soil excursion - wk 17

Today we head off on a soil excursion with Marcus and Laurie to improve our knowledge of the different types of rock and soils around the Hobart region. We will be taking soil samples from each area to analyse and assess at a later date.

Stop 1  - Mudstone

Our first convoy stop is a mudstone cutting on cascade rd heading towards the mountain. Here we look at the mudstone parent rock profile and the surrounding Silver Peppermint forest that is found on this soil type. Also here we find the Bush Cherry (Exocarpos cupressiformis) which is an hemiparasitic tree.  Marcus points out how we can recognise different rock/soil types based on the dominant plant growth in the area.

All the Eucalyptus tenuiramis 'Silver Peppermint' growing

in this area thrive in the mudstone soil type .

Mudstone is identifiable by it's white, chalky colour 
and how it breaks apart easily into layers. 
The mudstone here also has a sulphur smell due to 
being laid down in anaerobic conditions.

The groups checks out the roadside cutting.

Being a sedimentary rock type, it is laid down
over the years in layers that can easily be seen here.

We take a soil sample from the shallow profile to find what Laurie describes as a 'duplex' profile. This is where the soil colour changes suddenly.  At the deepest tip of the sample the soil turns from white to orange. Mudstone soils typically have a pH of around 5 - 6. Sedimentary parent rock layers such as mudstone, sandstone and limestone are laid down over thousands of years of melting and erosion.

Mudstone soil profile.

Stop 2 - Mudstone Mt Wellington

Next stop was further up the road at the base of Mt Wellington where there is a fantastic cutting in the roadside carpark. Here there is a much higher rainfall and so a different concentration of plants thrive here.

This is a great photo to highlight the years and
years of sedimentary layers that have built up .

Stop 3 - Dolerite

Next up we park on the side of Summerleas Rd to eye spy a large exposed dolerite hillside. Here we observe the rounded red dolerite boulders that Marcus explains are intruded igneous (volcanic) rock that was formed and cooled under immense pressure but have been forced up from deep below during volcanic action many hundreds of years ago. Dolerite crushed rock has a pH of 8-9 where the dolerite soil is around 6.5 - 6.8 pH.

Dolerite hillside.

Now the rocks are up on the surface and no longer under the same kind of pressure, they literally start to expand and peel away from the bluestone core like an egg. A process known as exfoliating.

A classic 'egg' shaped dolerite rock showing a hard bluestone centre,
with aged outer layers that will easily peel and break off. 

The outer layers of dolerite rocks are more red in colour because they have been exposed to the rain and the high iron content in the rock is beginning to oxidise or 'rust'. Dolerite soil colour is typically browny/black. Dolerite rock is high in minerals and the rock dust can be beneficially added to compost or to contribute to clay soils.

Dolerite profile.

We take a dolerite soil sample and Laurie explains that this profile is more of a 'gradational' one which means that there are no clearly defined layers, more of an even gradual mix of the profile as it goes down. Trees that are happy growing on this bedrock type include Eucalyptus globulus (Bluegum), Eucalyptus pulchella (Narrow-Leaf Peppermint) and the registered weed Spanish Heath (Erica lusitanica).

Stop 4 - Basalt & Sandstone

Our next stop is another cutting just up from the Kingston on the Huon Hwy. Here we can see a section of Basalt bedrock that stops abruptly and turns into Sandstone. The Verbascum weeds growing along the top of the bank that prefer the volcanic soil also stop suddenly.

Here the Basalt soil on the right stops in the middle
and the pink sandstone on the left begins.

Basalt rock.

Marcus explains that Basalt is also an Igneous rock but 'extruded' rather than the 'intruded' Dolerite. Extruded means it is formed on the surface -  basically it is quickly cooled volcanic lava. It will also exfoliate and has a high iron content. Very high in nutrients, the basalt soils are the rich red volcanic soils that dominate the agriculture north of the state. There is also a large dolerite pocket in the valley near Richmond heading out to Campania. This is a blacker soil because of the lower rainfall and has a higher salt content. When the soil dries out it will crack or become sticky in the rain. Will benefit from the addition of gypsum.

Basalt is clearly identified by it's porous appearance.

The soil we take here is also a even gradational profile and you can tell it is a much richer, more desirable type than our previous two samples. Basalt soil is typically 6.5 - 6.8 pH, brownish and crumbly in texture with good aggregates - ideal for growing. It is a bit dangerous to cross the highway and take a sandstone profile but Laurie assures us he will do this later.

Basalt soil profile.

Stop 5 - Alluvial

Just a bunch of soil nerds hanging out in Browns Rivulet.

Laurie and Marcus are not entirely happy with the profile we find here. There has been some flooding in previous weeks and a lot of debris has been washed up alongside the banks. A better spot for a sample is perhaps on the other side of the river and seeing as no one brought their waders (or swimmers!) we are making do with this one and pop it in the bag for testing later. 

The dark Alluvial profile.

So that's it. We have come to the end - it's been a great day all up and I feel like I have a MUCH better understanding of the different types and how to identify them. I'm looking forward to getting into all the samples and testing them. Will have to wait till next week though.........