Nearly two-thirds of hydroponically grown vegetables recently tested for potentially harmful pesticides were found to be in excess of the maximum residue limit, according to the Thailand Pesticide Alert Network (Thai-PAN).
The non-governmental food safety organisation tested 30 vegetable samples marked as hydroponic on sale at fresh markets and supermarkets in Bangkok and other provinces.
Nineteen, or about 63%, contained pesticides at levels above the maximum residue limit. Three others were found to have residue but not exceeding the limit, while the other eight samples were free of any residue.
The tests were conducted on vegetables grown at hydroponic farms -- where crops are grown in a water-based, nutrient-rich solution, without the use of soil.
The findings suggested hydroponic vegetables were not as safe as some consumers might have thought, Thai-PAN coordinator Prokchon Usap told reporters.
Thai-PAN conducted similar tests on fruit and vegetables grown in soil late last year and found that about 54% contained pesticides exceeding the limit.
"Some people think hydroponic vegetables are safe and use less pesticides or are even free of them. That's not true," she said.
Ms Prokchon said the tests found 25 types of toxic chemicals such as ametryn (a herbicide), azoxystrobin (a fungicide), carbofuran and methomyl (insecticides).
She said the findings would be presented to farmers to encourage them to better regulate the use of toxic chemicals in hydroponic vegetables.
Thai-PAN also conducted nitrate tests on hydroponic vegetables and used the European Commission's maximum level of 3,000 milligrammes/kg as a benchmark because Thailand lacks one.
Several lettuces including red coral, as well as red and green oak were below the limit. However, other vegetables such as Chinese kale, red spinach and morning glory were up to twice the limit. Thai-Pan expressed alarm at the results saying nitrate residues in food can cause cancer.
Kingkorn Narindharakul Na Ayudhaya, of BioThai, a group campaigning for sustainable farming policies, said yesterday many consumers mistake hydroponic as organic and believe no pesticides are used in hydroponic farming.
"I believe this study will help show that hydroponic vegetables are not safer than vegetables grown in soil despite the fact they can be made safer," she said.