Definition of Soil Texture: Soil texture refers to the relative proportion of particles or it is the relative percentage by weight of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles.
The proportion of each size group in a given soil (the texture) can not be easily altered and it is considered as a basic property of a soil.
The soil separates are defined in terms of diameter in millimeters of the particles. Soil particles less than 2 mm in diameter are excluded from soil textural determinations.
Stones and gravels may influence the use and management of land because of tillage difficulties but these larger particles make little or no contribution to soil properties such as WHC and capacity to store plant nutrients and their supply.
Gravels: 2 – 4 mm
Pebbles: 4 – 64 mm
Cobbles: 64 – 256 mm
Boulders: > 256 mm
Particles less than 2 mm are called fine earth, normally considered in chemical and mechanical analysis.
The components of fine earth: Sand, Silt and Clay (Soil separates. The size limits of these fractions have been established by various organizations. There are a number of systems of naming soil separates.
(a) The American system developed by USDA
(b) The English system or British system ( BSI )
(c) The International system (ISSS)
(d) European system
i) USDA
Soil separates | Diameter in mm |
Clay | < 0.002 |
Silt | 0.002 – 0.05 |
Very Fine Sand | 0.05 – 0.10 |
Fine Sand | 0.10 – 0.25 |
Medium Sand | 0.25 – 0.50 |
Coarse Sand | 0.50 – 1 |
Very Coarse Sand | 1 – 2 mm |
ii) BSI
Soil separates | Diameter in mm |
Clay | < 0.002 |
Fine Silt | 0.002 – 0.01 |
Medium Silt | 0.01 – 0.04 |
Coarse Silt | 0.04 – 0.06 |
Fine Sand | 0.06 – 0.20 |
Medium Sand | 0.20 – 1 |
Coarse Sand | 1 – 2 mm |
iii) ISSS
Soil separates | Diameter in mm |
Clay | < 0.002 |
Silt | 0.002 – 0.02 |
Fine Sand | 0.02 – 0.2 |
Coarse Sand | 0.2 – 2 |
iv) European System
| Soil separates | Diameter (mm) |
Fine clay | < 0.0002 mm |
Medium clay | 0.0002 – 0.0006 |
Coarse clay | 0.0006 – 0.002 |
Fine silt | 0.002 – 0.006 |
Medium silt | 0.006 – 0.02 |
Coarse silt | 0.02 – 0.06 |
Fine sand | 0.06 – 0.20 |
Medium sand | 0.20 – 0.60 |
Coarse sand | 0. 60 – 2.00 |
Sand:
Usually consists of quartz but may also contain fragments of feldspar, mica and occasionally heavy minerals viz., zircon, Tourmaline and hornblende.
Has uniform dimensions
Can be represented as spherical
Not necessarily smooth and has jagged surface
Silt:
Particle size intermediate between sand and clay
Since the size is smaller, the surface area is more
Coated with clay
Has the physico- chemical properties as that of clay to a limited extent
Sand and Silt forms the SKELETON
Clay:
Particle size less than 0.002 mm
Plate like or needle like in shape
Belong to alumino silicate group of minerals
Some times considerable concentration of fine particles which does not belong to alumino silicates. (e.g.) iron oxide and CaCO3
These are secondary minerals derived from primary minerals in the rock
Flesh of the soil
Knowledge on Texture is important. It is a guide to the value of the land. Land use capability and methods of soil management depends on texture.
Particle size distribution/ determination
The determination of relative distribution of the ultimate or individual soil particles below 2 mm diameter is called as Particle size analysis or Mechanical analysis
Two steps are involved
i) Separation of all the particles from each other ie. Complete dispersion into ultimate particles
ii) Measuring the amount of each group
Separation
| Sr. No. | Aggregating agents | Dispersion method |
1 | Lime and Oxides of Fe & Al | Dissolving in HCl |
2 | Organic matter | Oxidises with H2O2 |
3 | High conc. of electrolytes | Precipitate and decant or filter with suction |
4 | Surface tension | Elimination of air by stirring with water or boiling |
After removing the cementing agents, disperse by adding NaOH
Measurement
Once the soil particles are dispersed into ultimate particles, measurement can be done
i) Coarser fractions – sieving – sieves used in the mechanical analysis corresponds to the desired particle size separation for 2 mm, 1 mm and 0.5 mm – sieves with circular holes, for smaller sizes, wire mesh screens are used (screening)
ii) Finer fractions – by settling in a medium the settling or the velocity of the fall of particles is influenced by viscosity of the medium. Difference in density between the medium and falling particles, size and shape of object
Stokes’ Law:
Particle size analysis is based on a simple principle i.e. “when soil particles are suspended in water they tend to sink. Because there is little variation in the density of most soil particles, their velocity (V) of settling is proportional to the square of the radius ‘r’ of each particle.
Thus V = kr2, where k is a constant. This equation is referred to as Stokes’ law.
Stokes (1851) was the first to suggest the relationship between the radius of the particles and its rate of fall in a liquid. He stated that “the velocity of a falling particle is proportional to the square of the radius and not to its surface. The relation between the diameter of a particle and its settling velocity is governed by Stokes’ Law:
V = 2/9 gr^2 (ds – dw) / n
Where,
V – Velocity of settling particle (cm/sec.)
g – Acceleration due to gravity cm/ sec2 (981)
ds – Density of soil particle (2.65)
dw – Density of water (1)
n – Coefficient of viscosity of water (0.0015 at 4oC)
r – Radius of spherical particles (cm).
Assumptions and Limitations of Stokes’ Law
Particles are rigid and spherical / smooth. This requirement is very difficult to fulfill, because the particles are not completely smooth over the surface and spherical. It is established that the particles are not spherical and irregularly shaped such as plate and other shapes.
The particles are large in comparison with the molecules of the liquid so that in comparison with the particle the medium can be considered as homogenous i.e. the particles must be big enough to avoid Brownian movement. The particles less than 0.0002 mm exhibit this movement so that the rate of falling is varied.
The fall of the particles is not hindered or affected by the proximity (very near) of the wall of the vessel or of the adjacent particles. Many fast falling particles may drag finer particles down along with them.
The density of the particles and water and as well as the viscosity of the medium remain constant. But this is usually not so because of their different chemical and mineralogical composition.
The suspension must be still. Any movement in the suspension will alter the velocity of fall and such movement is brought by the sedimentation of larger particles (> 0.08 mm). They settle so fast and create turbulence in the medium.
The temperature should be kept constant so that convection currents are not set up.
Methods of Textural determination
Numerous methods for lab and field use have been developed
i) Elutriation method – Water & Air
ii)Pipette method
iii) Decantation/ beaker method
iv) Test tube shaking method
v) Feel method – Applicable to the field – quick method – by feeling the soil between thumb and fingers
Feel Method
Evaluated by attempting to squeeze the moistened soil into a thin ribbon as it is pressed with rolling motion between thumb and pre finger or alternately to roll the soil into a thin wire
η four aspects to be seen – i) Feel by fingers, ii) Ball formation, iii) Stickiness and iv) Ribbon formation
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