Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
Appendix 3. Major Soils used for Agriculture in Australia
Neil McKenzie, Ray Isbell, Katharine Brown and David Jacquier
CSIRO Land and Water
Soil test results are most useful when there is an appreciation of the general features of the complete soil profile. These features, together with climate, can have an overriding impact on plant growth. A great variety of soils are used for agriculture in Australia and this chapter provides a general account of 20 major types. The new Australian Soil Classification (Isbell 1996) is used as a frame of reference because of its practical focus. It is also the national standard for soil classification. However, there are many variants on the soil types described here and more detailed and local accounts should be consulted for specific guidance on land management.
General features of Australian soils
In recent geologic times, there has been a general absence across the continent of major processes that renew soils, for example, mountain building, volcanic activity and glaciation. Land surfaces across many parts of Australia are ancient and as a consequence the associated soils are strongly weathered and infertile. However, the agricultural lands have significant areas with younger land surfaces and more fertile soils.
Australian soils have many distinctive features. The surface layers usually have low organic matter levels and are often poorly structured, a condition made worse by most agricultural practices. Subsurface layers with a sharp increase in clay content are widespread (Kurosols, Chromosols, and Sodosols) and they can restrict drainage and root growth. In these soils, bleached layers with very low nutrient levels are also common. Soils affected by salt, either now or in earlier geological times (e.g. Sodosols), cover large portions of the arable lands of the continent and have various nutrient and physical limitations.
Soils lacking strong texture contrast generally pose fewer physical limitations to plant growth. Most notable are the deep red soils with high iron contents (Ferrosols) and closely related soild with structured B horizons (Dermosols). Australia is noted for the very large areas of cracking clays (Vertosols). These soils are relatively fertile but exhibit physical limitations. Soils formed in aeolian sands (Rudosols and Tenosols) fringe the southern cropping lands but are more extensive in the arid zone. A feature of the agricultural soils of parts of southern Australia is the widespread occurrence of highly calcareous soil types (Calcarosols). The remaining ancient land surfaces, particularly in northern Australia, have deep and strongly weathered soils (Kandosols) with very low levels of nutrients.
Correlations between soil nutrition and major soil types
Many useful inferences can be drawn about particular aspects of the nutrient status of major soil types. However, caution is required because direct or universal correlations between nutrient status and soil type cannot always be assumed. Strong correlations should only be expected when the field criteria used for classifying profiles have a logical physical connection with the nutrient of interest. An advantage of the new Australian Soil Classification (Isbell 1996) over previous systems is the introduction of chemical criteria for classification. However, the large body of literature on soil variability demonstrates that many chemical properties are spatially variable and may exhibit only limited correlation with other chemical, physical and morphological properties. The impact of previous management practices (e.g. application of fertiliser and ameliorants, loss of organic matter.) will often further reduce the association between a given soil type and its nutrient status.
The Australian Soil Classification
The Australian Soil Classification (Isbell 1996, Isbell et al. 1997) is a hierarchical, general purpose system that can be used at various levels of detail. In this appendix, soils are considered at the Order and Suborder level. A summary of the system at the Order level is presented in Figure 1. Most distinctions at the Suborder level are based on colour of the B horizon. The Organosol, Hydrosol, Rudosol and Anthroposol Orders are rarely used for extensive agriculture and are not represented here.
Figure 1. Schematic summary of the soil orders. Figures in parentheses refer to the number of soil profiles described in this chapter. The figure is not a key and readers are referred to Isbell (1996) if profile allocation is required.
|Human-made soils||ANTHROPOSOLS (0)|
|Dominated by organic materials||ORGANOSOLS (0)|
|Negligible pedological organisation||RUDOSOLS (0)|
|Minimal pedological organisation||TENOSOLS (1)|
|Bs, Bhs or Bh horizon||PODOSOLS (1)|
|Clay U 35% in all horizons, cracks, slickensides||VERTOSOLS (3)|
|Prolonged seasonal saturation||HYDROSOLS (0)|
|Strong texture-contrast between A and B horizons||pH < 5.5 in upper B horizon
|Sodic in upper B horizon with pH > 5.5
|Non-sodic B horizon with pH > 5.5
|Lacking strong texture-contrast between A and B horizons||Calcareous throughout profile or below A1 horizon
|High free iron B2 horizon
|Structured B2 horizon
|Massive B2 horizon
Selection of major soils and format
Twenty major soils used for agriculture were selected after consultation with State and Territory land resource assessment agencies. The information available on each soil varied substantially. The descriptions are based on the published literature and data held by CSIRO and the State agencies. A consistent format has been followed although some approximations were required and these are described in the following sections.
The various items covered in the format apply generally to the Suborders, whereas the soil profile descriptions and the laboratory data on which the graphs are based refer to specific example profiles shown by the images. In a few cases laboratory data were not available for the profile shown in the image. In these instances data from a similar soil was used and noted in the individual acknowledgments.
A brief account is provided of distribution, climate, dominant parent material, landform and native vegetation. Terms defined by McDonald et al. (1990) are used where appropriate.
A simplified description of profile morphology is presented for a representative soil—in most instances it corresponds to the adjacent image. The description includes: horizon type and depth, colour (moist soil unless otherwise indicated), field texture, coarse fragments (if present), grade and type of structure, consistence, pedogenic segregations and sharpness of the horizon boundary. All terms are defined by McDonald et al. (1990).
There may be some small discrepancies between horizon boundaries on the image and in the profile description. This is usually caused by variation across the pit face, diffuse horizon boundaries or placement of the scale.
Physical and chemical characteristics
Comprehensive soil physical data are lacking for many major groups. Particle size data for the fine earth fraction (< 2 mm) have been plotted wherever they were available. The percentage of clay, silt and sand gives an overall indication of the physical properties of a soil and sharp increases down the profile are often indicative of restrictions to root growth and water movement.
The total porosity of the soil and its capacity to store water are plotted on a profile basis. Soils with restrictions to plant growth often have a narrow range of available water (darker blue region in graphs on following pages) or aeration (pale brown region in graphs on following pages). The water retention data have in most instances been estimated using the predictive equations from Williams et al. (1992) or Cresswell and Paydar's (1996) two-point method.
The permeability of a soil profile is indicated by the saturated hydraulic conductivity in mm hr1. In general terms, soil layers with low hydraulic conductivity (e.g. less than the local rainfall intensities) will cause waterlogging and possibly generate runoff and erosion depending on the landscape setting.
Bulk density gives a general indication of limitations to root growth—bulk densities higher than 1.5 Mg m-3 are often limiting while values less than 1.0 Mg m-3 are very low and relatively uncommon. Further information on the interpretation of soil physical properties can be found in standard texts such as White (1997) or Hillel (1982). The source of data or estimation method used for each figure is denoted on the axis title using superscripts wherever direct measurements were not available.
A Water retention data estimated using Williams et al. (1992)
B Water retention data extrapolated from direct measurements using Cresswell and Paydar (1996)
C Estimate based on direct measurements of similar soils
D Estimate based on experience with similar soils
The values for soil properties at the immediate soil surface (i.e. 0.00 m depth) are rarely determined and they have been extrapolated manually using the near surface measurements (usually at 0.1 m).
Chemical characterization is restricted to the sum of exchangeable basic cations (Ca, Mg, K, Na) expressed as cmol(+) kg-1, exchangeable sodium percentage (ESP), pH (1:5 soil:water) and electrical conductivity (1:5 soil:water
dS m-1). The ESP is not presented for soils with a very low sum of exchangeable basic cations because the interpretation of its effect on physical properties is unclear. In the remaining soils, an ESP of 6% or more is associated with dispersive clays and soil structure less suited to root growth.
In most instances the analytical data are from the described profiles. Laboratory methods are generally consistent with those described in Rayment and Higginson (1992). More details can be obtained from the sources of the data listed in the individual acknowledgments.
Related soils and common names
Many informal names are used for soils and these vary greatly between districts. Common names often perpetuate misunderstanding and prevent clear communication. However, some of the more useful common names are included along with superseded class names from previous classification systems.
Soil qualities, occurrence and land use
Descriptive accounts of the main soil qualities relating to agriculture are presented. More specific rating systems for individual soils can be obtained from State and Territory land resource agencies. It should be appreciated that in a number of topics only broad generalisations are possible given the space constraints and the obvious difficulties in giving adequate brief accounts of, for example, the Australia wide occurrence and land use of widespread soils.
The major organisations that provided illustrations and data have been individually acknowledged. The authors sincerely thank the numerous individuals who contributed to this appendix.
Cresswell H.P. & Paydar Z. 1996, Water retention in Australian soils. I. Description and prediction using parametric functions. Australian Journal of Soil Research vol. 34, pp. 195–212.
Grealish G. & Wagnon J. 1995, Land Resources of the Bencubbin Area. Land Resources Series No. 12. Natural Resources Assessment Group, Department of Agriculture, Western Australia, Perth.
Hillel D. 1982, Introduction to Soil Physics, Academic Press Inc.
Isbell R.F. 1996, The Australian Soil Classification, CSIRO Publishing, Melbourne.
Isbell R.F., McDonald W.S. & Ashton L.J. 1997, Concepts and Rationale of the Australian Soil Classification, Australian Collaborative Land Evaluation Program, CSIRO Land and Water, Canberra.
McArthur W.M 1991, Reference Soils of South-Western Australia, Department of Agriculture, Western Australia.
McDonald R.C., Isbell R.F., Speight J.G., Walker J. & Hopkins M.S. 1990, Australian Soil and Land Survey Field Handbook, 2nd Edition, Inkata Press, Melbourne.
Oades J.M., Lewis D.G. & Norrish K. 1981, Red-Brown Earths of Australia, Waite Agricultural Research Institute, University of Adelaide and CSIRO Division of Soils, Adelaide.
Rayment G.E. & Higginson F.R. 1992, Australian Laboratory Handbook of Soil and Water Chemical Methods, Inkata Press, Melbourne.
Stace H.C.T., Hubble G.D., Brewer R., Northcote K.H., Sleeman J.R., Mulcahy M.J. & Hallsworth E.G. 1968, A Handbook of Australian Soils, Rellim Technical Publications, Glenside, SA.
White R.E. 1997, Principles and Practice of Soil Science: The Soil as a Natural Resource, Blackwell Science, Oxford.
Williams J., Ross P.J. & Bristow K.L. 1992, ‘Prediction of the Campbell water retention function from texture, structure and organic matter’, in Proceedings of the Int. Workshop on Indirect Methods for Estimating the Hydraulic Properties of Unsaturated Soils, M.Th. van Genuchten, F.J. Leij & L.J. Lund (eds), University of California, Riverside, CA, pp. 427–441.
* Reprinted with permission from K.I. Peverill, L.A. Sparrow & D.J. Reuter (eds) 1999, Soil Analysis an Interpretation Manual, CSIRO Publishing, Australia.
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