Structure of Soil

Structure of Soil

The grouping or arrangement of soil particles is called soil structure. The smaller soil particles aggregate and colloids form coatings over these larger grains, binding them together into structural units of various sizes called peds or fragments, except for soils where the texture is uniformly coarse. Soil structure describes arrangement of the primary soil separates (sand. silt and clay) into secondary groupings (peds or fragments),

Methods & Factors Affecting the Formation of Soil Structure

The causes and methods of formation of structural units or aggregates. Soil structures results from the following two processes:

Flocculation: It is the aggregation of individual particles into small clumps or clusters.

Granulation: It is the cementation of aggregates into masses.

The processes of flocculation and granulation depend upon many factors such as:

The amount of clay and type of adsorbed cations. High clay and adsorbed Na will give a coarse pattern because alternate wetting and drying of clay result in and shrinkage causing cracks and splits in the soil. On the other hand, low clay adsorbed Ca will produce finer-grained pattern.

When plant roots and root hairs grow, these break the soil mass or clods into granules. Developing roots exert pressure on these granules and induce aggregation. Sticky organic chemicals secreted by plant roots flocculate the colloids and stabilize or cement the aggregates. Microbes act upon dead roots and decay them. Organic acids produced by this decaying matter causes cementation or aggregates.

Micro-organisms present the topsoil causes formation of desirable type of structural units, particularly crumb. These microbes exert sticky substances that bind the soil particles into aggregates. The fungal hyphae and bacterial colonies present in the soil help in binding mineral particles.

Hydrated Na ions have large radius and disperse sod particles, white anhydrate ions Ca cause flocculation of soil particles.

The type of parent material also affects soil structure. Soil developing from sandstone will be sandy and will have single-grain or massive structure, while those formed from shale will be clayey and may develop platy, prismatic or columnar structure because of their higher clay content.

Types of Soil Structure

The structural units may either be:

Peds, relatively more permanent units.

Fragments, less permanent units originating through cultivation or frost action.

In general, three broad categories of soil structure are recognized:

Single-grained: The soil particles exist as individuals such as sand and do not form aggregates. These are usually found in parent material or C horizon.

Massive: The soil material clings together in large uniform masses. These are found in parent material or C horizon.

Aggregated: The primary particles group together to form clusters of aggregates.

On the basis of their shape the aggregates can be classified into four types as followings:

Spheroidal: These are rounded aggregates with curved or irregular surfaces that are not accommodated to the adjoining aggregates. Therefore, they usually lie loosely and are separated from each other. These are generally not larger than 2 cm in diameter. Relatively non-porous aggregates are called granular, whereas the porous ones are known as crumb. These aggregates are present near soil surface or A horizon, particularly when it is rich in organic matter. The spheroidal aggregates are prominent in grassland soils.

Platy (Plate-Like): These aggregates are flat and arranged in relatively thin horizontal plates or leaflets. The vertical dimension of the aggregate is limited and greatly less than the other two. The platy aggregates are usually found in the surface layers of virgin soils but may also be found in lower horizons as well and subsurface of A horizon of forests.

Prism-Like: These are vertically oriented aggregates or pillars that vary in height in different soils and may reach a diameter of 15 cm or more. Such structures are usually found in B horizon (sub. surface horizon) in arid and semiarid regions. They also occur in some poorly drained soils of humid regions.

When the tops of prisms are rounded, they are called columnar, when the tops of prisms are plane and level, i.e., flat, they are catted prismatic.

Block-like: These aggregates have all their three dimensions more or less equal. These are block-like or polyhedral and range from about 1 to 10 cm in thickness. Block-like aggregates are usually confined to the sub-soil and affect drainage, aeration and root penetration of the soil.

When the edges of the cubes are sharp and the rectangular faces distinct, these are designated as angular blocky. When the aggregates have, sides forming obtuse angles, with mixed rounded and flattened faces and many rounded vertices, these are termed sub angular blocky.

Importance of Soil Structure

Unlike soil texture the soil structure is not a permanent property and can be by changed by management practices. But it is very important to growth of the plants as followings

  • The formation of aggregates make the soil permeable to water, and root hairs.
  • Soil aggregates increase water- and nutrient-holding capacities of the soil.
  • The soil characteristics such as water movement, heat transfer and porosity are much influenced by the soil structure.
  • The well aggregated soils are not easily subjected to wind or water erosion.
  • Soil structure is one basis for characterization of soil horizons in a soil profile and classification of soils.

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