Plant material may become contaminated from the atmosphere by fission products that do or do not become diluted with soil substances. Direct contamination may occur on leaves, fruits and seeds. Depending on the nature of the plant, material not retained in this way may be partly absorbed by the basal parts and surface roots of the plant. Material also may enter the soil and then the plant via the roots. The soil route of entry, in addition to leading to dilution, allows material deposited before the plant developed to enter the plant. Soil entry is unimportant, however, with short-lived nuclides. If the soil is undisturbed, fission products will move downward only slowly. Surface rooted crops are therefore likely to absorb the largest amounts from soil deposits.
The extent to which different fission products are absorbed by plants depends on many factors. Radionuclides contaminating plants may reach man directly by way of foods of plant origin or indirectly through animal products. Sources of radioactive contamination are: fallout from nuclear explosion, movement of radioactive nuclides from the soil, radioactive soil dust borne on the wind, and irrigation contaminated by radioactive water.
The increase in the radioactivity of the soil due to artificial radioactive nuclides is small compared to the radioactivity from naturally occurring radioactive nuclides. The increase in radioactivity of crops is relatively greater than that in the soil and the concentration of radioactive isotopes is especially high in water plants.