Benambra (Symbol: Ba)

EG204 | EG211 | Soil Conservation Authority, Profile No. 212

Geology
Late Pleistocene to Recent alluvial sediments.

Landform
Gently undulating plain.

The Benambra map unit can be subdivided into four components. The level plains are the most extensive component and occur both east and west of Benambra. East of Benambra there are low sandy rises as well as a narrow flood plain associated with Morass Creek. The other component occurs south of Lake Omeo and is comprised of low rises, called lunettes, south-east of Lake Omeo.

Geological history
This unit occupies the relatively level area to the south-west and east of Benambra. Rowe (1984) and others believe these areas formed a once extensive lake system formed when the Morass and Deep Creeks were blocked by a basalt flow about 2.3 million years ago. Before down-cutting commenced, water must have ponded along Morass Creek for some 15 km above the basalt flow and flooded the basin now occupied by Lake Omeo up to about the 715 m contour, the approximate level of the surface of the basalt. Sediment accumulated in the basin, filling it to a maximum depth of about 80 m before Morass Creek breached the basalt. Now there is no ponding of the water except during major flood flows.

After the basin filled with sediment, geological faulting occurred resulting in a narrow long ridge running north-south through Benambra. This isolated Lake Omeo from Morass Creek. Now the plains east of Benambra are drained by Morass Creek but the plains to the south-west have no external drainage thus forming Lake Omeo. The elevation of clay plains on both sides of the ridge is very similar, about 695 – 700 m.


Alluvial plains east of Benambra

Original vegetation
Montane grassland with montane grassy woodland.

Soils
Component 1: Soils of the level plains
These soils form the majority of soils both south-west and east of Benambra. The surface soils are commonly dark grey brown fine sandy clay loams to light clays. At about 15 cm there is a clear change to dark grey medium clays with a coarse prismatic to coarse sub-angular blocky structure. Clay textures generally continue to at least 1 m with dark yellowish or greyish brown colours predominant. Sandier deep subsoils appear to adjoin the low sandy rises. Occasionally small pellets of lime (calcium carbonate) occur in the deep subsoil. Two pits were excavated for this report (EG204, EG211).

Component 2: Soils of the low sandy rises
East of Benambra there are low sandy rises. A soil pit (EG204) was excavated on the level plain adjoining one area which showed the sandier material extended below the adjoining greyer soils of the plain. This means the sandy to gravelly sediments predate the last phase of dark clay deposition. No pits were excavated on these rises. However there is a gravel pit off Limestone Road where a quarry face is exposed. The soils here are quite gravelly and show weak profile development.

Dark clays overlay earlier deposits of sands and gravels

Quarry face off Limestone Road

Component 3: Soils of the narrow flood plain associated with Morass Creek
The present flood plain of Morass Creek is confined to a narrow area adjoining either side of the creek which is lower than the adjoining level plain. No pits were excavated but shallow auger holes showed the surface soils are predominantly dark grey clays which crack during dry periods. The Morass Creek is now a drain and its present flood plain is rarely under water.

Component 4: Soils of the low rises south-east of Lake Omeo
A continuous narrow low rise, called a lunette, occurs along the immediate south-east part of Lake Omeo. Rowe (1984) describes the surface soils as dark clays with a well developed structure becoming olive grey heavy clay with depth. They are neutral to slightly alkaline in the surface but become more alkaline with depth with free lime usually present. A fragmented outer lunette comprising three low rises occurs about 700 m south of the Lake. These deposits are believed to wind-blow clays derived from the dry lake bed. Rowe (1967) has described a profile on the outer lunette and his information is used here (Soil Conservation Authority, Profile No 212). Auger holes on the level plain show a similar soil profile.

Site EG204

Location: Benambra (East of township)	Geo. Ref. 0564162E 5909929N
Australian soil classification: Sodic, Eutrophic, Black CHROMOSOL	Black texture contrast soil
General landscape description: Almost level plain
Geology: Pleistocene to recent alluvial deposits
Land use: Grazing on annual pasture
Native vegetation: Montane grassland
Mapping unit: Benambra (Component 1)

Soil profile morphology

Surface soil
A1	0 – 15 cm	Very dark greyish brown (10YR3/2); clay loam; moderate coarse (20 – 50 mm) polyhedral structure; firm consistence dry; abrupt change to:
Subsoil
B21	15 – 45 cm	Very dark grey (10YR3/1); medium clay; strong very coarse (> 50 mm) prismatic structure; very firm to strong consistence dry;
B22	45 – 80 cm	Brown (10YR5/3); medium clay; strong very coarse (> 50 mm) prismatic to lenticular structure; strong consistence dry; clear change to:
D	80 – 105+ cm	Yellowish brown (10YR5/6); sandy clay; apedal; strong consistence dry; becoming sandier with depth.

Key profile features

• Texture contrast between the surface (A1) horizon and subsoil (B21) horizon.
• Strongly structured subsoils.

Soil profile characteristics

	pH	Salinity rating	Sodicity	Dispersion	Slaking
Surface soil (A1 horizon)	Strongly acid	Very low	Non sodic	None	Water stable
Subsoil (B21 horizon)	Slightly acid	Very low	Non sodic	Slight1	Slakes partially
Deeper subsoil (B22)	Slightly alkaline	Very low	Sodic	None	Slakes considerably

¹ Moderate after remoulding

Horizon	Depth cm	Field texture	pH water	EC dS/m	Coarse sand %	Fine sand %	Silt %	Clay %	Field cap. % w/w	Wilt. point % w/w	PAW* water % w/w
A1	0-15	CL	5.3	0.09	17	35	18	20	31.4	11.1	20.3
B21	15-45	MC	6.1	<0.05	19	28.5	18	30	27.5	15.0	12.5
B22	45-80	MC	7.7	0.13	22.5	31	16	26	26.0	14.4	11.6

* PAW is plant available water. This is the difference between the soil’s water content at field capacity and wilting point.

Management considerations

Whole profile Plant available water is considered to be low, based on an effective rooting depth of 45 cm. Surface (A) horizon The surface soil is strongly acid. However, the level of exchangeable aluminium is quite low (<10 mg/kg) and is unlikely to affect the growth of aluminium sensitive species, particularly as the soil becomes alkaline with depth. Lime can be used to increase soil pH but a pH/aluminium test is best performed by taking samples across the paddock and bulked together. The use of test strips to observe lime response could be used as an aid in evaluating its cost/benefit. Potassium levels are moderate and plant responses to potassium fertiliser is unlikely. Salinity is not a problem in the surface soil. Deficiencies of molybdenum and phosphorus may occur in the strongly acid surface soils and increasing the pH by lime application should enable the phosphorus (from superphosphate) and molybdenum to become more available. The nutrient holding capacity of the surface (A1) horizon is moderate (based on the sum of the exchangeable calcium, magnesium, potassium and sodium cations). Subsoil (B) horizons The pH increases with depth allowing the successful growing of deep rooting aluminium sensitive species (e.g. lucerne).The coarse strong prismatic nature of the clay B horizon confines the roots to cracks and makes it difficult for the roots to fully exploit the soil (See photo opposite). The deep subsoil is also sodic and slakes considerably when wet. The coarse strong prismatic nature of the clay B horizon confines the roots to cracks and makes it difficult for the roots to fully exploit the soil (See photo opposite). The deep subsoil is also sodic and slakes considerably when wet.

Site EG211

Location: Benambra (East of township)	Geo. Ref. 0563650E 5912350N
Australian soil classification: Eutrophic, Subnatric, Black SODOSOL	Black sodic texture contrast soil
General landscape description: Almost level plain
Geology: Pleistocene to recent alluvial deposits
Land use: Lucerne
Native vegetation: Montane grassland
Mapping unit: Benambra (Component 1)

Photo taken from pit looking towards the north-east. The narrow present flood plain of
Morass Creek is in the middle ground

Soil profile morphology

Surface soil
A1	0 – 25 cm	Very dark greyish brown (10YR3/2); clay loam; strong medium (10 -20 mm) granular structure; weak consistence dry; abrupt change to:
Subsoil
B21	25 – 55 cm	Very dark greyish brown (10YR3/2); medium clay; coarse (20 – 50 mm) strong columnar structure; strong consistence dry; diffuse change to:
B22	55 – 90 cm	Very dark greyish brown (10YR3/2) mottled with yellowish brown (10YR5/4); medium clay; coarse (20 – 50 mm) moderate prismatic structure; strong consistence dry; diffuse change to:
B23	90 – 115 cm	Greyish brown (10YR5/2) and yellowish brown (10YR5/6) with distinct very dark greyish brown (10YR3/2) mottles; medium clay; moderate coarse (20 – 50 mm) prismatic structure; very firm consistence dry.

Key profile features

• Texture contrast between the surface (A1) horizon and subsoil (B21) horizon

Soil profile characteristics

	pH	Salinity rating	Sodicity	Dispersion	Slaking
Surface soil (A1 horizon)	Strongly acid	Low	Non sodic	None	Water stable
Subsoil (B21 horizon)	Slightly alkaline	Very low	Sodic	None1	Water stable
Deeper subsoil 90-115 cm	Moderately alkaline	Low	Strongly sodic	Slight1	Slakes considerably

¹ Complete after remoulding

Horizon	Depth cm	Field texture	pH water	EC dS/m	Coarse sand %	Fine sand %	Silt %	Clay %	Field cap. % w/w	Wilt. point % w/w	PAW* water % w/w
A1	0-25	CL	5.0	0.17	9	33	24	23	35.9	12.9	23.0
B21	25-55	MC	6.5	0.06	11	29	19	36	30.9	15.9	25.0
B22	55-90	MC	7.8	0.09	3	27	23	41	38.9	18.9	20.0
B23	90-115	MC	8.2	0.16	12	36	17	34	33.4	16.2	17.2

* PAW is plant available water. This is the difference between the soil’s water content at field capacity and wilting point.

Depth	OrgC#	Total N	Exch. Al	Exchangeable cations meq/100 g							Ca:Mg	Na %
cm	%	%	ppm	Ca	Mg	K	Na	Total	H	CEC
0-25	4.0	0.32	17	7.0	3.7	0.52	0.24	11	11.0	22.0	1.9	1
25-55	1.1	0.08		6.0	6.9	0.2	1.3	14	7.4	21.4	0.9	6
55-90	0.66			6.2	7.3	0.25	2.2	16			0.9	14
90-115	0.39			6.1	6.4	0.16	3.3	16			0.9	21

# Organic carbon

Management considerations

Whole profile

• Plant available water is considered to be moderate, based on an effective rooting depth of 55 cm.

Surface (A) horizon

• The surface soil is strongly acid. However the level of exchangeable aluminium is low (17 mg/kg) and is unlikely to affect the growth of aluminium sensitive species, particularly as the soil becomes alkaline with depth. Lime can be used to increase soil pH but a pH/aluminium test is best performed by taking samples across the paddock and bulked together. The use of test strips to observe lime response could be used as an aid in evaluating its cost/benefit. At a pH of 5.0, a pH response would be expected.
• Potassium levels in the surface soil are moderate and plant responses to potassium fertiliser is unlikely.
• Salinity is not a problem in the surface soil.
• Deficiencies of molybdenum and phosphorus may occur in the strongly acid surface soils and increasing the pH by lime application should enable the phosphorus (from superphosphate) and molybdenum to become more available.
• The nutrient holding capacity of the surface (A1) horizon is moderate (based on the sum of the exchangeable calcium, magnesium, potassium and sodium cations).

Subsoil (B) horizons

• The pH increases with depth, thus allowing the successful growing of deep rooting aluminium sensitive species (e.g. lucerne).
• The coarse strong prismatic nature of the clay B horizon confines the roots to cracks and makes it difficult for the roots to fully exploit the soil. As the subsoil is sodic and the deep subsoil strongly sodic, root and downward water movement may be restricted.

Soil Conservation Authority, Profile No. 212

Location: Church Road, Benambra	Geo. Ref. 560800E, 590900N
Australian soil classification: Mottled-Sodic, Calcic, Black CHROMOSOL	Black texture contrast soil
General landscape description: Low lunette
Site description: Road reserve
Geology: Quaternary alluvial/aeolian deposits
Mapping unit: Benambra (Component 4)

No photo available

Soil profile morphology

Surface soil
A11	0-7 cm	Black (10YR2/1); fine sandy clay loam; moderate 0.5-4 cm angular blocky structure; clear smooth boundary to:
A12	7-15 cm	Black (10YR2/1); fine sandy light clay; moderate 0.5-4 cm angular blocky structure; clear smooth boundary to:
Subsoil
B21	15-30cm	Mottled lack and dark yellowish brown; heavy clay; strong 0.5-1 cm angular blocky structure; occasional 1 cm carbonate concretions; clear smooth boundary to:
B22	30-60 cm	Mottled black and dark yellowish brown; heavy clay; strong 1-5 cm blocky peds arranged in columns; occasional 2 cm carbonate concretions; clear smooth boundary to:
B23	60-100 cm +	Mottled olive and black; heavy clay; strong 0.5-2 cm subangular blocky peds weakly arranged in columns; calcium carbonate concretions up to 25 cm common – decreasing below 100 cm.

Soil profile characteristics

	pH	Salinity rating	Sodicity	Dispersion	Slaking
Surface soil (A1 horizon)	Moderately acid	Very low	Non sodic
Subsoil (B21 horizon)	Strongly alkaline	Medium	Non sodic
Deeper subsoil 90-115cm	Moderately alkaline	High	Sodic

Horizon	Depth cm	Field texture	pH water	EC dS/m	Coarse sand %	Fine sand %	Silt %	Clay %	Field cap. % w/w	Wilt. point % w/w	PAW* water % w/w	Free Fe2O3 % w/w
A1	0-7	FSCL	5.5	0.09	11	36	17	27
B21	15-30	HC	6.2	0.06	6	22	6	58
B22	30-60	HC	8.6	0.80
B23	60-110	HC	8.4	1.20	1	11	9	74

* PAW is plant available water. This is the difference between the soil’s water content at field capacity and wilting point.

Depth	OrgC#	Total N	Avail P	Avail K	Exch. Al	Exchangeable cations meq/100 g							Ca:Mg	Na %
cm	%	%	ppm	ppm	ppm	Ca	Mg	K	Na	Total	H	CEC
0-7	3.5	0.32				8.8	5.0	2.1	0.3	16.2			1.8	2
15-30	1.3	0.12				8.3	10.5	2.5	0.8	22.1			0.8	4
30-60	0.2	0.04				10.2	20.2	2.2	1.3	34.9			0.5	4
60-110						9.1	21.7	1.9	3.8	35.5			0.4	11

# Organic carbon