Soil Structure: Classification, Genesis and Evaluation

After reading this article you will learn about:- 1. Classification of Soil Structure 2. Genesis of Soil Structure 3. Evaluation 4. Influence on Soil Physical Properties 5. Importance.

Classification of Soil Structure:

Soil structure is described under three categories:

I. Types,

II. Classes and

III. Grades.

I. Types of Structure:

It is determined by the general shapes and arrangements of peds.

There are mainly four types of soil structure:

(i) Plate like,

(ii) Prism like,

(iii) Block like and

(iv) Spheroidal.

(i) Plate Like:

The horizontal dimensions are much more developed than the vertical axis resulting a flattened, compressed or lens like appearance to the peds. When the units are thick, they are called platy and when the units are thin, they are called laminar.

The platy types are often inherited from the parent materials. In addition, frost heaving; fluctuating water tables, compaction and thin layering of different textured alluvium or lacustrine material can form platy type of soil structure (Fig. 4.3.)

(ii) Prism Like:

The vertical axis is more developed than others, with flattened sides, giving a pillar-like shape. It has also two sub-types: columnar—when the top of such ped is rounded, and prismatic—when the tops of the prisms are still plane, level and clean cut. The prism like structures are commonly found in sub-soil horizons in arid and semi- arid (Fig. 4.3.) soils.

(iii) Block Like:

All three dimensions are about the same size and the peds are cube like with flat or 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 (Fig. 4.3).

(iv) Spheroidal (sphere Like):

All axes are developed equally with the same length, curved and irregular faces. Generally all rounded or sphere like peds (aggregates) may be placed in this type of soil structure. Spheroidal type of soil structure has two structural sub­types: granular—simply the aggregates of this type are usually termed as granular and it is less porous, and crumb—when the granules are especially porous.

The term spheroidal more appropriately refers to those sizes of aggregates not exceeding 1/2 inch in diameter. Granular and crumb structures are characteristic of many surface soils. In spheroidal soil structure, the physical properties of soil like, infiltration, percolation and aeration etc. are not affected by wetting of soil (Fig. 4.3).

II. 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:

1. Very fine or very thin (in case of platy soil structure)

2. Fine or thin

3. Medium

4. Coarse or thick (in case of platy soil structure)

5. Very coarse or very thick.

III. Grades of Soil Structure:

Grade indicates the degree of distinctness and durability of the individual peds.

1. 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.

2. Weak:

Poorly formed, non-durable, indistinct peds that break into a mixture of a few entire and many broken peds and much un-aggregated material.

3. Moderate:

Moderately well-developed which are fairly durable and distinct.

4. Strong:

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.

According to soil survey staff, the classification of soil structure is given in table 4.3.

Genesis of Soil Structure:

The mechanism of structure formation is very complex. However, for the formation of aggregates the soil particles should coagulate or flocculate and should be held together or bound together into clusters by some binding and cementing materials.

Aggregate formation in soil is largely a function of:

(a) Silt and clay content

(b) Organic compounds present in the soil

(c) The microbial activity

(d) The concentration of irreversible soil colloids and

(e) The long-range and van der Waals forces acting between the charged clay minerals and the polymers present.

Besides these, many salts, parent material, climate and some of the soil forming processes also affect granulation.

Cation Effects on Soil Structure:

It is evident that lime and organic matter improve the physical properties of the soil. It is found that the flocculation may help in the aggregation process and so “granulation is flocculation plus”. The aggregate formation requires a cementation or binding together of flocculated particles.

The poor structural qualities of alkali soils can be changed into favourable physical condition if the sodium (Na⁺) is replaced by calcium (Ca²⁺), because calcium has profound influence on aggregation.

The relation of the adsorbed ions to alkali soil problems may be briefly described as follows:

It has been generally accepted that the flocculating effects of calcium (Ca²⁺) ion is the contributing factor for stable granulation of soil particles. The aggregate formation is dependent upon an interaction between exchangeable cations on the clay particle and the dispersion medium.

The Ca²⁺ ion becomes the link between the clay particle and the organic polymer and stable aggregates are formed. Many experimental evidences indicate that the effect of calcium upon aggregate formation is indirect, that means, it affects the production and decomposition of organic matter as well as the mechanisms of binding action between organic colloids and clay particles.

Effect of Colloids on Soil Structure:

The soil colloidal material is responsible for the cementation of primary particles into stable aggregates. Stable aggregate formation cannot take place in sands or silts in absence of soil colloidal material. The soil colloidal material may be divided into three distinct groups so far as its cementation effects are concerned: clay colloids, inorganic colloids like oxides of iron and aluminium and organic colloids.

Clay colloids by virtue of their cohesion and adhesion properties, stick the particles together to form aggregates. The greater the amount of clay in a soil, the greater is the tendency to form aggregates. The smaller clay particles (< 0.001 mm in size) can form aggregate very effectively and readily. Clay minerals having high Base Exchange capacity form aggregate readily than of clay minerals having low Base Exchange capacity.

Iron and aluminium oxides and hydroxides are irreversible or very slowly reversible colloidal material. This irreversibility of this colloidal material is the important factor in the production of stable aggregates in certain soils. This is especially true in lateritic soils.

The iron may serve a dual purpose in aggregation. The solution part of iron may act as a flocculating agent and the other more gelatinous part may exert a cementation action. Organic matter serves as a granulating agent in soils and it is also conducive for the formation of relatively large stable aggregates. Organic matter is of much importance in modifying the effects of clay.

An actual chemical union may take place between the decaying organic matter and the silicate molecules. During decomposition of organic matter, humic acid and other sticky substances produced which helps to form aggregates.

The products of microbial metabolism are the major cause for stabilization of soil aggregates except for the mechanical binding effects of fungi mycelia in the early stages of organic matter decomposition. Another view of aggregate formation is that clay particles are adsorbed by humus forming a clay-humus complex. This adsorption takes place on positively charged sites.

The mechanism by which organic colloids stabilize soil structure can be attributed to the bonding of organic polymers to clay surface by (i) cation bridges, (ii) hydrogen bonding, (iii) van der Waals forces and (iv) sesquioxide-humus complexes. The organic colloids compete with water molecules for space on the surfaces; reduce wetting and swelling and increase the strength of the aggregates through cementation effects.

A pictorial form of soil structure formation. The clay particles are shown as domains consisting of several clay particles held together face-to-face, and the domains may be held edge-to-face through aluminium bonds, or edge-to-face, edge-to-edge or face-to-face through organic polymers. These polymers can also bond clay particles to the surface of siliceous silt or sand particles.

Evaluation of Soil Structure:

Soil structure can be evaluated by determining the extent of aggregation, the stability of the aggregates and the nature of the pore space. All these characteristics change with tillage practices and cropping systems. The amount and distribution of pore-spaces are highly related with the aggregates and the susceptibility of the aggregates to water and wind erosion.

Aggregate Analysis of Soil Structure:

There are generally three techniques that can be followed for the aggregate analysis:

(i) Wet and dry sieving,

(ii) Elutriation and

(iii) Sedimentation.

Direct dry sieving of soils in the field is used to evaluate the distribution of clods and aggregates. Dry sieving of aggregates gives an important index for characterizing the susceptibility of soils to wind erosion.

In the wet-sieving technique, the soil is slowly wetted by capillarity for 30 minutes and is then transformed onto a nest of sieves immersed in water. The sieves are slowly raised and lowered in the water 30 minutes. The weight of soil on each sieve is then determined.

Elutriation may be used for the separation of aggregates with diameters between 1 and 0.02 mm. Sedimentation methods have been used to determine the aggregate distribution in the finer fractions that cannot be separated by sieving. They are limited to aggregate sizes less than 1 mm.

There are generally two limitations of sedimentation method: varying density of particles and possibility of flocculation during sedimentation because of the downward motion of the large aggregates.

Index of Soil Structure:

A number of indices of soil structure have been suggested that are given below:

(i) Percentage aggregates > 2.0 mm

(ii) Percentage aggregates > 0.25 mm

(iii) Mean weight diameter (MWD)—the proportion by weight W_i of a given size fraction of aggregates is multiplied by the mean (average) diameter x̅_i of that fraction. The sum of these products for all size fractions is called the mean weight diameter.

(iv) Geometric mean diameter (GMD)—the weight of the aggregates in a given size fraction is multiplied by the logarithm of the mean diameter of that fraction. The sum of these products for all size fractions is divided by the total weight of sample to give the GMD.

(v) Stability index (SI)

(vi) Structural co-efficient of soils (SS)

(vii) Organic carbon

(viii) Minimum bulk density (BD)

(ix) Hydraulic conductivity

Methods of Evaluation of Soil Structure:

There are four methods of evaluation:

(i)Stability against disruption during wet sieving

(ii) Stability against the impact of falling drops of water.

(iii) Stability against disintegration during leaching with dilute NaCl solutions.

(iv) Stability against slaking when pretreated with alcohol or other organic liquids.

Among these methods, wet sieving has been used extensively to determine the size distribution and the stability of aggregates. The relation between aggregate stability and length of oscillation time was exponential according to the equation.

log W = a- b log T

where, W = the weight of water stable aggregates

T = Oscillation time

a = log W at zero oscillation time initial stability

b =Slope of the regression equation or rate of disintegration.

Soils with high initial stabilities and low rates of disintegration had stable aggregates.

Influence of Soil Structure on Soil Physical Properties:

Soil structure influences the physical properties in a various ways:

(i) Aeration Status/Porosity:

Aeration or porosity of a soil is easily altered through the change of types of soil structure. As for example, in platy type of soil structure, soil aeration or porosity is less than that of other type of soil structure like crumby that contain more pore spaces.

(ii) Temperature:

Good soil structure like crumby provides a well-aeration and improves the water-holding capacity of the soil. Thus these characteristics help in maintaining the thermal regimes of the soil in comparison to other soil structure.

(iii) Density:

Bulk density, which is more important for the plant growth, is influenced with the changes in pore spaces resulting from the different types of soil structure. Platy soil structure with low total pore spaces has high bulk density whereas crumby structure containing more total pore spaces low bulk density.

(iv) Consistence:

Soil structure is influenced by the consistency of soil. Plate-like structure exhibit strong plasticity.

(v) Colour:

Platy structure generally hinders free drainage and percolation of water. So the colour of the soil changes with the soil structure. As for example, bluish and greenish colour of the soil is generally found in poorly drained soils.

Importance of Soil Structure:

Soil structure is the most important physical property in relation to plant growth, because it influences the’ amount and nature of porosity. The best structure for favourable physical properties of soil is spheroidal type of soil structure.

Some structures are mechanically stable and strong but when they absorb moisture and are wet, they become soft and lose their shape and size. Soils high in water stable aggregates are more permeable to water and air.

Soils tend to puddle under the condition of less stability of aggregation. Soil structure can be changed easily under different management practices namely ploughing, draining, liming, fertilizing and manuring etc. The application of organic matter also improves the soil structure. Grasses are most effective in promoting granulation as well as soil aggregation.