By Alex Kirby BBC News Online environment correspondent

Once a wetland, now a saltpan

The trouble dates from the introduction of European crops, whose shallow root systems did not reach the water table.

As a result, water levels slowly rose, bringing with them old salt deposits which are gradually poisoning the land.

Australians are increasingly divided on how to tackle the threat, whether by trying to flush the salt out to sea or by stopping it entering the rivers.

Eaten away

BBC Radio 4's environment programme Costing the Earth went to Australia to see the damage the salt is doing.

Too much salt is bad for emus and other wildlife

It reports on the way railways, roads and gas pipelines are all succumbing to corrosion, and says it is estimated that by 2050 there will be no water fit to drink in one of Australia's largest cities, Adelaide.

Outside the cities, huge tracts of the most productive farmland are being choked by the salt. The 13.7 million hectares (33.85 million acres) of agricultural land that are likely to be threatened by the middle of the century will exceed the current total area devoted to wheat, Australia's principal crop.

Already, waterholes used by Aboriginal people are contaminated, sheep are being killed by the salt, and some of the great wines of the Barossa Valley have been put at risk.

River conundrum

One of the most serious consequences of the salt's inexorable rise is its incursion into waterways.

Farmers look for signs of salt

The Murray river provides the water to grow 40% of Australia's food, as well as the drinking water for most of the southern part of the country. But it has become so saline that it is slowly dying.

There are solutions: many farmers are now planting tree and bush species native to Australia, as their root systems reach further down and do not draw up the salt.

Tougher wheat

That is not an option for the arable farmers of the wheat belt, but even for them there may be an answer.

The government research agency, the Commonwealth Scientific and Industrial Research Organisation, is breeding new varieties of wheat, modern strains crossed with ancient Persian wheats, which are being tested to see how salt-tolerant they are.

But there is no unanimity on how to revive rivers like the Murray. One proposal is to increase the river's flow, so as to wash more of the salt out to sea.

Against that, another school of thought argues that it makes more sense to try to prevent the salt leaching into the river in the first place.

Costing the Earth is broadcast in the UK on BBC Radio 4 at 2100 BST on Thursday 1 April