Quantification of pore size distribution and the movement of solutes through isolated soil blocks

Techniques to provide a quantitative description of soil structure in a reasonable time scale using equipment that is widely available are examined. Three complementary techniques for quantifying soil structure within the pore size range from 136 to 4000 mu m (equivalent diameter) are evaluated for two isolated, unsaturated soil blocks (0.85 m(3)). Profile tracing and binary transect methods were used to examine the pore sizes > 1000 mu m. Pore sizes < 1000 mu m were quantified using 2-dimensional image analysis of resinated core samples. Prior to collection of the blocks, the technique of mole ploughing was used to generate a range of pore sizes in the heavy clay soil (Hallsworth series) by shattering. Pore size distribution was related to observed changes in solute concentration following a miscible displacement of a 250 mg l(-1) Cl- solution applied at an irrigation rate of 3 mm h(-1). Analysis of soil matric potential data showed that solute movement was predominately limited to pores of between 136 and 1000 mu m equivalent diameter. Using a combination of Poiseuille's law and pore-size distribution data, it was inferred that the observed rate of flow in the soil could have been achieved through a range of pore-size distributions, between 300 and 1000 mu m as a sufficient quantity of pore space within the range of 136 to 1000 mu m (diameter) was recorded. It is concluded that the rapid movement of tracer to depth can occur within an unsaturated soil in which active porosity is limited to mesopores and this rapid movement can be described as preferential flow. (C) 1999 Elsevier Science B.V. All rights reserved.