What is soil water?

Soil water is important because water is essential for all the life on the earth, including the plants and organisms’ life in soil. Since most processes that release nutrients into the soil require water and air, the Proper balance between soil water and soil air becomes critical. Water is the medium from which all plant nutrients are assimilated by plants.  Water is also necessary for the weathering of soil.

Soil water, sometimes referred to as the soil solution, contains dissolved organic and inorganic substances.

The amount of water in the soil depends on amount and pattern of precipitation receives from climate. Secondly, the amount of water in the soil depends upon water holding capacity of the soil.

Capillarity or Capillary action

Water molecules behave under two forces namely cohesion and adhesion. Capillarity is a combination of cohesion/adhesion and surface tension forces. Capillary action is demonstrated by the upward movement of water through a narrow tube against the force of gravity.

Cohesion Force: Water molecules are attracted to one another through cohesion force. Cohesion causes water molecules to stick to one another and form water droplets. Water surfaces behave in an unusual way because of cohesion.

Adhesion Force: The attraction between water and solid surfaces is referred to adhesion force.

  • Capillary action occurs when the adhesive intermolecular forces between a liquid, such as water, and the solid surface are stronger than the cohesive intermolecular forces between water molecules.
  • Capillarity is the primary force that enables the soil to retain water, as well as to regulate its movement. Like water moves upwards through a tube against the force of gravity; in the same way, water moves upwards through soil pores, or the spaces between soil particles.
  • The smaller the soil pores, the higher the capillary rise. Fine-textured soils (clay soil) have smaller pores than coarse-textured soils. As a result, fine-textured soils have a greater ability to hold and retain water in the soil.
  • Small clay particles are micropores, while larger pore spacing between lager particles, such as sand are called macropores. In addition to water retention, capillarity in soil also enables the upward and horizontal movement of water within the soil profile, as opposed to downward movement caused by gravity.

Water holding capacity

Now, it is clear that water is held within the pores of the soil. Therefore,

  • The water holding capacity depends on capillary action and the size of the pores that exist between soil particles.
  • Sandy soils have large particles and large pores. However, large pores do not have a great ability to hold water. As a result, sandy soils drain excessively.
  • On the other hand, clayey soils have small particles and small pores. Since small pores have a greater ability to hold water, clayey soils tend to have high water holding capacity.

Classes and availability of Soil water

Gravitational water

  • Gravitational water is free water moving through soil by the force of gravity and percolates below the root zone and remains unavailable to plants.
  • Soil moisture tension is zero or <1/3 atm.
  • pF: <2.4

Capillary water

  • Water held by surface tension (Capillary action) in the pore space (micro-pores).
  • Capillary water is the main water that is available to plants.
  • Soil moisture tension1/3 to 31 atm.
  • pF range is 2.54 to 4.50.

Hygroscopic water

  • Hygroscopic water forms as a very thin film surrounding soil particles and is generally not available to the plant.
  • This type of soil water is bound so tightly to the soil by adhesion forces that very little of it can be taken up by plant roots.
  • Hygroscopic water is found on the soil particles and not in the pores.
  • Soil moisture tension ranges from 31to 10,000 atm.
  • pF ranges from 4.50 to 7.00.

Hygroscopic coefficient

  • It is the maximum amount of hygroscopic water absorbed by 100 gm. of dry soil under standard conditions of humidity (50% relative humidity) and temperature (15°C).
  • Water at this tension is not available to plant.
  • Soil moisture tension is 31 atm.
  • pF is 4.50.

Field capacity

  • It is the amount of water held in the soil when all pores are filled. The soil is said to be saturated.
  • It is the capacity of the soil to retain water against the gravity. At this stage only micropores or capillary pores are filled with water and plants absorb water for their use.
  • At field capacity water is held with a force of 1/3 atmosphere. Water at field capacity is readily available to plants and microorganism.

Wilting coefficient

  • At this stage, the water is so firmly held by the soil particles that plant roots are unable to draw it; as a result, plant begins to wilt.
  • At this stage even if the plant is kept in a saturated atmosphere it does not regain its turgidity and wilts unless water is applied to the soil.
  • This stage is termed as Wilting point and the percentage amount of water held by the soil at this stage is known as the Wilting Coefficient.
  • It represents the point at which the soil is unable to supply water to the plant. Water at wilting coefficient is held with a force of 15 atmospheres.

Available water

  • The amount of water required to apply to a soil at the wilting point to reach the field capacity is called the “available” water.
  • The available water is the difference in the amount of water at field capacity (- 0.3 bar) and the amount of water at the permanent wilting point (- 15 bars).

Read also…
Acidic, Saline and Alkaline Soil


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