Soil Structure PDF

Physical properties of soil  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 suffi...

Physical properties of soil  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.  Classification 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 Structure: There are four principal forms of soil structure a) Plate like b) Prism like c) Block like d) Spheroidal  Plate like 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 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 Structure: Each primary structural type of soil is differentiated into 5 size classes depending upon the size of the individual peds.  Soil Consistency Soil consistence is defined as “the resistance of a soil at various moisture contents to mechanical stresses or manipulations”. It combines both the ‘cohesive’ and ‘adhesive’ forces, which determine the ease with which a soil can be reshaped or ruptures.  Types of soil consistency Adhesion: Molecular attraction that holds the surfaces of two substances (eg. Water and soil particles) in contact Cohesion: Holding together: force holding a solid or liquid together, owing to attraction between like molecules. Decreases with rise in temperature Soil consistence is described at three moisture levels namely ‘wet’, ‘moist’ and ‘dry’.  Wet soils Consistency is denoted by terms stickiness and plasticity Stickiness is grouped into four categories namely i) non sticky, ii) slightly sticky, iii) sticky and iv) very sticky Plasticity of a soil is its capacity to be moulded (to change its shape depending on stress) and to retain the shape even when the stress is removed. Soils containing more than about 15% clay exhibit plasticity – pliability and the capacity of being molded. There are four degrees in plasticity namely i) non plastic, ii) slightly plastic, iii) plastic and iv) very plastic.  Moist Soils Moist soil with least coherence adheres very strongly and resists crushing between the thumb and forefinger. The different categories are: i. Loose-non coherent ii. Very friable - coherent, but very easily crushed iii. Friable - easily crushed iv. Firm - crushable with moderate pressure v. Very firm - crushable only under strong pressure vi. Extremely firm - completely resistant to crushing. (type and amount of clay and humus influence this consistency)  Dry soils In the absence of moisture, the degree of resistance is related to the attraction of particles for each other. The different categories are: i) Loose - non coherent ii) Soft - breaks with slight pressure and becomes powder iii) Slightly hard - break under moderate pressure iv) Hard - breaks with difficulty with pressure v) Very hard - very resistant to pressure vi) Extremely hard - extreme resistance and cannot be broken