Contrasting soil physical properties after zero and traditional tillage of an alluvial soil in the semi-arid subtropics
Introduction
Zero till is commonly advocated as a preferred cropping system to conventional, multicultivation practices. Zero till is particularly attractive on clay soils, both to minimise compaction and induce natural structure formation. In particular, the soil structure of Vertisols has strong potential to attain optimal conditions for plant growth through activation of their in-built resiliency via shrink–swell cycles (McGarry, 1996, McGarry, 1997; Pillai and McGarry, 1999). Articles and reviews on the benefits of reduced tillage systems are many. Thomas et al. (1997) provide a recent, comprehensive review that places concepts and practices of minimum tillage in context with the many types of conventional cultivation.
The aim of the current work was to identify reasons for the increased water storage in the untilled soil of a zero till field experiment. At the site, Radford et al. (1995) measured a 28% increase in plant-available soil water at sowing with zero tillage (ZT). Since water supply is a major factor limiting the yield of rainfed crops in the semi-arid, subtropical areas of central Queensland, a practice which improves soil water storage can potentially increase grain yields. At the trial site ZT outyielded traditional tillage by 1.2 t wheat grain per hectare per year during 4 years when fertiliser (N + S + Zn) was applied (Radford et al., 1995).
Radford et al. (1995) also showed there was a 4-fold increase in earthworm numbers with zero till compared to conventional tillage. Mele and Carter (1999) link the adoption of conservation tillage practices with increased earthworm activity; mulched stubble in particular favouring large increases in earthworm numbers. Retention of maximum levels of crop residues on the soil surface and lack of soil disturbance (ZT) apparently create a more favourable habitat for soil animals (Webb et al., 1997). Earthworm channels and termite galleries increase the volume of soil pores, which should increase aeration and the rate of water entry into the soil (Ehlers, 1975; Holt et al., 1993).
Image analysis of soil structure is gaining considerable use as both a pictorial and quantitative assessor of soil management effects. Moran et al. (1988) compared the effect of direct drilling and conventional cultivation for wheat in NSW, Australia. Cavanagh et al. (1991) compared the differences in pore structure attributes in a minimum till trial on a red-brown earth, in NSW, Australia. Wild et al. (1992) assessed the long-term effects of growing irrigated cotton on a Vertisol, with contrasting treatments of deep and shallow ripping and gypsum application. Douglas et al., 1992a, Douglas et al., 1992b assessed the effects of wheel traffic on the growth of perennial grass in Scotland. Lytton-Hitchins et al. (1994) compared binary images and derived structure attributes of adjacent bio-dynamic and conventionally managed dairy pastures in Victoria, Australia. Pillai and McGarry (1999) used binary images and derived data to compare soil structure repair of a compacted Vertisol with four breakcrops and a range of wet–dry cycles.
In this paper we present the results of soil physical determinations and image analysis of soil structure from the traditional and ZT treatments of the Radford et al. (1995) experiment. The aim is to describe the soil physical changes induced by these two contrasting fallow management practices.
Section snippets
Site
The site is located at the Biloela Research Station, Qld, Australia (latitude 24° 22′S; longitude 150° 31′E; altitude 173 m). Slope is negligible. The native forest was cleared in 1924, and crop and pasture were grown until 1983 when the experiment commenced. A meteorological station is located at the site. The soil is a black cracking clay (Vertisol) developed on alluvium; classed locally as Tognolini series (Shields, 1989). Selected soil physical and chemical properties are presented by
Sorptivity
The sorptivity of the surface soil in the TT and ZT treatments is shown in Fig. 1a. The distribution of sorptivity with supply tension was quite different for the two treatments. Sorptivity for ZT at the −10 mm supply tension was significantly (P = 0.05 and throughout the script, unless otherwise stated) greater than the sorptivity at each of the −20, −30 and −40 mm supply tensions of that treatment, though none of these three were significantly different from each other. In contrast, the
Conclusions
Zero till, relative to traditional tillage practices, has given marked differences in soil physical properties of a Vertisol in the semi-arid subtropics of Australia. In this environment and cropping system, high water storage in the soil profile during the fallow period is of paramount importance, particularly to increase the incidence of opportunity cropping. Previous work at this site has shown zero till achieved a 28% increase in plant-available soil water at sowing and an associated
Acknowledgements
We gratefully acknowledge Denis Orange, Alan Key and Alexandra Wilson for assistance in collecting and analysing the field data. Ian Sinclair and Steven Bray (QDPI) conducted the sample preparation and analysis of the images. Mrs. R. Kopittke (QDPI, Biometry) and Mrs. N. Abbott (QDNR) developed the data averaging routines for the image analysis attributes and conducted the statistical analysis of the image data. The assistance of David Freebairn and Robin Connolly with the rainfall simulator is
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