Theory & Procedure, Study of Physical Properties of Soil | Additional Study Material for NEET PDF Download

Our Objective

Our aim is to collect and study soil for texture, moisture content, pH and water holding capacity.

Theory

Soil is the upper humus, containing a layer of the earth, consisting of rock and mineral particles mixed with decayed organic matter.  Soil sustains plant life and contains numerous living organisms. Soil, along with air and water, is one of the three most important natural resources, which we cannot live without.  A productive soil contains approximately 46% mineral matter, 4% organic matter, 25 % water and 25% air.

An approximate composition of soil shown below

Let’s see the physical characteristics of soil.

The physical characteristics of a soil are due to the size of its soil particles.  Soils are classified according to their particle size as follows:

Texture of Soil

Soil texture is an important physical characteristic of soil which is used in both the field and laboratory to determine classes for soils based on their physical texture. The soil texture depends upon the proportion of the constituent solid particles of different sizes. The terms sand, silt, and clay refer to particle size; sand is the largest and clay is the smallest. The size of sand particles is 0.05–2 mm, silt particles are 0.002–0.05 mm, and clay is smaller than 0.002 mm.

The term loam refers to a soil with a combination of sand, silt, and clay sized particles.  Each texture corresponds to specific percentages of sand, silt, or clay.

The soil texture triangle is a tool used to visualise and understand the meaning of soil texture names.  The below figure shows each of the 12 textural classes based on the percentage of sand, silt, and clay in each.

If we know the sand, silt, and clay percentages of a soil, then the textural class can be identified from the textural triangle. Say for an example soil consists of 12% clay, 55% sand and 31% silt, we will see how to determine the textural class of the soil.

Here, the sample soil has 12% clay, so draw a line corresponding to percent clay. Similarly draw the lines for percent sand (55%) and percent silt (31%). The lines which intersect indicate the soil type we have. From the above sample, soil consists of 12% clay, 55% sand, and 31% silt; hence the soil type is sandy loam.

pH of Soil

The chemical property of the soil depends upon the presence of different types of nutrients and pH of the soil. The soil pH is an indication of acidity or alkalinity of soils. The soil pH is important in determining the availability of soil minerals.  Different plants have differing optimum soil pH requirements. The majority of plants prefer a pH of around 6 to 7, which is very slightly acidic.

Water Holding Capacity of Soil

One of the main functions of soil is to retain water and make it available for the plant to access.  All of the water in the soil is not available to plants. The amount of water available to plants is therefore determined by the number and size of the soil’s pore spaces. Water holding capacity of the soil is the amount of water held by the capillary spaces of the soil after the percolation of gravitational water into the deeper layers.  Fine sandy loam, silt loam and silty clay loam soil store the largest amount of water, whereas sand, loamy sand and sandy loam have limited water storage capacity.

Learning Outcomes:

• Students understand the physical properties of soil.
• Students understand texture, pH and water holding capacity of soil.
• Students do the experiment better in the real lab having gone through the animation and simulation.

1. To Study pH of Different Types of Soil

Materials Required:

Real Lab Procedure:

• Let’s first prepare the soil solution.
• Take roadside soil from the watch glass and dissolve it into the beaker containing water to make soil solution.
• Similarly, repeat the same procedure for other soil samples.
• Take a funnel, place a filter paper in it and keep it on a test tube.
• Take roadside soil solution and filter the solution through the filter paper and collect the filtrates in a test tube.
• Repeat the same procedure for other samples with fresh filter papers.
• The soil solution is now ready for testing ph.

Using pH Paper

Materials Required:

Procedure:

• Take a pH paper booklet.
• Tear pH paper strips from the booklet and place 4 strips on the tile.
• Using a dropper, take some roadside soil solution from the test tube.
• Put 1 to 2 drops of solution on the first pH strip on the tile.
• Using fresh droppers, do the same procedure for garden soil, humus rich and riverside soil.
• Wait for some time for the pH paper strip to dry.
• Note the colour and compare with the colour chart given on the broad range indicator paper and get a rough estimate of pH of the sample solutions.

Observations:

We can observe that the roadside soil has pH 7, garden soil and humus rich soil have ph 6 and riverside soil has pH 8.

Using Universal Indicator Solution

Materials Required:

Procedure:

• Using a dropper, take some universal indicator solution.
• Put 5 drops of indicator solution into the test tube that contain water samples from roadside soil, garden soil, humus rich and riverside soil.
• Note the colour developed and compare it with the colour chart.

Observation:

We can observe that the roadside soil has pH 7, garden soil and humus rich soil have ph 6 and riverside soil has pH 8.

2. To Study Moisture Content of Soil

Materials Required:

Real Lab Procedure:

• Take watch glass containing garden soil and put it into a crucible.
• Weigh the crucible with soil sample on a weighing balance.
• Take crucible and place it over the Bunsen burner.
• Heat the soil for some time till the soil becomes dry.
• Weigh the crucible again to record the weight of dry soil.
• Take watch glass containing roadside soil and put it into a crucible.
• Weigh the crucible with soil sample on a weighing balance.
• Take crucible and place it over the Bunsen burner.
• Heat the soil for some time till the soil becomes dry.
• Weigh the crucible again to record the weight of dry soil.

Observation

Record the initial and final weights of each sample and the difference between initial and final weights in the form of a table.

Conclusion

Garden soil shows higher difference between initial and final weight indicating higher moisture content in the garden soil than the roadside soil.

3. Water Holding Capacity of Soil

Materials Required:

Procedure

• Take watch glass containing garden soil and put it into a mortar.
• Using a pestle, grind the sample into fine powder.
• Take a filter paper and place it in the bottom of the tin box.
• Weigh the tin along with the filter paper and note its weight.
• Transfer the fine powdered soil sample into the tin box.
• Take a glass rod and tap the box gently several times, so that soil is compactly filled and forms a uniform layer at the top.
• Weigh the tin box along with soil sample and note its weight.
• Take a Petri dish filled with water.
• Take two small glass rods and place them parallel to and at a small distance from each other.
• Place the soil filled tin on the two glass rods in such a manner that its bottom is in contact with water.
• Leave the set up undisturbed till water appears on the upper surface of the soil. Wait till entire soil surface is wet.
• Remove the tin and allow all the gravitational water to flow out from the bottom.
• When no more water percolates, wipe the bottom to dry it with a filter paper.
• Weigh them again and note its weight.

Observation

Record all the values into the table and calculate the percentage of water holding capacity of the garden soil.

4. To Study the Texture of the Soil

Materials Required:

Procedure:

• Take 50 gm of soil sample and put it into a measuring cylinder.
• Take beaker containing water and pour some water into the measuring cylinder.
• Shake the measuring cylinder to mix the sample.
• Allow the soil particles to settle down.
• Record the thickness of the layers formed by different types of particles in the measuring cylinder.
• The thickness of clay particles in measuring cylinder is 21%, silt particles is 18% and sand particles is 61 %.
• Using a textural triangle, draw lines corresponding to the percentage of clay, silt and sand. 
• The lines which intersect indicate the soil type we have.
• Here, the sample soil consists of 21% clay, 61% sand, and 18% silt; hence, the soil type is sandy clay loam.

Simulator Procedure (as performed through the Online Labs)

You can select the experiment from the ‘Select the test drop down list (Moisture Content Test, Water Holding Capacity, pH Test and Texture Test).

1. Moisture Content Test
   • You can select the soil sample by clicking on the corresponding icon.
   • Click and drag the spatula towards crucible to transfer some soil sample into it.
   • Click and drag the crucible to place it on the weighing machine.
   • Note the weight of the sample, as the initial weight (x).
   • The initial weight (x) can be entered in the worksheet.
   • Click and drag the weighed sample from the weighing machine and drop it on a burner.
   • Click on the knob of the burner to turn it on.
   • A timer is shown and you need to wait till the timer stops.
   • Click and drag the soil sample back onto the weighing machine to weigh it again.
   • Note the weight of the soil sample, as the final weight (y).
   • The final weight (y) can be entered in the worksheet.
   • Based on the values entered, the worksheet gives the moisture content in soil.
   • You can redo the experiment by clicking on the ‘Reset’ button.
2. Water Holding Capacity
   • Click and drag the filter paper and drop it on the tin box.
   • Click and drag the tin box to place it on the weighing machine.
   • Note the weight of the tin box with filter paper (x).
   • The weight (x) can be entered in the worksheet.
   • Click and drag the weighed tin box from the weighing machine and place it on the table.
   • Click and drag the spatula towards tin box to transfer some soil sample into it.
   • Click and drag the glass rod towards the tin box to compactly fill the soil
   • Click and drag the tin box back onto the weighing machine to weigh it again.
   • Note the weight of the soil sample with tin box (y).
   • The weight (y) can be entered in the worksheet.
   • Click and drag the weighed sample from the weighing machine and place it on a Petri dish containing water.
   • Click and drag the tin upwards to allow all the gravitational water to flow out from the bottom.
   • Click and drag the tin box back onto the weighing machine to take the final weight.
   • Note the weight of the soil sample with water, as the final weight (z).
   • The final weight (z) can be entered in the worksheet.
   • Based on the values entered, the worksheet gives the water retained by the soil sample.
   • You can redo the experiment by clicking on the ‘Reset’ button.
3. pH Test
   • Choose any one of the soil solution in the beakers by clicking on it.
   • There are two ways of finding the pH value of the solution:
4. Using the pH strip:
   • Click and drag the dropper from the stand and move into the solution in the beaker to collect the soil sample.
   • Still holding the dropper, move it from the beaker over to the pH strip and release it.
   • To find the pH value of the solution, select the colour from the colour chart by clicking and dragging it to the pH strip and comparing it.
   • The colour that matches the spot on the pH strip indicates the pH value of the solution.
5. Using the Universal Indicator:
   • Click and drag the dropper from the Universal Indicator bottle and move it into the solution in the beaker to drop the Universal Indicator into it.
   • To find the pH value of the solution, select the colour from the colour chart by clicking and dragging it next to the solution in the beaker and comparing it.
   • The colour that matches the solution in the beaker indicates the pH value of the solution.
6. Note:
   • Once test is done using the Universal Indicator, you cannot do it with the pH strip. To do it with the pH strip, click the ‘Reset’ button and vice versa.
   • The ‘Reset’ button can be used to redo the experiment with other soil solutions.
7. Texture Test
   • Click and drag the spatula towards measuring cylinder to transfer some soil sample into it.
   • Click and drag the distilled water bottle towards the measuring cylinder to add water to the measuring cylinder containing the soil sample.
   • Click and drag to shake the solution properly to mix the soil sample.
   • Allow the soil particles to settle down.
   • A timer is shown and you need to wait till the timer stops.
   • Click on the check box “Soil Texture Triangle” to view the textural triangle.
   • You can identify the type of soil for selected sample by clicking on the corresponding box in the textural triangle.
   • You can redo the experiment by clicking on the ‘Reset’ button.