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 (ka) in situ, on-site (exhumed soil samples), and in the laboratory on a wide range of Danish agricultural soils. Two different sizes of sample rings were used (100 cm3 and 3140 cm3). The device was initially tested in the laboratory on repacked soil samples to evaluate dependency of ka on sample size. The results showed consistent values of ka for both sample sizes, indicating only little 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 for the shape factor developed by Liang et al. (1995) was used. The results from the two measurement methods compared well, indicating reliable air permeability values using the expression of Liang et al. for the soils studied. Air permeability in structured soil measured using exhumed samples of different size showed that small samples generally yielded lower values and higher variability in ka than large samples, in accordance with the concept of a representative elementary volume.

Key Points: 64 of 65 undisturbed soil columns exhibited preferential transport The strength of preferential transport was correlated with the bulk density The dominant transport process was inferred to be transport through macropores

Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, K_w) and air permeability (k_a, at a matric water potential of −50 cm H₂O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm³ and 6280 cm³). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship between K_w and k_a on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at −50 cm H₂O to validate a comparison between the two parameters. Predictive K_w (k_a) relations for the remaining three soils at the two scales compared favorably with a general K_w (k_a) relation proposed by Loll et al. (1999). This study supports the use of a general predictive relation between k_a near field capacity (at around −50 to −100 cm H₂O) and K_w, but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of −50 cm H₂O.