GP67

Location: Nambrok.	Australian Soil Classification: Vertic (& Mottled-Sodic), Eutrophic, Brown CHROMOSOL.
Great Soil Group: solodic.	Geology: Mid Pleistocene alluvial plain deposits.
General Landscape Description: Upper slope of an old alluvial plain from the mid-Pleistocene era.	Mapping Unit: Wooundellah.
Site Description: Grazing paddock.

Site GP67 Landscape

Soil Profile Morphology:

Surface Soil

A11	0-15 cm	Brown (10YR4/3); fine sandy clay loam; weak coarse blocky structure; very firm consistence dry; pH 5.6:	Site GP67 Profile
A12	15-30 cm	Brown (10YR4/3); sporadically bleached; with rusty root channel mottling; fine sandy clay loam; weak coarse blocky structure; very firm consistence dry; pH 5.9:
A2	30-40 cm	Dark yellowish brown (10YR4/4) sporadically bleached; fine sandy clay loam; moderate coarse polyhedral, parting to moderate fine polyhedral structure; very firm consistence dry; pH 5.9:
Subsoil
B21	40-60 cm	Dark yellowish brown (10YR4/4) with dark red (2.5YR4/8) and brownish yellow (10YR6/8) mottles; medium clay; strong coarse polyhedral, parting to strong fine polyhedral structure; shiny faced peds; strong consistence dry; pH 6.1:
B22	60-100 cm	Yellowish brown (10YR5/8) with dark yellowish brown (10YR4/4) and slight dark red (2.5YR4/8) mottles; medium clay; moderate coarse prismatic, parting to moderate coarse blocky structure; shiny faced peds; strong consistence dry; many slickensides evident; pH 7.3:
B23	100-120 cm	Yellowish brown (10YR5/6); medium clay (sandy); moderate coarse prismatic, parting to moderate coarse blocky structure; dark staining evident; contains few (10%) quartz grit; pH 6.9:
B24	120+ cm	Reddish brown (5YR5/4); sandy clay; massive structure; pH 7.7.

Key Profile Features:

Soil Profile Characteristics:

¹ Slight dispersion after remoulding.
² Moderate dispersion after remoulding.

	The surface soil is moderately acid. The upper subsoil is slightly acid becoming slightly alkaline at 1 m depth.		The surface and upper subsoil is non-sodic becoming sodic at 60 cm depth.
	The surface salinity rating is moderate. The subsoil is very low.		The clay content increases markedly at the A/B horizon interface

Horizon	Horizon Depth (cm)	Exchangeable Aluminium mg/kg	Exchangeable Acidity meq/100g	Field Capacity pF2.5	Wilting Point pF4.2	Coarse Sand (0.2-2.0 mm)	Fine Sand (0.02-0.2 mm)	Silt (0.002-0.02 mm)	Clay (<0.002 mm)


A11	0-15	16			8.7	16	39	20	22
A12	15-30	<10			7.7	14	41	231	24
A2	30-40	<10	4.8		8.7	16	38	19	29
B21	40-60	<10	9.4		20.8	8	19	12	62
B22	60-100				15.8	29	13	10	50
B23	100-120
B24	120+

Management Considerations:

Whole Profile

Plant Available Water Capacity (PAWC) is considered to be moderate (estimated at 110 mm) for this soil profile. This is based on an estimated effective rooting depth of 60 cm (i.e. surface soil and upper subsoil). The upper subsoil is well structured (parting to many fine polyhedral shaped peds) which should not restrict root growth as much as some of the more sodic and coarsely structured upper subsoils in the region. The dense and coarsely structured deeper subsoil is likely to restrict rooting depth. Some 50 mm of this estimated PAWC is in the upper two surface horizons (i.e. to 30 cm depth). PAWC has been estimated using a model developed by Littleboy (1995) which uses analytical data for clay%, silt%, fine sand%, coarse sand % and wilting point.

Surface (A) Horizons

The level of soluble salts in the surface is moderate and may restrict the growth of salt sensitive species. These levels may be unnaturally high due to the effects of the dry season at the time of sampling (i.e. summer 1997) and are likely to decline following heavy rains and/or irrigation.
The surface (A1) horizon has a relatively low nutrient holding capacity (based on the sum of the exchangeable calcium, magnesium and potassium cations). Organic matter is important for maintaining soil fertility and aggregation on surface soils such as these with high fine sand and silt contents.
The surface (A1) horizon is non-sodic but disperses slightly after remoulding. This indicates that tillage or over-stocking of the soil whilst in a moist to wet condition may result in some structural degradation (e.g. surface sealing, exacerbated hardsetting, increased cloddiness) occurring. Raindrop action on bare soil may have a similar effect, so it is important to maintain surface cover.
The presence of rusty root channel mottling and sporadic bleaching in the subsurface (A2) horizon indicates that periodic waterlogging can occur above the more slowly permeable subsoil.

Subsoil (B) Horizons

The upper subsoil is non-sodic and well structured (parting to many fine polyhedral shaped peds). Root and water movement will therefore not be as restricted as is the case for many of the Sodosols in the region. In the deeper subsoil (i.e. from 60 cm depth) the subsoil becomes sodic and more dense and coarsely structured [and has a low exchangeable calcium to magnesium ratio (0.3)]. As a result, root and water movement will be more restricted in the deeper subsoil.
Also at 60 cm depth the subsoil displays vertic properties (slickensides). This indicates that significant shrinking and swelling occurs during wetting and drying cycles. This may have some engineering implications (e.g. disruption to fence lines).

Profile Described By: Mark Imhof and Ian Sargeant (22/04/1998).

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