THE VARIETY OF SOILS IN BRITAIN - The Soil - page 39

free drainage and ease of cultivation make them the most generally favoured for ar-
able cropping.
In association with these freely drained soils, imperfectly drained Gleyed Brown
Earths occur on gentle slopes that receive run-off or groundwater from above, when
soil texture and structure cause a moderate check to through drainage. The Bg hori-
zons are of duller grey-brown colour, with fine rust-like mottling along root channels
brought about by alternation of anaerobic and aerobic conditions.
In aerobic conditions the iron-oxidizing bacterium Thiobacillus ferrooxidans
converts ferrous iron compounds to ferric compounds ( see here ). It is the ferric oxides
that give the brown colour to freely drained Brown Earths. Gleying is a process that
occurs during anaerobic conditions caused by saturation of soil pores by water. T. fer-
rooxidans is unable to operate and the soil has a grey colour produced by the ferrous
iron compounds. The resulting Gleys are found, therefore, where there is poor or very
poor drainage, both in lowland and moorland situations, often as one member of a
catena of soil types on sloping ground, as mentioned earlier.
Land form, regional climate and soil texture (the prominence of clay, and to a
lesser extent, silt) all affect the relative frequency of Gleys, Gleyed Brown Earths
and Brown Earths in lowland soil associations. The agricultural system of 'ridge-
and-furrow' created surface relief partly in order to improve the drainage of such
gleyed soils. The ecological effects of this system are discussed in chapter 7 . Al-
though, today, Gleys are mainly used for grassland farming, modern sub-soil drainage
can maintain the water-table lower than it would be naturally, and thus allow a wider
range of agricultural uses.
Peaty Gleys are prominent moorland soils in the uplands and hills. They fall
between rather better drained Peaty Podzols ( see below ) , and the more or less stand-
ing water conditions of deep peat Organic Soils. Their wet, generally very acid, O
horizons overlie strongly gleyed, pale grey mineral horizons.
Podzolization is a weathering and leaching process. It can be thought of as taking
place in two stages. The first is release of iron and aluminium compounds, the second
their transport and re-deposition within the profile. As mentioned above, iron and alu-
minium oxides released by mineral weathering in Brown Earths are retained in the A
and B horizons as persistent coatings around soil particles. Where there is a very acid
parent material, and/or strongly acidifying litter (e.g. pine needles), perhaps also com-
bined with high rainfall, the mull humus of Brown Earths is replaced by superficial
accumulations of moder or mor humus. As in the sand dune example described earli-
er, organic acids leaching from these surface horizons attack the soil minerals more
strongly and carry dissolved salts further down the profile.