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Feature Article - Agriculture and soils in Tasmania

FERTILITY

Despite usually having a moderate organic matter content, Tasmanian soils in their native state are not inherently fertile for productive agriculture. Phosphorus and molybdenum are commonly deficient, while soil acidity can limit the growth of some plants. As in most of agricultural Australia, superphosphate has been widely used in Tasmania and, as in many areas with high rainfall and acid soils, molybdenum and lime have periodically been added.

The establishment of improved clover with grass pastures on many soils has increased soil nitrogen. Many soils in Tasmania’s agricultural areas are low in potassium, so potash fertilisers are commonly used, particularly where rainfall is high or production and crop removal is intensive (e.g. dairy farming and vegetable cropping).

While initial soil fertility at the time of white settlement is largely unknown, it appears that the majority of soils have probably increased or maintained their fertility since the introduction of the fertiliser and cultural practices just described.

Unlike many other acidic soil areas in Australia, liming has been common in Tasmania, often to increase soil pH above its natural level. Lime is widely available and therefore relatively cheap to transport. Many crops grown in Tasmania respond to lime, including onions, poppies and barley.

STRUCTURE

Some soils, particularly the ferrosols (the red soils of the north-west and north-east, previously known as krasnozems) are of world-class quality because of their free drainage and good structure, but they can be easily degraded by compaction and water erosion.

Areas rated as potentially susceptible to severe soil-structure decline are those primarily used for vegetable cropping. Areas rated as moderate are those used for frequent cropping or forestry operations, and the remainder of the State’s private land, which is mainly used for pasture, is shown as having little or no soil structure decline hazard.

SOIL ACIDITY

There are a number of reasons why acidification of Tasmanian soils does not appear to be as rapid as elsewhere in southern Australia. Firstly, Tasmanian soils are already naturally acidic. Secondly, the generally moderate organic-matter in Tasmanian soils buffers them more against pH change. Thirdly, Tasmanian pastures generally contain a perennial grass that helps to restrict the loss of soil nitrate through leaching, a mechanism generally held to contribute to acidification in other parts of Australia.

SALINITY

Apart from the Tamar Valley, King Island and Flinders Island, salinity is restricted to land systems with annual rainfall less than 750 mm and land systems cleared for grazing and infrequent cropping. Confirmed saline areas are therefore less than 1% of the total agricultural area of Tasmania. This is a relatively small problem compared with mainland states such as Victoria, South Australia and Western Australia.

WATER EROSION

There has been no complete assessment of the extent, severity and rate of soil erosion in Tasmania. Areas of concern are:

• the red basalt soils of the north-west;

• the shallow soils on dolerite hills in the Midlands and Derwent Valley, where the north-facing slopes are denuded of vegetation in the summer and are then susceptible to sheet and rill erosion; and

• the soils derived from sandstone and mudstone in south-eastern and northern Tasmania; these have unstable subsoils conducive to both tunnel and gully erosion.