Advantages of the Pressure Pycnometer for Measuring the Pore Space in Soils

T blotter method of Learner and Shaw (3)* is rather widely accepted as the standard method for determining the pore space in soils. The greatest advantage of this method is that all soils are brought to a comparable moisture status for determination of the noncapillary or air pore space. In using this method, especially on heavier soils where low air pore space may be critical, several limitations of the blotter method become apparent. In the first place rapid draw down of saturated field soils to a tension of 40 or 60 cm is a condition very rarely found in field soils in which crops are growing. Plants may be and usually are growing in soils of quite different air and water status than that found in soil samples when taken off the blotter, yet no direct indication of field conditions is given by the data. In light of the easily demonstrable hysteresis which occurs in the soil' structure-wetting cycle, the use of blotter data to describe conditions under which plants are growing becomes open to serious doubt. Particular difficulty is experienced with soils that swell. Such soils if sampled in the spring of the year may show no further swelling upon saturation. Blotter data on these samples could be expected to be quite significant. Later in the growing season, however, such soils usually shrink. They then exhibit large cracks which are a distinctive feature, persisting usually throughout the remainder of the growing season. Upon saturating such samples considerable swelling usually occurs. If the determination is completed, extremely low air space porosity will be indicated even though observation of the soil may indicate healthy, vigorous plants and an apparently wellventilated soil because of the large number of cracks and checks extending throughout the soil. It is obvious in such extreme cases that for the convenience of bringing all samples to the same moisture condition, significance and accuracy of the data have been sacrificed. With many soils,-excepting those having porosity primarily due to coarse texture or those with extreme water-stability, data obtained by the blotter method must be interpreted with caution. A.direct method for determination of soil pore space based on a simple application of Boyle's law has been used by a few investigators (2, 5, 7, 8), but its advantages have apparently been overlooked by many workers in the field of soil physics. The paper by Kummer and Cooper (2) described a laboratory model air pycnometer for direct determination of soil pore space. Their results indicated considerable error in the blotter method due to failure to obtain complete saturation, but that after the blotter data were corrected for lack of saturation there was good agreement between the two methods. They suggested that a portable unit might be built and that it would be possible to determine pore space without the necessity of bringing the samples to a constant moisture status, although this suggestion was not developed further. With the appearance of the paper by Kummer and Cooper (2), construction of a portable unit was undertaken in this laboratory. It is the purpose of this paper to describe the apparatus developed, to present some of the data obtained, and to discuss what we feel are advantages .which should lead to a wider use of the method in soil physics research.