Soils Academy: Determination of Soil Texture

There are number of techniques that are used in the determination of soil texture. These techniques are broadly classified into field methods and laboratory methods. With the availability of these techniques, the textural class of a soil can be determined.

Field Determination of Soil Texture

In the field, soil texture is determined through a technique known as the “Feel Method”. This technique involves the addition of sufficient amount of water to a small quantity of soil and the attempt to make a cylinder by rubbing the moist soil between the thumb and finger.

The degree of smoothness of the cylinder made, the length of the cylinder and the extent to which the moist soil forms into different shapes, helps to determine the soil texture. The accuracy of the feel method depends on practice, and the experience of the soil scientist.

The following is a guide when using the feel method:

Ø If the moist soil breaks down and forms neither a ball nor a cylinder, it is SAND.

Ø If the moist soil forms a ball at any time, but breaks down when attempting to make a cylinder, it is LOAMY SAND.

Ø If the moist soil forms a ball and a cylinder, but the cylinder breaks down when bent into a U-shape, it is a LOAM.
Ø If the cylinder forms a U-shape but fails to form a circle, then the soil is a CLAY LOAM.

Ø If the cylinder forms a circle with cracks in it, then it is SANDY CLAY.
Ø If the cylinder forms a full circle without cracks, then it is CLAY.

Laboratory Determination of Soil Texture

The laboratory determination of soil texture employs the mechanical method. This method has several techniques, most of which usually follow this procedure:

Ø Complete removal of all roots and other non-soil materials from the soil sample.

Ø Complete removal of all organic matter, other cementing agents and dissolved mineral matter. (Though Iron oxide is an important cementing agent; it is not removed from tropical soils because removing it will increase the clay content of the soil).

Ø Dispersion of the soil sample - separation of the various separates (sand, silt and clay).

Ø Measurement and calculation of the proportions of the various soil separates.

Some of the techniques used in the mechanical method include sieving, sedimentation, centrifugal, and decantation techniques. The most common being the sieve and sedimentation.

Sieve Technique:

In this method, soil particles, up to those of approximately 0.05mm in diameter are, with the use of sieves, separated. This method is simple but it has limitations, which includes:

a) Non-uniformity in the shape of particles (for this reason, round hole sieves are better suited and are generally more preferred to square ones).
b) Unequal sieve openings.

Sedimentation Technique:

The hydrometer and the pipette methods are the most common of the sedimentation technique. The theory of both is similar but the manner of determination of soil texture is different.

Both methods depend on the fact that sedimentation eliminates in a given time, from a known depth, all particles having a particular settling velocity. This is based on the principle of stokes law which indicates that the terminal velocity of a spherical particle, settling under the influence of gravity in a fluid of given density and viscosity is proportional to the square of radius of the particle.

It is mathematically as: