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Abrasion
The rubbing away (erosion) of a rock due to the scraping of sand or large material carried by water, wind or ice.
Acid brown soil
A brown soil that has a pH of about 5. In this kind of soil there is not enough humus to bind
all of the topsoil together, and
the clays are washed down into the subsoil, making it heavy and waterlogged. (See also: Eluviation and Illuviation.)
Acid rock
A type of igneous rock that consists predominantly of light- coloured minerals and more than two-thirds silica (for example, granite).
Acid soil
A soil with a pH of less than 7. A slightly acid soil (from pH 7 down to pH 5.5) still has enough nutrients in it for good plant growth. Most farmland soils in the cool, humid parts of the world (those with brown soils
or grey–brown soils) are slightly acid. However, a strongly acid soil (with a pH below 5.5) has relatively few nutrients in it and so is, in general, an infertile
soil. This kind of soil will only support plants that are specially adapted for scavenging nutrients. These plants include heath plants and coniferous trees. Tropical rain- forest trees are also adapted to living on very acid soils.
In general, very acid soils occur for two reasons. The rock from which they develop may contain few nutrients (for example, a sandstone or a granite), or the
 Acid brown soil – A soil that produces a strong acid reaction, and in which clays move downwards, causing the pores in the subsoil to be blocked by clay, giving the subsoil a much heavier texture than the topsoil. However, no iron moves, and so the soil is brown throughout. A greater degree of acidity would lead to a podzol soil forming.
Acid soil
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