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A Diffusion-Equilibrium Method for Obtaining Soil Gases under Field Conditions¹

ABSTRACT

A soil gas sample obtained by mass flow of gases may not be representative of the point sampled because of mixing with gases from other soil depths or from the atmosphere. The authors have investigated a method which eliminates this error and which is based on the diffusion of gases between a point in the soil to a collection chamber via a tube tightly fitted in the soil.

A physical analysis and laboratory tests were made to determine the time required for the collected gas to be representative of the soil atmosphere at any depth. For all conditions, this time depends on initial differences between the partial pressure of a gas in the collection chamber and in the soil. When the percentage of water-free pores in soil is approximately 20 or greater, the time is strongly dependent on the volume of the collection chamber and on the diameter and length of the tube connecting the chamber to a point in the soil. For porosities between 0–15%, the effect of reduced diffusion rates in soil becomes significant and increases the time. Equations are given for making these calculations.

An evaluation was also made of the condition in which (1) approximate equilibrium had been previously established between partial pressures in the chamber and in the soil, and (2) partial pressures in the soil were changing due to release or absorption of a gas. It was shown that the pressure of a gas in the chamber will not differ from its partial pressure in the soil by more than 0.5% unless concentration changes in the soil exceed 15% by volume per day.

Oxygen analyses are shown for 2-years field results. The average coefficient of variation of oxygen concentration was a low value of 3% but was strongly dependent on soil moisture content. Some comparisons of soil oxygen concentrations obtained by pumping and by the diffusion-equilibrium method showed that the former method gave higher values.

¹ Jour. Paper No. 53-52 of the Ohio Agricultural Experiment Station. Publication with permission of the Director. Presented before Division I. Soil Science Society of America, Cincinnati, Ohio, Nov. 20, 1952.

² Assistant Professor and Research Assistant, respectively. The authors express appreciation to R. E. Shapiro for suggesting the possibilities of this method.

Received for publication January 5, 1953.