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    All About Soil: Soil Formation: According to the Natural Resource Conservation Service, soil refers to the loose surface of the earth as distinguished from rock. Soil is formed when weathering continuously breaks down rocks into pieces. These pieces are broken down into even smaller pieces to form soil. Soil Profile: Soil is made up of many layers called horizons, such as the humus, topsoil, subsoil, weathered rock fragments and bedrock. Leaching (drainage of minerals/chemicals away from soil, ash, or similar material by the action of percolating liquid) can occur from the humus to the bedrock.

    Factors Affecting Soil Profile (Layers), Soil Formation and eventually the Soil Composition:

    Soil Composition:

    Soil is composed of organic matter, mineral and nutrients. An average soil sample is 45% mineral, 25% water, 25% air and 5% organic matter (material that was once

    living or was formed by the activity of living organisms) Soil texture comes from different size mineral particles such as sand, silt and clay Pieces of weathered rock and organic material, or humus (Decaying organic material; becomes mixed with

    the top layers of rock particles, and supplies some of the needed nutrients to plants; also creates crumbly soil which allows adequate water absorption and drainage.), are the two main ingredients of soil.

    Factor: Explanation: Time

    - The longer a rock is exposed to the forces of weathering, the more it is broken down. - Mature soil is formed if all three layers have had time to develop.

    Climate - In areas with heavy rainfall and warm temperatures, weathering takes place more rapidly. - Heavy rainfall may wash much of the topsoil away. - But since organisms are more plentiful these areas, the soil is quickly replaced. - They speed up the chemical and mechanical weathering of rocks.

    Type of rock

    - Rocks that do not break down easily do not form soil rapidly. - In some climates it takes along time for granite to break down. So soil formation from

    granite is slow. - Sandstone can break easily and forms soil quickly.

    Surface features of the region

    - The surface features of the region also determine the speed at which soil is formed. - On very steep slopes, rainwater running off the land erodes the soil and exposes rock to

    weathering. - The presence of vegetation also speeds up biological weathering of rock.


    Rock particles form more than 80% of soil. The composition of soil varies from place to place. The type of rock broken down by weathering determines

    the kinds of minerals in the soil. The type of weathering also affects the composition of soil. Mechanical weathering produces soil with a composition similar to the rock being weathered. Chemical weathering produces soil with a different composition.

    Properties of Soil: Property: Description & Trend: Texture - Determined by the size of mineral particles within the soil.

    - Too many large particles leads to extreme leaching, too many small particles lead to poor drainage.

    Colour - Generally speaking, the darker a soil is, the more nutrient rich it is. The darker color often indicates

    an increase in decomposed organic matter known as humus. - Gray soils often indicate poor drainage, while red soils can indicate very poor soils. - These general rules about soil colors can however be misleading. Under certain conditions, a very

    poor soil can appear as dark black, while a rich healthy soil can appear as red. pH (Acidity/ Alkalinity)

    - If a soil has too much acid in it, the nutrients in the soil will be dissolved too quickly, and leached away as the water drains.

    - If a soil is too alkaline, or in other words, if there is not enough acid, than nutrients will not dissolve quickly enough.

    - Thus, a neutral soil, which is neither too acidic, nor too alkaline is the preferred type of soil for plant life to thrive.

    Structure - Dirt clods (small clump of soil that has formed a rock-like clump) are called peds by geologists. - Many soils form peds, as soil fragments bond together. These peds effect how easily water is able

    to move through the soil, as well as plant roots, and other organisms. (Related to particle size) - Peds tend to form more often in wet locations, and less frequently in drier locations. Sandy

    locations such as deserts do not form any peds at all. Types of Soil: Type: Description: Residual Soil:

    Sometimes soil remains on top of its parent rock, or the rock from which it was formed. This is called residual soil. Residual soil has a composition similar to that of the parent rock it covers.

    Transported Soil:

    Some soil is removed from the parent rock by water, wind, glaciers and waves. Soil that is moved away from its place of origin is called transported soil. Transported soil can be very different in composition from the rock it covers.

  • SOIL EROSION & MASS WASTING 3 Soil Erosion: Erosion is the process of detachment and transport of soil particles by erosive agents (Ellison, 1944), and is a natural geologic process. It is a global problem - 1/3 of the worlds arable land has been lost since 1950, mostly in Asia, Africa and South America at a rate of 13-18 t/a/yr (tons per year I think, I dont know what the a stands for). In the USA, 30% of the farmland has been abandoned through erosion, salinization and water-logging. 90% of USAs cropland is also losing soil faster than it is replaced, at a rate of >1 t/a/yr. Types of Soil Erosion: Type What it does:

    4 Stages/Types:

    1st Stage: Raindrop erosion - Begins with raindrops striking bare soil, hence dislodges particles - Intense rains seals off the surface - When the rainfall exceeds the maximum infiltration capacity, water is stored in small

    depressions on the surface of the soil. (Give a dimpled appearance to the surface) - Once the depressions are filled, surface runoff begins, bringing soil particles with them.

    2nd Stage: Sheet erosion - Removal of a layer of exposed soil by the action of raindrop splash and runoff. - The water moves in broad sheets over the land and is not confined in concentrated flow paths. - Amount of erosion increases with slope and distance.

    3rd Stage: Rill and gully erosion - Occurs after runoff flows concentrate into rivulets, cutting into the soil surface. - Amount of erosion increases with slope and distance. - Rills => Narrow and shallow incisions into topsoil layers, will evolve into larger fluvial features - Gullies => Gullies are larger than rills and cannot be fixed by tillage (agricultural preparation of

    the soil by mechanical agitation of various types, such as digging, stirring, and overturning.). Gully erosion is an advanced stage of rill erosion, just as rills are often the result of sheet erosion.

    Water Erosion

    4th Stage: Streambank and bed erosion Occurs with an increase in the volume and velocity of runoff. Water is a great erosional agent:

    - Corrasion/Abrasion: grinding against surface by materials - Attrition: colliding together to become smaller rocks - Solution: Dissolve in carbonic acid. (Eg. Limestone calcium carbonate dissolves) - Hydraulic Action: water hitting against the rocks, loosening.

    Wind Erosion - Wind erosion may occur on any soil whose surface is dry and unprotected by vegetation.

    - Saltation (the movement of hard particles such as sand over an uneven surface in a turbulent flow of air or water) detaches particles, and smaller particles are suspended while larger particles creep.

    - Sandy and silty soils are the most susceptible - This process causes soil accumulation in ditches and fence rows.


    - It cannot be divided into such distinct types. Surface texture is the best key to wind erosion hazard potential.

    Gravitational Erosion

    - Gravity is the principal force acting to move surface materials such as soil and rock.

    Tillage Translocation

    - Net downhill movement by tillage (basically, LAND) results in the smoothing of surface. - Combined with water erosion, it increases relief intensity and soil variability. - Boundaries stop the movement

    Mitigation Measures: Measure Description Picture: Contour Farming

    - Tilling at right angles to the slope of the land. - Rows formed slows water run-off during rainstorms,

    acting as a small dam, to prevent soil erosion and allows the water time to settle into the soil.

    Strip Farming

    - Alternating strips of closely sown crops to slow water flow.

    Terracing - Level areas constructed at right angles to the slope to

    retain water - good for very steep land.

    Windbreaks - Planting of trees or other plants that protect bare soil

    from full force of the wind. - Planting sufficiently dense rows or stands of trees at

    the windward exposure of an agricultural field subject to wind erosion. Evergreen species are preferred to achieve year-round protection; however, as long as foilage is present in the seasons of bare soil surfaces, the effect of deciduous trees may also be adequate.


    Mass Wasting: What is Mass Wasting?

    - Also known as slope movement or mass movement - Geomorphic process by which soil, regolith (loose uncemented mixture of soil and rock particles that covers

    the Earth's surface), and rock move downslope under the force of gravity - Not accompanied by a moving agent such a river, glacier or ocean wave. - Comprehensive term for any type of downslope movement of Earth materials