The effects on porous-material permeability characteristics of the absolute pressure level (and associated scale effects), choking of the flow, bending the material^ and other factors have been investigated. Samples of rolled 30-by 250-mesh Dutch weave Monel metal cloth and l/l6-inch-thick sintered bronze were calibrated with constant upstream pressures of 1 atmosphere and 2-atmospheres (varying downstream pressure) and with constant downstream pressure of 1 atmosphere (varying upstream pressure). Experiments showed permeability characteristics to be appreciably affected by absolute pressure level, flow choking, and thickness of the material. Moderate bending of the material caused no noticeable change in the permeability. Simple calculation and correlation procedures are presented for determining permeability characteristics with reasonable accuracy when experimental data are limited.

A simple filter paper method for estimating a wide range of moisture potentials has been tested for fifteen soils ranging in texture from sands to heavy clays. The method has given estimates of moisture potential from -0.1 to -900 bars with an accuracy that should be acceptable for many types of field experimentation. Relationships between 15-bar percentages determined by both the filter paper and pressure membrane methods, and biologically determined permanent wilting percentages for wheat, are discussed.

Experiments with Medicago tribuloides and Lactuca sativa confirm Sedgley's conclusion that the wetted area of contact is a factor controlling germination of the seed. The evidence suggests that this is of consequence at matric potentials near that of free water and is most important for germination of the last seeds in each population. Matric potential is shown to be an important factor in seed germination over and above its effect in controlling the wetted contact area of seed and medium. Prewetting the seed at matric potentials near that of free water for a few minutes increases the germination rate markedly and minimizes the normal effect of matric potential. The results and experimental conclusions are discussed and are shown to indicate the need for further investigation. They show that it is practicable to design experiments to distinguish the effects of the various facets of soil water energy on seed germination behaviour.

The germination (i.e. the first emergence of the radicle) of Medicago denticulata Willd., M. confinis Koch, M. tribuloides Desr., and Juncus vaginatus R. Br. under controlled moisture conditions has been examined. Moisture regime was defined by the soil moisture suction (pF). The influences of temperature and light, which would otherwise obscure the importance of soil moisture, have been minimized by comparing the number of seeds germinated under each physical regime with the maximum number germinated for each species under identical physical conditions when suction was approximately zero (i.e. standard cotton wool germination count procedure). The differing water regimes were obtained by using methods common in soil physics – the tension plate and pressure membrane methods – the seeds being in contact with the plate or membrane kept at a constant water suction. Additionally, two soil media, were used with the Medicago spp. It was found that: For all species an increase in suction produced a decrease in rate of germination until at 10 atm, germination practically ceased. Medicago spp. showed a smaller decline in rate as suction increased than did J. vaginatus. The moisture conditions to allow J. vaginatus to germinate at rates comparable with those for the Medicago spp. were quite restricted and corresponded to high water-tables or soils wetter than field capacity. The moisture condition of the soils controlled the germination rate not only by means of the suction effect, but also in terms of hydraulic conductivity (or permeability). A drier soil having less ability to transmit water than a wetter soil reduced the rate at which water could reach the seed, with a consequent decline in the germination rate. As the relationships between: (a) moisture content and suction, and (b) moisture content and hydraulic conductivity, are soil characteristics, it is necessary to define the soil moisture characteristic as well as moisture content in germination experiments.