The shape of the water release characteristic as affected by tillage, compaction and soil type

The shape of the water release characteristic was described by deriving two independent parameters. The first is the pore median diameter, which corresponds to the potential at which half of the effective pore volume is drained. The second is the water release index, which is the slope of the central portion of the water release characteristic; this is the maximum volume of water released for a ten-fold decrease in water potential. Water release characteristics were measured from saturation to − 1500 kPa on undisturbed cores taken from the topsoils of seven field experiments on tillage and traffic in southeast Scotland.

Both pore median diameter and water release index increased with particle median diameter (soil type effect) and with total porosity (compaction or tillage effect). On a fine-grained soil, compaction by conventional field traffic reduced pore median diameter from 48 to 4.5 μm and water release index from 115 to 85 mm m⁻¹. On a coarse-grained soil, severe compaction was required to give such large decreases in parameter values. Soils with pore median diameters of less than 6 μm were considered likely to be at risk from restricted soil aeration. These shape parameters appeared to be more sensitive indicators of changes in soil structure associated with soil management than individual water contents such as field capacity or wilting point. They are also less susceptible to errors in individual values because they are derived by effectively smoothing the complete characteristic.