The Influence of Soil Moisture Tension on Vapor Movement of Soil Water

The intent of this research was to study the role of soil moisture tension in water vapor movement within the soil. All studies were conducted in the laboratory. Vapor movement was effected by vapor pressure differences obtained through the application of a sharp temperature interface at the midpoint of horizontal soil columns. The influences of such factors as soil porosity, initial moisture content, and vapor pressure gradients were determined. The movement of water vapor in the two soils and four soil separates studied increased rapidly with moisture tension up to a certain level, then decreased sharply. The pF at which vapor movement was initiated and at which maximum movement occurred increased with a decrease in particle size. The volume of unsaturated soil pores, not their size, seems to govern the soil moisture content at which vapor movement begins. The volume of vapor movement in disturbed soil samples under a given vapor pressure gradient is regulated by the balance of unsaturated pore space, evaporating surfaces, and moisture reserve existing in the soil. Maximum vapor movement occurred at moisture tensions slightly below the wilting point suggesting a mechanism by which plants may resist drought injury. Temperature induced vapor pressure gradients seem to be mainly responsible for water vapor transfer in soils.