Question

Which fraction of soil water is available to plants for absorption by roots?

Answer 1

Hint: Land plants absorb water mainly from the soil. The soil plays an important role as water storage and medium for root growth but it is also a reservoir of mineral nutrients and provides anchorage for plants. The chief source of water to the soil is rain. The other sources are water table, melting of snow or ice, and irrigation of fields. Water is a universal component of soil present from trace to sufficient saturation, but customarily water in the soil is known as soil water, soil solution, or the water relation of the soil.

Complete answer:
Five stages of water in the soil which differ in their availability to plants. Gravitational water, capillary water, hygroscopic water, runaway water, and chemically combined water.
-Gravitational water: The gravitational water is not readily available to the plants and penetrates below the reach of the roots within a few days after rain. Plants absorb this water only when rain showers follow one another. If the gravitational water remains indefinitely the soil is said to be waterlogged.
-Capillary water: It comprises the bulk of water remaining in the soil after gravitational water has drained away. The capillary water fills the spaces between non-colloidal soil particles. It occurs in the form of films coating smaller soil particles and is retained by the gravitational pull. The capillary water is readily available to the plants and is the main source of practically all the water absorbed by them.
-Hygroscopic or imbibed water: The hygroscopic water is held by colloidal soil particles due to cohesive forces (the force of molecular attraction). Plants can absorb a very small quantity of this water.

Note: The runaway water is the major part of rainwater flowing on the surface of the soil. Plants fail to avail of this water. Some of the water molecules are chemically combined with the soil minerals (e.g., silicon, iron, aluminum, etc.).This water is not available to plants.