Question

Define the following: (a) Gravitational water, (b) Hygroscopic water, (c) Combined water, (d) Capillary water

Hint:

Capillary water is the primary source for plant uptake, while gravitational water causes leaching of nutrients.

Answer 1

(a) Gravitational water: Water in soil that moves downward due to gravity, filling large pores after heavy rain or irrigation, unavailable to plants as it drains quickly. (b) Hygroscopic water: Water tightly bound to soil particles by adsorption, held at high negative matric potential (e.g., -31 bars), unavailable to plants. (c) Combined water: Water chemically bound in soil minerals (e.g., hydrated salts like gypsum, CaSO$_4 \cdot 2$H$_2$O), unavailable to plants as it’s part of the crystal structure. (d) Capillary water: Water held in soil micropores by capillary forces, available to plants as it’s retained against gravity but accessible to roots.