Measuring air-filled porosity changes in an irrigated swelling clay soil

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Abstract
An in-situ method for measuring air filled porosity (ªa) of soils using a neutron meter was developed and evaluated for a layered, swelling clay soil. Bulk density-volumetric moisture content relationships of soil layers were used to calculate the regressions of volumetric moisture content on ªa. These regressions were combined with calibration equations of the neutron meter to obtain relationships between ªa and neutron counts. The ªa values predicted from measured neutron count ratios show good agreement with ªa values calculated from the measured bulk densities and moisture contents in a transitional red-brown earth. The method provides a rapid and non-destructive measurement of ªa. Soil aeration in a transitional red-brown earth under two irrigation treatments was assessed using neutron counts to monitor the changes in ªa profiles. Large soil cores which were flooded had lower profile ªa values than did cores which were not flooded. However, differences were small, owing apparently to the very low rates of infiltration and redistribution. The potential use of this method to continuously monitor changes in ªa profiles allows frequent calculation of aeration stress indices, which may be used to predict crop responses and yield losses due to poor aeration.