Soil texture refers to the
relative proportion of particles or it is the relative percentage by we ight of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles.
We cannot easily alter Soil texture and consider it as a basic property of a soil.
The soil separates are defined in terms of diameter in millimeters of the particles like
Soil separates and Diameter (mm)
1. Clay < 0.002 mm
2. Silt 0.002 – 0.02 mm
3. Fine sand 0.02 – 0.2 mm
4. Coarse sand 0.2 – 2.0 mm
We call Particles less than 2 mm as fine earth are normally consider them in chemical and mechanical analysis and exclude them from soil textural determinations for physical properties of soil.
Particles more than 2mm in diameter are generally termed as Stones and gravels. These larger particles make little or no contribution to important soil properties such as Water Holding Capacity and capacity to store plant nutrients and their supply.
The different sizes and names of such larger particles are as follows
Gravels : 2 – 4 mm
Pebbles : 4 – 64 mm
Cobbles : 64 – 256 mm
Boulders : > 256 mm
It Usually consists of quartz but may also contain fragments of feldspar, mica and occasionally heavy minerals viz., zircon, Tourmaline and hornblende
Sand has uniform dimensions
It is represented as spherical
Not necessarily smooth and has jagged surface
Particle size intermediate between sand and clay
Since the size is smaller, the surface area is more
Has the physico- chemical properties as that of clay to a limited extent
Sand and Silt forms the Skeleton of soil
Particle size less than 0.002 mm
Plate like or needle like in shape
Belong to alumino silicate group of minerals
Clays are secondary minerals derived from primary minerals in soil
Clay is called as Flesh of the soil
Soil textural classes
According to the proportion of sand, silt and clay in the soil, it is given a name to indicate its textural composition. Such a name gives an idea not only of the textural composition of a soil but also of its various properties in general.
On this basis we classify soils into various textural classes like sands clays, silts, loams etc
The sand group includes all soils in which the sand separates make up at least 70% and the clay separate 15% or less of the material by weight.
The properties of such soils are therefore characteristically those of sand in contrast to the stickier nature of clays.
Two specific textural classes are recognized in this group sandy and loamy sand although in practice two subclasses are also used Loamy fine sand and loamy very fine sand.
The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt. Only one textural class – Silt is included in this group.
To be designated a clay a soil must contain at least 35% of the clay separate and in most cases not less than 40%. In such soils the characteristics of the clay separates are distinctly dominant, and the class names are clay, sandy clay and silty clay.
Sandy clays may contain more
sand than clay. Likewise, the silt content of silty clays usually exceeds clay fraction.
The loam group, which contains many subdivisions, is a more complicated soil textural class.
An ideal loam may be defined as a mixture of sand, silt and day particles that exhibits the properties of those separates in about equal proportions.
Loam soils do not exhibit dominant
physical properties of sand, silt or clay.
Loam does not contain equal percentage of sand, silt and clay. However, exhibit approximately equal properties of sand, silt and clay.
How the above textural classes are determined?
In the American system as developed by the United State Department of Agriculture twelve textural classes are proposed with the help of the textural triangle.
It is used to determine the soil textural name after the percentages of sand, silt, and clay are determined from a laboratory analysis.
Since the soil’s textural classification includes only mineral particles and those of less than 2mm diameter, the sand plus silt plus clay percentages
equal 100 percent without including the organic matte.
Knowing the amount of any two fractions automatically fixes the percentage of the third one.
Use of the diagram of triangle
To use the diagram, locate the percentage of clay first and project inward parallel to sand line.
Do likewise for the per cent silt and project inward parallel to clay line and for sand, project inward parallel to silt.
The point at which the projections cross or intersect will identify the class name.
Some times, the intersecting point exactly fall on the line between the textural classes.
Then it is customary to use the name of the finer fraction when it happens. (eg). Soil containing 40% clay, 30% sand
Importance of Soil Texture
Presence of each type of soil particles makes its contribution to the nature and properties of soil
as a whole. The soil texture gives us the following important knowledge in judging the soil and crop suitability
Texture has good effect on management and productivity of soil.
Sandy soils are of open character and usually loose and friable. Such type of the texture is easy to handle in tillage operations.
Sand facilitates drainage and aeration and allows rapid evaporation and percolation.
Sandy soils have very little water holding capacity. Such soils can not stand drought and unsuitable for dry farming.
Sandy soils are poor store house of plant nutrients
Contain low organic matter
Leaching of applied nutrients is very high.
In sandy soil all to grow few crops such as potato, groundnut and cucumbers.
Clay particles play a very important role in soil fertility.
Clayey soils are difficult to till and require much skill in handling. When moist clayey soils are exceedingly sticky and when dry, become very hard and difficult to break.
There are fine pores, and have poor drainage and aeration.
Such soils have a high water holding capacity and poor percolation, which usually results in water logging.
Soils are generally very fertile soils, in respect of plant nutrient content. Rice, jute, sugarcane can be grown very successfully in these soils.
Loam and Silt loam soils are highly desirable for cultivation
Generally, the best agriculture soils are those contain 10 – 20 per cent clay, 5 – 10 percent organic matter and the rest equally shared by silt and sand and 30% silt – called as clay rather than clay loam