In situ, on-site and laboratory measurements of soil air permeability: Boundary conditions and measurement scale

The measurement of soil air permeability is a fast and easy method that can be used in different ways to characterize the soil. An air permeameter was constructed in order to measure air permeability (k(a)) in situ, on-site ( exhumed soil samples), and in the laboratory on a wide range of Danish agri cultural soils. Two different sizes of sample rings were used (100 cm(3) an d 3140 cm(3)). The device was initially tested in the laboratory on repacke d soil samples to evaluate dependency of k(a) on sample size, The results s howed consistent values of k(a) for both sample sizes, indicating only litt le scale effect. In the field, air permeability was measured in situ and on -site using large sample rings. Air permeability in situ was determined by using a "shape factor" taking into account boundary conditions at the lower end of the ring while assuming isotropic soil conditions, An expression fo r the shape factor developed by Liang et al. (1995) was used. The results f rom the two measurement methods compared well, indicating reliable air perm eability values using the expression of Liang et al, for the soils studied. Air permeability in structured soil measured using exhumed samples of diff erent size showed that small samples generally yielded lower values and hig her variability in k(a) than large samples, in accordance with the concept of a representative elementary volume.