Soils 08

Soils: 

Soils

What is Soil?: 

What is Soil? Task 1: Watch the animation and make notes. 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.

How is soil formed?: 

How is soil formed? Part A 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 Part B True soil (topsoil) forms due to the addition of water, gas, living and dead organic matter.

The 5 main factors!!!: 

The 5 main factors!!! Passive Factors Parent material Topography Time Active Factors Climate Organisms (biota) (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 In some areas this is more important than others. In Britain it is a major factor. In the tropics, less so due to the intensity of weathering. The actual form the soil takes may depend on the dominant weathering process on the parent material. E.g. Granite. Case study: Isle of Purbeck

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Topography Large mountain ranges – will relate to climatic changes and thus weathering and length of growing season. Important localised changes include aspect, drainage, soil depth and slope gradient (affects erosion or succeptability to waterlogging) A CATENA is where soils are related to topography of a hillside. Case Study 1: A Catena in the Isle of Arran

Climate 1: 

Climate 1 Describe and explain the diagram to the left.

Climate: 

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. west of Rockies. Affects length of growing season Activity of soil organisms. Case study extension: Podsols (later) how exactly climate affects formation and structure. Case study 2 – rainforest climate and ferralitic soil v.imp.

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Case Study : Describe and explain how soil factors cause this pattern?

Organisms (biota): 

Organisms (biota) Organic matter is a basic component of soil In top 30 cm of one hectare of soil there is average of 25 tonnes of soil organisms 10 tonnes bacteria + 10 tonnes fungi Four tonnes worms – these ingest up to 40 tonnes soil per day and pass it to the surface. One tonne 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: 

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

Soil Profiles: 

Soil Profiles

The Soil Profile: 

The Soil Profile How soils develop over time (animation) A horizon – B horizon – C horizon - What is elluviation and illuviation Task: Draw a diagram of a profile and explain what each of the different horizons is. Describe why this is a simplified idea. (page 262)

Soil Profiles: 

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

Soil Profiles - The O Horizon: 

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: 

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: 

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 - Regolith: 

Soil Profiles - 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: 

Soil Classification Soils can be classified simply into 3 types Zonal Soils: These are soils 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: 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: Which have developed only recently and show little horion development. These include alluvial and volcanic soils which form independently of climate or vegetation.

Zonal and Intrazonal Examples: 

Zonal and Intrazonal Examples Before this we need to know some of the processes at work in soils.

Soil Forming Processes: 

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: 

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.: 

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.

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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.

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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 moors becomes acid which dissolves many bases, silica and also 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

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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 have Surface water or groundwater gleying. Think of your Catena on the Arran case study. What type of gleying is occuring. What about in podsols

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4, CAPILLIARY ACTION Calcification 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. From US case study – where would these occur? Interesting few pages to look at

Zonal Soil Examples: 

Zonal Soil Examples Case Study: Podsol, Churchill, Canada Task: learn how to draw an annotated diagram of this case study. Explain exactly why its profile has developed in such a way and why it has such a definate horizon. Location – found in wide belt of northern hemisphere associated with coniferous forests. Vegetation: pine needles release acid humus. Annual precipitation may be low but spring snowmelt releases water which cause heavy leaching. Organic acids remove iron and aluminium oxides form topsoil leaving high silica content. Illuviation of oxides in subsoil can cause an iron pan which impedes drainage. Soil horizons clearly defined as cold climate restricts action of soil biota and little mixing occurs.

Zonal Soil 2 – Ferralitic Soils of the rainforest: 

Zonal Soil 2 – Ferralitic Soils of the rainforest Very deep due to intense weathering. Small humus layer Rapid recycling of nutrients Bases leached rapidly in acidic conditions Non soluble oxides of fe and Al remain in top layers giving red colour Be able to draw a simple profile and explanation of the ferralitic profile Task: what effects could deforestation have on rainforest soils?

Intrazonal Soils: 

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. Tasks: Draw and annotate a profile of a Rendzina Describe why rendzinas form in areas of the isle of Purbeck. Intrazonal Soils

Intrazonal Soils: 

Intrazonal Soils 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. Tasks: Describe the characteristics of rendzina and the processes occurring on it. Be able to draw a profile Using the case study of Dartmoor describe how intrazonal soils develop on a catena.