Radioactive caesium deposited on upland Britain following the Chernoby
l accident in 1986 remains available for uptake by plants, despite the
potential of the contaminated soils to fix Cs. The minimum concentrat
ion of Cs+ required to cause Cs+ fixation is 0.60 to 0.75 mM, and this
is unlikely to be reached in any contaminated upland soil, It is sugg
ested that the fixation is caused by interlayer collapse of the illiti
c clay. The observed Cs+ fixation in lowland mineral soils and its abs
ence from acidic upland soils is explained by the action of K+ ions, w
hich can also induce interlayer collapse. Although Cs+ ions are unlike
ly to be fixed in acid organic soils, they can be strongly sorbed on a
ny unoccupied Cs-specific sorption sites in the narrow parts of illiti
c wedge zones. A method for determining the number of such sites is de
scribed. For two of the soils studied the number of sites ranged betwe
en 8x10(-8) and 1x10(-5) mmol kg(-1); for two others there appeared to
be no unoccupied Cs-specific sites, Although Cs+ ions sorbed on such
sites do not exchange with other ions, they can be desorbed if the con
centration of Cs+ in solution is decreased, Thus, radioactive Cs in su
ch soils will remain available for plant uptake, unless interlayer col
lapse can be induced.