Water Available to Roots for Absorption

Even though a root is surrounded by soil, not every drop of water in that soil is actually available for the plant to absorb. Soil water occurs in several distinct physical forms, and only one of them is usable by roots:

• Gravitational (free) water percolates deep down through the soil under the pull of gravity and drains away past the root zone before roots can take it up, so it is not available for absorption.
• Hygroscopic water is water held tightly, by adsorption, onto the surfaces of very fine soil particles; because it is bound so firmly, roots cannot pull it away and it too is unavailable.
• Combined water exists chemically bound within hydrated oxides of elements such as silicon and aluminium in the soil; being chemically locked up rather than free, it is likewise not available to roots.
• Capillary water is the water held, by simple capillarity, in the fine pores between neighbouring soil particles. Because it is neither draining away under gravity nor locked onto particle surfaces or into mineral structures, capillary water is the only category of soil water that plant roots can actually draw on for absorption. …