Another method of deactivation, based on the ability of plants to store up radioisotopes in their aerial parts, appears to be more promising. The land should be sown for several consecutive years with specially selected herbaceous mixtures. These should be mown and the hay burnt in field incinerators provided with special traps for radioactive ash and aerosols; finally, the radioactive ash should be buried in concrete containers. Such a technique might greatly facilitate decontamination of extensive agricul tural areas.
This method may be suitably modified in each case, according to the agricultural practices and ecology of the different plants in the various soils, and the reaction of plants and soils toward various isotopes /36-41/. It was demonstrated, for example, that in light sandy loamy soils the build up of certain radioisotopes (including by any plant species, and especially by some legumes, is much more intensive than in light clayey soils or in heavy clay soils. The difference has been tentatively explained by the mechanical make-up and other properties of the various soils.
This explanation seems inadequate. Attention is drawn to the fact that the microbial population is denser in heavy soils than in light ones; con sequently the absorption of isotopes by competing microorganisms is more intensive in the heavier soils.
The part played by soil microorganisms in the assimilation and fixation of radioisotopes was recently reconfirmed by Krasil'nikov /42/. The soil fauna (such as worms) is also richer in heavier soils. As mentioned earlier, worms facilitate the assimilation of isotopes by plants /35/. It should be recalled that earthworms have lime (oesophageal) glands which accumulate calcium from the soil and then release it; thus, earth worms are most effective in the translocation of calcium, and its cycle in soil. These glands may also appropriately be called strontium gland s, and special attention should be focused on the role of earthworms in the cycle of radioactive strontium and in soil contaminated with the latter. There are several other conceivable circumstances, such as strong fixation of isotopes by soils rich in organic matter, different degrees of fixation according to the pH of the soil, and the presence in soil of cations or anions chemically similar to those of the isotope. Consequently, the
degree of assimilation by plants of isotopes from the soil is the resultant of several factors, and the role of these factors must be well understood in order to obtain positive results with the technique of sowing selected grasses and their subsequent removal by mowing, in the process of de contaminating farmlands.
The method under consideration will require further modifications, since radioisotopes are prevented by other factors from remaining fixed in the soils; for instance, earthworms and various burrowing soil-inhabitants transfer radioisotopes, and the root systems gradually contaminate the neighboring, originally clean soils. This process can probably be stopped fairly easily by sufficiently deep ditches (with embankments facing the contaminated side) dug along the boundaries of the contaminated area so as to prevent migration of the fauna, spreading of radioisotopes from root systems and soil solutions. This technique seems more effective than the plowing of a wide strip along the boundaries of the contaminated sector, in order to destroy its plant cover, followed by the depositing of sand, gravel, or slags for obstructing the spreading of roots and migration of soil animals and solutions. After decontamination it will certainly prove easier to fill the ditches with the soil from the embankments, and level them, than to remove the sand, gravel, or slag which will then interfere with the tilling of the field.
Some authors have recommended liming of the soils, or addition of other calcium or potassium compounds for stopping the translocation of radio active strontium in plants /41/; we, however, regard this method as invalid. Instead of effecting a rapid deactivation, as can be achieved by mowing, liming will only protract deactivation over a longer period. Strontium will nevertheless migrate into plants and then to farm animals and humans, whereby the build up of undesirable doses will inevitably occur, although over a longer period than without liming. Moreover, liming will mainly affect assimilation of strontium but not of other radio isotopes. This technique is ineffective in soils rich in calcium. Finally, radioisotopes are assimilated by plants not only from the soil— they can also be assimilated through the foliage.