Soil Water

Soil Water

The availability of water in soil is essential for plant growth. It is also essential to microorganisms that grow in soil and decompose organic matter. It is important in the weathering process which involves the breakdown of rocks and minerals to form soil and release plant nutrients. Water is the solvent that together with the dissolved nutrients make up the solution from which plants absorb nutrients mainly through the roots. Soil water can provide control over both soil air and soil temperature. Water is generally held in the soil by micro pores. The force of gravity causes water to move downward through the soil particularly in the larger pores.




Soil Water Potential

                                                The difference between the free energy of soil water and that of pure free water in a standard reference state is known as soil water potential. The soil water potential can be expressed in bars, atmosphere or Kilo Pascals.

Soil Water Classification

The most useful classification of water content in soil is the biological classification as it relates water to plant growth. There is a definite relationship between water retention and its use by plants. These water contents are classified as gravitational water, field capacity, permanent wilting point and plant available water.


Gravitational Water: water present in excess of field capacity or water held at a potential greater than -33 Kilo Pascal is called gravitational water. It is available as it moves through the plan roots if adequate aeration is maintained. It is of limited use to plants as it is present in the soil only short period of time. It can affect plant growth due to poor aeration.


Field Capacity: the contents of water remaining in a soil two to three days after having been saturated with water and after the free water has been allowed to drain away is called field capacity. It is the percentage of soil water that is held at -10 to -33 Kilo Pascal water potential and is a measure of the greatest amount of water that a soil can store or hold.

Permanent Wilting Point: it is the amount of water at which plants growing in that soil will wilt and not recover when placed in a humid chamber. It is estimated at about -1500 Kilo Pascal water potential or less (more negative). When is held so strongly that plants are not able to absorb it fast enough for their needs.

Plant available water: it is defined as the weight percentage of total soil water held with a water potential between -10 to -1500 Kilo Pascal and is said to be usable by plant. It is estimated by subtracting the percentage of water held at the permanent wilting point from the percentage held at field capacity.



Location: Pakistan

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