), whereas the yellow colour may indicate the presence of this same iron compound combined with water (hydrated iron oxide). Red colour indicates that the soil is well drained, but locally the colour may be derived from a red source rock such as a red shale or sandstone.
Grayish and bluish colours in soils of humid climates often mean the presence of reduced iron compounds (such as FeO) in the soil and indicate poor drainage or bog conditions. Grayish soils in dry climates mean a meagre amount of humus: a white colour may be a result of the deposit of salts in the soil. Although some recently formed soils retain the colour of the parent overburden or bedrock, the colour of fully developed soils is independent of what lies beneath.
Soil texture, a major characteristic of the soil, refers to particle sizes composing the soil. Particles are classified as various grades of gravel, sand, silt, and clay, in decreasing order of size.
The U.S. Department of Agriculture has set up standard definitions of soil-texture classes in which the proportions of sand, silt and clay are given in percentages. Rather than to attempt to list these classes and give the limiting percentages for each, the information is given in a triangular diagram which enables the percentages of all three components to be shown simultaneously. The comers of the triangle represent 100% of each of the three grades of particles-sand, silt, or clay. The word loam refers to a mixture in which no one of the three grades dominates over the other two. Loams therefore appear in the central region of the triangle. A particular soil whose components give it a position at Point A in the triangle has 65% sand, 20% silt, and 15% clay: it falls into a texture class known as sandy loam. Another soil, whose texture is represented by Point B has 33 1/3% sand, (33 1/3% Silt) and 33 1/3% clay; it falls into the class of a clay loam.
Texture is important because it largely determines the water retention and transmission properties of the soil. Sand may drain too rapidly; in a clay soil the individual pore spaces are too small for adequate drainage. Where clay and silt proportions are high, root penetration is difficult. Generally speaking, the loam textures are best for plant growth.
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