Soil suction is defined as the state of the soil when it is under reduced pressure. It is measured in terms of the height of the water column (h) suspended in the soil.

The main features and factors affecting soil suction are explained briefly.

Soil Suction Formula

Soil suction can be represented as common logarithm of the height of the water column (h). The value of ‘h’ is substituted in centimeters.

For example, the height of the water column is 100cm. Hence, the Pf value is given by :

Similarly, for a height of columm=1cm. Hence the Pf value is given by :

Soil suction can also be represented in atm, kPa, kg/cm2, and bar. Different values of soil suction for different units are explained in table-1 below.

Table.1. Relationship between Different Units of Soil Suction; Image Courtesy: Nptel

Features of Soil Suction

Soil suction is formed due to a complex energy state that is created by the interaction between the soil, water and the air present in the soil. This complex energy state creates a negative pressure which is called as the suction of soil. Hence suction is also defined as the energy that is required to extract unit volume of water from the given soil mass.

Soil suction is studied in unsaturated soil compared to saturated soil. In unsaturated soil, the presence of suction affects the behaviour of parameters like water potential, permeability, strength, etc which are constant in the case of saturated soil. Suction in the unsaturated soil makes these parameters to be transient in nature.

Thus, the unsaturated soil is studied based on a graph drawn between the suction and the water content. This curve is also called as soil-water characteristic curve (SWCC) or water retention characteristic curve (WRCC).

Fig.1. Water Retention Characteristic curve (WRCC). ; Image Courtesy: Nptel

The WRCC curve is obtained by combining the desaturation and saturation curve of the soil mass. Desaturation curve is obtained by drying the soil mass and saturation is obtained by wetting the soil mass. A typical drying and wetting of the soil mass give a WRCC which forms an S curve with a hysteresis relation. The WRCC curve conveys the following points:  

The slope of the WRCC curve gives the specific capacity or differential water capacity of the soil. This is an important property which is essential to describe the water availability and the water storage to the plants.

Factors Affecting Soil Suction

The suction in soils is dependent on the following factors:

  1. The particle size of the soil grains: Smaller the particle size, higher is the amount of pores present which results in higher capillary action. This, in turn, results in suction.
  2. History of drying and wetting cycle: The soil suction is greater during the drying cycle compared with the wetting cycle for the same water content.
  3. The angle of contact: With the increase in the angle of contact, the soil suction decreases. The soil suction is maximum with an angle of contact equal to zero
  4. Soil Structure: The size of the interstices in the soil is the factor governing the soil structure. With the change in the size of interstices, the soil suction also changes.
  5. The temperature of the soil: With the increase in temperature, the soil suction decreases. This is because higher temperature reduces the surface tension of the water.
  6. Water Content of the soil: Soil suction increases with the decrease in water content of the soil. The suction value is maximum when the soil is dry.
  7. Dissolved salts in the soil: The presence of dissolved salts results in the increase in surface tension. This facilitates the increase of soil suction.
  8. The denseness of the soil: A loose soil has pores of larger radius thus decreasing the value of soil suction. Hence, denser the soil, more is the soil suction.