GP49

Location: Giffard West	Australian Soil Classification: Mottled, Subnatric, Brown SODOSOL
Geology: Late Tertiary deposits (later modified by wind).	General Landscape Description: Level plain (Giffard Plain).
Soil Mapping Unit: Giffard	Site Description: Grazing paddock.

Site GP49 Landscape

Soil Profile Morphology:

Surface Soil

A11	0-12 cm	Dark greyish brown (10YR4/2); light sandy loam (organic); weak structure; very weak consistence dry; contains few (5%) subrounded quartz fragments (2-6 mm in size); pH 5.0; gradual change to:	Site GP49 Profile
A12	12-25 cm	Loamy sand; firm consistence dry; contains few (5-10%) subrounded quartz fragments (2-6 mm in size); gradual change to:
A2	25-40 cm	Pale brown (10YR6/3) conspicuously bleached (10YR8/2d); loamy sand; weak consistence dry; pH 6.1; sharp and wavy change to:
Subsoil
B21	40-80 cm	Yellowish brown (10YR5/6) and brownish yellow (10YR6/8), with pale brown (10YR6/3) and strong dark grey (10YR4/1) mottles; heavy clay; moderate coarse columnar structure; strong consistence dry; pH 6.9; gradual change to:
B22	80-110 cm	Yellowish brown (10YR5/8) with dark red (2.5YR4/8) and light grey mottles (10YR7/1) (associated with root channels); medium heavy clay; smooth-faced peds; moderate coarse prismatic, parting to moderate coarse blocky structure; dark stains along prism faces; strong consistence dry; pH 7.9; clear and wavy change to:
BC	110-130 cm	Light grey (10YR7/1) with dark red (2.5YR4/8) and brownish yellow (10YR6/8) mottles; sandy loam (clayey); contains few (5-10%) sub-rounded quartz fragments (2-5 mm in size); pH 8.0.

Key Profile Features:

Very strong texture contrast between sandy surface (A) horizons and subsoil (B) horizons.
Accumulation of quartz fragments within the upper surface horizons.
Conspicuously bleached sub-surface (A2) horizon.
Coarse columnar structure in upper subsoil.

Soil Profile Characteristics:

¹ Complete dispersion after remoulding.


The surface soil is strongly acid. The upper subsoil is slightly acid becoming slightly alkaline at 60 cm.	The salinity rating is low throughout the profile.	The clay content increases markedly at the A/B boundary (i.e. 40 cm depth).

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)


A1	0-12	15			7	48	32	8	7
A12	12-25
A2	25-40				1	50	37	10	4
B21	40-80		8.8		23.5	15	12	6	65
B22	80-110		3.2		16.8	33	15	6	47
BC	110-130					57	19	2	23

Management Considerations:

Whole Profile

Plant available water capacity (PAWC) is considered to be very low (estimated at 40 mm) for the top metre of the soil profile. This is based on available laboratory data and assumes an effective rooting depth of 40 cm. Effective rooting depth will be restricted in the dense and coarsely structured subsoil. 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 value.

Surface (A) Horizons

The surface (A11) horizon is strongly acid which indicates that aluminium toxicity may occur. However, levels of exchangeable aluminium measured at this site are not high (i.e. 15 mg/kg). A pH/aluminium test sampled across the paddock would be most appropriate to determine whether lime is needed to raise soil pH. However, other factors need to be considered before lime is recommended (e.g. pasture species grown, method of application, local trial responses, soil surface structure and likely cost/benefit).

The inherent fertility of the surface (A1) horizon (based on the sum of the exchangeable calcium, magnesium and potassium cations) is very low. The high levels of organic matter at this site are important for maintaining soil fertility, water holding capacity and enhancing surface soil structure (especially with the high sand content of 80%).

The sandy surface horizons (80% sand) will be quite permeable and have a low water storage capacity. The low wilting point value (i.e. 7 %) indicates that plants will be able to utilise light rains falling on dry soil. However, due to the low water storage capacity, plants will soon suffer moisture stress unless further rain occurs. Organic matter is an important source of nutrient holding capacity in sandy soils. It is also important in these sandy soils to enhance water holding capacity and fertility.

Deficiencies of the trace element molybdenum (Mo) are likely to occur in acid sandy soils (soil adsorption of Mo increases as pH decreases, leading to reduced availability to plants). Any deficiencies can be confirmed by plant tissue analysis. Deficiencies can be remedied with molybdenum-enriched fertiliser and foliar application. Lime application on acid soils will also make molybdenum more available to plants.

Manganese toxicity may also occur in strongly acid soils, particularly when poorly drained (as waterlogging may bring manganese into solution).

The sub-surface (A2) horizon is extremely infertile, so building up the surface (A) soil should be a prime objective.

The presence of conspicuous bleaching within the (A2) horizon indicates periodic waterlogging occurs above the more slowly permeable subsoil.

Subsoil (B) Horizons

The dense and coarsely structured subsoil is sodic (and has a low exchangeable calcium to magnesium ratio) becoming strongly sodic with depth. Strong dispersion occurs as a result. This indicates that water and root movement will be significantly restricted in the subsoil. Water will build up on top of the subsoil during wet periods and waterlogging may result.

Root movement will also be restricted by the hard capping of the coarse columnar structure.

Profile Described By: Mark Imhof, David Rees & Ian Sargeant (10/12/97).

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