Jiří Hálas textbook states that soils vary greatly in their ability to bind radioisotopes, the clay particles and humic acids can alter the distribution of the isotopes between the soil water and the soil.  The distribution coefficient K is the ratio of the soils radioactivity (Bq g) to that of the soil water (Bq ml).  If the radioactivity is tightly bonded to by the minerals in the soil then less radioactivity can be absorbed by crops and grass growing in the soil.
* Cs-137 K = 1000
* Pu-239 K = 10000 to 100000
* Sr-90 K = 80 to 150
* I-131 K = 0.007 to 50