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Currently, 90% of Australia's land is inhospitable desert. What geographic changes need to occur for its southern side to become fertile soil?

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When you state "its southern side to become fertile soil" do you mean the southern coastal region or the southern half of the continent from the center of Australia to the coasts?

Either way getting fertile soils in most of Australia is difficult.

To begin with, the landmass of Australia is old and it has changed little in millions of years. Except for the east coast, and in more recent times (some 5000 years ago), it has had little volcanic activity. Volcanic activity can result in soil fertility.

Being so old and weather affected, Australia is the most weathered land mass on Earth. Much of what is referred to as soil in Australia would be called sub soil elsewhere. A lot of soil has just blown away over time.

The photograph below was taken in the 1930s. It shows two women standing next to an exposed tree root, from a tree that has long since died. What the photograph shows is the depth of soil erosion (approximately 2 m) that has occurred in that part of the country since European settlement, less than 200 years.

enter image description here

Over millions of years of weathering with no new additional soil from volcanic activity of erosion of mountains Australian soils have been leached of minerals and nutrients that most plants require. Most Australian soils are severely lacking in phosphorus and trace elements.

Despite all of this, Australian agriculture is successful because of the use of fertilizers designed for local soils in the regions.

In the central part of the south coast of Australia is the Nullarbor Plain. It is a vast region of limestone country (200,000 square kilometers), with very little soil.

If you want natural soil fertility in southern Australia you will need a mountain building region and possibly some volcanic activity. A wetter climate would also be required as well as an accumulation of soil humus.

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  • $\begingroup$ Nulla**r**bor. Means "Zero Trees" in Latin. $\endgroup$
    – AlexP
    Commented May 23, 2022 at 22:16
  • $\begingroup$ I meant the southern coastal region. Where should the mountains be built? Should it run parallel to the south coast and then the east coast? $\endgroup$ Commented May 24, 2022 at 10:33
  • $\begingroup$ The Great Dividing Range already exist along the east coast & along the south coast of the south eastern corner. It needs to be higher. Mt Kosciuszko, within the Range, is only 2228 m high. It's the highest point of the Australian mainland. A tall east-west range parallel to the south coast either near the coast or further inland would help. The Mac Donnell Ranges ... $\endgroup$
    – user81881
    Commented May 24, 2022 at 13:28
  • $\begingroup$ ... in central Australia where once as high as the Andes of the Himalayas, but have eroded to mere stumps. Similarly for the Petermann Ranges of central Australia. See Australian Landforms and their History. $\endgroup$
    – user81881
    Commented May 24, 2022 at 13:32
  • $\begingroup$ @Fred do you mean a chain along the southern coast connected to the great dividing range? $\endgroup$ Commented May 25, 2022 at 14:58
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Move the continent. At present, Australia is on top of the 30th parallel south which is where the air circulation will make it a desert. Move it up on the equator and it will get a lot more rain.

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  • $\begingroup$ Could you suggest any technology that would be available in real life in order to move a continent like that? $\endgroup$ Commented May 28, 2022 at 18:47
  • $\begingroup$ @Bangerblade there is no such technology that humans can use. However, the process is happening due to continental drift. Australia is moving north. But the reason for mentioning the continent location is to point out that no matter what you do to the soil, the rainfall is determined partly due to the location. All around the world at the 30th parallel, we have great deserts. This is due to how air moves from the equator to that parallel. $\endgroup$
    – David R
    Commented May 29, 2022 at 14:19
  • $\begingroup$ Well the setting is close to the actual 21st century so it's not a possibility. $\endgroup$ Commented May 29, 2022 at 15:15

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