The Australian Enigma

[ahosey01]

I discovered recently, for the first time in my life, that most of the Australian continent - despite being dry and arid - actually features neutral / acidic soils.  As far as I knew, dry climates were nearly always alkaline and wet climates were almost always acid.  Even in the <15" rainfall areas of Australia, there is limited alkalinity or salinity in the soil.

Can someone explain this phenomenon to me like I am five?

[SeanK]

Perhaps @happypalms can explain.

[tropicbreeze]

As you say, high/low rainfall are big factors. That said, in places with more vegetation there's more fuel load where you have seasonal fires. Fire tends to burn off volatile elements that form acidic compounds, such as sulphur, nitrogen, etc. The alkaline forming elements, particularly potassium, tend to stay behind. Of course, where you have high organic content in, and on, the soil the absence of seasonal fires leaves the soil more acidic.  

 But there is also the chemical composition of the soil. A lot of places have underlying limestone rock. This will also influence soil pH. Some areas have old underlying marine sediments which can increase pH.

Rainfall patterns can be a bit misleading. The central arid regions record average rainfall totals of 100 to 300mm per annum. However these areas occasionally get remnants of decaying tropical cyclones which can bring heavy flooding rains. For the next 10 years they may get zero rain. There are some areas which will get some rain most of the year. Others with high rainfall for part of the year and less for the rest. And also places with high rainfall for half the year and zero for the rest. These are some of the variables that have an influence.

That's some of the story as I see it. This is a study done on the subject of soil pH in Australia. Sorry, it's aimed at 5+ year olds 😉 but you still might find some interesting bits.

https://www.publish.csiro.au/sr/fulltext/SR10121

http://www.bom.gov.au/climate/maps/averages/rainfall/

[Tyrone]

On 6/18/2025 at 3:32 AM, ahosey01 said:

I discovered recently, for the first time in my life, that most of the Australian continent - despite being dry and arid - actually features neutral / acidic soils.  As far as I knew, dry climates were nearly always alkaline and wet climates were almost always acid.  Even in the <15" rainfall areas of Australia, there is limited alkalinity or salinity in the soil.

Can someone explain this phenomenon to me like I am five?

Australia is a huge place. From my experience on the west coast, most of the coastal soils are derived from limestone and old calcium deposits from when the area was ocean. The soils there are high in pH, some into the mid 8’s, which technically is pure limestone. Once you go inland the soils become more acid as these soils get increasingly more ancient and weathered tending toward the acid side. If you come to ancient swamps etc those soils are very acid. As for salinity Australia has a huge problem with that. Many of the freshwater streams and rivers are now more salty than the ocean since settlement brought land clearing and slash and burn farming. 

[ahosey01]

19 minutes ago, Tyrone said:

Australia is a huge place. From my experience on the west coast, most of the coastal soils are derived from limestone and old calcium deposits from when the area was ocean. The soils there are high in pH, some into the mid 8’s, which technically is pure limestone. Once you go inland the soils become more acid as these soils get increasingly more ancient and weathered tending toward the acid side. If you come to ancient swamps etc those soils are very acid. As for salinity Australia has a huge problem with that. Many of the freshwater streams and rivers are now more salty than the ocean since settlement brought land clearing and slash and burn farming. 

According to this link here, salinity in Australian soils in four large study areas was highest in WA at an average of 6-8 dS/m: (PDF) Soil salinity assessment through satellite thermography for different irrigated and rainfed crops

For a country whose climate delivers <15" of rain on average over 2/3rds of its land area, and features little topography that would facilitate drainage, that number is suprising to me.  Maybe I'm just not that smart.

Likewise, here is a soil pH map of Australia, showing (despite significant aridity) a predominantly acid-neutral soil pH over most of the country: 

For comparison, here is a U.S. soil pH map.  As you can see, huge swaths of the west (comparable annual rainfall totals to the bulk of Australia) have >8.0 pH, whereas such areas cover only a tiny fraction of land in Australia:

There could certainly be something I'm missing here, but this - to me at least - seems interesting and also confusing.

[JohnAndSancho]

My county is all yellow! 

But I have a question about another Australian enigma. Do y'all's toilets really flush in the opposite direction as ours?

[Tyrone]

1 hour ago, JohnAndSancho said:

My county is all yellow! 

But I have a question about another Australian enigma. Do y'all's toilets really flush in the opposite direction as ours?

Didn’t you watch that Simpson’s episode about that exact question. 

[Tyrone]

7 hours ago, ahosey01 said:

According to this link here, salinity in Australian soils in four large study areas was highest in WA at an average of 6-8 dS/m: (PDF) Soil salinity assessment through satellite thermography for different irrigated and rainfed crops

For a country whose climate delivers <15" of rain on average over 2/3rds of its land area, and features little topography that would facilitate drainage, that number is suprising to me.  Maybe I'm just not that smart.

Likewise, here is a soil pH map of Australia, showing (despite significant aridity) a predominantly acid-neutral soil pH over most of the country: 

For comparison, here is a U.S. soil pH map.  As you can see, huge swaths of the west (comparable annual rainfall totals to the bulk of Australia) have >8.0 pH, whereas such areas cover only a tiny fraction of land in Australia:

There could certainly be something I'm missing here, but this - to me at least - seems interesting and also confusing.

They’re interesting maps.
All the farmers down here spread lime on their fields. I know that the gardening shows all based on the east coast often recommend a bit of lime in the hole when planting trees. However on the coastal areas of WA you certainly wouldn’t do that or you would just wreck your soil. Sulfur and sulfates are what is needed in that case. 
I remember my place in Perth where when the bore hole was drilled we busted through metres of limestone and brought up sea shells intact. Who knows when the sea was there. However the gutless sand on that property had a pH of 6.5 which was actually perfect when organics were added. 

[JohnAndSancho]

On 6/20/2025 at 2:58 AM, Tyrone said:

Didn’t you watch that Simpson’s episode about that exact question. 

I did. Just seeking confirmation 😂