Soil friability in relation to management history and suitability for direct drilling

Friability was determined for a selection of soils from the southern wheatbelt of New South Wales. Samples were taken from the surface of a range of soils that included a red-brown earth, a red earth, and a non-calcic brown soil, and covered a range of management histories. Differences in soil management history explained most of the variation in friability. Uncultivated woodland soils were highly friable in the top 20 mm [mean friability index (k) = 0.66], with intensively tilled soil the least friable (mean k = 0.05). Soils under direct drilling (DD) had friability similar to that from woodland sites (mean k = 0.55). Intensive cultivation of hardsetting soils over a number of seasons decreased their friability. Aggregates of 50 mm effective diameter had tensile strengths that ranged from 18 to 1087 kPa at woodland and DD sites, while the tensile strengths for the same size aggregates of soils under traditional tillage (TT) were between 1061 and 4419 kPa. Aggregates of 2 mm diameter had tensile strengths of 17 884-63 432 kPa for woodland and DD soils, and 2012-5268 kPa for TT soil. Significant relationships were detected between friability and the tensile strength of aggregates of 2, 20 and 50 mm size and this might present an easier means of determining friability. Friability was found to be significantly related to a number of other soil structural properties, including hydraulic conductivity, bulk density, organic carbon, aggregate stability, and sorptivity. Friability might be a useful indicator of the suitability of soil structural condition for crop establishment by direct drilling. Soils with low friability appear difficult to direct drill successfully; this reduces the management options available for their amelioration. Further work is necessary to relate friability directly to seedbed conditions following direct drilling and subsequent plant emergence and growth, and to establish firmly the wider applicability of relationships between friability and other soil structural properties.