Soil Structure

 

Soil Structure: Definition, Types.

After reading this article you will learn about Soil Structure:-

1. Definition of Soil Structure 

2. Types of Soil Structure 

3. Classes

4. Grades 

Definition of Soil Structure

The arrangement of soil particles and their aggregate into certain defined patterns is called structure. The primary soil particles "sand, silt, and clay" usually occur grouped together in the form of aggregates. Natural aggregates are called peds, whereas clod is an artificially formed soil mass.

The 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

(a) Plate-like:

In this structural type of aggregates are arranged in relatively thin horizontal plates. The horizontal dimensions are much more developed than the vertical. When the units are thick, they are called platy, and when thin, laminar (Fig. 2.1).

Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil. Although most structural features are usually a product of soil-forming forces, the platy type is often inherited from the parent material, especially those laid down by the water.

(b) Prism-like:

The vertical axis is more developed than the horizontal, giving a pillar-­like shape. When the top of such a ped is rounded, the structure is termed as columnar, and when flat, prismatic. They commonly occur in sub-soil horizons in arid and semi-arid regions.

(c) Block-like:

All these dimensions are about the same size and the peds are cube-­like with flat or rounded faces. When the faces are flat and the edges 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.

(d) Spheroidal (Sphere-like):

All rounded aggregates (peds) may be placed in this category, although the term more properly refers to those not over 0.5 inch in diameter. Those rounded complexes usually lie loosely and separately [Fig. 2.2 (a), 2.2 (b) and 2.2 (c)].

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, the term used is crumby.

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. Very fine or very thin

2. Fine or thin

3. Medium

4. Coarse or thick

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. The 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 are no noticeable peds, such as conditions exhibited by loose sand or a cement-like condition of some clay soils.

2. Weak structure

Indistinct formation of peds that are not durable.

3. Moderate structure

Moderately well-developed peds which are fairly distinct.

4. Strong structure

Very well-formed peds which are quite durable and distinct. For naming a soil structure the sequence followed is grade, class and type; for example, strong coarse angular blocky (soil structure).

Examples of sphere-like soil structure

Often compound structures are met within the soil under natural conditions. For example, large prismatic types may break into medium blocky structure, constitute the compound structure.