Friday, July 20, 2012

My Favourite Soil: Floodplain Soils


Being invited to join the soilduck blog is hugely exciting and I firstly have to thank Jess for the privilege! As way of an introduction to me and my interests I thought the best way would be to share My Favourite Soil.

My Favourite Soil is floodplain soil. Specifically the floodplain soils found in the upper catchments of south east Australia. These are soils that, while not covering much area compared to the typical red and yellow duplex clay soils of the region, can be highly productive and were prised pasture lands for early settlers. They are usually highly organic (owing to the slow decomposition of organic matter in the seasonally to permanently waterlogged conditions) and have a high clay percentage (owing to the low-energy deposition environment of the floodplains).  Importantly, these soils form in a very particular set of landforms known as swampy meadows and/or chain-of-ponds. It’s these unique landforms and the soils that result that make them my favourite soil.

Tasty black soil and the famed lush pasture growth

Swampy meadows and chain-of-ponds have mostly been severely degraded due to inappropriate grazing and vegetation management. The vegetation that originally prevented major erosion in these floodplains was lost and the subsequent channel erosion drained the swamps and/or ponds. This left incised stream channels too deep to allow overbank flooding and the loss of the shallow groundwater. This means that there is little or no connection between the stream/groundwater and the floodplain soil. The soils of these degraded floodplains can remain productive, but without the connection to both the shallow groundwater and the stream the highly organic and nutrient rich nature of the soils are compromised and degraded over time. 

Relatively minor channel erosion. The disconnection between groundwater and soil is only about half a metre.  Channel erosion or incision can cut down many metres if the floodplain sediments are deep.

But too much talk makes my head hurt and leaves me with a burning desire to dig holes and get soil everywhere. So what better way to investigate these soils than with a technical classification key! Note that I will be foregoing the formal definitions here (as sometimes floodplain soils are technically just *gasp* sediments!). Below is “Nathan’s Slightly Facetious Guide to Identifying a Floodplain Soil in South East Australian Upper Catchments.”

1. Are you in south east Australian upper catchments? They can look like this:
Time for a nap next to the chain of ponds methinks. Note the orange hue to the water indicating a strong iron presence (from all the saturated sediments in the floodplain)

Or this:
A bit cold for a nap today, but this pond is about 10 m by 20 m and is home to a family of terrapins.
2. Walk down into the valley (you may need waders for this as these small floodplains are sometimes under 6-18 inches of water) and dig a hole/soil pit. Does it look like this?



My left ankle is not broken or dislocated despite the photographic evidence suggesting so. Tasty soil soup retrieved whilst mapping the floodplain surface sediments/soil. Note the shallow groundwater presence in the soil pit/hole.

Or this: 

Borderline self-mulching soil, if it ever got a chance to dry out completely that is.
3.  Do a field texture test/make mud pies/wait around for the water-logged soil to dry out. Is the soil anything from clay loam to a heavy clay (i.e. you can mould little sheep out of it if you want).
4. You have found a south east Australian upper catchment floodplain soil.
5.  Enjoy!

Thus concludes a small introduction byway of My Favourite Soil and I look forward to sharing more posts on soilduck!



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