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

52

Class 4 land on Clays, Sands and Gravels

Large areas of Tertiary age clay, sands and gravel deposits occur on the Tamar map.  The

major localities are East Sassafras-Port Sorell, Parkham Basin, Saxons Creek-Franklin

Rivulet, Birralee-Selbourne-Rosevale-Westwood areas, and terraces either side of the

Tamar River (Kelso, Clarence Point, Bell Bay, Ilfraville, Beaconsfield, Rowella, Hillwood,

Paper Beach, etc).

Younger deposits mapped as Quaternary age also occur at Doctors Flats, along Bridport

Road east of George Town, and include windblown coastal sands at Northdown Beach and

on the Cimitiere Plain, and windblown alluvial sands which occur in pockets along the

Meander River around Selbourne.

Soils are variable depending on the parent material on which they are formed.  The major

soil associations mapped in the Beaconsfield Soil Survey (Dimmock, unpublished), are the

Legana, York Town and Robigana Associations.

The majority of soils mapped in the Legana Association are duplex yellow podzolic soils

which have a strong lateritic influence.  Typical profiles may have a dark grey sandy loam

surface; overlying a pale compact sandy loam or sand A2 horizon containing ironstone

gravel; over a yellowish brown mottled clay.

The York Town soils are also yellow podzolics but have many quartz gravels associated

with them.  Typical profiles may have a dark grey loamy sand A1 horizon; over a light grey

sand A2; and a mottled yellowish brown clay B with organic staining and coatings in the

upper part.  Quartz gravels occur throughout the profile, concentrated in the A2 horizon.

The Robigana Association incorporates a wide range of soils including yellow podzolics,

podzols and black cracking clays.  In some areas basalt has contributed to, or underlies the

sandy parent material.  Laterite and siliceous gravels are also associated with many of these

soils.

Soils on windblown alluvial sands are similar to the Panshanger soils, and there are small

areas usually on terrace remnants that have soils which resemble the Brickendon,

Woodstock and Cressy soils which occur in greater extent south of the Tamar map in the

Launceston Tertiary Basin (refer to Quamby Soil Map, Nicolls, 1959).

In the East Sassafras-Port Sorell area typical soil profiles have a dark grey fine sandy loam

A1 horizon; over a bleached fine sand A2; over sandy clay.  These areas are often used for

cropping in conjunction with adjacent areas on basalt; however these soils are less

versatile, require higher fertiliser inputs and are more prone to erosion than the krasnozem

soils.

Topography is generally flat and easy rolling terrace country, although some slopes up to

around 15% do occur.  Annual rainfalls range between 700 and 1 000 mm.  Soil structural

decline on the sandy soils, low fertility, the potential for sheet and rill erosion, and the

shallow and gravelly nature of some of the soils, limit the cropping potential of this land.

Photos 8 and 9 show examples of this type of Class 4 land.

---

53

Photo 8:

Class 4 land on Tertiary clay, sand and gravel (foreground).  Classes 3 and 4 on basalt in

background.  Tamar map 603328.  Chapel Road.

Photo 9:

Class 4 land on terraces of Tertiary clay, sand and gravel.  Class 6 land on dolerite in

background.  Tamar map 900425.  North west of Rowella.

---

54

Class 4 land on sandstones, mudstones and siltstones

Class 4 land has been mapped on Permian age sandstones, mudstones and siltstones, and

Triassic age sandstones.  Slopes are variable, but can range up to 18%.  They occur on the

footslopes of steeper hill country, or as undulating slopes within larger valleys or basins

where erosion has resulted in low relief landforms.  Average annual rainfall ranges between

900 and 1 000 mm.

Localities where this type of Class 4 land occurs are: Winkleigh, Glengarry, Holwell,

Frankford, West Frankford, Wings Flats, Franklin Rivulet, Parramatta Creek, Parkham

Basin, Beaconsfield, and low rises within the Exeter Basin.

Soil associations mapped on the Beaconsfield Soil Survey (Dimmock, unpublished), are the

Warrina and Tatana Associations.  Soils can vary depending on which parent material they

have developed from, but are predominantly yellow podzolic soils.  Sandy podzols have

also been mapped within the Tatana Association.  The difference between the podzols and

podzolics depends on the degree of development of the organic B horizon - where it is well

developed, soils are classified as podzols; and where it is not strongly developed, but

organic coatings do occur, the soil is classified as a yellow podzolic.  Podzols are more

commonly developed on sandy or siliceous parent materials, where clay, iron and organic

matter are more easily transported and redeposited down the profile.

The Warrina Association soils are yellow podzolics with either duplex or gradational

profiles.  Duplex profiles are more common and tend to be shallower than gradational

profiles.  Siliceous gravels and stones can be common throughout and are often found

scattered over the soil surface.

A typical soil profile of the Warrina Association may have a dark grey friable fine sandy

loam A1 horizon; over a grey massive fine sandy loam A2; over a yellowish brown massive

fine sandy clay loam A3; and a yellowish brown mottled clay B horizon.  Quartz gravels

and waterworn siliceous pebbles are common in the surface horizons.

The Tatana Soil Association occurs on sandstones of both Triassic and Permian age.

Typical profiles have a dark grey loose sand A1 horizon; over a light brownish grey loose

sand A2; over dark brown cemented sand (organic B horizon); over a brown massive sandy

clay loam B horizon.

In poorly drained areas soils may be gradational with sandy loam topsoils, or humus

podzols (with well developed humus and iron B2 horizons).

Boundaries between the Tatana and Warrina Soil Associations are ill defined and complex

in areas of low relief.  The Tatana, Warrina and Supply Association soils form a complex

pattern within the Exeter Basin.

Because this type of land occurs on low angled slopes, the majority of it has been

developed for agricultural use.  However its potential for frequent cropping is limited

because of low fertility, poor soil structure particularly on the sandier soils, and the

potential for sheet, gully and rill erosion under cultivation.  Intensive cultivation on these

soils breaks down the structure of the A horizons, making them more vulnerable to erosion.

Photo 10 shows an example of this type of Class 4 land and Figure 8 illustrates the

sequence of land capability classes on these deposits.

---