Accumulation of Cs-137 and Sr-90 by wild herbaceous plants is shown to depe
nd on the contents of radionuclides in the labile fraction of soil humus. T
he biological availability of radionuclides from various humus fractions ha
s been estimated using pot experiments. It is shown that water-soluble inog
anic forms Cs-137 and soluble cesium-fulvic associations are the fractions
most available for the root uptake. Sr-90 is more available from the clay-h
umus compounds. Peculiarities of the biological availability of cationic (e
xchangeable) radionuclides and soluble radionuclide-organic compounds sugge
st that exchangeable forms play the dominant role in the process of radionu
clide uptake by roots.