Economic strategists are monitoring the sales of zha cai - a kind of pickled mustard tuber which is a workers' staple - to find out exactly how many people are moving between provinces, reports suggest. The authorities have been tracking sales spikes around the country to determine employment hotspots, says Hong Kong's South China Morning Post . Sales of the spicy vegetable have apparently slumped over recent years in China's industrial southern cities, while consumption has steadily increased in other regions, suggesting that migrant workers are returning to their home provinces.