Different types of soil structures –
Soil structure is described under three categories viz., 1. Type, 2. Classes, and 3. Grades.
Type: Depending upon the presence or absence of interconnection between soil pores the aggregates are divided into two groups. Each group is further subdivided into two sub groups depending upon the regularity of the size and shapes of the pores. Each sub group is named as under.
(i) Pores interconnected
(a) Spongy, if pores are irregular in shape and size
(b) Cellular, if pores are regular in shape and size
(ii) Pores not interconnected
(a) Vesicular, if pores and cavities are small, round and smooth inside.
(b) Tubular, if pores and cavities are regular in size and connected to form tubes.
2. The classification is based on the shape, size and other physical features of soil aggregates. The aggregates are first classified into four groups according to main shape of aggregates or fragments.
(i) Plate like
(ii) Prism like
(iii) Block like
(i) Plate like – The horizontal dimensions are much more developed than the vertical axis resulting a flattened compressed or lens like appearance. When the units are thick, they are called platy. When the units are thin, they are called laminar. The platy is often inherited from the parent materials. In addition, frost, fluctuating water table, compaction and this layering of different textured alluvium or lacustrine can form platy type of soil structure.
(ii) Prism like – The vertical axis is more developed than others, with flattened sides, giving and pillar like shape. It has also two sub types. Columnar–when the top of such ped is rounded and prismatic–when the top of the prisms are plane, level and clean cut. The prisms like structure are commonly found in sub soil horizons in arid and semi arid regions.
(iii) Block like – All three dimensions are about the same size and the peds are cube like with flat rounded faces. Block like structure has also two sub types: angular blocky–when the faces are flat and edges of the cubes are sharp angular and sub–angular blocky–when the faces and edges are mainly rounded. The block like soil structures are usually found in the sub-surface horizons and their other characteristics have much to do with soil drainage aeration and root penetration.
(iv) Spheroidal – All axes are developed equally with the same length, curved and irregular faces. All rounded or sphere like peds may be placed in this type of soil structure. This type has two structural sub types
(a) granular simply the aggregates of this type are usually termed as granular and it is less porous, and
(b) crumb, when the granules are especially porous.
Classes of soil structure: Each primary structural type of soil is differentiated into five size classes based on the size of the individual peds. They are as follows:
• Very fine or very thin
• Fine or thin
• Coarse or thick
• Very coarse or very thick.
Grades of Soil Structure: Grades of soil structure indicates the durability of the individual peds.
Structure less – There are no visible peds or aggregates. If the appearance is coherent as in compact clay the term massive is used and if non-coherent as in loose sand it is called single grain.
Week – Poorly formed, non-durable, indistinct peds that break into a mixture of a few entire and many broken peds.
Moderate – Moderately well developed peds, which are fairly durable and distinct.
Strong – Very well formed peds, which are fairly durable and distinct.
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