The ground was still hot and smoke hung in the air when Saparina set out to plant her spinach in the ashen remains of rainforest on Indonesia's Sumatra island, where raging fires triggered Southeast Asia's worst smog crisis in years.

The farmer waded through ankle-deep ash as she laid out her crops in fire-blackened earth among charred tree stumps on land cleared by the illegal method of slash-and-burn.

"I give thanks to God, now I can easily plant vegetables and oil palms," said the 36-year-old, who only gave one name, her feet still dirty after planting the crops in her half-hectare (1.2-acre) plot of land in Riau province.

While the blazes last month cloaked Singapore and Malaysia in toxic haze and provoked howls of outrage from environmental groups, many on Sumatra, from plantation workers to villagers like Saparina, are die-hard supporters of using fires to clear land.

It is the quickest and cheapest method of clearance for cultivation -- far less expensive than using mechanical excavators or bulldozers -- and the ash from fires is also a natural fertiliser.

As the haze clears, authorities are turning their attention from firefighting to trying to catch the culprits. For many, the focus remains firmly on big palm oil and pulp and paper firms.

Global demand in particular for palm oil -- used in everyday goods from soap, to lipstick to biscuits -- is booming, and rapid expansion of plantations is behind much of Sumatra's deforestation.

Nevertheless, the common acceptance of slash-and-burn clearances among smallholders suggests that blame is widely spread, even if big companies often buy the palm oil fruit produced on the smaller, private farms.

Small farmers clearing their own land, people paid to quietly flick a match onto a concession owned by a big company, and major companies themselves are all starting fires, activists say.

This year's fires pushed haze to record levels in Singapore, forcing residents to don facemasks and stay indoors. They also raised diplomatic temperatures, with both Singapore and Malaysia calling on Indonesia to do more to stop the problem.

But many on Sumatra see little alternative.

"Burning is obligatory," said Herman, the owner of a small palm oil plantation who declined to give his full name.

"Who would want to cut huge trees with their own hands to clear land? The trees are enormous."

Once the fires start they often burn deep underground in deposits of carbon-rich peat, and are notoriously difficult to put out.

Firefighters have had to resort to sticking hoses deep into the ground to douse blazes that have spread across thousands of hectares.

"It takes only a flick of a cigarette butt to create a big fire, especially in the dry season," Herman said.

"The fire travels like water flowing beneath our feet -- you have no idea where it might resurface and burn the land above."

The continued enthusiasm for slash-and-burn comes despite chronic health problems -- nearly 20,000 people in Riau suffered breathing difficulties in June due to the haze, according to a local health official.

Saparina, who insisted she does not start fires herself, conceded her children were "coughing at home" while she was out planting.

With the annual haze from forest fires on Sumatra the worst this year since 1997-98, Jakarta is under pressure to take action.

Police have so far named 24 small farmers suspected of starting the fires. Authorities have not said that any of them are from a major plantation company but they are looking into possible links.

Government officials have said some fires took place within the boundaries of concessions owned by big companies and are investigating eight firms.

Many companies have insisted they have strict "zero burning" policies and that any fires in their concessions must have crept in from outside.

But proving who really set the fires is a daunting task for police in a huge province where many plantation workers and residents seem to have a cigarette permanently dangling from their lips, 50 percent of the land is peat, and using fires to clear land is part of life.

Some now argue that the law banning land clearance by fire is unrealistic and should be replaced with government-regulated controlled burning.

"I believe law enforcement alone does not work," said Willem Rampangilei, the deputy minister for people's welfare in the national government. "We tried to stop the tradition but it's impossible."

Environmental groups such as Greenpeace insist the government must enforce existing laws banning slash-and-burn more effectively.

"The continuing practice of clearing land with fire is just the tip of the iceberg of Indonesia's flawed natural resources management," said Yuyun Indradi, a forest campaigner with Greenpeace Indonesia.