Why do soils in Australia become salty or saline?

Answer 1

Excess irrigation is the short answer. If you consider a body of fresh water penetrating the sub strata, it is clear that any salt at all that it encounters will be quickly 'absorbed' into the fresh water - in order to equalize the solution. And when that water is evaporated at the soil surface, the dissolved salts will be left behind. Making the soil more saline.

If the rate of transpiration is greater than the rate of irrigation - as a long term average - then you'll have increasingly saline soils.

In the historic fertile soils of Mesopotamia were used by the Romans for wheat growing, they found that they could increase the natural yield by irrigation. But the salinity of the soils increased and yields decreased. One solution was to grow barley instead which is more salt tolerant, but the irrigation practices still increased the salinity until the soils became too salty even for barley.

Now, the Brits in India knew this from their classic scholars, and when they introduced irrigation into India, they warned that it would have a life time of about 150 years. In fact it lasted longer than that because of better drained soils, but eventually salination appeared.

So Australia must listen to its classic scholars.