MEASUREMENT OF SOIL MACROPORE AIR PERMEABILITY

Soil air-permeability measurement is a promising tool for determining pore geometric influences on transport processes. The inability of previous systems to allow air-flow measurement on undisturbed soil samples at and near saturation, where only macropores will conduct air, has prevented routine use of this technique for investigation of macropore geometry. The device described here allows simultaneous measurement of air permeability (Ka) and the soil water-retention curve in the 0- to -6.0-kPa matric-potential range. This is accomplished by counteracting the effect of the gravitational-potential gradient within the sample using controlled air pressure. Use of a wettable porous plastic of high conductance as the tension-control medium allows equilibration times of <4 h during desaturation for each step change in matric potential (ψm). Operation of the device is illustrated with results from two undisturbed soil samples having contrasting macroporosity.