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GP50

Location: Devon.Australian Soil Classification: Humose-Acidic, Dermosolic, Oxyaquic HYDROSOL
Geology: Recent alluvial deposits.General Landscape Description: Tarra River flats.
Soil Mapping Unit: RiverineSite Description: Grazing paddock.

Photo: Site GP50 Landscape
Site GP40 Landscape


Soil Profile Morphology:

Surface Soil

A110-20 cmVery dark greyish brown (10YR3/2); light silty clay loam; strong fine polyhedral structure; pH 5.0; clear change to:

Photo: Site GP50 Soil Profile
Site GP50 Profile
A1220-50 cmBlack (10YR2/1) with faint rusty root channel mottling; light fine sandy clay loam (organic and peaty); moderate polyhedral, parting to strong fine polyhedral structure; pH 4.7; smooth change to:

A250-90 cmVery dark grey (10YR3/1) with minor rusty root channel mottling; heavy fine sandy clay loam; pH 5.0; contains organic reed material; water comes into this layer; smooth change to:

Subsoil

B2190-110 cmVery dark grey (2.5Y3/1) with minor rusty root channel mottling; fine sandy light clay; weak medium polyhedral structure; minor organic reed content; pH 4.9.

Key Profile Features:
  • Seasonal saturation (for a number of months each year) of much of the soil profile.

Soil Profile Characteristics:


pH
Salinity Rating
Surface
(A1 horizon)
Strongly Acid
Moderate - High
Non-Sodic
None1
Subsoil
(B21 horizon)
Strongly Acid
Low - Moderate
Non-Sodic
None
Deeper subsoil
(at 1 m)
Very Strongly Acid
Low
Non-Sodic
None
1 Slight dispersion after remoulding.



Photo: Site GP50 pH
Photo: Site GP50 Salinity
Photo: Site GP50 aluminium
Photo: Site GP50 clay
The surface soil is strongly acid
becoming very strongly acid
at 20 cm.
The salinity rating in the surface
soil is moderate to high and
decreases with depth
to relatively low levels.
The levels of exchangeable
aluminium are high throughout
the sub-soil.
The clay content is fairly
constant throughout the
profile.


Horizon
Horizon Depth
(cm)
pH
(water)
pH
(CaCl2)
EC 1:5
NaCl
Exchangeable Cations
Ca
Mg
K
Na
meq/100g
A11
0-20
5
4.6
0.63
0.07
2.8
3.1
0.4
1
A12
20-50
4.7
4.3
0.4
0.05
1.2
1.6
0.3
0.7
A2
50-90
5
4.9
0.24
5.8
5.3
0.34
0.81
B21
90-110
4.9
4.3
0.1
4.6
4.1
0.31
0.48

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
0-20
52
25
26.8
10
10
31
31
A12
20-50
290
29
32.5
2
5
25
48
A2
50-90
300
31
30.6
10
9
30
32
B21
90-110
190
19.9
1
10
41
39

Management Considerations:

Whole Profile
  • Plant available water capacity (PAWC) is considered to be moderate (estimated at 110 mm) for the top metre of the soil profile. This is based on available laboratory data. 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.
  • Much of the soil profile is saturated for prolonged periods (i.e. several months) in most years (i.e. Hydrosol). Subsurface drainage may be a viable management option for such soils.
Surface (A) Horizons
  • The levels of exchangeable aluminium are moderate in the strongly acid surface (A11) horizon (50 mg/kg) but become very high in the very strongly acid subsurface horizons (300 mg/kg) These levels are likely to restrict the growth of aluminium sensitive species e.g. lucerne, phalaris. A pH/aluminium test sampled from across the paddock would be most appropriate to determine whether lime is needed to raise soil pH. 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.
  • Deficiencies in molybdenum and phosphorus are likely to occur in strongly acid soils. If lime is required, an application may assist in alleviating molybdenum deficiency and will increase phosphorus availability, as well as increasing soil pH. Phosphorus tends to become 'fixed' by high levels of exchangeable aluminium.
  • Manganese toxicity may also occur in strongly acid soils, particularly when poorly drained as waterlogging may bring manganese into solution.
  • The organic matter levels are very high for the surface (A11 and A12) horizons and extremely high for the subsurface (A2) horizon. Organic matter is important for maintaining aggregation on such soils.
  • The surface soil is likely to suffer structural degradation e.g. pugging, compaction, if overstocking occurs whilst the soil is wet.
  • The relatively high wilting point value (i.e. 27%) indicates that plants will not be able to utilise light rains falling on relatively dry soil. If the soil dries out, then small falls of rain may not be sufficient to wet the soil above permanent wilting point. The high wilting point also indicates that water is ‘held tightly’ by the soil, decreasing its availability to plants as the water content approaches this point.

  • The salinity rating is moderate for the surface (A11) horizon which may restrict the growth of highly salt sensitive species.
Subsoil (B) Horizons
  • The upper subsoil (B21) horizon has a moderate inherent fertility based on the sum of the basic exchangeable cations.
  • Exchangeable aluminium levels remain high throughout the very strongly acid subsoil which has implications for aluminium sensitive species (e.g. lucerne, phalaris).
  • Root growth may also be restricted by seasonal saturation of the subsoil.
Profile Described By: Mark Imhof, David Rees & Ian Sargeant (10/12/97).
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