February 12, 2010

Excessive use of nitrogen fertilisers 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.

In the south, heavy use of fertilisershas pushed the pH to three or four in some places and crops cannot be grown, said Zhang Fusuo, a professor on plant nutrition at China Agricultural University in Beijing.

Most plants grow best in neutral soil with pH from 6-8 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 said, 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 fertilisers in farming intensive countries particularly in the last few decades.

China''s grain production and nitrogen fertiliser use hit 502 million tonnes and 32.6 million tonnes in 2007, up 54% and 191% 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 fertilisers like urea and ammonium bicarbonate in the 1980s.

"The average pH in all of China has decreased by 0.5 units 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 fertiliser 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. He offered alternatives to address the problem. "The first option is to reduce the use of nitrogen fertilisers, the second is return straw, or crop residuals to the land to reduce acidity," Zhang said.