Fundamentals of Soil Science (1+1)

Methods of Textural Analysis

Methods of textural analysis:

• 1. Rapid feel method
• 2. Hydrometer method
• 3. International pipette method.

1. Soil texture by feel method:
The common field method for determining the textural class of a soil is by its feel. It is of great practical value and depends upon the skill and experience of the worker. The principle underlying the determination of soil texture by this method is based on the properties, which the soil components exhibit. These are being described below:

• Clay: (Particles < 0.002 mm in diameter) Confers cohesion, stickiness and plasticity to the ball and increases its resistance to deformation.
• Silt: (between 0.02 and 0.002 mm in diameter) Confers a silky smoothness to the ball.
• Sand: (Particles between 0.02 and 2 mm in diameter) Confers grittiness.
• Organic matter: Imparts cohesion to sandy textures, greasiness to clayey textures and tends to produce a short thick ribbon from the balls.

2. Hydrometer method (Using Bouyoucos hydrometer):

• The hydrometer method is based on the principle that the density of the suspension at a given depth decreases as an initially homogenous dispersed suspension settles. The rate of decrease in density at any given depth is related to the velocities of settling particles, which in turn, is related to their sizes.
• The time required by the particles of a given size to settle can be calculated using Stokes law.
• It however, gives approximate values that too for silt and clay only. Hydrometer method cannot be used for saline or organic soils or for soils which are known to be difficult to disperse.
• It also gives anomalous results with calcareous soils as the method does not involve any pre-treatment with hydrochloric acid to remove the calcium carbonate.

3. International pipette method:

• Pipette method is a standard method for particle size analysis of soils because of its accuracy, but it is time consuming and cannot be employed where large numbers of samples have to be analyzed.
• Particle size analysis is done by using sieves to separate out coarse sand from the finer particles. The silt and clay contents are then determined by measuring the rate of settling of these two separates from the suspension in water.
• The time required by the particles of a given size to settle can be calculated using Stokes law.

• Stoke’s law: It states that the terminal velocity of a spherical particle settling under the influence of gravity in a fluid is directly proportional to the square of its radius and is expressed as:

This method is widely used. Using a pipette, samples are drawn at a given depth (10 cm) after specific time and then dry matter is determined.

Assumptions of Stoke’s law:

1. The particles must be large enough, so that Brownian movement will not influence their rate of fall.
2. There must be no slipping between the liquid and the particles.
3. The velocity of fall must not exceed a certain critical value.
4. The particles fall independently. This requires a suspension concentration of less than 5 per cent.
5. The particles must be rigid and smooth.

Limitations of Stoke’s law:

1. It is necessary to maintain a constant temperature during analysis in the vessel filled with suspension. Otherwise, there will not be uniform settling of the particles and results will be wrong. This is not possible most of the times.
2. Since the density of clay particles decreases with the particle size, hence it may affect the accuracy of the stoke’s law.

Textural Triangle:

• The horizontal lines mark percentages of clay (by mass). The lines angle upwards to the right mark percentages of silt. The lines angled upwards to the left mark percentages of sand.
• The percentage of sand, silt and clay are used to draw lines. When three lines so drawn intersect in a compartment then it is the textural class of the soil under study.

Range of soil separates in different textural classes:

Textural class	Ranges of soil separates (%)
	Sand	Silt	Clay
Sand	85 - 100	0 - 15	0 - 10
Loamy sand	70 - 90	0 - 30	0 - 15
Sandy loam	40 - 80	0 - 50	0 - 20
Loam	23 - 52	28-50	7-27
Silt loam	0 -50	50-88	0-27
Silt	0-20	80-100	0-12
Sandy clay loam	45-80	0-28	20-35
Clay loam	20-45	15-53	27-40
Silty clay loam	0-20	40-73	27-40
Sandy clay	45-65	0-20	35-45
Silty clay	0-20	40-60	40-60
Clay	0-45	0-40	40-100