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The Significance of Air Adsorption by Soil Colloids in Picnometric Measurements¹

ABSTRACT

Pressure deficits from calculated values in picnometric measurements on dry soils have been observed by the author and others. The explanation given is gas adsorption by dry soil colloids. The magnitude of this source of error in picnometric soil porosity measurements and its relationship to soil moisture was studied. A dual chamber picnometer adapted to measuring gas adsorption over a wide range of pressures was used. For the soils used the results were essentially the same for air and oxygen. At room temperatures (23° C) and atmospheric pressure, oven-dried Hurricane clay adsorbed approximately 15.3 cc (one atm., 0° C) per 100 gm, while oven dried Lloyd clay adsorbed only 3.7 cc per 100 gm. However, adsorption fell to near zero at about 10% moisture (100 atm. moisture tension) for the Hurricane clay but did not approach zero for the Lloyd clay until at about 20% (15 atm. moisture tension). Quartz sand, Lakeland sand, and the organic colloid separated from Lakeland sand showed no measurable adsorptive capacity for oxygen or air. The adsorption found at ordinary temperatures does not approach the surface saturation values measured at very low temperatures by Emmett, Brunauer, and Love (2) but are consistent with their measurements at 0° C. The air picnometer cannot be used to estimate porosity or moisture content of soils near or below air dryness unless a method based on direct calibration of fixed fresh soil weights is used.

¹ Contribution from the Division of Soil Management, BPISAE, USDA in cooperation with the USDA Tillage Machinery Laboratory at Auburn, Ala. Presented before Division I, Soil Science Society of America, Cincinnati, Ohio, November 20, 1952.

² Soil scientist.

Received for publication December 15, 1952.