MODELLING OF SOIL PORES AS TUBES USING GAS PERMEABILITIES, GAS DIFFUSIVITIES AND WATER RELEASE

Two models are presented describing the air-filled continuous pores in soil and how they change with soil water potential. In the first model (A), the pores are represented by tortuous tubes of uniform radius. The radius, length and number are calculated from air permeability, relative diffusivity and air-filled porosity measured at each soil water potential. In the second model (B), the pores are represented by tortuous tubes of three radii joined at random in series. The radii and total lengths of the tube sections are estimated by comparison of air permeability, diffusion coefficient and air-filled porosity at each water potential with values calculated for a large number of theoretical systems. The models were applied to the results from undisturbed cores of a silt loam taken from 30 to 80 mm depth. For both models, the sequences of continuous pores were estimated to be 2 to 7 times as long as the sample but shortened as the sample dried. From the second model the average pore radius in direct drilled soil, 0.3 mm, was half that in ploughed soil and the minimum radius, 0.1 mm, was one-quarter that in ploughed soil.