Summary
Nathan Odgers
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A number of experiments were carried out at a site at the Australian Cotton Research Centre near Narrabri, to determine various soil physical properties. Results were varied. Some experiments showed large variation in the data obtained while others were much more uniform.

Certainly, there are large differences between the pasture site and the cultivated site. At the most cursory level, the pasture site exhibited large cracks descending to the subsoil whereas no cracks were found in the cultivated area—crusting was starting to develop, though. Vegetative groundcover was essentially 100% in the pasture, compared to 0% in the cultivated area.

Hydraulic conductivity: A number of methods were used to estimate saturated hydraulic conductivity (Ksat)—the amoozemeter, single-ring infiltrometer, double-ring infiltrometer, falling-head lined borehole and the tension and ponded disc permeameters. There was a large variation in the results, ranging from 3.42 mm hr-1 to 577.93 mm hr-1 in the pasture and from 0.09 mm hr-1 to 28.17 mm hr-1 in the cultivated area. Generally, Ksat tended to be larger in the pasture than the cultivated area.

Water content: The capacitance probe and a time-domain reflectometry experiment were used to estimate soil water content. The time domain reflectometry experiment was carried out on both the cultivated and pasture areas, measuring water content at six depths to enable the construction of moisture profiles. The behaviour of the moisture profiles were quite different—the pasture soil was initially much drier at depth than the cultivated soil. This is possibly due to the lack of vegetative cover on the cultivated site which means that deep drainage is not prevented.

Bulk density: Bulk density was not significantly different between pasture and cultivated sites. Mean bulk density was 1.0 g cm-3 in the cultivated area and 1.2 g cm-3 in the pasture. Penetrative strength measured using a cone penetrometer was statistically higher in the pasture than in the cultivated area.

The difference in soil temperature between mulched and unmulched soil was easily noticeable (see below)—diurnal variation in temperature decreased markedly. Unfortunately, soil temperature was only monitored in the cultivated area so no comparison with the pasture could be made.

Soil penetrative strength—comparision between cultivation (c) and pasture (p).
Effect of mulch on soil temperature

Conclusion

To conclude, the pasture is more indicative of a "natural profile" or one that we would expect to find under some sort of vegetated, undisturbed state—soil has better structure and the influence of cracks on soil water movement can clearly be seen from the data recorded. The difference in soil water content at depth between cultivation and pasture reflects the effect of vegetation on reducing deep drainage, and the mulch experiment gives an indication of the effect of cropping on soil temperature.

In terms of sustainability, the above effects should be taken into account. The cultivated soil may store more water, but leakage from the bottom of the profile will be much greater. Cultivated soil does not have the same resistance to compression, so will become compacted more easily via vehicle or livestock traffic.

So overall, the status of the pasture soil is more desirable in terms of sustainability, but is likely difficult to sustain under an agricultural regime.