Soil/Landform Unit 92

Gentle undulating plains with defined dissection at the base of the Otway Ranges near Irrewillipe

Area:

10 367 ha

0.78% of CMA region

Adjacent to the ferruginised plateau around Simpson and at a similar elevation, a gently undulating plain without ferruginous ironstone extends eastwards towards Barongarook. The parent material is mainly Neogene sand and clay, with some minor redistribution on sand veneers in parts, and outcrops of deeply weathered

Cretaceous

sandstone along the sides of some of the drainage lines. The soils exhibit similar mottling and deep weathering to those found in Unit

181

, and are prone to nutrient deficiencies and phosphate fixation. Open forests of

Eucalyptus obliqua

occur over most of the landscape, although

E. baxteri

tends to dominate on the polygenetic soils with hardpans.

Acacia mucronata

acts as a strong indicator of the presence of Cretaceous outcrops. Most areas remain uncleared and are selectively logged for hardwood timber, although most timber is of insufficient size to provide good sawlogs.

Cleared land combined with dispersible subsoils are particularly prone to gully erosion
	Native vegetation of lower slopes

Component	1	2	3	4	5
Proportion of soil-landform unit	45%	7%	25%	15%	8%
CLIMATE Rainfall (mm)	Annual: 850-950, lowest January (40), highest August (20)
Temperature (^oC)	Annual: 13, lowest July (8), highest February (18)
Seasonal growth limitations	Temperature: less than 10^oC (av.) June-August Precipitation: less than potential evapotranspiration late October-March
GEOLOGY Age and lithology	Paleocene marine unconsolidated clay, silt and sand				Lower Cretaceous feldspathic sandstone and siltstone
Geomorphology	3.3.2 Dissected rolling low hills of the Southern Uplands
LANDUSE	Uncleared areas: Hardwood forestry for sawlogs, posts and poles; water supply; nature conservation; gravel extraction Minor cleared areas: Beef and cattle grazing; dairy farming
TOPOGRAPHY Landscape	Undulating plain in the north of the Gellibrand River catchment
Elevation range (m)	120-340
Local relief (m)	30
Drainage pattern	Parallel and dendritic
Drainage density (km/km²)	1.2
Landform	Undulating plain
Landform element	Crest and upper slope	Colluvial fans and depressions	Slopes	Lower slopes	Lower slope and drainage line
Slope and range (%)	7 (0-12)	4 (0-7)	7 (1-16)	10 (4-14)	10 (4-14)
Slope shape	Convex	Concave	Convex	Linear	Linear
NATIVE VEGETATION Structure	Open forest	Open woodland	Open forest	Open forest	Open forest
Dominant species	E. obliqua, E. radiata, E. baxteri, occasionally E. ovata, E. viminalis, E. aromaphloia	E. baxteri, E. ovata, E. nitida	E. baxteri, E. radiata, E. ovata, E. obliqua, occasionally E. aromaphloia	E. obliqua, E. radiata, E. ovata, E. baxteri	E. obliqua, E. ovata, E. radiata, E. aromaphloia
SOIL Parent material	Clay, silt and sand	Colluvial sand on sand, silt and clay	Colluvial sand on sand, silt and clay	Clay, silt and sand	In situ weathered sandstone and siltstone
Description (Corangamite Soil Group)	Mottled yellow and red gradational soils (29)	Grey sand soils, weakly structured clay underlay (29)	Grey sand soils, structured clay underlay (29)	Yellow-brown gradational soils, coarse structure (29, 17)	Yellow-brown gradational soils, coarse structure (29, 17)
Soil type sites	-	OTR742	OTR737	OTR750	OTR750
Surface texture	Sandy loam	Sandy loam	Sandy loam	Sandy loam	Fine sandy clay loam
Permeability	Moderate	Low	Low	Low	Low
Depth (m)	>2	>2	>2	>2	1.5
LAND CHARACTERISTICS, POTENTIAL AND LIMITATIONS	Low inherent fertility and phosphorus fixation lead to nutrient decline.	Low permeability and perched watertables lead to seasonal waterlogging and soil compaction.	Low inherent fertility and leaching of permeable surfaces lead to nutrient decline. Low permeabilities lead to seasonal waterlogging and soil compaction.	Dispersible clay subsoils of low permeability are prone to gully erosion. Steeper slopes are prone to sheet erosion.	Dispersible subsoils receiving runoff from adjacent areas are prone to gully erosion.

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