Orange sandy loam Stony dark brown sandy loam Yellow-brown sandy loam Yellow-brown and gray mottled plaatic clay Friable decaying rock Orange loam Black loam Yellow-gray friable loam Red, yellow, brown mot Lied plastic clay Red and yellow decaying rock Georgevilte loam Black loam Red-brown friable stony loam Red very friable stony clay Red and yellow decaying rock Georgcville sandJ clay loam Dark brown loam Red-brown sandy clay loam Red very friabte clay Red and yellow deca~ ing rock Georgeville-Tirzah loam Dark brown loam Dark red•brown loam Red friable clay loam Reddish docaying rock 300 PHILIPPE BOURDEAU

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The relative importance of soil and water in the indirect contamination of flooded rice with radiocaesium was investigated in model rice fields, using a double tracing technique.Roots were found to contain relatively much more 134Cs (coming from the soil) than 137Cs (from the water), whereas the reverse was true for the other plant parts and especially for the caryopsis. Further experiments are needed to explain this lack of isotopic equilibrium within the plant. Flooded rice absorbed 30 to 100 times more Cs from the water than from the soil.The 137Cs activity of the plants was related to the average activity of the water, both in time and in space. At the end of the experiment (13 weeks) the soil of the 5 plots had fixed from 41 to 64% of the 13'Cs brought with the water. Concentration factors between plants and water are quite similar to those observed in nutrient solution culture which confirms the importance of water as a vector of Cs contamination to flooded rice.The upper layer of soil was depleted in Cs to a much greater extent by the surface roots than the lowerlayers by the normal Eoot system.

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