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Soils Geography 12 2005-6 Stages in soil forming processes Soil - is the top layer of the earth s crust that has been physically and chemically weathered into small ... – PowerPoint PPT presentation

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Title: Soils

  • Geography 12
  • 2005-6

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Stages in soil forming processes
Soil - is the top layer of the earths crust
that has been physically and chemically weathered
into small particles and in which vegetation
grows if temperature and precipitation are
  • Soil forming processes
  • 1. Decaying vegetation forms humus. Soils high in
    humus are generally very fertile.
  • 2. Leaching. Water infiltrates into the ground
    and dissolved minerals are carried deep into the
    sub soil and away. Common where rainfall is
  • 3. Capillary Action. When surface layers are dry
    water is transferred from deep in the ground to
    the surface. As transpiration occurs and
    evaporation water is dran up through the roots to
    the leaves and through the soil. Capillary action
    brings water and dissolved minerals close to the
    surface. Great for plants. (but) Common in dry
    desert regions.
  • 4. Translocation. This is the movement of solid
    material from on place to another by water or by
    animals. Helps to mix the soil.

  • Soil forms over many thousands of years from
    weathered rock fragments by physical and chemical
    weathering and decayed remains of living
    organisms. This is referred to as parent
  • As soil develops, it forms distinct layers, known
    as horizons.
  • Each horizon has a specific colour, texture, and
    mineral content
  • The number and type of horizons in a particular
    soil vary, but in general the uppermost horizon
    of soil forms the nutrient-rich topsoil.
  • Beneath the topsoil lies the subsoil, which
    contains minerals that have trickled down from
    the topsoil.
  • Rock fragments reside below the subsoil
  • The horizon forming the foundation of soil
    consists of unweathered parent rock.

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Laterite Soils
Latosols or larerite soils arise in the tropical
rainforest biome in the equatorial zone where
high temperatures and high precipitation occur
throughout the year. Climatic conditions permit
the highest net primary productivity of all
the terrestrial biomes, and extensive chemical
weathering leads to the development of deep
soils, often reaching 20 m to 30 m (65 ft to
100 in depth. Tropical soils have a loose
structure and if there is deforestation that
removes vegetation cover and roots they suffer
rapid erosion because of the heavy rainfall.
This may result in loss of fertility and many
attempts at cultivation of latosols have, in
fact, been unsuccessful.
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Laterites are among the most infertile soils in
the world!
The tropical soil is very deeply weathered and
red due to its high iron-oxide concentration.
A continual drop of leaves from the broadleaf
evergreen forest adds nutrients to the soil.
Without this continual leaf litter, the
nutrients would be quickly leached down..
The trees shallow root system allows them to
quickly capture these nutrients before they are
washed away.
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Podzols Are often acidic and heavily leached .
They have a thin humus layer of decaying
evergreen needles at the surface.
  • Podzol soils are mainly found in the taiga
    (boreal forest) biome at high latitudes and at
    higher altitudes in temperate latitudes.
  • Podzols form under a harsh, cold climate where
    growth is slow during the winter months and snow
    accumulates and stays on the ground for long
  • The vegetation consists largely of coniferous
    trees, which are specially adapted to the
    climatic conditions,
  • Productivity is low due to the climatic
    conditions. The soils are impoverished and the
    climate is unsuitable for agriculture, but is
    suitable for commercial forestry

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  • Brown Chernozem
  • Boreal Sub-Arctic of Alberta and Saskatchewan
  • Often referred to as the Breadbasket areas of
    the world.

  • Chernozem soil of temperate grassland areas
  • Fertile with thick humus layer
  • Due to climate leaching is not a problem
  • Capillary action is good and cold winters force
    small animal down.

  • Chernozem soils occur under the temperate
    grasslands of the steppes of Russia, the prairies
    of North America, in Australia, South Africa, and
    the Pampas in South America (all approximately
    30-40 north and south of the equator). T
  • The vegetation is mainly grasses and herbaceous
    plants, which have become dominant following
    natural and accidental fires and extensive human
    modification of these regions over time.
  • Although initial ploughing is difficult because
    of a dense mat of roots, once cultivated, these
    soils are regarded as the best in the world for
    agriculture. They have high nutrient levels, good
    humus content, and good texture and structure.
  • Chernozem soils support the major grain growing
    belts of the world.

  • Grey-Brown
  • Rapid accumulation of leaf litter into the A
    horizon is due to the high earthworm activity

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  • Light colour of the A horizon indicates a very
    dry climate and little or no humus content
  • The light brown A horizon at the surface occurs
    because of the lack of organic matter or moisture
    which usually darken the soil. There is very
    little vegetation growing here, and so organic
    matter is not returned to the soil. When rainfall
    does occur in this environment, it carries
    materials downward into the profile to form B
    horizons. The white streaks near the bottom of
    this profile are formed from deposits of calcium
    carbonate which become very hard as they
    accumulate over time

  • Tundra occurs in the extreme northern parts of
    Alaska, Greenland, Russia, and Canada to the
    north of the taiga.
  • The climate is very harsh, and the temperatures
    rarely rise above 0 C, so the ground is
    permanently frozen.
  • Precipitation accumulates as snow for most of the
    year. In the short summer temperatures rise
    sufficiently for a few centimetres at the surface
    of the soil to thaw.
  • Plant growth is limited by the cold temperatures
    and lack of available water lichens, mosses, and
    low shrubs predominate.
  • The cycles of freezing and thawing with the
    changing seasons causes the weathering of rock by
    frost shattering. Frost heave can occur, whereby
    broken rock fragments are brought to the surface.
  • No clear horizons are developed.