Australian Agriculture Assessment 2001
Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
Appendix 3 - Chromosols
The essential feature of Chromosols is the strong texture contrast between the A and B horizons. They are distinguished from other texture contrast soils by not being strongly acidic (cf Kurosols) or sodic (cf Sodosols) in their upper B horizons. In their natural condition, these soils may have favourable physical and chemical properties but many now have hardsetting surface layers with structural degradation caused by long-term cultivation.
Red Chromosols
Although these soils have clayey B horizons they tend to be well drained. A typical example of a Haplic, Calcic, Red Chromosol is given below.
Environment
Distribution: A widespread soil in the cropping lands of eastern and southern Australia and to a lesser extent south-west Western Australia.
Climate: Broad mean annual rainfall range, approximately 300 mm to 1200 mm.
Parent materials or substrate: These vary widely from alluvial to aeolian sediment and less basic metamorphic and igneous rocks.
Landform: Undulating plains to rolling hills.
Native vegetation: Eucalypt woodland and sclerophyll forests.
Land use
Red Chromosols are prominent in the wheat belt of southern New South Wales, northern Victoria and the mid-north of South Australia where they are widely used for cereal and oil seed growing. In southern Queensland they are used for mixed farming but in the tropics mainly for cattle grazing of native pastures.
Common variants
Red Chromosols commonly lack an accumulation of carbonate in higher rainfall regions although base status is usually moderate to high. Bleached A2 horizons may occur indicating restricted drainage. Such variants are often sodic at depth and grade into Sodosols.
Nomenclature
Also known as Red - Brown Earths, Non-Calcic Brown Soils, Red Podzolic Soils or Red Duplex Soils.
Soil qualities
Water availability: Storage varies greatly but usually adequate and between 100 - 200 mm.
Drainage: Imperfectly to well drained but the B horizons can be an impediment.
Aeration: Generally adequate although temporary saturation can occur in bleached A2 horizons if present.
Physical root limitations: Main restrictions are caused by strong and dense B horizons and structurally degraded surface layers
Erosion hazard: Low to moderate depending on slope but increasing with degradation of the A horizons. Susceptible to surface slaking upon rapid wetting, resulting in hardsetting if organic matter is low.
Nutrient availability: Low contents of phosphorus and nitrogen with good responses to fertilizer.
Toxicities: Boron in areas of Western Australia, South Australia and Victoria. Less commonly, aluminium associated with induced acidification.
Workability: Degraded, hardsetting surfaces have poor workability but this can usually be overcome with increased organic matter.
Acknowledgment
Photo from CSIRO Land and Water. Soil description and laboratory data from CSIRO Land and Water and Oades et al. (1981). Urrbrae Loam.
Typical profile
Waite Institute, Adelaide, South Australia. |
Soil description of a typical profileA1 0 - 0.15 m Dark reddish brown (5YR3/4) fine sandy loam; moderate subangular blocky structure; soft consistence; diffuse boundary to: A2 0.15 - 0.26 m Red (2.5YR 5/8) sandy clay loam; moderate subangular blocky structure; soft consistence; sharp boundary to: B1 0.26 - 0.30 m Dark red (2.5 YR 3/6) light medium clay; moderate subangular blocky structure; soft consistence; clear boundary to: B22 0.30 - 0.80 m Dark red (2.5 YR 3/6) heavy clay; subangular blocky parting to strong subangular blocky structure; firm consistence; clear boundary to: B2k 0.80 - 1.10 m Yellowish red (5YR 4/6) medium clay; weak subangular blocky structure; 2 - 10% carbonate nodules; very hard consistence; diffuse boundary to: BCk 1.10+ Yellowish brown (10YR 5/6) medium clay; moderate subangular blocky structure; firm consistence; 10 - 20% soft carbonate segregations. |
Laboratory data for the typical profile
Brown Chromosols
These soils are characterised by brown clay loam or clay B horizons. An example of a Bleached-Mottled, Mesotrophic, Brown Chromosol with a conspicuously bleached A2 horizon is described below.
Environment
Distribution: A common soil of eastern, southern and south-western Australia but is less extensive than those with a dominantly red colour class.
Climate: Broad mean annual rainfall range, approximately 300 mm to 1200 mm.
Parent materials or substrate: Wide range of rocks and sediments other than more basic materials.
Landform: Level to undulating plains, some hilly to high, hilly lands.
Native vegetation: Eucalypt woodland and open forest.
Land use
The largest areas of Brown Chromosols used for agriculture are in the western part of the Western Australian wheatbelt where they are used for cereals and some lupins. In south-eastern Australia the soils are used mainly for improved pastures with some cropping (usually where the surface soil has a suitable depth, texture and workability).
Common variants
Although an A2 horizon is commonly present it may not always be bleached. This usually implies better internal drainage. Base status varies widely and subsoil pH is more commonly neutral to slightly acid. Some profiles become sodic in the lower B horizons and thus grade to Sodosols. Ironstone nodule horizons (> 20% nodules) may also occur, usually below the A1 horizon.
Nomenclature
Otherwise known as Brown Duplex Soils, Brown Podzolic Soils or Lateritic Podzolic Soils.
Soil qualities
Water availability: Low to moderate in the rootzone.
Drainage: The bleached A2 horizon and mottled B horizon suggests the soil is imperfectly drained.
Aeration: Moderate. Temporary saturation may occur in the A2 horizon.
Physical root limitations: Slight restriction by some clay subsoils.
Erosion hazard: Moderate water erosion potential.
Nutrient availability: Needs organic matter and lime to maintain moderate fertility. Phosphorus content is usually low.
Toxicities: Low pH in deep subsoil - probable aluminium toxicity.
Workability: Good due to lack of stone and a relatively well structured surface.
Acknowledgment
Photo, soil description and laboratory data from Primary Industries and Resources, South Australia. Site CH 013.
Typical profile
Adelaide Hills, South Australia. |
Soil description of a typical profileA1 0 - 0.15 m Very dark greyish brown (10YR 3/2 d) sandy loam; weak granular structure; moderately moist, very soft consistence; 10% ironstone and sandstone gravel; clear boundary to: A2 0.15 - 0.33 m Pink (7.5YR 8/3 d) sandy clay loam; massive structure; moderately moist, very soft consistence; 10% sandstone and ironstone gravel; clear boundary to: B21 0.33 - 0.48 m Strong brown (7.5YR 5/8) sandy light clay with red mottles; strong polyhedral structure; moderately moist, soft consistence; gradual boundary to: B22 0.48 - 0.85 m Brownish yellow (10YR 6/8) light clay; moderate polyhedral structure; moderately moist, firm consistence; diffuse boundary to: B3 0.85 - 1.20 m Brownish yellow (10YR 6/8) fine sandy clay loam; massive structure; moderately moist, firm consistence; diffuse boundary to: Cr 1.20 - 1.80 m Yellow (10YR 8/6) sandy clay loam; moderately moist, firm consistence; highly weathered kaolinitic sandstone. |
Laboratory data for the typical profile
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