The pollution of arable land had led some middle-class people in Guangzhou and Shenzhen to rent farmland in remote rural areas and hire local farmers to grow organic vegetables for them, Peng said. They would eat only vegetables grown on their own land.

The trend represents a turning point in how the province, and the mainland as a whole, thinks about how to feed itself. A generation ago, most of the population were either farmers or ate food grown nearby. But the drive to industrialisation has put a huge strain on the mainland's farmland. The central government, concerned about food security, has set a minimum national level for the amount of arable land. But local authorities often find it hard to resist the lure of the developer's dollar, just as the dwindling number of farmers would prefer giving up toiling in the fields to take a payout or work in a factory. Such competing forces exist throughout the chain, and the stakes are high.

'The problem is very serious,' said Wang Xiaoying, a professor at the Chinese Academy of Social Sciences' rural development research institute. 'The damage we are doing to agricultural land is permanent. Once concrete is poured on the soil, the land will not be able to revert back to farmland. The loss is forever.'

The situation is especially severe in Guangdong. A Ministry of Lands and Resources report three years ago said the province was one of a few facing a severe shortage of grain and that problems with the supply of staple foods would last for a long time. A survey by the Guangdong Institute of Eco-Environment and Soil Sciences last year suggested heavy metals had contaminated about 40 per cent of the province's arable land, 10 per cent of that seriously.

The province's arable land produces only a third of the food that its population of more than 100 million people needs, according to the official Guangzhou Daily. Imports from other provinces account for 80 per cent of the shortfall, and the rest comes from overseas. Guangdong will need an extra five million acres to provide enough food for itself, according to one of Guangzhou's development research institutes.

The province used to play a major role in supplying vegetables and other staples to the nation, thanks to its abundant fertile land. The situation changed in the 1970s, when unprecedented industrialisation, urbanisation and population growth began. Huge swathes of farmland have been lost to factories, housing, highways and railway tracks. This has left the province with about 313 square metres of arable land per capita - a third the national average.

The city's changing landscape over the past 30 years illustrates the degree of urbanisation. Vegetable fields at Qingcai Gang (green vegetable knoll) have long been replaced by rows of low-rise housing. Standing on the former rice paddies of Tianhe are office towers, houses, banks and other businesses. Vegetables used to be grown on the site of the city's central business district, Zhujiang New City. Pantang in the city's west was home to fish ponds.

'Farmland used to be everywhere,' said Chen Bingwu, a Guangzhou taxi driver. 'Now I have no idea where we could possibly find farmland in Guangzhou. It has been replaced by factories and skyscrapers. With less and less farmland, how are we going to feed ourselves?'

Now Chen has to travel to Zhongluotan , 50 kilometres north of Qingcai Gang, before he finds any farmland. It is national policy that keeps agriculture alive in Zhongluotan; 800 hectares of arable land there is graded as prime land, which means it cannot be used for anything else.

Bu the pressure to build continues to intensify. Late last month, Guangzhou saw the highest price yet for a piece of land in the city. Guangzhou R&F teamed up with Agile Property Holdings and Country Garden to buy sites for Guangzhou Asian Games City for 25.5 billion yuan (HK$28.9 billion). The Panyu site is 4.38 million square metres, about 11 times the size of Cyberport in Pok Fu Lam, in Hong Kong. It is part of farmland the government took over in recent years to build venues for the Asian Games, to be held in November.

Beijing has vowed to keep at least 1.2 million square kilometres of arable land available for agriculture - the minimum needed to grow enough to feed the country. Of that, 1.04 million sq km should be prime land, and 1.05 million sq km should be growing grain by 2020. Grain yield should reach 525 tonnes per sq km. Efforts to increase grain output should include upgrading low-yield farmland, promoting technology, advancing agricultural mechanisation and strengthening the prevention of diseases and insect pests. Beijing also wants to improve agriculture infrastructure and water facilities.

A plan to take over farmland in Zhongluotan, near Guangzhou, was aborted some years ago, said 60-year-old vegetable farmer Zeng Hong, who owns a 4.5-hectare lot in the village. 'It was some years ago. I was once told there are people interested in renting land from us to build something, but the plan was banned because the land here is not allowed to be used for development.'

He is not against the idea of converting his farmland over to factory use. 'Farming is a difficult job. I get up at 2 or 3 o'clock in the morning throughout the year. The vegetables are not worth that much money. If I could choose, I'd prefer to work in a factory where I can earn more money. As long as the price is reasonable, I have no problem letting them take my land.'

For Zeng, reasonable means 100,000 yuan in a one-off compensation plus annual rent for using his land.

His neighbour Zeng Rong, who owns 1.2 hectares on which he grows vegetables, has a similar attitude. 'I earn more working in factories in Guangzhou. But I'm too old now; I can't work in the factories any more. It is why I've come back. Farming is not bad, I have more freedom farming on my own land, but I can't get much money by growing vegetables.'

For both Zengs, food security is an alien term they have not heard before and cannot comprehend. For them, farming is just about how much they will earn from it.

For Wang, the academy professor, the attitude of the two Zengs in indicative of why agricultural land is continually losing out to industrialisation. 'It is difficult to stop local governments from converting arable land to urban land use. Their concern is economic development, not the nation's food security. Building factories and residential development are too lucrative to resist compared to the minimal revenue from agriculture. Food security is the central government's business.'

Even though Guangdong has lost nearly all of its arable land, the central government has set targets for further urbanisation.

State Council guidelines for the development of the Pearl River Delta to the year 2020 requires the per capita gross domestic product of Guangdong to reach 80,000 yuan by 2012 and 135,000 yuan by 2020. The service industry should account for 53 per cent of the delta's economy by 2012 and 60 per cent by 2020. Guangdong's per capita GDP was 37,588 yuan in 2008, while contributions from the service sector amounted to less than 45 per cent of the economy. The blueprint also demands that hi-tech industry play a bigger role. To satisfy the demand for more development without violating the state's order to conserve agricultural land, Guangdong decided in 2000 to turn to less developed parts of the province - the east and the north. It pays farmers in the poor areas to open up more arable land in the mountainous areas, so developed areas can continue to expand.

Wang believes that although the government can turn to the poorer areas for extra arable land, yields are low. 'The newly explored arable land is infertile land; farmers have to rely heavily on fertiliser for agricultural output. If soil is fertile and suitable for farming, the land has long been in agricultural use already.'

The solutions are not easy. Wang suggests the best way to protect arable land is to increase the cost of land resumption by demanding local governments increase compensation to farmers. But there also needs to be a change in thinking.

The government should promote respect for agriculture but 'changing thinking is very difficult', Wang said. 'Since the country embarked on economic reform 30 years ago, the people deeply believe that without business, the economy will not be active, without industries, the people will not get rich, and without agriculture, the country will be unstable. Everyone wants to get rich, so no one wants to be a farmer and no one respects agriculture.

But not everyone shares his fears. 'I'm not particularly worried about food security,' Peng said. 'We're living in a globalised time with clear division of labour. We earn money by producing manufactured goods; we can use the money we earn to buy food from other countries. Actually, Chinese companies are renting land in Serbia, Africa and South America to grow food for our consumption. I don't see any reason why we have to slow our urbanisation and industrialisation.'