Soil and Plant Analysis

(iii) Water vapour movement

The movement of water vapour from soils takes place in two ways

(a) Internal movement—the change from the liquid to the vapour state takes place within the soil, that is, in the soil pores and
(b) External movement—the phenomenon occurs at the land surface and the resulting vapour is lost to the atmosphere by diffusion and convection.
The movement of water vapour through the diffusion mechanism takes place from one area to the other soil area depending on the vapour pressure gradient (moving force), this gradient being simply the difference in vapour pressure of two points a unit distance apart. The greater this difference, the more rapid the diffusion and the greater is the transfer of water vapour during a unit period.

Soil conditions affecting water vapour movement. There are mainly two soil conditions that affect the water vapour movement namely moisture regimes and thermal regimes. In addition to these, the various other factors which influence the moisture and thermal regimes of the soil are like organic matter, vegetative cover, soil colour etc. The movement takes place from moist soil having high vapour pressure to a dry soil (low vapour pressure). Similarly the movement takes place from warmer soil regions to cooler soil region. In dry soils some water movement takes place in the vapour form and such vapour movement has some practical implications in supplying water to drought resistant plants.