Soil/Landform Unit - St Helens gentle plains

Gentle plains north-west from Mount Zero

Area: 32 072 ha

1.37% of CMA region

Gently undulating plains that lie either side of the Wimmera River valley centred on Horsham are known as the St Helens plains. This simple plain unit is part of the North West Dunefields and Plains; clay plains and subdued depressions divisions (5.4). These plains occupy the outwash slopes of the Grampians in the south and are bound by flats and drainage depressions. This unit also occupies the boundary between the clay plains to the south, the Wimmera River valley to the west and north-east of Horsham. Minor dunes occur on the plains. Grey cracking clays (

Vertosols

) with sodic red (occasionally brown) texture contrast soils (mainly on the dunes) are the major soil types. Waterlogging is a potential issue on the clay soils, while sheet and rill erosion is more significant on the dune slopes. This unit is used for dryland cropping, grazing (sheep, beef cattle) and some water supply function in some areas. Remnant vegetation communities recorded include woodlands.

Raised bed trials west of Horsham
	Cracking self-mulching soil surfaces

Component	1	2
Proportion of soil-landform unit	5%	95%
CLIMATE Rainfall (mm)	Annual: 480

Temperature (^oC)	Minimum 8, Maximum 21
Precipitation less than potential evapotranspiration	October–April
GEOLOGY Age and lithology	Quaternary fluvial silt, sand and minor gravel (Shepparton Formation); Neogene marine sand and silt (Parilla Sand); Quaternary alluvium gravel, sand and silt

Geomorphology	5.4 Clay plains with subdued ridges of the North Western Dunefields and Plains
LANDUSE	Uncleared areas: water supply; nature conservation Cleared areas: dryland cropping; sheep grazing; water supply

TOPOGRAPHY Landscape	Gentle plains

Elevation range (m)	113–155
Local relief (m)	2–3
Drainage pattern	Dendritic
Drainage density (km/km²)	0.4
Landform	Gently undulating plains
Landform element	Dune slope	Gently undulating plain
Slope and range (%)	4 (1-6)	2 (0-5)
Slope shape	Convex	Straight
NATIVE VEGETATION Ecological Vegetation Class	Plains Woodland (28.6%), Riverine Chenopod Woodland (23.7%), Plains Savannah (8.5%), Other (22.4%)

SOIL Parent material	Aeolian sand	Marine sand and silt, fluvial gravel, sand and silt

Description (Corangamite Soil Group)	Sodic red texture contrast soils (39)	Cracking clay soils (34)
Soil type sites	NA174	WIA6, WIA9, WIA5
Surface texture	Sandy clay loam	Medium clay
Permeability	Moderate	Slow to moderate
Depth (m)	> 3	> 3
LAND CHARACTERISTICS, POTENTIAL AND LIMITATIONS Critical land features, processes, forms	Hardsetting upper soil susceptible to wind erosion when exposed. Dispersive and deep subsoils susceptible to gully erosion on steeper slopes. Compaction of upper soil variable depending on soil depth and moisture content. Upper soil is susceptible to acidification.	High clay content results in restricted soil drainage, but can be variable due to micro relief (gilgai). Slightly susceptible to surface sheet erosion where slopes occur and gully erosion where dispersive subsoil is exposed. Surface soils are friable and only slightly susceptible to wind erosion due to self-mulching nature. Soils may be susceptible at critical moisture contents and have variable resilience to compaction.

Page top