Tamar report land Capability Survey of Tasmania k e noble 1992 Tamar Report

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Photo 10:

Class 4 land on Permian sandstones, mudstones and siltstones.  Class 5 land on sandstones in

background.  Tamar map 828212.  Frankford Road.

Figure 8:

Relationship between land capability classes on sedimentary rock types.

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Class 4 land on Alluvium

Alluvial flats adjacent to streams and rivers which are subject to occasional flooding or

have poor internal drainage, have been mapped as Class 4.  The frequency of flooding can

vary, but is usually around two to three times per year.  The time of year when flooding

occurs is critical, as is the length of time the water stays on the land surface.  The risk of

flooding in these locations combined with either high water tables or slow internal soil

drainage, limit the cropping potential of this land.  Refer to Photo 11 for an example of

Class 4 land on alluvium.

Localities where this type of Class 4 land occur are: river flats of the Rubicon River and its

tributaries at Dunorlan, the Avenue Plains and Parkham Basin; Meander River flats and

tributaries at Weetah, Reedy Marsh, Shoulder of Mutton Plain, Egmont, Selbourne, Pipers

Lagoon and Pipers Lagoon Creek; Supply River flats and tributaries draining into the

Exeter Basin; Middle Arm Creek south west of Beaconsfield, Saxons Creek (West

Frankford), Westford and Eastford Creeks at Thirlstane, Johnston Creek (Flowery Gully)

and at Holwell.

Class 4 land on alluvium occurs on a wide range of soils depending on the nature of the

alluvial deposits.  On higher terraces soils can be deep with brown silty to sandy loam

textures.  Areas with very sandy textures are freely drained and can dry out rapidly during

dry periods.  In more poorly drained lower areas soils can be grey clay soils with mottled

and gleyed subsoils, or dark soils with heavy clay textures.  These soils are more prone to

surface ponding of water because of slow internal drainage.

Photo 11:

Class 4 land on alluvium.  Tamar map 787065.  Shoulder of Mutton Plain.

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The Supply Association is the major soil type mapped on this type of land in the

Beaconsfield Soil Survey (Dimmock, unpublished).  These soils generally have gradational

profiles with a dark grey, well developed granular silt loam, silty clay loam or fine sandy

clay loam surface; over mottled dark grey, massive silty clay or fine sandy clay.

Cropping is limited to occasional crops such as cereals and forage crops because of the soil

drainage characteristics.  Drainage (where economically feasible) would substantially

improve the present condition of some of the heavier clay soils, and would result in more

rapid removal of surface water and more intensive use of these areas.  Annual rainfall

ranges between 800 and 1 000 mm.

Other Class 4 land

Other areas of Class 4 land occur on dolerite, and sedimentary sandstones and siltstones of

the Mathinna Bed sequences.

Between Elizabeth Town and Christmas Hill an area of Class 4 land on dolerite has been

mapped.  Slopes are variable in this area, however the major limitation to agricultural use is

the amount of dolerite stones and rocks which can severely hinder cultivation.  The soils

are red krasnozems which are freely drained and friable.

Outcrops of Mathinna Beds occur in the north eastern corner of the map, around Lefroy and

Beechford.  Areas of this type of land are more extensive on the adjacent Pipers map.

The soils on these deposits are mostly grey podzolic soils.  They have a very sandy and

loose topsoil, which is highly susceptible to water erosion if the soil surface is disturbed

and left unprotected.  The soils have been leached of nutrients, and require high fertiliser

inputs including phosphorus and potassium.  Trace elements of molybdenum, copper and

selenium are also required.  These soils are generally very acid, with topsoil pH less than

5.0.  Areas around Lefroy have large amounts of quartz gravels present in the soils.  Nearer

the coast some lower areas have been covered by a veneer of windblown sand.

Slopes can range up to 18%.  Above this slope, the potential erosion hazard is considered

too severe for this type of land to be safely cultivated for cropping purposes, and has been

mapped as Class 5 land.  On the steeper slopes, intensive soil conservation measures such

as major earthworks would be needed, if cropping was undertaken.

This land is used mainly for grazing, with occasional crops such as root and green fodder

crops grown (turnips, chou moellier, etc).  The poor soil structure, low fertility and high

erosion hazard severely limit the cropping potential of this type of land.

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10.5 Class 5 (36 773 ha; 20.18%)

Class 5 land on Basalt

Steeper slopes on basalt (generally 30%) have been mapped as Class 5 land in the

Dunorlan-Gannons Hill, and Rosevale-Selbourne areas.  Photo 12 shows an example of this

type of Class 5 land.

These areas are usually steeper scarp edges around the more subdued and elevated

topography of the basalt flows.  In some areas the basalt colluvium, erosion debris and

associated soils form a veneer over underlying older rocks such as Permian mudstones and

sandstones or Tertiary clays and gravels.  Although the slopes are steep (in excess of 30%),

the soils are generally more stable than those on other rock types, and can be used for

grazing.  These slopes can be subject to sheet and mass movement erosion, but this does

not pose a major limitation to grazing use, as the fertile basalt soils tend to regrass rapidly.

Some areas also are prone to seepage which can increase the potential for landslide events,

particularly on cleared, steeper slopes, in areas recently cleared from forest, or undercut by

access tracks.

As well as the limitation of slope and erosion hazard, some areas also have significant

amounts of rock outcrops.  Average annual rainfall is around 800 - 900 mm at Rosevale,

and 1 000 mm at Dunorlan.

Photo 12:

Classs 5 land on basalt.  Tamar map 605133.  Railton Road.

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