Soil Structure: Types, Grades and Importance | Soil Science

In this article we will discuss about:- 1. Types of Structures 2. Classes of Soil Structure 3. Grades 4. Importance 5. Factors 6. Effect 7. Soil Structure and Farming Practices 8. Structural Management 9. Soil Structure and Plant Growth.

Contents:

1. Types of Soil Structures
2. Classes of Soil Structure
3. Grades of Structure
4. Importance of Soil Structure
5. Factors Affecting Soil Structure
6. Effect of Soil Structure on the Physical Properties of Soil
7. Soil Structure and Farming Practices
8. Structural Management of Soil
9. Soil Structure and Plant Growth

1. Types of Soil Structures:

Soil structure may be divided into two groups on the basis of shape of aggregates as follows:

(a) Simple Structure:

In this structure, the soil particles are not coagulated or flocculated.

The two groups are as follows:

(i) Single Grained Structure (i.e. Structure Less Soil):

This occurs normally in sandy and silty soils of low organic matter content. The binding influence of organic matter often modifies this original form to certain extent by building up a weak aggregates.  

(ii) Massive Structure:

Similar to single grain structure except that it is coherent e.g. dense soil crusts, plough pans and fargi pans.

(b) Compound Structure:

This structure can be divided into four classes depending on shape and characteristics of their surface i.e. the relative length of the vertical and horizontal axis and by the contour of their edges as follows –

(i) Platy or Plate like Structure:

In this structure, the horizontal axis is longer than vertical axis. Platy aggregates look like plate and peds are in the thin plates. If the units are quite thin, they are called laminar while if the units are thick, they are called platy. Platy structure is most noticeable in the surface layer of virgin soil but may present in sub soil and they are subjected to wide and rapid change.

Although most structural features are usually a product of soil forming process, the platy type is often inherited from the parent materials, especially those laid down by water or ice.

2. Prism like Structure:

In this structure, the vertical axis of aggregates is longer than horizontal axis, giving a pillar like shape.

Prism like structure has been subdivided into two subtypes as follows:

(i) Columnar – When the tops of the prism is rounded, the term columnar is used. This may occur when the profile is changing and certain horizons are degrading.

(ii) Prismatic – When the tops of the prism are still plane, level and clean cut, the structural pattern is designed prismatic.

The pillar varies in length with different soils and may go upto 15 cm or more. They commonly occur in sub-soil horizons in arid and semiarid region and when developed are a very striking feature of the profile.

3. Blocklike Structure:

In blocklike structure, the aggregates develop almost equally in all direction. The original aggregates have been reduced to blocks, irregularly six faced and with their three dimensions more or less equal. In size, these fragments range from a fraction of an inch (2.5 cm) to 3 or 4 (7.5 or 10 cm) inches in thickness. In general, the design is so individualistic that identification is easy.

Blocklike structure is sub-divided into two groups as follows:

(i) Angular Blocky:

In angular blocky structure, the edges of the cubes are sharp and the rectangular faces are distinct. When the term blocky is used alone, it refers to angular blocky.

(ii) Sub-Angular Blocky:

In this structure, the aggregates are similar to angular blocky, but the faces and edges are rounded. These types usually are confined to the sub-soil and their stage of development and other characteristics have much to do with soil drainage, aeration and root penetration.

4. Spheroidal (Sphere like Structure):

In this type of structure, the aggregates are rounded and the two axis (i.e. vertical and horizontal axis) are more or less equal and the aggregates are not in close contact with each other. Ordinarily the aggregates are called granules.

Spheroidal structure has been subdivided into two groups as follows:

(i) Granular – The granules are relatively less porous.

(ii) Crumb – The granules are especially porous. Granular and crumb structure are characteristic of many surface soil, especially those high in organic matter. This type of soil structure occurs in furrow slice or surface soil.

From the agricultural point of view, crumb structure is most desirable as this type of structure is helpful in root penetration, aeration and retention of water which are essential for the satisfactory growth of the plant.

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2. Classes of Soil Structure:

Soil structure is differentiated into five size-classes depending on the size of peds as follows:

(i) Very fine or very thin structure.

(ii) Fine or thin structure.

(iii) Medium structure.

(iv) Coarse or thick structure.

(v) Very coarse or very thick structure.

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3. Grades of 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. Grades also depends on organic matter, texture etc.

Four terms are commonly used to describe the grade of soil structure as follows:

(i) Structureless – There is no noticeable peds, such as condition exhibited by loose sand or cement like condition of some clay soil.

(ii) Weak structure – Indistinct formation of peds (i.e. natural aggregates of soil particles) which are not durable.

(iii) Moderate structure – Moderately well-developed Peds which are fairly durable and distinct.

(iv) 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.

Often compound structures are meet within the soil under natural conditions. For examples, large prismatic types may break into medium block structure constitute the compound structure.

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4. Importance of Soil Structure:

Single grain, granular and crumb structure are generally found in surface soil. The other three types are generally found in sub-soil. The single grain structure is not desirable. Because the sand particles do not remain in combination. As a result of which, the water and nutrient will be leached out from the sandy soil when irrigation is given or rainfall occurs. In sandy soil, the size of porespace is big which is responsible for leaching loss of water.

But the aeration and drainage is very good in single grain structure. In sandy loam and clay loam soil, the aggregation is occurred and the porespace is blocked by fine particles of soil. As a result of which, the soil becomes impervious and drainage also be poor and the soil also form a complex type of structure in which plant root cannot penetrate in deep and water­logging will be a problem. The plant suffers from oxygen supply as the soil becomes impervious and water-logging.

In order to improve this structure, sufficient amount of organic matter in the form of Compost, F.Y.M. Green-manure, oil cakes etc. should be applied. Crumb structure is desirable for agricultural soil so as the retention of nutrient and water holding capacity are concerned. In this type of structure, macrospore and microspore remain almost in equal proportion.

Macrospore helps in proper aeration and drainage and microspore helps in retention of nutrient and water in the soil. The plant root do not face much difficulty in penetrating in these soil. The crumb structure is more desirable as it is more porous than granular structure. A good structure is essential for proper growth and functioning of the plant root.

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5. Factors Affecting Soil Structure:

The structure of soil is mainly influenced by soil texture.

The factors affecting the soil structure are as follows:

(i) System of Soil Management:

The implements that are used for cultivation to break down the large clods into smaller fragments and aggregates. The structure becomes granular and crump, when the land is ploughed at an optimum moisture level. In these soil, aeration and drainage become good and the plant absorbs the nutrients from the soil by penetrating their roots. Tillage has both favourable and unfavourable effects on granulation. The soil tilled in high moisture content produces large clods on drying. Tillage is mainly responsible for aggregation of soil particles. It is possible to keep the soil in aggregation by adopting proper crop rotation.

(ii) Absorbed Cation:

Aggregate formation is definitely influenced by the nature of the cations absorbed by soil colloids. If the soil contains sodium (Na) and potassium (K) of positively charged cation, the particles are dispersed. On the other hand, if the soil contains Hydrogen (H) and Calcium, (Ca), the particles remain in aggregate. A soil saturated with Calcium has crumb structure. Calcium helps directly and indirectly in the formation of soil aggregation by effecting the decomposition of organic matter and influences the activity of soil micro-organism.

(iii) Quantity and Type of Clay Minerals:

Clay acts as a cementing agent and thereby binds floccules. The cementing effect of soil are the clay minerals, oxides of iron (Fe) and aluminium (Al), slimy products of the microorganism, decay products of organic matter and these will help to produce stable soil aggregates. In case of kaolinite type of soil, generally platy structure will develop but in case of montmorillonite type of soil, the aggregate will develop in all direction.

(iv) Activities of Soil Microorganism:

Soil organism decomposes soil organic matter to form dense, gummy substances which bind the clay, sand and silt particles to form the soil aggregates. In clay soil, organic matter modifies the properties of clay by reducing its cohesive power. So organic matter is of much importance in modifying the effect of clay. Soil microorganism and animals help in keeping soil aggregates stable.

Among the soil fauna, small animals like earthworms, moles and insect etc. that burrow in the soil are the chief agents that take part in the aggregation of finer particles. Algae, fungi, actinomycetes and other bacteria form living matter that binds soil particles together more effectively than any exchangeable ion. During passing of soil through the intestine of earthworm, particles are intimately mixed with organic matter. This process binds the finer particles into aggregate.

Besides the soil organism, plant roots also binds the compound particles of soil as well as sand and silt particles to form soil aggregates. The secretion of plant root may also act as cementing agents in binding soil particles. The plant roots, on decay, may also bring about granulation due to production of sticky substances.

(v) Variation of Soil Moisture:

Water causes swelling and shrinking and freezing and thawing of soil which help in the formation of soil structure. When the soil dries up, the chain of water molecule shortens more and more and the clay minerals are ultimately united. When a dry soil is wetted, the soil colloids swell on absorbing the water. Ploughing a wet (i.e. water stagnation) clay soil also destroys soil structure. For this reason, care should be taken in drainage and moisture preservation of a land.

(vi) Climate:

Climate has considerable influence on the degree of aggregation as well on the type of structure. Rainfall and temperature are the main factors of climate. The soil aggregation increases with the increase of rainfall if the temperature is constant. With the increase of the temperature in humid region, there will be increase in percentage of soil aggregation. In arid and semi-arid region with the increase of temperature, there will be decrease of soil aggregation. In semi-arid region, the degree of aggregation is greater than arid region.

(vii) Application of Fertilizers:

Fertilizer like Sodium Nitrate destroys granulation by reducing the stability of soil aggregates. Because when sodium is a prominent adsorbed ion, the soil particles are dispersed and results very undesirable soil structure. On the other hand, calcium ammonium nitrate help in development of good structure as the calcium may encourage granulation by a phenomenon called flocculation.

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6. Effect of Soil Structure on the Physical Properties of Soil:

The soil structure have some effect on the physical properties of soil as follows:

(i) Density:

Bulk density depends on the structure of the soil and it varies with total pore space present in the soil. Structure chiefly influences the porespace. Platy structure with less total porespace has high bulk density. But the crumb structure with more total porespace has low bulk density. Sandy soils have high bulk density as the particles of this soil generally lie in close contact.

(ii) Consistence:

Consistence of soil also depends on structure. Plate like structure exhibits strong consistency whereas granular and crumb structure exhibits loose consistency.

(iii) Porosity:

Porosity of soil depends on the structure of soil and it is easily changed. In crumb structure, porespaces are more.

(iv) Colour:

Soils which are imperfectly and poorly drained are nearly always mottled with various shades of grey, brown and yellow, specially within the zone of fluctuation of water table. Platy structure normally hinder drainage.

(v) Temperature:

Crumby structure provides good aeration and percolation in the soil. These characteristics help in keeping optimum temperature in comparison to platy structure.

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7. Soil Structure and Farming Practices:

Farming practices (i.e. tillage, cropping pattern etc.) greatly affects the soil structure, specially on the surface soil as follows:

(i) Tillage:

Soil aggregation is destroyed by faulty tillage i.e. tillage at very high moisture contents. A fine seedbed which destroys the soil structure is not necessarily desirable. Formation of crusts after rains in a fine seedbed creates difficulties for the germination of seed as well as emergence of seedling. As a practice, tillage should be carried out as minimal as possible and in soil containing moisture in tillage range.

(ii) Cropping Pattern:

Crops such as Maize, (Zea mays) Soyabean (Glycine max), vegetables etc. require more intertillage, tend to destroy the aggregates of soil. Crop rotation (e.g. Leguminous crop-cereals/oilseeds) and green manuring are some of the practical way to achieve and maintain favourable soil structure.

Plant canopy should be provided on the surface of the land as they protect the land from bombarding action of raindrops. Maintenance of good structure may be effected by protecting the surface soil from beating action of raindrops.

(iii) Pudling:

When fine textured soil (i.e. clay) is tilled when wet, it pores become 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 becomes usually hard and dense. Puddling is the international process of breaking soil aggregates into uniform mud and is commonly practiced in low land paddy cultivation to reduce the loss of water by percolation.

Wet system of paddy cultivation needs puddled condition which involves ploughing of land under waterlogged condition (i.e. Wet condition). The land is ploughed and re-ploughed, followed by planking after each ploughing, to bring the soil to a fine puddle condition. Paddy grows well on puddle soil. It is important to maintain standing water in the field as paddy is semi-aquatic crop.

Advantages of Puddling:

(i) Less power is required for tillage.

(ii) Puddling hastens transplanting operation easily and smoothly as well as the establishment of seedlings.

(iii) Puddling creates an impervious layer below the surface to reduce deep percolation of water and to provide soft seed bed for planting paddy, wheat, onion etc. Puddling increases the moisture retention capacity of a soil.

(iv) Puddling is an important method of controlling some weeds such as Cynodon dactylon, Cyperus rotundus etc.

(v) Puddling increases the availability of nutrients, especially Phosphorus (P), Iron (Fe) and Manganese (Mn).

(vi) Wet tillage is only the means of land preparation for transplanting semi-aquatic crop plant such as Paddy (Oryza sativa).

Disadvantages of Puddling:

(i) Puddling causes almost complete destruction of soil aggregates and destruction of macropores. The original structure can be restored by alternate wetting and drying and tillage at right moisture content.

(ii) A large quantity of water is required for puddling the soil.

(iii) The toxic substances like Hydrogen sulphide (H₂S), Ferrous oxide (FeO) etc. are formed in the puddled soil due to absence of air.

(iv) Puddling creates hard pan below the plough depth which affect the drainage.

(v) Gas exchange, especially of oxygen between Soil and atmosphere is strictly restricted in submerged Puddle Soil.

(vi) Puddling can also Produce a more open structure and bulk density decreases and total porosity increases.

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8. Structural Management of Soil:

A well-structured soil provides an ideal environment for healthy plant growth.

Structural management of soils are as follows:

(i) Coarse Textured Soil:

In coarse textured soil, use of organic manure (i.e. compost, F.Y.M. green manure, oil cake etc.) is the only ways of improving structure. As a result, the water holding capacity of this soil increases because organic materials will not only act as a binding agents for the particles but will also increase the water holding capacity. Sod crop, for example corn, blue grass etc. also help in improving the structural condition of sandy soils.

(ii) Fine Textured Soil:

In fine textured soil, organic manure is helpful for improving the structure of soil as they help in granulation of soil particles. Growing of sod crops also improves the granulation of soil particles. The tillage of clay soil must be carefully timed. If ploughed too wet, the structural aggregates are broken and unfavourable structure results. On the other hand, if ploughed too dry, big clods are turned up which are difficult to work into good seedbed.

The lime should be applied in acidic soil to improve the structure, because lime stimulates the growth of microorganisms which are responsible for the improvement of soil structure. It is found in fertilizer experiment in India that nitrogenous fertilizer applied alone has a deteriorating effect on soil structure. But phosphatic fertilizers used in conjunction with nitrogenous fertilizers improve and maintain soil structure. Phosphatic fertilizers increase the activity of plant-root and microorganism which are the indirect contributory factor determining good soil structure.

The surface of the land should be kept covered with waste organic materials like grasses, straws, leaves etc. which are known ‘mulches’. Mulches keep the surface soil moist and cool, where microorganisms multiply rapidly and improve soil structure. Mulches also protect the soil from the beating action of raindrop.

Soil conditioners such as krilium, polyvinyl alcohol (PVA), polyacrylic acid (PAA) and polyacrylamide (PAM) which are synthetic polymers, can help in stabilization of artificially formed aggregates in the soil, provided small doses (0.1 per cent) are used. These synthetic polymer behaves like natural polymers such as polysaccharides and polyuronides which are the products of microbial decay of organic matter in the soil. Since synthetic polymers are costly, they are impracticable for improving soil structure.

Water repellent material to form a monomolecular layer around each aggregate requires only a small quantity compared to soil conditioner, needed to treat the entire soil mass. Because of the presence of water repellent material around an aggregate, water comes in contact with the aggregate only slowly and without distracting the aggregates. However, a practicable water repellent has still to be developed. Dispersed soils can be corrected by the application of amendments such as gypsum, press mud etc.

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9. Soil Structure and Plant Growth:

Soil structure influences plant growth rather indirectly. Porosity of soil depends on the structure of soil. The aeration and movement of water in the soil govern the growth of a plant. In crump structure, porespaces are more and this structure is favourable for germination of seed and emergence of seedlings.

So the soil in seed bed should have a crumb structure. In crumb structure, the peds are soft and porous. As a result, the roots of seedlings can penetrate in the soil easily. On the other hand, the plant roots fail to penetrate in the highly compacted soil as the hard compact layer impedes the root growth.