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Soil Structure

  • Soil conditions and characteristics such as water movement, heat transfer, aeration, and porosity are much influenced by structure. In fact, the important physical changes imposed by the farmer in ploughing, cultivating, draining, liming, and manuring his land are structural rather than textural.
    Definition of Soil Structure: The arrangement and organization of primary and secondary particles in a soil mass is known as soil structure.
    Soil structure controls the amount of water and air present in soil. Plant roots and germinating seeds require sufficient air and oxygen for respiration.
    Bacterial activities also depend upon the supply of water and air in the soil.
    Formation of soil structure: Soil particles may be present either as single individual grains or as aggregate i.e. group of particles bound together into granules or compound particles. These granules or compound particles are known as secondary particles. A majority of particles in a sandy or silty soil are present as single individual grains while in clayey soil they are present in granulated condition. The individual particles are usually solid, while the aggregates are not solid but they possess a porous or spongy character. Most soils are mixture of single grain and compound particle. Soils, which predominate with single grains are said to be structure less, while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure.
    Mechanism of Aggregate Formation: The bonding of the soil particles into structural unit is the genesis of soil structure. The bonding between individual particles in the structural units is generally considered to be stronger than the structural units themselves.
    In aggregate formation, a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloids. The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminium hydroxides and decomposing organic matter. Whatever may be the cementing material, it is ultimately the dehydration of colloidal matter accompanied with pressure that completes the process of aggregation.
    Colloidal clay: By virtue of high surface area and surface charge, clay particles play a key role in the formation of soil aggregates. Sand and silt particles can not form aggregates as they do not possess the power of adhesion and cohesion. These particles usually carry a coating of clay particles; they are enmeshed in the aggregates formed by the adhering clay particles. Colloidal particles form aggregates only when they are flocculated. There is vast difference between flocculation and aggregation. 
    Flocculation is brought about by coalescence of colloidal particles and is the first step in aggregation.
    Aggregation is some thing more than flocculation involving a combination of different factors such as hydration, pressure, dehydration etc. and required cementation of flocculated particles. The cementation may be caused by cations, oxides of Fe and Al, humus substances and products of microbial excretion and synthesis. Clay particles form aggregates only if they are wetted by a liquid like water whose molecules possess an appreciable dipole moment.

    Clay – – +Water – – +Cation+ – -Clay – – +Water – – +Cation+ – -Clay –

    The aggregation also depends upon the nature of clay particles, size and amount of clay particles, dehydration of clay particles, cations like calcium and anions like phosphate.
    Fe and Al oxides: The colloidal Fe oxides act as cementing agent in aggregation. Al oxides bind the sand and silt particles. These act in two ways. A part of the hydroxides acts as a flocculating agent and the rest as a cementing agent.

    Organic matter: It also plays an important role in forming soil aggregates.

    1. During decomposition, cellulose substances produce a sticky material very much resembling mucus or mucilage. The sticky properly may be due to the presence of humic or humic acid or related compounds produced.

    2. Certain polysaccharides formed during decomposition.

    3. Some fungi and bacteria have cementing effect probably due to the presence of slimes and gums on the surface of the living organisms produced as a result of the microbial activity

    Classification of Soil Structure: The primary particles sand, silt and clay usually occur grouped together in the form of aggregates.

    Natural aggregates are called peds where as clod is an artificially formed soil mass. Structure is studied in the field under natural conditions and it is described under three categories
    1. Type – Shape or form and arrangement pattern of peds 
    2. Class – Size of Peds 
    3. Grade – Degree of distinctness of peds

    Types of Soil Structure: There are four principal forms of soil structure

    Plate-like (Platy): In this type, the aggregates are arranged in relatively thin horizontal plates or leaflets. The horizontal axis or dimensions are larger than the vertical axis. When the units/ layers are thick they are called platy. When they are thin then it is laminar.
    Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil.

    This type is inherited from the parent material, especially by the action of water or ice.

    Prism-like: The vertical axis is more developed than horizontal, giving a pillar like shape. Vary in length from 1- 10 cm. commonly occur in sub soil horizons of Arid and Semi arid regions. When the tops are rounded, the structure is termed as columnar when the tops are flat / plane, level and clear cut prismatic.

    Block like: All three dimensions are about the same size. The aggregates have been reduced to blocks. Irregularly six faced with their three dimensions more or less equal.
    When the faces are flat and distinct and the edges are sharp angular, the structure is named as angular blocky. When the faces and edges are mainly rounded it is called sub angular blocky. These types usually are confined to the sub soil and characteristics have much to do with soil drainage, aeration and root penetration.

    Spheroidal (Sphere like): All rounded aggregates (peds) may be placed in this category. Not exceeding an inch in diameter. These rounded complexes usually loosely arranged and readily separated. When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition.

    Therefore in sphere like structure, infiltration, percolation and aeration are not affected by wetting of soil. 
    The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous, it is termed as crumb. This is specific to surface soil particularly high in organic matter/ grass land soils.

    Classes of Soil Structure: Each primary structural type of soil is differentiated into 5 size classes depending upon the size of the individual peds.

    The terms commonly used for the size classes are:

    1. 1. Very fine or very thin

    2. 2. Fine or thin

    3. 3. Medium

    4. 4. Coarse or thick

    5. 5. Very Coarse or very thick

    The terms thin and thick are used for platy types, while the terms fine and coarse are used for other structural types.

    Grades of Soil Structure: Grades indicate the degree of distinctness of the individual peds. It is determined by the stability of the aggregates. Grade of structure is influenced by the moisture content of the soil. Grade also depends on organic matter, texture etc. Four terms commonly used to describe the grade of soil structure are:

    1. Structure less: There is no noticeable aggregation, such as conditions exhibited by loose sand.

    2. Weak Structure: Poorly formed, indistinct formation of peds, which are not durable and much unaggregated material.

    3. Moderate structure: Moderately well developed peds, which are fairly durable and distinct.

    4. Strong structure: Very well formed peds, which are quite durable and distinct.

    Soil Structure Naming: For naming a soil structure the sequence followed is grade, class and type; for example strong coarse angular blocky, moderate thin platy, weak fine prismatic.

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