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 À1 , and the rice of more than half of the households contained Cd levels higher than the critical level of 0.4 mg Cd kg À1 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 À1 (total Cd) and 0.030 to 13.3 mg Cd kg À1 [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 À1 , and that in the soybean grains ranged from 0.07 to 0.80 mg Cd kg À1 . The soil extractable Cd concentration was well reflected in the soybean grain Cd levels (r 2 ¼ 0.581), but not in the rice grain levels (r 2 ¼ 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.

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In this study sequential extraction was used to fractionate cadmium (Cd) and zinc (Zn) from soils into six operationally defined groups; water soluble, buffer-exchangeable, carbonate, FeMn oxide, organic, and residual. Soil samples from agricultural areas surrounding Pha Te village, Mae Sot District, Tak Province, Thailand, were classified into four categories; forest soil, upland soil, upper-paddy soil and lower-paddy soil. Total soil Cd and Zn concentrations ranged from 0.63 to 30.4 mg kg-1and 14.4 to 594 mg kg-1, respectively. Cd and Zn concentrations were higher in the upper-and lower-paddy soil (5.93 to 30.4 mg kg-1for Cd and 286 to 594 mg kg-1for Zn). These soils are considered as polluted. Cd in the polluted soil was dominantly associated with the buffer-exchangeable and carbonate-bound (40 to 70 % of total Cd), while in non-polluted soils; the residual fraction was dominant (50 to 80 % of the total Cd). The major proportion of Zn (37 to 46 % of total Zn) in the non-polluted soil and the upper-paddy soil occurred in the residual fraction. On the other hand, the major proportion of total Zn in the lower-paddy soil was associated with FeMn oxides (36% of total Zn). The results show that mobility and potential bioavailability of Cd and Zn (61 and 25 %) in polluted soil were higher than in non-polluted soils (15 and 19 %in Cd and Zn, respectively). Metal distribution in different chemical fractions in these soils depended on the respective total metal concentrations.

To identify rice cultivars with low grain cadmium (Cd) levels, 42 cultivars of Thai rice (Oryza sativa L.) were cultivated in a Cd-contaminated field in the Pha Te village, Mae Sot district, Tak province, Thailand, from 2009 to 2011. Among non-glutinous and late-ripening cultivars, lower levels of Cd accumulated in the grains of RD5, RD15, and Sang Yod than in Khao Dawk Mali 105, a prevailing and popular cultivar in this area. Among glutinous and late-ripening cultivars, Khao' Niaw Ubon 1 and Khao' Niaw Ubon 2 accumulated lower Cd levels than a prevailing cultivar RD6. The findings suggest that human cadmium intake can be reduced by selecting low-Cd rice cultivars.

Over 50 years of the Green Revolution since the 1960s, the global population has increased by 2.5 times, cereal production by 3.3 times and the use of N, P and K fertilisers by 9.4, 4.2 and 4.3 times, respectively. Information is still limited, however, on the influence of these impacts on the fertility status of agricultural soils. Here we investigated the influence of the Green Revolution on 142 paddy soils in three tropical Asian countries, that is, Thailand, the Philippines and Malaysia, during the given period by repeated soil sampling in the 1960s and 2010s at or near the same locations. We revealed that the phytoavailablility indices of three macronutrients – N, P and K, that is, available P, exchangeable K and total N, showed 743% (p < 0.01), 12% and 1% increase on average, respectively, while total C showed 9% decline. Comprehensive investigation of overall fertility status by factor analysis using 11 soil parameters suggested that only the factor scores associated with ‘available P status’ increased drastically in all the three countries (p < 0.01) whereas those associated with ‘organic matter and N contents’ and ‘inherent potentiality’ did not exhibit any consistent changes among the countries. In conclusion, intensive soil/fertiliser management systems under the Green Revolution have successfully improved the nutrient status, especially P status, of paddy soils with slight decrease of soil organic matter over the last 50 years, while a large amount of nutrients applied, especially N and K, has been released from soil to the outer environments. Conversion to a high‐efficiency system of external nutrient inputs with organic matter‐conserving strategies is, therefore, urgently required to secure sustainable food production while restoring the environment during the coming decades. Highlights Paddy soils from 142 locations in three tropical Asian countries were repeatedly surveyed in the 1960s and 2010s. Available P increased by 8.4 times, that of exchangeable K and total N showed 12% and 1% increase, while organic matter level showed 9% decline. Factor analysis indicated significant increase of the factor scores for those associated with ‘available P status’. A high‐efficiency system with organic matter‐conserving strategies is to be established for both sustainable food production and restoration of the environment.