Measurement of Soil Macropore Air Permeability

Abstract

AbstractSoil 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. The soil core with visible macropores (> 1‐mm diameter) had a higher Ka and a greater change in water content at each ψm step throughout the 0‐ to −3.0‐kPa range, compared with the core without visible macropores. The positive linear slope of the log air‐filled porosity vs. log Ka relationship was greater for the sample containing visible macropores, indicating greater pore continuity. Air permeability of the macroporous sample remained >0 at ψm = 0, but the opposite was true for the soil sample without visible macropores.