There are three main British soils; podzol, brown earth and gley.
Podzols are easily recognisable by their distinct layers or horizons. A grey or light-coloured 'E' horizon is the result of severe leaching, or eluviation, which washes out everything but quartz grains.
Eluviation is the removal of soil, clay, silt or fine organic matter in suspension from a soil horizon.
The iron and aluminium oxides collect in the 'B' horizon where the iron oxides can accumulate to form a thin layer of hardpan, which impedes drainage through the soil.
Illuviation is the process of deposition of soil material removed from one horizon to another, usually from an upper to a lower horizon as material is washed down profile by percolating water. Some iron and aluminium oxides get through the iron hardpan, giving this 'B' horizon its dull orange colour.
These soils are found where there is good drainage and soil water is strongly acidic. They tend to be found on the upper slopes of upland areas where precipitation is heavy or where the vegetation is coniferous forest, producing an acid humus.
The acidic conditions are not liked by soil organisms which would normally merge the boundaries of the horizons.