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HONG KONG, Feb 12 (Reuters) - Heavy use of nitrogen fertilizers in China since the 1980s has resulted in severe acidification of its soil and some cropland in the south of the country can no longer be used, a Chinese expert said.
"In the south, heavy use of fertilizers has pushed the pH to 3 or 4 in some places. Maize, tobacco and tea cannot be grown. This is a long term effect," said Zhang Fusuo, a professor on plant nutrition at China Agricultural University in Beijing.
Acidity is measured by pH, which ranges from zero (strongly acidic) to 14 (strongly alkaline). Most plants grow best in neutral soil with pH from 6 to 8 because the availability of essential nutrients is usually optimal in this range.
"PH that is under 5 is very serious, under 4 a lot of trees cannot grow," Zhang told Reuters by telephone, adding that the problem was serious in the southern province of Hunan.
Soil acidification occurs naturally from factors such as acid rain, but this problem has worsened with the overuse of nitrogen fertilizers in farming intensive countries particularly in the last few decades.
China's grain production and nitrogen fertilizer use hit 502 million tonnes and 32.6 million tonnes in 2007, up 54 percent and 191 percent compared to 1981, according to Zhang and his colleagues, who published their findings in the journal Science.
They examined two soil surveys in the 1980s and 2000s and found that soil throughout the country had become more acidic since farmers started using cheap nitrogen fertilizers like urea and ammonium bicarbonate in the 1980s.
"The average pH in all of China has decreased by 0.5 unit in the last 20 years. Left to nature, a single unit change needs hundreds of years or even over 1,000 years, but we have got this change now due to fertilizer overuse," Zhang said.
Soil acidification can be reversed quickly with lime, but that is an expensive and labour intensive process that farmers in China are reluctant to undertake.
"The government doesn't subsidise but we hope that through our work the government can help. However, lime is only the third option," Zhang said.
"The first option is to reduce the use of nitrogen fertilizers, the second is return straw, or crop residuals to the land to reduce acidity," Zhang said.
"Now burning (of residuals) is still used because returning it is labour, machinery intensive. The government should subsidise machinery costs." (Reporting by Tan Ee Lyn; Editing by Sanjeev Miglani)