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

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10.3 Class 3 (10 061ha; 5.52%)

Class 3 land on basalt

Areas of Class 3 land on basalt occur in the following localities: Thirlstane-Harford-East

Sassafras, Moltema-Dunorlan-High Plains, and in the Selbourne-Westwood area.  Refer to

Photo 6 and Table 8.

As with Class 2 land on basalt, the areas in the north western corner of the map have a

more favourable climate for crop growing than areas in the south.

The areas mapped as Class 3 land on basalt in the Thirlstane-Harford-East Sassafras area

are part of the Tertiary basalt flows, and soils are similar to those on Classes 1 and 2 on

basalt.

The soils on basalt in these areas are deep krasnozems with well drained profiles and good

soil structure.  Refer to Section 10.1 for a description of krasnozem soils.  They are suitable

for cash cropping but because of the higher slope angles, cultivation and crop rotations

should be limited in order to maintain soil structure, prevent loss of top soil by erosion, and

to preserve the long term sustainability of this land for cropping.

Slopes are steeper than those on Class 2 land, ranging between 12 and 18%, with the result

that the land is more susceptible to water erosion under an intensive cropping regime.

Therefore more intensive soil conservation works are required than that on Class 2 land,

and careful soil management practices are necessary when cropping is carried out.  These

measures would include grassed waterways and cut off drains at frequent intervals to

intercept surface water flow, and the use of cover and green manure crops to maintain

organic matter content and to protect soils from sheet, rill and gully erosion.

At present most of the areas in the Thirlstane-Harford-East Sassafras area on krasnozem

soils are being used intensively for cropping purposes.  Rainfalls in this area are between

800 and 900 mm.

The areas of Class 3 land on basalt in the southern half of the map are also part of the

Tertiary basalt flows, but are a little more variable in terms of the range of slope angles,

stone content, soil depth and soil type, than those nearer the coast.  These differences may

be a reflection of different mineralogical composition, rates of weathering, climate or

drainage properties.  Stony krasnozems tend to be more common in areas inland from the

coast, and some are relatively shallow.  In some areas the amount and size of stone is a

limitation to cultivation.

Soils are variable with areas of typical krasnozem soils as well as brown and black soils on

basalt.  These brown and black soils reflect poorer site drainage due to run-on and ground

water seepages.

The effects of climate in this area (frosts and shorter growing seasons), reduce the range

and yields of crops in comparison to the areas in the north.  Rainfall ranges between 700

and 800 m in the Selbourne-Westwood area, and around 1 000 mm in the Moltema-

Dunorlan area.

Slopes can range up to 18%.  Because of the additional limitation of stoniness and climate,

some slopes less than 12% are mapped as Class 3 land.

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50

Photo 6:

Class 3 land on basalt.  Tamar map 593329.  Chapel Road.

Other Class 3 land

Around the margins of the basalt flows and outcrops in the Thirlstane-Harford-East

Sassafras area, and in small areas at Selbourne and Westwood, are areas of shallow

krasnozems, and basalt colluvium admixed with other soils, which form a complex mosaic

(Refer to Figure 8).  The soils in this area are not true krasnozems, and although they can

be used for cropping, they cannot withstand the same intensity of cropping as the

krasnozem soils because of their inferior soil structure. Soil profiles are variable in depth

and texture. They are not as free draining as krasnozems and often have impeded

subsurface drainage, particularly on flatter areas.  Slopes can range up to 18%, with some

areas on the lower basalt outcrops occurring as flat or undulating terraces.

Other areas of Class 3 land occur around the Quamby Bend-Westwood area on terraces

with lateritic podzolic soils of the Cressy Association (Refer to Quamby Soil Map, Nicolls,

1959).  The Cressy soils have a loam to clay loam surface texture, overlying friable clay.

Ironstone gravel can occur throughout the profile.  The Cressy soils in this area often merge

with the basalt soils, and may be  difficult to differentiate across the boundaries.  The

Cressy soils have the potential to be used for cropping, and in the past some areas have

suffered from over cultivation.  Therefore care needs to be taken in the management of

these soils, particularly in the area of improving soil structure.  Rainfall in this area is

around 700 mm and climate (low rainfall and winter frosts) is an additional factor in the

capability assessment of this land.

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51

10.4 Class 4 (56 953 ha; 31.26%)

Class 4 land on Basalt

Class 4 land on basalt occurs in the Moltema-Dunorlan, Egmont Bridge, and Sassafras-

Harford-Thirlstane areas.  The major limitations to cropping in these areas are slope, rock

outcrops or stone content, and/or climate (Refer to Table 8 and Photo 7).

Small areas on steeper slopes (18-30%) occur in the East Sassafras-Harford-Thirlstane

areas.  The area that they occupy is limited and they often occur alongside areas of

intergrade soils which are more erosion prone and require more careful management, than

the basalt soils.

If cultivation is carried out on these steeper slopes, major soil conservation works and

careful soil management practices are necessary.  This type of land is only suitable for

occasional cropping (one to two years in ten).  Although the basalt soils have excellent

structure, frequent cropping on these steeper slopes may result in a deterioration of soil

structure and induce high levels of soil erosion (sheet, rill and gully).  Annual rainfalls in

this area are between 800 and 900 mm.

The areas of this type of land that occur in the Moltema-Dunorlan area (e.g. Blackamoor

Head, Brumbys Folly, Brooklyn Road) have a combination of slope, stoniness and climate

limitations.  Slopes can vary up to 30%.  The stone content of the soil is also variable and

limits cultivation in some areas.  The cropping versatility of land in this area is more

restricted than areas nearer the coast, because of frost hazard and shorter growing seasons.

Rainfalls are around 1 000 mm per annum.

Photo 7:

Class 4 land on basalt (background).  Class 3 land on basalt in foreground.  Tamar map

595103.  Brooklyn Road.

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