An epic journey carries phosphate rock from the Western Sahara desert to New Zealand farmland.

New Zealand's economy is reliant on a mineral at the centre of an ugly refugee crisis, deep in the Sahara desert. National Correspondent Charlie Mitchell reports in the first part of 'Growing Pain', a Stuff series examining our dangerous addiction to fertiliser.

The cycle that underpins New Zealand's economy starts with a town in the desert, shielded by a minefield. A white, dusty rock is stripped from the earth and put on a conveyor belt, which rumbles slowly across the Sahara - flat, desolate and silent - for 100km. It leaves a cloud of dust so vast it can be seen from satellites, etching a scar across Northern Africa.

When it reaches port, the rocks topple off the belt and are loaded onto trucks, which are driven to awaiting cargo ships. They sail a tedious route south, ducking beneath the horn of South America to avoid nations that may confiscate the cargo.

The ships arrive at their destination: It could be Bluff, Dunedin, Lyttelton, Napier, or Tauranga, depending on who is buying the cargo. The rock is driven to a factory and blended with other rocks, mixed with sulfuric acid, and compressed into small, white balls. The balls are dropped with precision from planes over the countryside and bleed into the soil, releasing nutrients into plants, which are eaten by sheep and cows, which are eaten by us.

The scale of the operation is enormous. The conveyor belt is the longest in the world, as is the minefield protecting it. The trade is worth tens of millions of dollars each year, which has gone on for decades.

Many countries use phosphate fertiliser, but few have hitched their wagons to it like New Zealand.

New Zealand is uniquely reliant on pastoral farming, which revolves almost entirely around grass. Animals need grass, which grows with nutrients from the soil. To put nutrients in the soil, farmers use fertiliser.

Most of the phosphate rock dug up from that mine in the Sahara ends up as Superphosphate, the most commonly used fertiliser in New Zealand. If farmers stopped using Superphosphate, agricultural productivity would halve, and the export economy as a whole would contract by 20 per cent, according to some estimates.

The problem with phosphate is that there's a finite supply - it can only be stripped from the earth.

Most of the world's phosphate reserves are near the Sahara desert, in mines controlled by Morocco.

Morocco controls 70 to 80 per cent of the world's phosphate rock reserves. The country with the next largest supply, China, has around 3 per cent.

JBDODANE/FLICKR The world's longest conveyor belt in the Sahara Desert.

Some of that phosphate isn't in Morocco proper.

It's in a bitterly disputed territory known as Western Sahara, which was ruled by Spain but which Morocco has since claimed as its own, against the wishes of the indigenous Saharawi people.

It is one of the most significant and long-running territorial disputes anywhere in the world. There are hundreds of thousands of Saharawi, many of whom fled to Algeria during a war decades ago, where they live in sprawling refugee camps in the desert near the border, desperately wanting to return to their homeland.

That homeland, similar in size to New Zealand, is now split in two. Morocco built a sand wall thousands of kilometres long, flanked on both sides by millions of explosives. The side it manages, called the occupied territories, has 80 per cent of the land, all of the sea, and all of the phosphate. The eastern side, the liberated territories, has nothing.

More than a dozen international human rights groups have condemned the Moroccan government, which they say is brutally occupying a territory it has no right to. The ugliness of the situation has led to a backlash against those trading in the region. Some investment funds and banks have blacklisted those who trade there, and many companies dropped their imports.

First the Australian companies stopped, then the American ones. Earlier this year, the Canadian fertiliser giant that was by far the largest phosphate trader said it would stop by the end of this year.

A decade ago, New Zealand was the fourth-largest importer. Then it became the third largest, then the second.

DEAN KOZANIC/STUFF A pile of phosphate at the Ravensdown fertiliser manufacturer in Sockburn, Christchurch, pictured in 2015.

As of next year, only three companies in the world are known to be actively in the market for Western Saharan phosphate. One is an India-based subsidiary of the mine's operator, the Moroccan state-owned Office Cherifien des Phosphates (OCP), which is effectively selling the phosphate to itself.

It leaves two independent buyers: Ballance Agri-Nutrients and Ravensdown, the New Zealand farmer cooperatives that supply 98 per cent of New Zealand's fertiliser.

If you are a farmer in New Zealand and you use Superphosphate fertiliser, it is likely the raw materials came from the western side of the world's longest minefield, and made the long, slow journey to the sea on the world's longest conveyor belt.

When it arrives in New Zealand, records will say the phosphate came from Morocco, but the many thousands of people in Algerian refugee camps would say it was stolen from them.

"New Zealand farmers are the only clients in the whole world of the Moroccan plunder of Western Sahara," says Erik Hagen of Western Saharan Resource Watch, an NGO that monitors trade from the region.

"New Zealand stands alone now as the main funder of the illegal occupation... They are buying stolen goods."

For the co-ops importing the phosphate, the geopolitical drama is a major concern, but the alternatives are no better.

"It's very, very difficult to find a phosphate supply that's clean of any issues," says Mark Wynne, Ballance chief executive.

"Unfortunately, where most of the phosphate is in the world, there are political issues."

GOOGLE Satellite imagery shows phosphate blowing in the wind as it travels along the world's longest conveyor belt.

THE DEVIL'S GARDEN

The refugees in 'The Devil's Garden' dream of a life by the sea.

Temperatures in the Algerian desert soar to 50C in the summer and fall below 0C overnight in the winter. Persistent sand storms trap people indoors for days on end, dust filling their lungs and their tents and their tiny, battered shelters built of red adobe brick.

There is a permanent water shortage because it barely rains. Each person is allocated 18 litres per day, below the humanitarian standard. A common diet includes rice, onions, and pasta, resulting in high rates of diabetes. Children are fed a nutritional paste because they have limited access to fruits and vegetables. With no natural water or food sources, and no resources to trade to make money, the refugees are entirely reliant on foreign aid.

Its moniker, 'The Devil's Garden', is apt - few places on Earth are as inhospitable to life. But that is where most of the world's Saharawis have lived for decades, in sprawling camps that have been there so long they resemble rickety cities, with a unique and self-functioning society. The camps have police, hospitals, and schools, which are all self-run.

Before the Moroccan forces moved in, causing many Saharawis to flee deeper into the desert, many had lived by the sea, on the North African coast.

The war between Morocco and the Polisario Front, the Saharawis' political representatives, lasted 16 years, and resulted in thousands of deaths. A ceasefire has been in place since 1991.

The Saharawis are nomadic, a society of roamers, but for decades they have been trapped. The former UN secretary-general Ban Ki-moon, who visited the camps in 2016, called the situation "heartbreaking," "unacceptable," and "one of the forgotten humanitarian tragedies of our times."

Because of the brutal environment, few westerners visit the camps in the desert. Those who have say conditions are harsh.

One recent visitor, UK teacher Beccy Allen, said those in the camps had a longing to return to their homeland.

TOUFIK DOUDOU/AP A Sahrawi woman washes clothing in the Smara refugee camp near Tindouf in 2016.

She recently returned from a six-month stint teaching English in the camps. She says she lived with an older woman who wanted nothing more than to return to her childhood home by the sea.

"Their aim is to always go back to their homeland, rather than to accept that the small amount of land is all they're ever going to get," Allen says.

"They're aware they had houses and people used to have real jobs. People lived by the sea, as well as living in the desert."

Because the conflict has been going on for so long, generations of Saharawis have only known life in 'The Devil's Garden'.

For some of the younger generation, it has caused immense frustration, Allen says. But most Sahrawi remain committed to a peaceful solution through the international system.

"Their parents were born in the camps, they were born in the camps, they only see their children born in the camps," Allen says.

"Because I've become so involved in it, I find it hard to understand how countries can do that ... I find it quite difficult that there are international governments that would go against some of their morals, just to buy some excellent phosphate."

With the peace process at a loggerheads, and their plight long ignored, the Saharawis have been testing a new strategy to get back home - and one of its targets is New Zealand.

* In part two of Growing Pain, a Stuff series, Charlie Mitchell reports on New Zealand's tarnished reputation as a humanitarian nation in the eyes of those stuck in refugee camps in Algeria.

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* This story had been edited to clarify that most of the world's phosphate reserves are near - rather than in - the Sahara desert, in mines controlled by Morocco.