Rolston 1986

Rolston DE. 1986. Gas diffusivity. Pp 1089-1102 in Methods of Soil Analysis Part 1: Physical and Mineralogical Methods 2nd Ed. ed. A Klute. American Society of Agronomy Inc. Soil Science Society of America Inc, Madison, WI.

This author presents a review of the principles and measurement methods of soil gas diffusion, including formulae and relevant calculations. At the heart of all considerations of soil gas diffusion is Fick’s law and the measurement of Dp, the soil gas diffusion constant for a particular gas. There are a wide range of laboratory methods for measuring gas diffusion, but all are based on measuring the passive movement of a target gas through a volume of soil, often by measuring the accumulation of the target gas in a chamber that initially lacks that gas.

Many variables will impact rates of gas diffusion; perhaps of greatest importance is the moisture content of the soil. Wet soils make take hours to measure, while dry soils only a few minutes. Temperature also has a strong effect, and this author urges the reporting of temperatures with all measures of soil diffusion. Additionally, a simple formula is presented that relates diffusion at one temperature to diffusion at a second temperature.

Further calculations surround the determination of Dp, and correcting for errors associated with the details of the measurement chamber. For example, a correction factor can be applied when the ratio of soil air volume to chamber volume is more than about 0.005; this corrects for the gas “stored” in the soil of the measurement apparatus.

This paper is the first of two adjacent chapters in this edited volume by this author, both dealing with soil gas movements. The calculations and formulae here will be very useful in attempts to calibrate the FTIR and the flux chambers.