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GP30

Location: Maffra.Australian Soil Classification: Sodic, Eutrophic, Red CHROMOSOL.
Great Soil Group: solodic.Geology: Quaternary prior stream deposits.
General Landscape Description: Level plain.Mapping Unit: Tinamba.
Site Description: Irrigated dairy grazing paddock (non laser graded).

Photo: Site G30 Landscape
Site GP30 Landscape


Soil Profile Morphology:


Surface Soil
Photo: Site G30 Profile
Site GP30 Profile
A10-40 cmReddish brown (5YR4/3); fine sandy clay loam; pH 5.2; gradual change to:
A240-60 cmReddish brown (5YR5/4); sporadically bleached; fine sandy loam; macropores evident; pH 6.0; sharp change to:
Subsoil
B2160-100 cmYellowish red (5YR4/6) with dark stains (manganese oxide) on some ped faces; light medium clay; coarse prismatic, parting to strong coarse to medium blocky structure; strong consistence dry; some manganese flecks; pH 6.5; gradual change to:
B22100 + cmReddish brown (5YR5/4); light medium clay; coarse prismatic, parting to weak to moderate blocky structure; pH 7.7.

Key Profile Features:
  • Strong texture contrast between the surface (A) horizon (13% clay) and the subsoil (B21) horizon (49% clay).

Soil Profile Characteristics:

Horizon
pH
Salinity Rating
Surface
(A1 horizon)
Strongly Acid
Very Low
Non-Sodic
None
Subsoil
(B21 horizon)
Slightly Acid
Very Low
Non-Sodic
None1
Deeper Subsoil
(at 100+ cm)
Slightly Alkaline
Very Low
Sodic
None
1 Complete dispersion after remoulding.


Graph: pH levels in Site G30





The profile becomes increasingly
alkaline with depth.
Graph: Sodicity levels in Site G30



The upper soil profile is non-sodic.
The deeper subsoil becomes sodic.
Graph: Salinity levels in Site G30



The level of soluble salts is very
low throughout the profile.
Graph: Clay% in Site G30



The clay content increases significantly
at the A/B horizon interface.



Horizon
Horizon Depth
(cm)
pH
(water)
pH
(CaCl2)
EC 1:5
NaCl
Exchangeable Cations
Ca
Mg
K
Na
meq/100g
A1
0-40
5.2
4.4
<0.05
1.6
0.39
0.1
0.13
A2
40-60
6
4.8
<0.05
1.2
0.49
0.08
0.08
B21
60-100
6.5
5.4
<0.05
3.6
6.2
0.35
0.48
B22
100+
7.7
6.6
<0.05
2.6
6.1
0.2
0.5

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
0-40
51
5.7
24
5
2
55
28
13
A2
40-60
18
4
2
56
32
13
B21
60-100
29
16
1
27
23
49
B22
100+
23
12
3
41
20
36



Management Considerations:

Whole Profile
  • Plant Available Water Capacity (PAWC) is considered to be low-moderate (estimated at 70 mm) for this soil profile. This is based on an estimated effective rooting depth of 60 cm (i.e. surface soil). The denser and more coarsely structured upper subsoil is likely to restrict rooting depth, although not to the same extent as some of the more sodic subsoils in the region. Much of the plant available water (i.e. 50 mm) will be in the surface (A1) horizon. 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 surface soil is relatively deep, providing a good environment for plant growth.
  • The surface soil is strongly acid. This indicates that aluminium and manganese toxicity may occur. However, the level of exchangeable aluminium measured for this pit site is quite low (<10 mg/kg) and unlikely to restrict the growth of aluminium sensitive species. A pH/aluminium test is best performed from samples taken across the paddock and bulked together. Other factors also 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 may occur in the strongly acid surface soil. Increasing soil pH by lime application should enable phosphorus (from superphosphate) and molybdenum to become more available. If lime is required, and pH is increased, then the availability of major nutrients should improve.
  • The surface soil has a high fine sand content (55%). When cultivated in a dry condition it will become ‘powdery’ and subsequent rainfall may result in surface sealing occurring. Soils such as these rely to a large extent on organic matter for maintaining aggregation. The levels of organic carbon measured at the pit site (which may or may not be representative of the whole paddock) is low. Organic matter levels will build up under pasture but will decline if cropping takes place. Practices such as residue retention, minimum tillage and including pasture rotations could be utilised if cropping takes place. This will build up organic matter, improve aggregation, soil fertility and water holding capacity.
  • The low wilting point value (i.e. 5%) of the surface horizon indicates that plants will be able to utilise very light rains when the soil is dry. However, due to the low water storage capacity, plants will soon suffer moisture stress unless further rainfall occurs. This is not likely to occur as these soils are irrigated.
  • The presence of sporadic bleaching in the subsurface (A2) horizons indicates that periodic waterlogging occurs above the more slowly permeable subsoil.
Subsoil (B) Horizons
  • Water and root movement will be restricted by the coarsely structured upper subsoil. This horizon is non-sodic but does completely disperse after remoulding. It is likely to allow more water movement compared to the more sodic and dispersive subsoils in the region. Disturbance to this horizon by cultivation whilst moist should be avoided.
Profile Described By: Mark Imhof, Ian Sargeant (19/12/96).
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