Geographical Distribution of Potential Problem Areas with Micronutrient Anomalies in Tropical Asian Paddy Soils

Abstract

A total of 407 paddy soil samples were analyzed for the extractable essential trace elements, boron, cobalt, copper, manganese, molybdenum, and zinc, by inductively coupled argon plasma atomic emission spectroscopy (ICAP-AES). The extractants used were 0.1N hydrochloric acid, 2.5% acetic acid, and water. Results showed that considerably higher amounts of trace elements were extracted by 0.1N hydrochloric acid than 2.5% acetic acid, which in turn extracted more trace elements than water. To obtain information on areas with threshold levels of trace elements, the acetic acid-extractable values deviating from the means by two standard deviations or more after log-transformation were considered to be abnormal. In the case of molybdenum, the same criterion was applied to the original data without logtransformation. Values falling outside the normal range were plotted on maps. The most prevalent toxicity problems were found to be abnormally high boron and molybdenum contents. The former abnormality was principally recognized in India, the latter in Thailand and also in India. About 24% of the paddy soils examined in Tropical Asia contained abnormally low levels of various trace elements. Abnormally low levels of molybdenum, zinc, and copper and cobalt are respectively evident in Indonesian, Cambodian, and West Malaysian soils. East Malaysian soils contained abnormally low levels of manganese and zinc, Thai soils, abnormally low levels of copper, molybdenum, and manganese. Abnormally low levels of molybdenum were also found in Indian soils. The findings in this study have provided valuable baseline information that may be used for further assessment of the geographical distribution of trace element abnormalities in other parts of Tropical Asia. Moreover, this study has delineated areas where trace element problems are evident.