Abstract
The lower and the upper limits of soil water content for plant growth were reviewed in terms of water and gas transmission in the soil. Plants can survive for about one month after a lot of rainfall by extracting water held in the soil, but plants exhaust soil oxygen within a few days if soil oxygen was not replenished by gas exchange between the soil and the ambient air. This is a distinct difference between soil water and soil air for plant growth. In case of water, both water potential and hydraulic conductivity are expressed as a function of soil water content, which enables to translate from the water potential to the hydraulic conductivity and vice versa. The lowest limit of soil water called wilting point corresponds to —1.5 MPa for soil water potential or to a value of the order of 10-11 cm/s for hydraulic conductivity, and this limitation was explained well by a single root model. The highest soil water content, which impedes respiration has been studied by use of air porosity, oxygen concentration, oxygen diffusion rate (ODR) or relative gas diffusion coefficient. Even though these studies have proposed critical values of root respiration such as 0.1 to 0.2 for air porosity, 0.1 for a critical oxygen concentration,10 for a critical ODR and 0.005 for relative gas diffusion coefficient, we still have little knowledge of the relation among these physical factors. factor to be uppermost layer in the field supplies the largest amount of water to roots by this time. It for research to confirm either water potential or hydraulic conductivity governs the break potential transpiration. Under field conditions, water transmission ability of the soil should by the amount of water above the break point in the surface soil. Aa gas exchange is inevitable for soil aeration and air porosity is the easiest physical Transpiration begins to decrease at soil water potential between —0.1 and —1.0 MPa and the is important point of the be evaluated measured in the field, relative gas diffusion coefficient as a function of air porosity must be studied further. It is important for gas diffusion to evaluate the amount of isolated air filled pores at saturation and the reflection point or the bubbling pressure in the soil water characteristic curves.
