Study of Physical Properties of Soil

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.

 

Cite this Simulator: