The authors have developed a practical radioactivation analytical method for the determination of I and Br concentrations. Most soils in Japan contain high levels or both elements (average values, I : 43 mg/kg dry wt., Br : 102 mg/kg dry wt.) except for paddy soils. The high concentrations are ascribed to the environmental conditions (wet and mild climate, acidic soils) that are conducive to the capture of both elements originating from the atmosphere. The contents of both elements in plants (aerial parts) are at a slightly higher level than the worldwide average, suggesting that both elements in soils occur in insoluble forms that cannot be easily absorbed by the plants. The very low concentrations of I and Br in paddy soils (average content, I : 1.8 mg/kg dry wt., Br : 5.3 mg/kg dry wt.) were attributed to rapid dissolution and leaching. Under flooded conditions, the dissolution ratios of both elements increased remarkably, especially, that of I increased by one thousand times along with the change of the chemical forms. Under the oxidizing conditions of non-flooded soil, IO3- (oxidation no.5) accounted for 90% of the total I, a value comparable to that in rain water. Under the reducing flooded conditions, - (oxidation no.-1) accounted for 90%. The I content did not vary appreciably among plant species but was markedly different among plant organs. The average I content (mg/kg dry wt.) in the organs of various plants in Japan was: leaves, 0.46＞ fruits, 0.14 ＞edible roots, 0.055＞seeds, 0.0039. Active intake of green vegetables for the prevention of goiter was suggested.