Why are Plants and Soils in the Narrogin Area so Diverse?
Summary
The world's most productive agricultural soils are generally on relatively new soils on deep sediments from rivers, glaciers, wind deposits, or rapid breakdown of new fertile rock from volcanos and mountains.
For hundreds of millions of years the wheatbelt has been an undulating upland plain overlying granite bedrock, without mountains, glaciers or large rivers to deposit fertile sediments. A layer of ironstone laterite formed on uplands, beginning about a hundred million years ago. Since then wetter and dryer climate phases and mild geological uplift caused the plateau to be dissected to form a range of (mostly infertile) soils. Changes were gradual enough to allow our diverse plant community to evolve.
The world's most productive agricultural soils are generally on relatively new soils on deep sediments from rivers, glaciers, wind deposits, or rapid breakdown of new fertile rock from volcanos and mountains.
For hundreds of millions of years the wheatbelt has been an undulating upland plain overlying granite bedrock, without mountains, glaciers or large rivers to deposit fertile sediments. A layer of ironstone laterite formed on uplands, beginning about a hundred million years ago. Since then wetter and dryer climate phases and mild geological uplift caused the plateau to be dissected to form a range of (mostly infertile) soils. Changes were gradual enough to allow our diverse plant community to evolve.
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Geology
This region is underlain by a stable piece of continental rock called the Yilgarn Craton, which is mainly composed of granite and gneiss, some of the world's oldest (up to 3 billion years) rocks. Since then the craton has joined and separated from other continents in the supercontinent cycle causing faults and stresses. Mountains associated with these movements have long since eroded away to leave a subdued landscape, but rock stresses have influenced our rivers, ridges and soils. The radiometrics image of Narrogin north area reveals faults and dykes in the granite bedrock and their influence on river direction. |
Granite has a high quartz content and weathers to sandy soils. Over a billion years ago, liquid mafic rock from the earth's mantle squirted up cracks in the granite to form lines of heavy black rock called mafic dykes. Dolerite is a common mafic rock, which weathers to red-brown clay soils. Foxes Lair straddles the huge Binneringie Dyke, which ranges from Quindanning to Coolgardie. Red soils from this dyke are exposed on the southern edge of Foxes Lair.
Granitic sandy soil at Arboretum picnic spot
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Mafic red brown loam near the Claypit
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Australia separating from Greater India and Antarctica
Later other land masses joined the Yilgarn Craton to form the Gondwana supercontinent, and it stayed above sea level. About 300 million years ago glaciers flattened our landscape down to the bedrock. Without mountains, volcanoes, active rivers or sedimentary deposits, mostly sandy soils slowly formed from the bedrock and were transported into surrounding basins and seas.
When Australia separated from adjoining India and Antarctica, parts of Western australia rose or fell, which redirected rivers that eroded the old plateau to form areas of active rivers and salt lakes.
From about 100 million years ago, laterite formed as a layer over the landscape. Subsequent climate cycles and land movements have created our present pattern of riges and waterways. More resistant laterites formed from mafic rock often coincide with uplands, and the very angular waterway patterns follow ancient cracks in the underlying bedrock.
When Australia separated from adjoining India and Antarctica, parts of Western australia rose or fell, which redirected rivers that eroded the old plateau to form areas of active rivers and salt lakes.
From about 100 million years ago, laterite formed as a layer over the landscape. Subsequent climate cycles and land movements have created our present pattern of riges and waterways. More resistant laterites formed from mafic rock often coincide with uplands, and the very angular waterway patterns follow ancient cracks in the underlying bedrock.
Biology
Vegetation is a good indicator of the underlying soil. For example, Marri (Corymbia calophylla) grows on sands and gravels, Brown Mallet (Eucalyptus astringens) on mottled clays around ironstone ridges, Wandoo (Eucalyptus wandoo) on the sand over clay (and other soils), and York Gum (Eucalyptus loxophleba) prefers fertile soils that have formed from fresh granite and dolerite bedrock.
Recent research has shown that most soils in the reserve have been created by distinctive native vegetation through root secretions. These stimulate microbes and fungi to form clays and laterites, which favour the host plants and restrict competitors.
Two examples are:-
Vegetation is a good indicator of the underlying soil. For example, Marri (Corymbia calophylla) grows on sands and gravels, Brown Mallet (Eucalyptus astringens) on mottled clays around ironstone ridges, Wandoo (Eucalyptus wandoo) on the sand over clay (and other soils), and York Gum (Eucalyptus loxophleba) prefers fertile soils that have formed from fresh granite and dolerite bedrock.
Recent research has shown that most soils in the reserve have been created by distinctive native vegetation through root secretions. These stimulate microbes and fungi to form clays and laterites, which favour the host plants and restrict competitors.
Two examples are:-
Laterites
Many upland soils in Foxes Lair are lateritic (gravelly or sandy) with brown round stones, which are high in iron and aluminium. These soils are underlain by bauxite, which is mined in the Darling Range futher to the west.
Laterites support diverse and colourful wildflowers, particularly the Proteaceae (Banksias, Hakeas, Grevilleas etc) and Casuarinaceae (tammas, sheoaks) families. These plants dominate here because they can extract phosphorus (a vital nutrient), which is unavailable to most plants on these infertile soils. They do this with special cluster roots, which release an organic acid in to the soil in winter. The acid releases phosphorus from soil particles for the plants to use. Iron that causes the red and brown colours in soil, and aluminium are also released. Soil bacteria then use the remaining acid for food, and in doing so, cause iron and aluminium to become solid again (precipitate) on gravel stones in the topsoil or down plant root channels. If you look inside gravel stones, you will often see rings of deposited iron.
Many upland soils in Foxes Lair are lateritic (gravelly or sandy) with brown round stones, which are high in iron and aluminium. These soils are underlain by bauxite, which is mined in the Darling Range futher to the west.
Laterites support diverse and colourful wildflowers, particularly the Proteaceae (Banksias, Hakeas, Grevilleas etc) and Casuarinaceae (tammas, sheoaks) families. These plants dominate here because they can extract phosphorus (a vital nutrient), which is unavailable to most plants on these infertile soils. They do this with special cluster roots, which release an organic acid in to the soil in winter. The acid releases phosphorus from soil particles for the plants to use. Iron that causes the red and brown colours in soil, and aluminium are also released. Soil bacteria then use the remaining acid for food, and in doing so, cause iron and aluminium to become solid again (precipitate) on gravel stones in the topsoil or down plant root channels. If you look inside gravel stones, you will often see rings of deposited iron.
closeup of a cluster root
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iron layers in gravel
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Iron deposited in root channels
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Over thousands of years, a deep laterite profile formed. Ancient laterites here can be likened to a layer cake,with an often a gravelly topsoil underlain by ironstone, a mottled iron rich clay (mottled zone), a white clay (pallid zone), and finally the granite or dolerite). Over geologic climate cycles laterites have degraded and reformed
Laterites vary greatly in age and type. They range from dark heavy gravel and red-brown soil to others with pale light sandy gravels, and even ironstone pipes.
laterites have also developed on sands and previous laterites.
Mesas are lateritic gravel flat-topped hills with steep sided slopes on one or all sides called breakaways.
Mesas have a dense ironstone cap that is underlain by pink to white clay that erodes rapidly (a source of ochre), causing ironstone blocks to be undercut and fall down the slope. Mallet or powderbark eucalypts often grow on these slopes.
The diagram below depicts a cross section showing an eroded 'layer cake' lateritic mesa and soils formed from the layers.
Laterites vary greatly in age and type. They range from dark heavy gravel and red-brown soil to others with pale light sandy gravels, and even ironstone pipes.
laterites have also developed on sands and previous laterites.
Mesas are lateritic gravel flat-topped hills with steep sided slopes on one or all sides called breakaways.
Mesas have a dense ironstone cap that is underlain by pink to white clay that erodes rapidly (a source of ochre), causing ironstone blocks to be undercut and fall down the slope. Mallet or powderbark eucalypts often grow on these slopes.
The diagram below depicts a cross section showing an eroded 'layer cake' lateritic mesa and soils formed from the layers.
 Reddish clay nodules formed around roots
Clay subsoils
Many eucalypts secrete organic chemicals from their roots, which stimulate microbes and fungi to produce clay subsoils. The image from Chillinup shows nodules of reddish clay formed around mallee roots in the subsoil. Most eucalypt tree and mallee soils are on duplex soils, with sand or loam topsoil overlying a clay subsoil. |
Exposed domed clay subsoil near the claypit
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Climate
Laterite first developed when the climate was wetter and warmer than now. For the past two million years our long term climate has alternated between warm wet and cold dry phases. Wet cycles have more vegetation, which favour gentle landscapes and laterite formation. Dry cycles coincide with cold weather. barer soils, infrequent flash floods, and very strong winds that cause salt lakes and adjoining dunes. The steep valleys and lateritic mesas and breakaways seen on the Breakway Walk probably formed then.
We are currently in a wetter geological period (excluding man-made climate change).
Laterite first developed when the climate was wetter and warmer than now. For the past two million years our long term climate has alternated between warm wet and cold dry phases. Wet cycles have more vegetation, which favour gentle landscapes and laterite formation. Dry cycles coincide with cold weather. barer soils, infrequent flash floods, and very strong winds that cause salt lakes and adjoining dunes. The steep valleys and lateritic mesas and breakaways seen on the Breakway Walk probably formed then.
We are currently in a wetter geological period (excluding man-made climate change).
Further Reading
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