Salmon-coloured clouds of flamingoes sweeping overhead is a common sight at east Africa's Rift Valley lakes, but the mounds of mud where they lay their eggs are found only here.

The caustic waters of Lake Natron form the only breeding ground for east Africa's endangered lesser flamingoes, but the Tanzanian government is determined to revive plans to build a soda ash plant at the lake.

The Royal Society for the Protection of Birds (RSPB) estimates that three quarters of the world population of lesser flamingos live and nest in east Africa.

"All depend on Tanzania's Lake Natron as a breeding site. Food is plentiful, nesting sites abound and above all, the lake is isolated and undisturbed," The RSPB says.

Lake Natron, whose maximum depth is 50 centimetres (20 inches) lies at the foot of Ol Doinyo Lengai, which means the Mountain of God in the local Maa language.

It is the lake's proximity to the volcano that accounts for the caustic nature of its water. Ol Doinyo Lengai is the world's only active volcano whose lava is a type of rock called natrocarbonatite.

That rock creates caustic water keeping predators at bay for the flamingoes, who typically lay one egg on a small volcano-shaped mound of mud on the salt pans in the lake.

Tanzanian President Jakaya Kikwete in April spoke of the need to fast-track the soda ash plant plan, already shelved once over environmental concerns.

"There is no need for further delay of the project, which will give the country's economy a big boost," Kikwete told officials at the trade and industry ministry.

He said Lake Natron's reserves of soda ash deposits were big enough to make Tanzania the world's leading producer of the product.

Sodium carbonate is used in water softening agents for laundry and in the manufacture of certain types of glass.

Ibrahim Matovu, acting executive director for Longido District welcomed the planned soda ash plant with enthusiasm.

"We expect it'll raise the standard of living of the people here and create jobs and support social services -- education and health," he told AFP in Longido - a town desperately short of formal employment or visible prospects.

Under pressure from environmentalists the Tanzanian government mooted moving the actual plant several dozen kilometres (miles) away from the lakeshore.

-- 'Traditions will be affected' --

However, the pipes that will pump the soda to the plant would still disturb flamingo breeding, environmentalists say.

There is the infrastructure and the workers required to build them and the fact the pipe will have to be moved in order to keep channelling soda from different parts of the lake.

"It takes very little disturbance to cause an entire breeding colony to abandon its nests," said the RSPB.

Breeding is further complicated by the fact flamingoes, who live to be as old as 40, only breed once every five or six years.

Here at Lake Natron many residents say they do not want the plant to be built.

"For my part I dont like the idea," said Lemra Kingi, who looks after the basic "tourist office" in Engaresero village.

"The birds will move from this area," he predicted, perched on a tree branch.

He wears traditional Maasai blankets in orange and purple, topped off with a necklace of shells and silver, with tennis socks and hiking boots as a concession to modernity.

But Lucas Lemole, 23, and secretary of the association of guides, said: "We dont know how the industry will work but we want the jobs. We'll get jobs, development, a hospital, education."

Engaresero and the entire lakeside area have no mobile phone network. The main road along the western shore of the lake is a boulder-strewn track.

A host of small mammals -- mongooses, jumping hares and baboons come scampering across it.

Sarupe Koileken, a livestock officer turned local official told AFP he also feared that local traditions would be diluted if there was an influx of workers from elsewhere.

"The population here is 95 percent Maasai. Traditions will be affected because there will be a mix of cultures," he said.

Joseph Seuri, a community NGO coordinator whose home village Pinyinyi lies just 29 kilometres (18 miles) from Magadi, a soda lake on the Kenyan side of the border that has a functioning soda ash plant, warned his fellow villages to be wary of any promises made.

"At Magadi .. they told the people they'd get jobs. Then they chased the people away and stopped the cattle grazing near the factory," he said bitterly.