SoilsPankaj Kumar Singh

What is Soil?

How is soil formed?• A layer of loose broken up parent

material (rock) accumlutates.• This is known as the regolith• This can come from breakdown of rock

by weathering or be brought from elsewhere. E.g. rock falls, glacial debris.

The 5 main factors!!!

• Passive Factors1. Parent material2. Topography3. TimeActive Factors1. Climate2. Organisms (biota)3. (human influence)

Professor Hans Jenny came up with the following

Soil=f(pm+cl+to+org+time)

However its easier to remember

CLORPT

Parent Material.

Parent Material.• Soils can develop from underlying rock or rock that has been

transported to an area.• The supply of minerals in the soil is dependent on that rock• It controls depth, texture, drainage colour and quality of the soil • The actual form the soil takes may depend on the dominant

weathering process on the parent material. E.g. Granite.

Topography • Large mountain ranges – will relate to climatic changes and thus

weathering and length of growing season.• Important localised changes include drainage, soil depth and slope

gradient (affects erosion or succeptability to waterlogging)

ClimateDescribe and explain the diagram to the left.

Climate• Climate influences soil type on a global scale and

links also to vegetation patterns.• It affects rate of chemical and biological reactions.• Case study: The relationship of rainfall to potential

evapotranspiration affects whether water moves down or up in the soil. This affects leaching or capillary action. Net downward movement = Pedalfer, e.g. east of Rockies. Net upward movement=pedocals e.g. Rajasthan.

• Affects length of growing season• Activity of soil organisms.

Organisms (biota)• Organic matter is a basic component of soil• soil organisms• bacteria + fungi• worms – these ingest soil per day and pass it to the surface.• others eg spiders, snails.• Organisms are more active in warm, well drained, aerated soils• They perform 3 functions• Decomposition: - detrivores eg termites eat and bury leaf litter

(detritus). Fungi and bacteria break it down further releases nutrient ions for plants. Rest stays as humus.

• Fixation: some bacteria fix nitrogen from air to nitrates• Develop structure: fungi bind soil particles. Burrowing animals

create pathways to aerate and allow drainage.• Man as an animal has effects relating to farming, deforestation

and extractive industries.

Time

• Usually take up to 400 years for 10mm to form.

• Newly formed reflect parent material but older ones may reflect other factors relating to climate and vegetation.

• Time taken for mature soils to develop depend on parent material and climate. Faster on sands and hot wet environments.

• Not a causative factor•Allows soil processes to operate and for soil to evolve.•Dependent on parent material, sandstones weather faster than granites.

The Soil Profile• How soils develop over time (animation)• A horizon – • B horizon –• C horizon -

• What is elluviation and illuviation

Soil ProfilesA soil profile is a vertical section through the soil showing its different horizons.

Soil Profiles - The O Horizon

•This is the top part of the A horizon.

•This is the organic layer. It is made up of leaf litter at various different stages of decomposition.

•Organic matter that has been completely decomposed forms a black, jelly-like substance called humus.

•It contains a lot of valuable nutrients.

Soil Profiles - The A Horizon

•This is where biological activity and humus content are at their maximum.

•This horizon is most affected by the leaching of soluble material and by the downward movement or eluviation.

•The A horizon is a combination of weathered rock from below and organic matter from above.

•These two constituents are brought together and mixed up by the actions of organisms living in the soil (earth worms)

•The A horizon is usually the most fertile.

Soil Profiles - The B Horizon

•This is the zone of accumulation or illuvation where clays and other materials removed from the A horizon are redeposited.

•The A and B horizons together make up the true soil.

•The B horizon is generally less fertile than the A horizon because it is further from the source of organic matter.

Soil Profiles – The C Horizon Regolith

• The C horizon consists of recently weathered parent material (regolith) resting on the bedrock.

• The weathered rock provides the soil with its mineral matter.

Soil Classification• Soils can be classified simply into 3 types• Zonal Soils: It is the soil which is well

developed soil in the process of time and having latitudinal distribution.This soils well associated with broad zones of climate and vegetation. It is simplistic but these are mature soils that have had maximum time for effect of climate and living matter on rock.

• Intrazonal Soils:Soils with in Zonal soil. These have been strongly influence by parent material and can develop in more than one climate zone e.g. rendzina soils (shallow and containing clacium caarbonate)

• Azonal Soils: Horizons are not developedWhich have developed only recently and show little horion development. These include alluvial and volcanic soils which form independently of climate or vegetation.

Soil Forming Processes

• Processes involve:- the gains and losses of material to the profile- The movement of water through the horizons - Chemical transformations within each individual horizon

• Soils Must be considered a open systems in a state of dynamic equilibrium varying constantly as the factors and processes that influence them alter.

Processes

• 1. Weathering. • Minerals released as inputs into • the soil store.• Name some of the weathering processes?• What factors may affect the rate of

weathering?

2. Humification and Cheluviation.Humification is breakdown of organic matter to humus

by soil organisms. Occurs in the H and upper A horizon.

Cheluviation: organic matter breaks down forming nutrients and organic acids – chelating agents.

They attack clays and other minerals and release Fe and Al

Chelating agents combine with cations in Fe and Al and form organic-metal compounds called CHELATES. These are soluble and move down through soil – CHELUVIATION

These may redeposit further down profile in less acidic soil.

3. Organic sorting. Lots of processes occur

which move organic material and minerals into horizons.

• a) Leaching. This involves the downward movement of water with minerals in solution Processes also associated are

• b) Eluviation –going down of particles not in solution.

• C) Illuviation – redeposition of minerals

Removal of soluble material. Where ppt exceeds evpt and drainage is good.

Rainwater (with acids from soil) causes chemical weathering and breaks down clays and soluble bases . Ca and Mg eluviated from A horizon making it more acid.

Podzolisation:

Cool climates where ppt a lot in excess of pevpt.Soils are well drained and sandy.

It is the removal of Fe and Al oxides with humus.

Often under coniferous forest or becomes acid which dissolves the sesquioxides of fe and iron. Leaves drained A horizon and reddish brown B horizon of illuviated sesquioxides. Often has an iron pan.

4. b) podsolisation and Gleying:

• Occur in poor drainage

Gleying: bad drainage gives anaerobic conditions. Pore spaces filled and de-oxygenised. Red colorued Fe3+(ferrous oxide) reduces to Fe2+ (ferric iron) which is grey blue. Can haveSurface water or groundwater gleying.

4, CAPILLIARY ACTIONCalcification• Low rainfall areas with

ppt =or just higher than pevpt.

• Some leaching but calcium accumulates in B horizon e.g. chernozems.

• Salinisation• When pevpt greater than

ppt. and where water table is near surface.

• Moisture evaporates from surface and salts move up by capillary action.

• May be caused by irrigation.

Interesting few pages to look at

• Reflect the dominance of a local factor

• Calcimorphic: - soils developing on limestone

• Hydromorphic: Having constantly high water content. E.g. gley soils

• Halomorphic: - soils with high levels of salt.

Intrazonal Soils