Current situation of cadmium-polluted paddy soil, rice and soybean in the Mae Sot District, Tak Province, Thailand

Abstract

Since 2002, a joint research team from the International Water Management Institute and the Department of Agriculture, Thai Government, has reported that there are cadmium (Cd)-contaminated paddy fields in northern Thailand. We evaluated the current situation of the Cd-polluted paddy fields in this report. Home-consumed rice grains were collected from all 23 households in the village of Pha Te, Mae Sot District of Tak Province, Thailand. The Cd concentration in unhusked rice (Oryza sativa L.) grains ranged from 0.04 to 1.75 mg Cd kg−1, and the rice of more than half of the households contained Cd levels higher than the critical level of 0.4 mg Cd kg−1 polished rice, which is CODEX standard. Among the paddy fields, we selected one plot (1.2 ha) and analyzed Cd concentrations of the soil, and rice and soybean (Glycine max L.) grains. In this area, a rotation cropping system of wet-season rice and dry-season soybean is common practice. The soil Cd concentration ranged from 0.31 to 13.9 mg Cd kg−1 (total Cd) and 0.030 to 13.3 mg Cd kg−1 [extracted with 0.1 M hydrochloric acid (HCl)], the Cd concentration in the rice grains ranged from 0.12 to 1.27 mg Cd kg−1, and that in the soybean grains ranged from 0.07 to 0.80 mg Cd kg−1. The soil extractable Cd concentration was well reflected in the soybean grain Cd levels (r2 = 0.581), but not in the rice grain levels (r2 = 0.015), suggesting that rice grain Cd levels are influenced not only by the soil Cd concentration, but by other factors as well, such as soil water regime and soil pH. However, a significant difference in the grain-Cd concentration was found; that is, lower Cd in the cultivar “Khao’ Khaeng” and higher in the “Khao Dawk Mali 105”, which suggests a possibility of selecting a rice cultivar having low-grain Cd.