January Mottled Brown Sodosol - Richard MacEwan CPSS-3 Federal President 2009 to 2012
During the International Year of Soils, each month we ask a soil researcher to select his or her favourite soil. First up is Richard MacEwan a past president of the Soil Science Australia and a member of the Victorian Branch. Richard is a pedologist who did his training in Reading, UK and has worked as a government research scientist and university lecturer in Australia since 1989.
If you had to choose a special Australian soil type what would it be?
Richard MacEwan – The Mottled Brown Sodosol (the State soil for Victoria)
What are the main properties of this soil type?
The Mottled Brown Sodosol is a ‘texture contrast soil’ that has an obvious boundary (sharp, abrupt or clear) between a lighter textured surface horizon (e.g. sandy loam or clay loam) and a heavier textured (clay) subsoil horizon. The subsoil horizons are dominantly brown to yellow and typically have mixed mottling of yellow, grey, brown or red; they are sodic and have massive or coarse structure (columnar or prismatic).
These soils also generally have a bleached subsurface (A2) horizon that has been leached of most of its nutrients. It is not uncommon, in some parts of Victoria, to have abundant ferro-manganiferous gravels and buckshot in the A2 horizon. The Mottled Brown Sodosol belongs to the Brown Duplex and Yellow Duplex (or Dy3.4) soils of the former Northcote soil classification key.
Why do you find this soil type particularly interesting?
Since coming to Australia, much of my field work and research into soil management has been on these and closely related soils in the Chromosol order and I find all Australia’s texture contrast soils fascinating. Coming from Scotland, I had never encountered such extreme profiles before although the classic Podzol, Spodosol or Podosol, so common in the cool temperate regions of the Northern hemisphere, is very striking visually and is usually everybody’s favourite soil and has some visual parallels, particularly the bleached A2. Because of the A2 presence, early classifications of the texture contrast soils borrowed the terminology of the Europeans and called some of these ‘podzolic’, although the pedogenesis of the European podzols and the Australian texture contrast soils is very different. There continue to be many debates concerning the genesis of the strong texture difference between surface (A) and subsurface (B) horizons, the origins of buckshot and Fe-Mn gravels and the development of sodicity. A wonderful soil to debate – one pit, two pedologists, three opinions. Many happy hours have been spent picking away at profiles and wondering at their formation. They are also highly variable at local and regional scales so there is always something new to look at.
Do the properties of this soil type have consequences for its management, e.g. in terms of land use, soil quality, conservation, …
Of all of the texture contrast soils the Mottled Brown Sodosol is a particularly nasty beast in certain conditions and especially if you don’t treat it nicely, so it is a really good subject for the researcher interested in solving management problems, dealing with soil conservation and helping to improve agricultural production. It certainly presents plenty of challenges. The A2 horizons are particularly pernicious being very hard set when dry and weak and ‘spewy’ when wet, causing them to belong to a broader group of ‘SundaySols’, too wet to cultivate on Saturday, too dry on Monday. It is also a very common soil in Australia, particularly in the south-east and south-west, extensively represented in the higher rainfall agricultural land of Victoria and Western Australia. Land uses include dairy, sheep and beef, and cropping.
Waterlogging, sheet, rill, gully and tunnel erosion are common problems in landscapes with these soils, particularly as their occurrence is associated with higher rainfall (>500 mm MAR). The clay subsoil presents a throttle to downward water movement so the A2 horizons quickly become saturated after rain. Water ponding in the A2 moves laterally so the waterlogging becomes more severe as you traverse downslope. A nick or weakness in the soil surface can initiate rill erosion which can quickly develop into gullying in heavy rainfall events. The sodicity and dispersive behaviour of the clay subsoil exacerbates this and tunnel erosion may also develop. Good surface drainage, maintenance of ground cover and care during wet conditions are critical management requirements for this soil.
Can you tell us your most memorable story concerning this soil type?
There are several memorable stories that I have of this soil type and most of them involve waterlogging and getting bogged. And although I have strong memories, somehow the lessons are not really learned! In 1990 while doing mole drainage research we had occasion to totally bog our large tractor while demonstrating how the mole plough worked. In 1995 doing soil survey I managed to bog our land cruiser on the return trip down a slope. We tried every trick to get out of trouble but in the end decided we should call on the farmer. I walked a couple of km to the farm but nobody was there, however the 4WD tractor was there, with a key and a radio. I called the farmer and he said to go ahead and borrow the tractor. So I set off back to rescue our bogged cruiser. Half an hour later I was on the radio again… ‘Kevin, I am very sorry but now I have your tractor bogged’. Obligingly Kevin turned up and rescued us with his second tractor, but not without some difficulty, and of course a lot of embarrassment on my part. In 2010 the long drought broke and the grass needed mowing on our block, so at the most hydropedologically predictable position in the landscape I got my Yanmar bogged. I called on the neighbour to help me out….. (see picture below) .. it was another 10 days before we freed both tractors.