The internal air contents of subsoil aggregates in some clay soils

Abstract

Abstract Subsoil aggregates from several soils of high clay content in Auckland province were cut and sieved to form test aggregates from 2,0 to 3.35 mm in diameter. These were wetted and drained to various tensions and their bulk densities determined by displacement of kerosene. Bulk densities were combined with particle densities to give the air contents of the aggregates. At the greatest water contents used, corresponding to a tension of 0.02 bars, the test aggregates were denser than core samples of the soils from which they came by an average amount of 0.1 g/cm3. On drainage to a tension of 15 bars test aggregates from soils on basalt and old andesitic ash shrank by 5-8% and the internal air contents of most aggregates increased by 5-12% of aggregate volume. Aggregates from soils on sedimentary rocks shrank by well over 10% without admitting significant amounts of air. The internal aeration of aggregates in the volcanic soils is likely to favour a more even spread of roots than would be found in soil...