· The name podsol derives from the Russian for “ash like”.
· Podsols are more typical of the more northern areas, where it is associated with the boreal forests, but can also be seen where heath plants occupy soils within the deciduous forest areas.
· These soils are characterised by the presence, just below the surface, of an ash coloured horizon. On world soils maps, podsols are shown as extending in a circumpolar belt from the Arctic Circle to about 50°N in Eurasia. This is distinctly associated with the coniferous forests of the northern latitudes.
· A podsol soil is one in which the soil constituents are redistributed by downward percolating rainwater. The water contains decomposition products, and can dissolve and extract further constituents when it passes through the soil.
· In acid conditions, fine particles of organic matter are capable of forming organo-mineral complexes which are carried down the profile.
· Because of the removal of the iron and aluminium and the breakdown of clays, there is a relative increase in the amount of silica remaining in the A horizon, and an increase in the amount of sesquioxides of iron and aluminium in the B horizon.
· This process results in the distinctive horizons of the podsol profile.
· Humus is limited by plant and tree type. Any humus formed is very acidic (mor), and provides acids to help dissolve the Fe and Al. Cold climate discourages organisms and the soil is too acidic for worms.
· Downward percolation causes the leaching of bases, the translocation of clays and organic matter, and the eluviation of sesquioxides of iron and aluminium.
· The A horizon is therefore narrow, ash grey, bleached and composed mainly of quartz sand and silica.
· The dark coloured organic matter is redeposited at the top of the B horizon.
· Beneath this, the sesquioxides of Fe then Al are deposited as a rust coloured hard pan.
· Often a convoluted shape, this acts as an impermeable layer, restricting downward moisture and plant root movement.
· Can cause waterlogging in the E horizon to give a gleyed podsol.
· The lower B horizon has an orange-brown colour, and lies above the regolith.
· Throughflow from here will usually contain bases in solution.
· Calcimorphic/calcareous soil like terra rossa.
· Rendzina develops where limestone/chalk is the parent material and where grasses form the surface vegetation, e.g. the English Downs.
· Grasses produce leaf litter rich in bases. This encourages considerable organism activity which aid the nutrient cycle.
· Dark brown/black mull humus.
· Alkaline (pH 7-8) because of calcification and associated lack of hydrogen ions.
· Calcium saturated clays, with crumb or blocky structure tend to limit water movement and consequently, there is little leaching.
· None; no leaching occurs.
· Underlying limestones are affected by chemical weathering and leave very little soluble residue.
· In addition, the bedrock is very permeable.
· This results in a thin soil with limited moisture reserves.
(3) Terra Rossa
· “deep red clay accumulated from the weathering of limestone of karst areas to form a distinctive parent material for soil formation”
· Name derives from the Italian for “red earth”, after the red colour of the soil.
· It is found in areas of heavy (seasonal) rainfall where the calcium carbonate parent rock is chemically weathered and silicates are leached to leave a residual deposit which is rich in iron hydroxides.
· Usually occurs in depressions within limestone and Mediterranean areas with garrigue vegetation.
(4) Gley Soils;
· Occurs when pore spaces are water filled to the exclusion of air.
· Lack of oxygen generates anaerobic conditions, which reduced iron compounds from ferric (Fe3+) to ferrous (Fe2+) form.
· Resultant soil has a grey/blue colour, with a scattering of red mottles. The red mottles represent pockets of aerobic activity.
· Gleying is a result of drainage, and almost independent of climate. Therefore it can occur in any zonal soil.
· Surface gleys are caused by slow infiltration rates through topsoil.
· Groundwater gleys result from a seasonal rise in the water table or impermeable parent rock.
(5) Brown Earths
· Brown earths are related to podsols in the sense that they are both leached soils.
· A typical brown earth should have the following features;
· Should be leached of carbonates. Some may be present in the C horizon, or as added lime.
· Should be neutral to moderately acidic (pH 4.5 -6.5) with base saturation and pH increasing with depth from the surface.
· Humus should be of the mull type - ranging from acidic to calcareous, or moder in podsol transition type.
· Composition of the clay fraction should be constant throughout - shown by the silica:sesquioxide ratio.
· Profile should be freely drained, although transitional gleys do occur.
· In Europe, the parent material is variable. Usually formed from deposits left after the Pleistocene. Non glacial brown earths are found on sandstones, siltstones, clay and loess.
· Brown earths are associated with deciduous woodland; oak, beech and ash in Europe.
· Climate is not as extreme as the podsol zone of the north. Winter temperatures average zero for less than three months, and summer temperature rarely exceed 21-26°C. Rainfall is evenly distributed (autumnal max.). Water received is enough to cause moderate leaching, but not podsolisation.
· Leaf litter is more varied that that of the coniferous forest. It is also of a higher nutrient content, and is more easily digested by the soil living fauna. An efficient breakdown of plant tissue results. Large numbers of fauna make a mull humus.
· Profile lacks the superficial horizons of organic matter.
· A Horizon:
· Enriched by the presence of organic matter to between 3 and 5% by weight in organic woodland soils.
· Most brown earths are used for agriculture, the original forest is long gone.