Dirt by definition is a filthy or soiling substance such as mud, dust, or grime; it is a form of excrement. Whereas soil is “the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles”, which is a sentence but it doesn’t really explain where it comes from. 

Australia has some of the oldest and most weathered soils in the world, and you may wonder how dirt can be different ages? Well, the age of soil relates to how it is formed, which in turn is dependent upon the geology of where it is found.

A Quick Geology Lesson.

Earth is like a tiramisu, it is made up of distinct layers. The thin outer layer is called the crust, which is what we walk on and includes the soil beneath our feet but also deeper down varying in thickness from 5 to 70 km depending where you are on the planet.

And where you can stand on the planet is largely defined by which tectonic plate you’re on. Tectonic plates are the geomorphic mechanism that moves land masses around. If you want to know more, Google ‘breakup of Pangaea’. The most difficult conceptual element of geology is the time scale at which things occur. 

In terms of human existence, we have been on the planet for a blink of an eye. Tectonic plates move, and will continue to move, until our closest star (aka The Sun) eventually implodes in a few billion years and destroys our solar system, which really begs the question: 

Why are we working? What is money? How do we sleep while our beds are burning?

Anyway, the movement of tectonic plates is important because it dictates what the climate is like which controls what the environment is like around us. A prime example of this movement is the collision occurring between continental India with the Asian plateau. This collision has formed the Himalayas, which continue to grow each year as India is slowly uplifted to form the mountain ranges.

The proof of this movement comes from a type of rock found high in the Himalayas called an Ophiolite, which is oceanic crust that is brought up above the continental crust and can be identified by the minerals and fossilised sea creatures of the rock. The Northern part of modern day India was once an ocean, and as the tectonic plate moved north it lifted up the oceanic floor up so that now you can see remnants of fish at 8000 metres above sea level.

What do tectonic plates have to do with soil? 

The cultures of India have been surviving for millennia as a result of the subcontinent being a large alluvial (a general term for clay, silt, sand and gravel) plain for the mountains. In simple terms, this means that all the rain and snow that falls on the Himalayas which form into rivers carry sediment from the mountains down into the valleys. 

Rocks are a collection of minerals, with the type of rock defined by the minerals it contains. So as the water flows down the rivers it collects the minerals. When the minerals clump together it is called sediment. Once the water evaporates, it forms sedimentary deposits containing an array of minerals that are essential for plant growth. The most important of these are nitrogen (N), phosphorus (P), and potassium (K). This is what we call soil. 

Floodplains are the most fertile places on the planet because of this density and diversity of minerals. Meaning that a variety of plants will grow from the soils. The rivers coming off the Himalayas deposit large amounts of sediment in the valleys, which is why the region can support such a large population. 

Back to Australia, in comparison to India the crust which forms Australia is on a very stable tectonic plate meaning it isn’t moving much and we don’t experience earthquakes like New Zealand which is at the junction of three tectonic plates. However, a consequence of this stability is that the rocks that form the country are very old. 

Some of the oldest rocks in the world can be found in the deserts of central Australia. They are old because they formed a long time ago and for millions of years were submerged beneath the surface. This is in contrast to a place like Hawaii which has regular volcanic extrusions where lava spreads across the surface creating a new layer - this is the definition of a young rock.

In geologic terms, Hawaii is known as a ‘hot-spot’ - a point at which the Earth’s crust is so thin that magma (liquid rock which hasn’t reached the surface), pushes up beneath the land mass to form the island chains in the Pacific. When the magma breaks through that is a volcano and the rock is called lava (liquid rock that has reached the surface). This is a very simplified version of the geophysics but you get the point.

Australia’s soils contain low levels of organic matter, which is the term used to describe the breakdown of vegetation and other debris such as manure, decomposing animals, sticks, leaf litter, and anything else you can think of that was once living and when it is no longer it returns to its original composition. We like to think we’re complex, but in the end we’re all just hydrogen, carbon, and oxygen with a few other minerals.

One of the main reasons Australia’s soils contain low amounts of organic matter is due to the river systems being very small compared to other countries. When a river floods the waters deposit sedimentation onto the surrounding areas which makes up the dark, nutrient rich, soil. This is why our most productive farming areas are flat and near rivers - the water is important but the soil is more important, and as aforementioned soils come from mountain ranges. 

What is the relationship between rivers and soil?

The longest and most significant mountain range in Australia is the Great Dividing Range, which stretches from Dauan Island off the northeastern tip of Queensland all the way through New South Wales and ending at the Grampians in Western Victoria.

Winds carrying moisture from the warm waters of the Pacific Ocean travel westwards until hitting the range where they meet cool air and form precipitation that primarily falls as rain on the Eastern side of the mountains.

This is why 90% of the Australian population lives to the East of the Great Dividing Range.

The Murray River, known to those of the lower river it was Murrundi and upstream near Echuca it was Millewa, and near its source in the mountains around Corryong it was called Indi. As it did for the old people, the river sustains a majority of the life around with a number of tributaries flowing into it along the 2500 km journey from the mountains to the sea in South Australia.

The Darling River, known as Barka in the Barkindji language which literally means the people of the Barka (the river), begins in the high-lands of Northern New South Wales and travels 1400 km before its confluence at the township of Wentworth.

These two rivers form the Murray-Darling Basin, also known as the ‘food bowl’ of Australia with around 40% of Australia's agricultural produce comes from the Basin, including 100% of our rice, 74% of our grapes and 30% of our dairy.

Prior to colonisation, the soils of the Murray-Darling Basin would’ve been soft and spongy, having evolved over million years to absorb the most amount of water available due to the intermittent rainfall on the continent.

There are no native Australian animals that have hard feet, or hooves, instead they all have soft padded feet which is an evolutionary biology feature of animals from dry places (think of the camel) but is also a result of the island geology of the continent. The dingo was the first introduced species with a placenta, prior to their arrival it was the Marsupial that dominated the terrestrial mammals.

Why this is important to know is because when the First Fleet arrived they brought with them seven horses, seven cattle and twenty nine sheep along with the first batch of convicts. Since the arrival of Europeans and the introduction of ungulates, hard-hooved animals, to the environment with devastating effects.

The once spongy soils capable of absorbing water and storing it for long, dry periods, have now been compacted by herds of hard hoofed animals. This compaction has increased surface runoff and reduced the productivity of our soils.

This has set off a cascade of detrimental consequences; from increased reliance on fertilisers, which are made up of nitrogen, phosphorus, and potassium (NPK) to a reduction in biodiversity and has had a dramatic impact on the ability of Australia’s soils to sequester carbon.

Our soils have gone from carbon positive to carbon negative, which means they now emit more CO2 than they absorb.

But from the depths of despair comes hope. Because our soils are so degraded, it means we can have a dramatic impact on how they function. If we collectively focus on making our soils healthier then we can draw down more pollution from the atmosphere utilising their carbon potential. 

Read our other articles to learn how this can be done!