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

39

Nye, P.B. & Blake, F., 1938,  The Geology and Mineral Deposits of Tasmania.

Department of Mines, Geological Survey Bulletin, No. 44.  Gove,rnment Printer, Hobart.

Pinkard, G.J., 1980,  Land Systems Survey of Tasmania, Region 4. Tasmanian Department

of Agriculture, Hobart.

Scanlon, A.P., Fish, G.J. & Yaxley, M.L., 1990, Behind the Scenery - Tasmania's

landforms and geology.  Department of Education and the Arts, Tasmania.

Spry, A. & Banks, M.R., 1962, The Geology of Tasmania. J. Geol. Soc. Aust., 9(2).

Sutherland, F.L., 1969, The mineralogy, petrochemistry and magmatic history of the Tamar

lavas, Northern Tasmania.  Pap. and Proc. Roy. Soc. Tasm. Vol. 103:17-33.

Sutherland, F.L., 1971, The Geology and Petrology of the Tertiary Volcanic Rocks of the

Tamar Trough, Northern Tasmania.  Records of the Queen Victoria Museum, Launceston,

No. 36.

9.5

Soils

The soils of the area are very diverse, resulting from the highly variable geology.  They

have a complex distribution pattern and have variable profile types which range in colour,

depth, structure and texture.

Soil information is available in a number of maps and reports.  The published George Town

area soil map covers the area between Low Head, Bell Bay, Lefroy and Beechford (Nicolls,

1957).  The unpublished Beaconsfield Soil Survey covers the Beaconsfield Municipality,

which includes approximately one third of the map area (Dimmock, unpublsihed).  A

reconnaissance extension of this survey has been mapped covering the Tamar and part of

Pipers 1:100 000 map sheets, however this information is also unpublished as work on this

map was never completed (Kershaw, unpublished).  The unpublished soil survey of part of

the Parish of Lewis covers the north eastern corner of the map, east of Beechford

(Stephens, unpublished).

Generally speaking, the majority of soils on the Tamar map are poor for cropping use

because of poor structure, low fertility or stoniness.  However the areas on red basalt soils

around Thirlstane - Sassafras, Moltema - High Plains and Selbourne - Westwood have good

structure which can withstand regular cropping, although they have low to moderate

chemical fertility.  (See Section 10.1 for further description of krasnozem soils.)

In the Thirlstane - Sassafras area where the basalt is interlayered between the Tertiary

clays, sands and gravels, a very complex soil pattern occurs (refer to Figure 7).  Below the

basalt outcrops, areas of basalt colluvium are mixed with grey podzolic soils, resulting in a

mosaic of kraznozem soils, intergrade or transitional soils which are not true krasnozems,

and grey podzolic soils.  This soil pattern is visually very evident when these areas are

cultivated.  An example is shown in Photo 3.

Podzolic soils are fairly extensive throughout the Tamar map area.  Podzolic soils are

characterised by a sandy, leached A2 horizon, with clay accumulation in the B horizon, and

are also referred to as duplex soils (soils with clayey subsoils).  They occur on a wide range

of parent materials including sandstones, siltstones, quartzites, slates and sand, gravel and

clay deposits.  These duplex soils are relatively infertile and are low in nitrogen,

phosphorus, potassium and molybdenum.  Some areas also have copper and selenium

deficiencies which can affect animal production.  They are generally very acid with topsoil

pH less than 5.0, and require liming and relatively high fertiliser inputs to be farmed

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successfully.  They also have poor soil structure and low organic matter levels, which

render them prone to wind and water erosion and unsuitable for general cropping use.

Soils on mudstones have high clay contents in subsoils with poor structure which tend to

make them prone to waterlogging.  In addition topsoils can also be highly erodible and B

horizons are often dispersive.

Soils on dolerite are variable in terms of soil depth and the amount of stones and boulders

present throughout the profile.  Topsoils are generally grey or grey-brown sandy or silt

loams, over heavier clay subsoils.  Most of the dolerite soils are imperfectly drained with

slow permeability.  The major limitation to cropping of the soils on dolerite is the amount

of rock outcrops, the presence of surface and subsurface stones and shallow soil depths.

The dolerite soils are moderately acid with topsoil pH generally between 5.0 and 6.0.  The

majority of soils on dolerite are grey-brown podzolic soils.  Small areas of lateritic podzolic

soils also occur, which have an accumulation of ironstone gravel in the A2 and upper B

horizons.  Some areas of krasnozem soils occur under higher rainfall, and these soils have

strong red colours, deep profiles, and are friable and well drained.

Podzols occur on the windblown sand deposits near the coast. These soils usually have a

dark peaty topsoil overlying a strongly leached light grey sand (A2 horizon), and a

brownish black organic-iron layer which may form a cemented pan.  Iron and organic

matter has been leached from the A2 horizon and deposited in the B horizon.  The sand

may continue for many metres in depth below this pan.  These soils are highly susceptible

to wind erosion and are unsuitable for cropping because of poor soil structure, low moisture

holding capacity and low fertility.  Many of these soils, especially those in interdune

depressions or drainage lines, are also waterlogged during winter.

Podzol soils may also occur on Tertiary sands and sandy gravel deposits, Triassic

sandstones, and Ordivician and Cambrian quartz sandstones and quartzites.

Recent alluvial soils which occur along the major river and stream valleys, are generally

deep and have a range of textures from sandy loams to heavy clays.  Some of these alluvial

soils would be well suited to cropping in areas not subject to regular flooding or frosts, and

many areas would benefit from the installation of drainage.  The alluvial soils are usually

moderately acid, with pH around 5.5 - 6.0.

Some of the soils in the Selbourne - Rosevale - Westwood area, and on a remnant terrace

near George Town, are similar to the Woodstock, Brickendon, Cressy and Brumby Soil

Associations which are found more extensively further south in the Launceston Tertiary

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

Some of the soil associations referred to in the text relate to soils from the Quamby Soil

Map.  Table 7 summarises the major characteristics of the Soil Associations mapped within

the Beaconsfield Municipality.  The unpublished report which accompanies the

Beaconsfield Soil Survey gives further information on the Soil Associations, including

profile descriptions and laboratory data.

A copy of the draft Soil Association map of the Beaconsfield Municipality is included in

the rear of this report.  Special attention should be given to the fact that this soil map was

prepared and printed as a draft, and was never published.

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