Agriculture for Engineers 4 (3+1)

Lesson 5. Soil Physical Properties and Soil Particle Distribution - I

5.2 SOIL TEXTURE

          Soil texture refers to the relative proportion of particles of various sizes in a given soil or it refers to the relative percentage of sand, silt and clay in a soil.

A. Classification of soil separates

          There are a number of systems of naming soil separates. They are: (a) The American system developed by USDA, (b) The English system, and (c) The International system. The international system which is commonly followed in India is given in Table 5.1.

Table 5.1: International system of naming soil separates

Soil Separate	Diameter range (mm)
Coarse sand	2.00 to 0.200
Fine sand	0.200 to 0.020
Silt	0.020 to 0.002
clay	< 0.002

 

Particle Size Analysis (Mechanical Analysis)

The mineral component constitutes the soil mass. This mineral portion consists of particles of various sizes. According to the size, the soil particles are grouped into gravels, sands, silts and clays which are termed as soil separates. The process of determining the amounts of individual soil separates below 2 mm in diameter is called a mechanical analysis. It is based on Stocke's law (1851) i.e. when soil particles are suspended in water they tend to sink and their velocity (V) of settling is proportional to the square of the radius (r) of each particle. The relation between the diameter of a particle and its settling velocity is given below:

To make a mechanical analysis, a sample of soil is crushed lightly in a wooden mortar. The material is next passed through the sieve is taken for mechanical analysis. The organic matter and other binding materials are removed from the soil before the mechanical separation.

There are several methods of mechanical analysis viz., sieve method, sedimentation method, decantation method, centrifugal method, pipette method and hydrometer method. Pipette method is universally employed for carrying out mechanical analysis of soil.

B. Characteristics and physical nature of soil separates

Sands are large sized particles and have the large size of pore spaces. Hence, they facilitate percolation and encourage aeration. Their water holding capacity is low. They do not possess plasticity. Clay particles are smaller in size and possess fine pore spaces. With decreasing particle size, there is decrease in aeration and percolation rate. The water holding capacity of clay is very high. Properties such as plasticity, swelling, cohesion etc. are very high. Silt particles are intermediate in size. Silts also show properties somewhat intermediary between sands and clays (Table 5.2).

Table 5.2: Comparative characteristics of different soil separates

S. No.	Characteristics	Sand	Silt	Clay
1	Size (mm)	2.0-0.02	0.02-0.002	<0.002
2	Visibility	Visible by naked eye	Visible by ultra microscope	Visible by microscope
3	Water holding capacity	Low	Medium	High
4	Total pore space	Least	Medium	Highest
5	Size of pore	Large	Medium	Very small
6	Air & water movement	Very rapid	Moderate	Slow
7	Plasticity, swelling, cohesion etc.	Very low	Moderate	Very high
8	Feel on rubbing between thumb and fingers	Gritty	Feel Like flour or talcum powder	Feel very plastic and sticky when wet and become hard under dry condition
9	Tillage	Easy	Moderate	Difficult
10	Fertility	Very low	Moderate	High
11	Minerals	Quartz dominates	Feldspar, Mica, hematite, Quartz	Kaolinite, montmorillonite, illite
12	Chemical activity	Chemically inactive	Slightly active	Chemical activity

C. Textural classes

Textural names are given to soils based on the relative proportion of sand, silt and clay. So that are preponderantly clay are called clay (textural class); those with high silt content are silt (textural class); those with a high sand percentage are sand (textural class). A soil that does not exhibit the dominant physical properties of any of those three groups is called loam. The textural triangle (proposed by USDA) is used to determine the soil textural name after performing mechanical analysis in the laboratory (Fig. 5.1).

Fig. 5.1 Triangular textural diagram

5.3 SOIL STRUCTURE

The arrangement of soil particles and their aggregate into certain defined patterns is called soil structure. The primary soil particle sand, silt and clay usually occur grouped together in the form of aggregates. Natural aggregates are called peds where as clod is an artificially formed soil mass. Soil structure is studied in the field under natural conditions and it is described under three categories (Fig.5.2).

(1) Type: Shape or form and arrangement pattern of peds (in details)

(2) Class: Size of peds

(3) Grade: Degree of distinctness of peds.

A. Types of soil structure

There are four principal forms of soil structure -

Fig.5.2: Types of soil structure

 (i) Plate like structure: In this structural type of aggregates are arranged in relatively thin horizontal plates. The horizontal dimensions are much more developed than vertical. When the unit is thick, they are called platy, and when thin, laminar (fig.7). Platy soil structure is most notice able in the surface layers of virgin soils but may be present in the sub soil. Platy type is often inherited from the parent material, especially those laid down by water (fig.8).

(ii) 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 soil structure is termed as columnar, and when flat, prismatic. They commonly occur in sub soil horizons in arid and semi arid regions.

(iii) Block like: All these dimensions are about the same size and the peds are cube like with flat or rounded faces. When 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.

(iv) 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 inches in diameter. These rounded complexes usually lie loosely and separately. The aggregates of this group usually termed as granular which are relatively less porous, when the granules are very porous, the term used is crumby (fig.9 a,b,c).

B. Structure Formation

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 colloids to form a compound or secondary particles. It is mainly the arrangement of these secondary particles that imparts structure to a soil mass. The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminum 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 aggregates very readily.

Iron and aluminum hydroxide act as cementing agent in binding the soil particles together. These are also responsible for the formation of aggregates by cementing sand and silt particles. Organic matter plays an important part in forming soil aggregates. The humic acid and other sticky product produced during the decomposition of organic matter help in aggregate formation.

 C. Factors affecting Soil Structure

The following factors influence the formation of soil structure in the arable condition.

(1) Climate: Climate influence the aggregate formation, in arid region, there is very little aggregation of primary particles while in semi arid region, the degree of aggregation is greater than arid region.

(2) Organic matter: Organic matter improves the soil structure in both sand and clay soils. During the decomposition the byproduct produced help in binding the various particles, while in clay soils it modifies the cohesion properties and their by improve the soil structure.

(3) Tillage: Cultivation or use of implements influence the soil aggregate, if at high moisture content tillage is carried out big clods may come out while at too low moisture will break the aggregates. Use of heavy implements will break the soil structure.

(4) Plant roots: The root secretions may also act as cementing agents and binds various soil particles. The plant roots, on decay, may also bring about granulation due to the production of sticky substances.

(5) Fertilizers: Certain fertilizer like sodium nitrate destroys the soil aggregates while calcium ammonium nitrate helps in the development of soil structure.

(6) Wetting and drying: When a dry soil is wetted, the soil colloids swell on absorbing water. On drying, shrinkage produced strains in the soil mass give rise to crack which break it up into clods and granules of various sizes.

 D. Importance of Soil Structure

Soil structure influences indirectly by the formation of an array of pores of various shapes and sizes. These pores are controlling factors governing water, air and temperature in soil. The roles of soil structure in relation to plant growth are as under:

1. Soil structure influences the amount and nature of porosity.

2. Structure controls the amount of water and/air present in soil.

Not only the amount of water and air dependent on soil structure, but also their movement and circulation are also controlled by soil structure.

3.  It affects tillage practices.

4.  Structure controls runoff and erosion.

5. Platy structure normally hinders free drainage whereas sphere like structure helps in drainage.

6. Crumby and granular structure provides optimum infiltration, water holding capacity, aeration and drainage. It also provides good habitat for microorganism and supply nutrients.