IYS Soil of the Month - September

Jason Hill, Soil Science Australia member, began his soil landscape career in the Territory in 1995 where he has been based since. He is currently the Northern Territory representative on the National Committee on Soil and Terrain.

Floodplain soil pitA flooded Top End coastal floodplain (insert) and the same environment (Aquic Vertosol) during the late dry season. Note the saturated gleyed marine clay deep in the profile. There is never a dull moment on the coastal floodplains of the Top End. The teeming wildlife including abundant birdlife, saltwater crocodiles, huge Melaleuca forests combined with excellent barramundi fishing (after a hard day down a humid soil pit of course) makes it a pleasurable soil landscape to spend time.  Soil field work on the coastal floodplains is generally restricted to a 3-4 week window of opportunity very late in the dry season (October) when the soils have dried sufficiently and before the onset of the next wet season.

If you had to choose a special Australian soil type what would it be?

The sodic Aquic Vertosols and Hydrosols of the Top End coastal floodplains have a special place for traditional owners, pastoralists, fishing enthusiasts, bird watchers, geomorphologists and soil-landscape scientists alike. These dynamic floodplains can be inundated to 1-2 metres for weeks at a time during the height of a monsoon, but then over many months the floodwaters retreat leaving in some parts a cracked parched “bulldust” plain. The Holocene floodplain soils have developed over a former mangrove shoreline when it extended inland during the last post glacial melt. These unique soils now only occur below 5 m above sea level, yet can still extend 60 km inland from the coast due to extremely low slope gradients.

What are the main properties of this soil type?

These soils vary but are generally alluvial black clays, sometimes organic overlying an oxidised brown clay layer (formally marine) and saturated gleyed mangrove clay at depth.   In the back swamps and paleo channels, organic peat can sometimes accumulate and as a result Organosols have developed over the gleyed marine clay.

Map NT Coasta Floodplains


Why do you find this soil type particularly interesting?

The dark organic clays are a sharp contrast to the typically sandy, gravelly upland cretaceous landscapes of the Top End. The floodplain soils also provide an example of relatively recent geomorphic events with dark alluvial clays over gleyed marine sediments in which shells and old mangrove roots several thousand years old are common.

Organic ProfileThree common variations of coastal floodplain soils (from L to R), (i) a Sulfuric Hydrosol in a floodplain depression with jarosite and dark organic material at depth, (ii) Peat with approximately 10-15% OC immediately overlying gleyed mangrove clay and (iii) black organic clay (Peaty Hydrosol) over brown oxidised clay (formerly marine) further inland.

Do the properties of this soil type have consequences for its management, e.g. in terms of land use, soil quality, conservation, …

The diverse wildlife dependant on these coastal floodplain soils has made this landscape very important for Indigenous people, the pastoral and tourism industries, conservation programs as well as a large recreational fishing community.

Floodplain WildlifeA coastal floodplain with abundant wildlife early in the dry season (May)Lessons learnt from past attempts at rice farming combined with annual flooding, salinity and acid sulfate soils have probably ensured these soil-landscapes will continue to be used for low impact land uses. The biggest threat in recent times has been salt water intrusion from the coastline.

Can you tell us your most memorable story concerning this soil type?

There is definitely no shortage of stories from soil scientists working on the coastal floodplains. We’ve had field staff get vehicles so seriously bogged that as a last resort they lit a signal fire. The trouble was the fire got out of control and ended up burning out a large part of the floodplain.

I remember describing and sampling a soil adjacent to a floodplain channel when we suddenly heard all this splashing nearby. We jumped out of the pit and saw two large saltwater crocodiles fighting in the channel. And of course there is always the charging wild buffalo and feral pig stories …

I also remember reading an old floodplain soil site card from the 1960s in which the field notes stated “structure difficult to describe probably due to the gelignite used”. Well I guess in days gone by when OH&S wasn’t as important as it is today this was one way of exposing a hard setting profile.

Finally, while digging a soil pit late in the dry with an excavator specially designed for floodplain operation we noticed the ground vibrating. We quickly realised that although the surface soil was dry we were effectively standing on a “jelly” soil base consisting of a saturated marine clay.

Distraught Soil ScientistA distraught soil scientist bogged on a treeless coastal floodplain in the early 1980s. These soils are often still saturated even though the surface is dry and cracking, catching out many unsuspecting soil scientists over the years.





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Soil Science Australia is a not-for-profit organisation that serves as the peak body for soil scientists across Australia, and seeks to advance soil science in the professional, academic and technical fields.  It is a member of the International Union of Soil Sciences



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