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Soils

Soils. Soil forms the thin surface layer of the earth’s surface. Soil composition. 4 main components of Soil Mineral Matter Organic Matter Air Water Mineral and Organic matter = 50% Remaining 50% = Air and Water. Soil Characteristics.

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Soils

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  1. Soils Soil forms the thin surface layer of the earth’s surface.

  2. Soil composition 4 main components of Soil • Mineral Matter • Organic Matter • Air • Water Mineral and Organic matter = 50% Remaining 50% = Air and Water

  3. Soil Characteristics The four major components are interlinked and determine the value of soil to agriculture--- they determine • Texture 2. Colour 3. Structure 4. Moisture / water retention 5. Humus / Organic matter 6. PH Value

  4. Mineral Matter > is the largest component > Minerals develop from weathered bedrock or material deposited by rivers. • Organic Matter > Comes from plants and animals – micro-organisms – earthworms, slugs and mites – after death plants and animals are reduced to dark brown organic matter called “Humus” • Air and Water proportions vary depending on climate and drainage.

  5. Texture The Texture of a soil is controlled by the amount sand, silt and clay particles in it. • Different soils have different amount of pore spaces, which affects soil aeration and drainage Three main textures: • Sandy • Silt • Clay

  6. Sandy Soils Positive Sandy Soil Properties • The coarse particles in these soils, whether wet or dry , are generally loose, making them easy to cultivate. • Large air spaces between the particles give good drainage and aeration. Negative Sandy Soil Properties • Rapid Loss water loss, minerals are leached and fertilisers need to be added regularly

  7. Silty Soils • Finer particles > pore spaces smaller > retaining more moisture • Lack organic nutrients > which can restrict seedling germination – easily eroded by heavy rainfall

  8. Clay Soils • Clay particles fit closely together resulting in poor drainage and aeration. • Prone to water logging and described as heavy. • They expanded when wet and tend to be sticky and shrink when dry and become hard. • Clay soils retain their nutrients but it is difficult for plant roots to penetrate them.

  9. Loam Soils • No single size dominates and consists of approx equal measures of sand, silt and clay. • Good drainage and aeration. • These soils are preferred by farmers and gardeners and are easily cultivated.

  10. Determining Soil Texture • Fill screw top jar two thirds. • Pour water into jar until almost full. • Shake jar for a minute. • Leave to settle for 24 hrs. • You will then be able to see the different sizes of particles > Heaviest at the bottom

  11. Structure • Structure refers to the shape of the soil grains calledPeds • Individual particles often gather in Peds due to cementing action of clay water and humus

  12. Fourtypes of Ped • Crumb – small rounded particles (breadcrumbs) 1 – 5mm • Platy – Thin horizontal particles often overlapping. • Blocky – roughly cube shaped close fitting particles. • Prismatic – column shaped particles with rounded tops

  13. PH Value • This is a measure of its degree of acidity or alkalinity

  14. PH Value • This is a measure of its degree of acidity or alkalinity • Low PH Value – Acidic • These soils lack calcium, magnesium and potassium • These soils have been leached heavily by rainfall. • Increased acidity discourages living organisms – reducing the rate of breakdown leading to Peat formation

  15. High PH Value = Alkaline • These soils have a high concentration of calcium (develop on chalk and limestone landscapes) • Best PH Value – 6.5…. most suitable for farming…..favours the growth of bacteria vital for the release of nutrients (sulphur and phosphorous) from organic matter.

  16. Organic Matter / Humus • The dark colour of humus is responsible for making the upper layers of soil dark, rich and fertile • Black or dark brown jelly like substance. • Formed from organic matter – decaying plants/animals. • Worms and insects digest organic matter to create humus (also mix and aerating soil) 

  17. Why Humus is important to soil: • Helps the soil to retain moisture. • Source of plant nutrients (carbon and nitrogen). • Improves soil structure > binding soil particles together > reducing risk of erosion. • It absorbs minerals that would otherwise be washed away.

  18. Water Content / Retention • Soil water is essential for plant growth. • The amount of water a soil can hold depends on the humus content, texture (sandy, silty, clay or loam) and structure (shape of Peds) • Soils rich in humus can hold more moisture than those which lack humus. • Sandy soils have low retention properties – water drains between the sand grains quickly. • Clay soils tend to hold more water due to small grains which have a large surface area which hold more water around them. • The presence of moisture is important as: • Enables plants to receive nutrients in solution • Supports micro-organisms • Helps to develop soil horizons • Helps to slow down rate of erosion

  19. Soil Horizons • A soil profile is a vertical section taken from the surface down to the parent material. The letters O, A, B and C are used to identify the horizons. • O Horizon – mainly organic matter • A Horizon – Called the topsoil (may have a dark brown colour (Brown Soils). Most biological activity takes place here due to roots etc • B Horizon – Called subsoil (may be light brown in colour) Many of materials that move down from above accumulate here • C Horizon – sit directly on the bedrock. It has been little affected by soil forming process.

  20. Air • Air fills pore spaces that are not occupied by water. • This air contains more water vapour and carbon dioxide than atmospheric air (released by plant roots and organisms). Fertile soil requires adequate supply of air because: • Essential for seed germination. • It is needed for the breakdown of organic matter in the soil.

  21. Colour • Soil‘s colour can affect its temperature • Darker soils (warm soils) absorb heat more than light coloured soils. • Warm soils will have a rapid breakdown of organic matter – more nutrients. • Warm soils better for seed germination. • Warm soils tend to have higher humus content

  22. Factors that affect soil formation

  23. Parent Material – source of mineral matter – can be unconsolidated material (e.g. alluvium or glacial drift loess) Consolidated material (solid rock) • If unconsolidated (is the source) soil formation will progress more rapidly. • Soils will inherit the parent material’s physical characteristics.

  24. Climate – Two key climatic influences temperature and precipitation. • They affect the type and rate of weathering. (Mechanical or Chemical) • They affect the rate of growth and decay of organic material. • They affect the degree of leaching.

  25. Topography • Soil formation is influenced by gradient, altitude and aspect • Soils tend to accumulate on flat gently sloping landscapes • Flat landscapes pose risk of water logging. • Soils are thinner on steeper slopes – good drainage here. • The higher the altitude the more precipitation and decrease in temps which impact length of growing season. • Aspect is important – north and south facing slopes develop different soils.

  26. Living Organisms • Plants/Roots bind soil & cover soil (protection)/ They take and return nutrients from soil. • Insects/animals aerate soil. • Insects fungi and bacterial help in breakdown and decay of organic matter breaking down into humus

  27. Time • Time allows processes to operate. • Soils can take a long time to form (can take 400 years per centimetre)

  28. Factors influencing soil characteristics

  29. No two soils have the exact same characteristics. • This is because a number of physical, chemical and biological processes operate. • The major processes are

  30. Weathering • Physical breakdown of rock / Chemical weathering breaks down and changes minerals by various processes including oxidation, carbonation, hydration, hydrolysis. Weathering key to release of essential nutrients for plants The reddish-orange colour of this oxide soil shows is aresult of oxidation, a chemical weathering process.

  31. Humification • Process where organic matter at the surface is decomposed to form humus

  32. Leaching • This occurs when rainwater carries soluble mineral and organic matter from the A horizon downwards through soil profile.

  33. Podzolisation

  34. Podzolisation • Surface vegetation is moorland or coniferous forest - rain percolating down becomes acidic, dissolving almost all the soil constituents including clay particles. Organic matter and mineral are leached to a lower horizon resulting in a podzol soil that is often infertile.

  35. Laterisation • Leaching process widespread in tropical and equatorial regions. Intense rainfall then leaches down to the lower layers. Rapid decay in vegetation means that there is little acid in the soil. Large concentrations of iron and alluvium remain in the soil close to the surface. The soil becomes reddish and may harden on exposure.

  36. Laterisation

  37. Salinisation • This occurs in regions where precipitation is low and evapotranspiration is high. Ground water drawn upward and evaporated leaving the soluable salts in the upper layers of the soil that form a hard toxic crust

  38. Calcification • Calcium accumulates in soils where rainfall is low and little leaching takes place. It provides a rich environment for plants.

  39. Gleying • This soil is waterlogged for all or part of a year. Occurs on gentle slopes where water table rises following heavy rain. Theses soils are therefore poorly aerated and the lack of oxygen retards decomposition.

  40. Classifying Soils

  41. Three basic groups under the zonal system (based on climatic zone) • Zonal Soils • Intrazonal Soils • Azonal Soils

  42. Zonal soils • These develop where the landscape and climate have been stable for a long time. • They are mature soils with distinctive profiles and clear horizons.

  43. Examples of zonal soils • Latosols From Tropical/Equatorial climatic zone and Tropical Rainforest Vegetation zone. • Desert and semi-desert soils From Arid / semi-arid climatic zone and Desert Vegetation zone. • Brown Earths From Cool temperate maritime and mixed / deciduous forest zone

  44. Intrazonal Soils • Within zonal soil belts some local soil forming factors (such as parent material or drainage conditions etc.) exerts a stronger influence than climate and vegetation. • These can be strong enough to modify the zonal soil to form an Intrazonal soil.

  45. Examples of Intrazonal soils • Gley These have been modified by poor drainage resulting in water logging. • Peat Cold wet climate and extreme water logging

  46. Azonal soils • These are immature soils that have not had enough time to develop. They do not have a well-developed profile. Examples of Azonal soils • Regosols Recent deposition of material by rivers or wind. • Lithosols Mass movement and erosion are too rapid to allow for soil development

  47. Other topics to investigate • Irish Soils • Causes of desertification in the Sahel • Soil Conservation

  48. Exam Brief - Geoecology Option Examine the general characteristics of any one soil type that you have studied (2006 HL) • It is better to treat of three or four aspects of the theme in some detail, rather than to give a superficial treatment of a large number of points . • 4 well – developed paragraphs > 6 SRPs in each paragraph

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