Soils Geography 12 2005-6
Soil - is the top layer of the earth’s crust that has been physically and chemically weathered into small particles and in which vegetation grows if temperature and precipitation are favourable. • 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 heavy. • 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 material. • 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.
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.
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.
Podzols: Are often acidic and heavily leached . They have a thin humus layer of decaying evergreen needles at the surface.
Podzols • 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 periods. • 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
Chernozem • 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
Sierozem • 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.