Soil Structure

Soil structure is most usefully described in terms of grade (degree of aggregation), class (average size) and type of aggregates (form). In some soils, different kinds of aggregates may be found together and they are then described separately. The following paragraphs will briefly explain the various terms which are most commonly used to describe soil structure. This will help you to judge better the quality of the soil where you plan to construct fish-ponds. It will also enable you to learn how to define the soil structure yourself when examining a soil profile.

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

Platy structure is most noticeable in the surface layers of virgin soils but may be present in the sub-soil. 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 water.

(b) Prism-like:

The vertical axis is more developed than 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

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. 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 which 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.

Formation of Soil Structure:

The mechanism of structure (aggregate) formation is quite complex. 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 colloidal clay, iron and aluminium hydroxides and organic matter.

The mineral Colloids (colloidal clay) by virtue of their properties of adhesion and cohesion, stick together to form aggregates. Sand and silt particles cannot form aggregates as they do not possess the power of adhesion and cohesion.

The amount and nature of colloidal clay influence the formation of aggregates. The greater the amount of clay in a soil, the greater is the tendency to form aggregates. Clay particles smaller than 0.001 mm aggregate very readily. So also clay minerals that have high base exchange capacity form aggregate more readily than those which have a low base exchange capacity. Iron and aluminium hydroxides act as cementing agent is binding the soil particles together. These are also responsible for forming aggregates by cementing sand and silt particles.

Organic matter plays an important part in forming soil aggregates. During decomposition of organic matter, humic acid and other sticky materials are produced which helps to form aggregate. Some fungi and bacteria taking part in the decomposition have also been found to have a cementing effect.

Another view of structure formation is that clay particles adsorbed by humus forming a clay-humus complex. It seems that humus absorbs both cations and anions. In normal soil, calcium is the predominant cation and forms calcium humate in combination with humus.

Factors Affecting Soil Structure

The development of structure in arable soil depends on the following factors:

1.Climate: Climate has considerable influence on the degree of aggregation as well as on the type of structure. In arid regions there is very little aggregation of primary particles. In semi arid regions, the degree of aggregation is greater.

2. Organic matter: Organic matter improves the structure of a sandy soil as well as of a clay soil. In case of a sandy soil, the sticky and slimy material produced by the decomposing organic matter and the associated microorganism cement the sand particles together to form aggregates. In case of clayey soil, it modifies the properties of clay by reducing its cohesiveness. This helps making clay more crumby.

3.Tillage:Cultivation implements break down the large clods into smaller fragments and aggregates. For obtaining good granular and crumby structure, optimum moisture content in the soil is necessary. If the moisture content is too high it will form large clods on drying. If it is too low some of the existing aggregates will be broken down.

4.Plants, Roots and Residues: Excretion of gelatinous organic compounds and exudates from roots serve as a link. Root hairs make soil particles to cling together. Grass and cereal roots vs other roots. Pressure exerted by the roots also held the particles together.Dehydration of soil strains the soil due to shrinkage result in cracks lead to aggregation

Plant tops and residues shade the soil prevent it from extreme and sudden temperature and moisture changes and also from rain drop impedance.

Plant residues serve as a food to microbes which are the prime aggregate builders.

5. Animals: Among the soil fauna small animals like earthworms, moles and insects etc., that burrow in the soil are the chief agents that take part in the aggregation of finer particles.

6.Microbes:Algae, fungi, actinomycetes and fungi keep the soil particles together. Fungi and actinomycetes exert mechanical binding by mycelia, Cementation by the products of decomposition and materials synthesized by bacteria.

7. Fertilizers: Fertilizer like Sodium Nitrate destroys granulation by reducing the stability of aggregates. Few fertilizers for example, CAN help in development of good structures.

8.Wetting and drying:When a dry soil is wetted, the soil colloids swell on absorbing water. On drying, shrinkage produces strains in the soil mass gives rise to cracks, which break it up into clods and granules of various sizes.