Soil Structure: Definition, Types and Formation

After reading this article you will learn about Soil Structure:- 1. Definition of Soil Structure 2. Types of Soil Structure 3. Classes 4. Grades 5. Formation 6. Factors Affecting 7. Effects 8. Structural Management of Soils.

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.

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 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 [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. 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 011 the type of structure. In arid region, there is very little aggregation of primary particles. In semi- arid regions, the degree of aggregation is greater than arid regions.

2. Organic matter:

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

3. Tillage:

Cultivation implements break down of large clods into smaller fragments and aggregates. For obtaining good granular and crumby structure, an 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. Plant roots:

Large number of granules remain attached to roots and root hairs which help to develop crumb structure. Plant root secretions may also act as cementing agents in binding the soil particles. The plant roots, on decay, may also bring about granulation due to the production of sticky substances.

5. Soil organism:

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. Fertilizers:

Fertilizer like Sodium nitrate destroys granulation by reducing the stability of aggregates. Few fertilizers, for example, Calcium Ammonium nitrate, help in development of good structures.

7. Wetting and drying:

Wren a dry soil is wetted, the soil colloids swell on absorbing water. On drying, shrinkage produced strains in the soil mass give rise to cracks which break it up into clods and granules of various sizes.

Effects of Soil Structure on Other Physical Properties of Soil:

Soil structure brings change in other physical properties of soil—like porosity, temperature, density, consistency and colour.

1. Porosity:

Porosity of a soil is easily changed. In plate-like structure pore spaces are less whereas in crumby structure pore spaces are more.

2. Temperature:

Crumby structure provides good aeration and percolation in the soil. Thus, these characteristics help in keeping optimum temperature in comparison to plate-like structure.

3. Density:

Bulk density varies with the total pore space present in the soil. Structure chiefly influences pore spaces. Platy structure with less total pore spaces has high bulk density whereas crumby structure with more total pore spaces has low bulk density.

4. Consistence:

Consistence of soil also depends on structure. Plate-like structure exhibits strong plasticity.

5. Colour:

Bluish and greenish colours of soil are generally due to poor drainage of soil. Platy structure normally hinders free drainage.

Structural Management of Soils:

(a) Coarse-textured Soil:

Sandy soils are commonly too loose and lack the capacity to adsorb and hold sufficient moisture and nutrients. They lack fertility and water- holding capacity. There is only one practical method of improving the structure of such soil- the addition of organic matter. Organic matter will not only act as a binding agent for the particles but will also increase the water-holding capacity. Sod-crops, for example, corn, blue grass etc., also help in improving the structural condition of sandy soils.

(b) Fine-textured Soil:

The structural management of a clay soil is difficult than sandy soil. In clay, plasticity and cohesion are high because of the presence of large amount of colloidal clay. When such a soil is tilled when wet, its pore space becomes much reduced, it becomes practically impervious to air and water and it is said to be puddled. When a soil in this condition dries, it usually becomes hard and dense.

The tillage of clay soil should be done at right moisture stage. If ploughed too wet, the structural aggregates are broken down and an un-favourable structure results. On the other hand, if ploughed too dry, big clods are turned up which are difficult to work. The granulation of fine-textured soil should be encouraged by the incorporation of organic matter. Growing of sod-crops also improves granulation in the soil.,

(c) Rice Soil:

Puddling of the soil is generally beneficial to the production of rice. In preparation for the planting of rice, the soil is flooded with water and then puddled by intensive tillage. Puddling destroys the structural aggregates. Rice seedling is transplanted into the freshly prepared mud.

Such soil management helps control weeds and also reduce the rate of water movement down (percolation) through the soil. This is important to maintain standing water in the rice through out the growing season. By reducing water percolation, puddled soil markedly decreases the amount of water needed to produce a rice crop.

Semi-aquatic characteristics of the rice plant account for its positive response to a type of soil management that destroy aggregate. Rice survives flooded conditions because oxygen moves downward inside the stem of the plant to supply the roots. This characteristic permits rice to stand well in the water-logging condition. Rice can be grown successfully on un-puddled but flooded soil.