Soil Texture

Introduction

Procedure for mechanical analysis of soil

Solving problems on soil texture

Introduction

This program explains the soil texture determination by the hydrometer method. You will calculate the percentage of sand, silt, and clay in a soil sample.

Based on these percentages, you will determine the texture of the soil by using a textural triangle. In this exercise you will get help if you go wrong in your calculations.

Mechanical analysis of a soil sample is based on the principle that a particle-settling rate is a function of the radius of the particles.

Hence, the mechanical analysis of a soil requires that each sand, silt and clay particle be separated to permit independent settling of these particles.

To ensure this the samples are treated with hydrogen peroxide to destroy the organic matter and also with sodium hexametaphosphate to disperse the clay particles. The sample is then stirred and the column filled to 1 L.

Since the particle settling rate is a function of the radius of the particles, sand being largest in size settles first in about 40 sec.

Hence the hydrometer reading (g/L) represents silt and clay. The silt size particles will settle in about 2 h. Now only clay particles remain in suspension. The hydrometer reading (g/L) after 2 h of suspension will represent clay. One should note that the room temperature will have an effect on the settling rate of the particles in a liquid.

Temperature influences the density and viscosity of water through which the particles settle. Hence a correction factor for the hydrometer reading is required if the temperature deviates from 20^oC.

We need to add to the hydrometer reading 0.4 g per L for each degree above 20^oC. Similarly if the temperature is below 20^oC, then subtract 0.4 g per L for each degree below.

 

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Procedure for mechanical analysis of soil

The following summarizes the procedure for the mechanical analysis of a soil.

Step 1: Weigh a 50 g sample of the soil.

Step 2: Disperse the soil in 1 L of water.

Step 3: After suspending the soil, take the hydrometer reading at 40 sec and at 2 h.

Step 4: Apply the correction factor if the room temperature is above or below 20^oC.

Step 5: Using the hydrometer readings, calculate the sand, silt, and clay percentages.

Step 6: Using the textural triangle find the texture of the soil.

Now let us try this exercise and identify the texture of a soil sample. In this exercise, we assume the room temperature to be 20^oC and the mass of the soil sample to be 50 g.

The mass of the soil sample to be 50 g.

Time : 40 sec

Hydrometer reading: 7 g per L

Time: 2 h

Hydrometer reading: 7 g per L

What is the percentage of sand?

The hydrometer reading at 40 sec represents silt and clay in the suspension. Hence, to determine the amount of sand, subtract this reading from the sample mass. Use the following equation to calculate the percentage of sand in the soil sample.

                      Sample mass   - 40  sec  hydrometer reading
Sand (%)  = --------------------------------------------------------------------------- x 100
                                    Sample mass

 

                   40  sec  hydrometer reading
Clay (%)  = -------------------------------------------x 100
                      Sample mass

Silt (%)  =    100  -  ( Sand %  +  Clay %)

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Solving problems on soil texture