Introductory Crop Physiology

ABSORPTION OF WATER

Soil water is the most important constituent of soil as it provides the medium for the absorption of nutrient elements and organic matter by the roots and activates various physiological processes. There are five categories of water depending upon their availability.

Gravitational water:

After a heavy rainfall, surface soil is temporarily saturated and it displaces air from large spaces between soil particles and gradually penetrates up to the root zone within few days under the influence of gravity. This gravitational water is not available to the plants. Plants can absorb this water only when there is continuous shower.

Capillary water:

It refers to the bulk of water remaining in the soil after gravitational water has drained away. It occurs in the form of film coating smaller than colloidal soil particles and retained by the forces of surface tension. It is readily available to the plants and is the main source of water to the plants.

Hygroscopic or imbibed water:

It is held by colloidal soil particles due to the adhesive forces. Plants absorb only very small quantity of this water.

Runaway water:

The major part of water that flows on the surface of the soil is called runaway water. Plants fail to avail this water.

Chemically bound water:

Some of the water molecules are chemically combined with soil minerals like iron, aluminium etc. This is also not available to the plants.

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