36 Buried organic horizons contain more cst3? than adjacent — soil layers. Roots are generally more radioactive than the surrounding soll except at the soil surface. Pumice particles in the soil adsorb radionuclides. Mechanical mixing by animals in old soils and by erosion in young soils results in a redistribution of radionuclides in the surface layers. es??? and sr?? are the principal radionuclides entering a cycle within the soil-plant system. UL