Soil Texture: Characteristics, Determination and Role | Soil Science

In this article we will discuss about:- 1. Characteristics of Soil Separates 2. Determination of Soil Texture 3. Role.

Characteristics of Soil Separates:

Soil separates have some characteristics as follows:

(i) Physical Nature of Soil Separates:

Sands are large sized particles and have large size of porespace. Hence the aeration and drainage capacity are good in sandy soil. But the plasticity, adhesion, cohesion, water holding and nutrient retention capacity are poor in sandy soil. Clay particles are smallest in size and possess fine porespace.

The aeration and percolation rate decreases with decreasing the particle size. The properties such as cohesion, plasticity, water holding capacity and nutrient retention compaction of clay soil are very high. Silt particles are intermediate in size. Silt has the properties somewhat intermediary between sands and clays.

(ii) Mineralogical Characteristics:

Quartz commonly dominates the finer grades of sand as well as the silt particles. In addition, variable quantities of other primary minerals usually occur, such as feldspar and micas. Hematite and limonite minerals may present. These impart various shades of red and yellow colour if present in sufficient quantities. Coarser clay fractions are composed of minerals such as quartz and hydrous oxide of iron (e.g. hematite and limonite) and aluminium. Another is the complex aluminosilicates. Three main mineral types such as kaolinite, illite and montmorillonite are at present recognized.

(iii) Chemical Make-Up:

Since sand and silt are dominantly quartz (SiO₂), these two fractions are generally inactive chemically. On the other hand, clay particles are very active. Clays consist principally of secondary products of weathering. Chemically kaolinite and montmorillonite are aluminium silicate. They also carry iron and magnesium in addition to sodium. Illite is hydrous mica, a potassium aluminium silicate.

Soil Texture and Growth of Plant:

The growth of plant depends on the texture of soil. A knowledge of texture helps in the management of soils. In coarse textured (i.e. sandy soil) soil aeration and drainage capacity is very good as they are very permeable but they cannot retain enough water and nutrients for the growth of the crop. The coarse textured soil needs frequent application of fertilizer and light irrigation to meet the demand of the cultivated crops.

In coarse textured soils, more fertilizer will be needed and it would be necessary to increase the organic matter content for holding water and nutrients and frequent light irrigation may be more useful as water is not held for long periods. The plant which requires good drainage will thrive well in coarse textured soil.

The medium textured soils (i.e. loamy soils) are most desirable, because they are well drained and aerated and can retain nutrients and enough water for the growth of most plants.

The fine textured soils (i.e. clay soil), have the capacity to retain plant nutrient and water because they are negatively charged and they have large surface area per unit volume and micropore respectively. But a soil consisting predominantly of clay is not permeable to air and water. This soil should not be worked upon when they are too wet as they produce hard clods on drying.

So ploughing should be done in an intermediate soil moisture content when it is dry enough to loss its stickness, yet moist enough not to produce hard clods. More power is required to plough a clayey soil than the sandy soil. So the clayey soils are called ‘heavy soil’ and sandy soils are called ‘light soil’. The use of the terms ‘heavy’ and ‘light’ refer to ease of tillage and not to weight of soil. The plant which requires stagnation of water throughout their life-cycle will thrive well (e.g. paddy) in a fine textured soil.

Determination of Soil Texture:

The texture of soil is determined by:

(a) Feel method and

(b) Laboratory method (Mechanical method)

(a) Feel Method:

In the field, texture of soil is commonly determined by ‘feel method’. For this, some soils are collected from the field and then rubbing it between thumb and finger, it is easy to say the soil is sand, silt, clay and loam. Usually it is helpful to wet the samples in order to estimate plasticity more accurately.

(b) Mechanical Analysis of Soil:

Mechanical analysis is the analytical procedure of determining the percentage of sand, silt and clay in the laboratory. The particles of soil separated from one another by the process of sedimentation in water by a law called ‘Stokes Law’ shows the relationship between the size (radius) of particles and their rate of fall down in the liquid medium. The velocity of fall of a particle in a liquid is directly proportional, to the square of its radius.

G.G. Stoke (1851) stated that “The resistance offered by a liquid medium against the fall particles down the same liquid medium varies with the radius of the particles”. The sedimentation method of measuring particle size fraction is based on ‘Stokes law’. It states that the viscos drag (F) for a spherical body of radius (r) moving with velocity (v) in a medium of coefficient of viscosity (ƞ) is given by –

In arriving at this result, Stoke’s made assumptions as follows:

(i) The medium through which the body falls is infinite in dimensions. This is achieved by reducing concentration of particles in the suspension to less than 0.5 per cent.

(ii) Particles may be large compared to the molecules of the liquid so that they cannot follow Brownian movement.

(iii) Particles must be spherical, smooth and rigid. But soil colloidal particles are plate shaped and fall slower than spherical particles of the same mass.

(iv) Particle must be of uniform density.

(v) The medium should be still and no turbulence should set in due to particle movement. Particles larger than 0.08 mm settle quickly and cause turbulence and therefore cannot be analysed by this procedure.

Role of Soil Texture in the Genesis and Fertility of a Soil:

The texture of soil is determined by the physical and physicochemical properties of a soil. By knowing the texture of soil, it is possible to approximate its mineralogical and chemical composition, its physicochemical properties and the content of nutrients.

Soils with sandy or loamy sand texture are mainly made from quartz, though they contain a small quantity of feldspar and other minerals. Sandy soils have large non-capillary pores and loose structure. They are thus poor water retainers, water ‘seeps’ through them as through a sieve. Sandy soils are poor in nutrients and need additional clay, and minerals and organic fertilizers.

Soils with clay textures have a densely packed structure and capillary pores. They can retain large quantities of water and can swell. They resist the passage of water because their capillaries are small in diameter and they may be plugged by swelling. The composition of clay minerals is very important when assessing the properties of a clay.

This composition governs the formation of macro and micro-aggregates and the degree of swelling. For example, with a smectite clay, the soil swells but does not lose its aggregation (granulation). Despite its poor water permeability, there still is some migration of clay, yielding so called planes of sliding, as is the case in vertisols.

Although they have unfavourable physical properties, clay soils can contain large quantities of nutrients. To improve their physical properties, large amounts of organic substances, should be applied or frequent loosening or sanding can be done.

Soils with large quantities of fine silt are often unfavourable because their clay content is insufficient to produce aggregates. Silt itself has no aggregating abilities and as a consequence its water permeability and aeration deteriorate. The structure of a silty soil is almost unnoticeable, particularly when the humus content is low. That is why such soils are vulnerable to water and wind erosion.

Many tropical soils are characterized by the domination of a fine silt composed of kaolinite which can be attributed to the long and continuous weathering and soil formation in the tropics.