soil
thin surface layer of the earths crust and the foundation for plant life
underlying bedrock
parent matrial
unweathered rock type found at the bottom of the soil
regolith
weathered rock
top layer of bedrock which has begun to be broken down by the elements of the weather
mineral matter
minerals derived from breaking down of bedrock by weathering
oraganic matter
soil
material from decaying organic matter e.g. leaves, plants, roots, organisms
eluviation
prcoess of rainwater washing organic matter down through the soil
leaching
process of rainwater washing minerals down through the soil
e.g. iron
illuviation
process of depositing mineral matter lower down in the soil
capillary action
upward movement of water within a soil
soil profile
name for a vertical cross section through a soil from the usrface to the underlying bedrock
factors affecting soil formation
- underlying rock type
- vegetation type
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soil properties
(other than their profile)
- acidity (pH)
- grain size
typical soil profile
- Ao - organic layer
- A - topsoil
- B - subsoil
- C - regolith (weathered rock)
- D - parent material (unweathered rock)
podsol
soil profile
- commonly found in taiga (coniferous forest)
- Ao - partial slow decay of vegetation due to cold climate; trees have shallow roots so dont bring materials up from weathered bedrock; pine needles decompose to form an acidic infertile MOR humus
- A - high precipitation and snow melt mean leaching takes place and minerals are washed from the topsoil leaving it infertile and grey in colour
- redisposition of minerals (illuviation) leaves hardpan layer which can sometimes impede drainage making topsoil wet
- B - reddish-brown layer due to leached minerals being redeposited (illuviation)
- C - parent material
- few organisms in soil means very little mixing and therefore distinct layers
brown earth
soil profile
- commonly found in temperate climate zones
- Ao - deap leaf litter which decays in warm climate to provide a rich fertile MULL humus (moderate rainfall helps this process)
- A - long tree roots bring up materials from weathered regolith producing fertile topsoil; precipitation > evaporation so some leaching occurs, but minerals remain in topsoil leaving it dark in colour
- B - the subsoil is also brown in colour due to some leaching of minerals (e.g. iron and aluminium) from the topsoil
- C - underlying rock type often sedimentary, this weathers easily releasing minerals into the soil
- no distinct horizons due to presence of biota, well aerated soil
gley
soil profile
- commonly found in tundra climate zones
- Ao - partially decayed peat like MOR humus - cold climate so slow decay of shrubs and heaths
- A - heavy rainfall/snow melt leaches minerals out of the soil, leading to a greyish infertile topsoil
- B - gleying occurs - pore spaces filled with water so lack of oxygen, this means oxygen is taken from iron compounds turning them from a red ferric colour to a grey ferrous colour; some red flecks (mottling) where iron compounds havent been fully broken down - due to small air pockets where reoxygenation takes place when soil dries out in summer
- C - frozen subsoil/permafrost/high water table (if flood plain) means water cannot drain away leading to waterlogging
- few organisms due to cool anaerobic conditions so little mixing occurs therefore distinct horizons
