The Behavior of RADIOACTIVE FALLOUT in SOILS and PLANTS National Academy of Sciences- National Research Council Publication 1092 ,„;,f5«!5»«t»>^'.{5% National Academy of Sciences — National Research Council COMMITTEES ON THE BIOLOGICAL EFFECTS OF ATOMIC RADIATION In 1955, Dr. Detlev W. Bronk, President of the National Academy of Sciences, appointed a group of scientists to conduct an extended appraisal of the effects of high-energy radiations on living organisms. Since the beginning, the studies have been supported from funds provided by the Rockefeller Foundation. The over-all study has been divided into six parts, each being considered by a separate Com- mittee. The areas under consideration are (1) genetics, (2) pathology, (3) agriculture and food supplies, (4) meteorology, (5) oceanography and fisheries, and (6) disposal of radioactive wastes. The Committees themselves do not perform research; like many other NAS-NRC committees, they maintain appropriate surveillance within their own fields; evaluate, in the light of their own experience and judgment, the significance of reported findings; and recommend effective programs of action. In consequence, the published reports not only summarize present knowledge but may also recommend needed research, reveal areas of concern or confidence, and project larger prob- lems associated with potential hazards of the future. The reports vary greatly in the extent of technical detail they contain. Some are intended for the lay reader, to tell the citizen what science has learned about the potential effects of atomic radiation on himself, his progeny, and the race as a whole, so that he may participate more intelligently in making decisions about atomic energy. Others contain the results of specialized studies, made by the Committees, of various aspects of the problems. This study will be a continuing one, since many of the problems involve basic scientific questions that will take many years to answer. New questions may be expected to arise as the uses of atomic energy continue to expand. The members of the Committees, numbering more than 100, are among the most distinguished scientists in their fields in the United States. They have given generously of their time and talents in making these analyses. They serve as individuals, contributing their knowledge and judgment as scientists and as citizens — not as representatives of any institution, company, or Government agency with which they may be affiliated. The studies have been greatly assisted by consultations with many authorities in private and Government organizations. Following is a list of the Committees participating in this study and their chairmen: Committee on Genetic Effects of Atomic Radiation James F. Crow, University of Wisconsin Committee on Pathologic Effects of Atomic Radiation Shields Warren, New England Deaconess Hospital, Boston Committee on Effects of Atomic Radiation on Agriculture and Food Supplies A. G. Norman, University of Michigan Committee on Meteorologic Aspects of Effects of Atomic Radiation Lester Machta, U. S. Weather Bureau, Washington Committee on Effects of Atomic Radiation on Oceanography and Fisheries Roger Revelle, University of California Committee on Disposal and Dispersal of Radioactive Wastes Abel Wolman, Johns Hopkins University THE BEHAVIOR OF RADIOACTIVE FALLOUT IN SOILS AND PLANTS A Review Prepared for the Committee on Effects of Atomic Radiation on Agriculture and Food Supplies National Academy of Sciences— National Research Council by M. H. FVere and R. G. Menzel /'Soil Scientists U. S. Soils Laboratory Agricultural Research Service U. S. Department of Agriculture Beltsville, Maryland i CO I ru I ^ I '-' i □ ! m ; CD : CD K. H. Larson Chief Environmental Radiation Division Laboratory of Nuclear Medicine and Radiation Biology University of California Los Angeles, California Roy Overstreet Professor of Soil Chemistry Department of Soils and Plant Nutrition University of California Berkeley, California R. F. Reitemeier Soil Scientist Division of Biology and Medicine U. S. Atomic Energy Commission Washington, D. C. Publication 1092 National Academy of Sciences— National Research Council Washington, D. C. 1963 MEMBERS OF COMMITTEE ON EFFECTS OF ATOMIC RADIATION ON AGRICULTURE AND FOOD SUPPLIES A. G. Norman, Chairman University of Michigan Ann Arbor, Michigan C. L. Comar, Cornell University, Ithaca, New York George W. Irving, Jr., U. S. Department of Agriculture, Washington, D. C. James H. Jensen, Oregon State University, Corvallis, Oregon J. K. Loosli, Cornell University, Ithaca, New York R. L. Loworn, North Carolina State College, Raleigh, North Carolina Ralph B. March, University of California, Riverside, California George L. McNew, Boyce Thompson Institute for Plant Research, Inc. , Yonkers, New York Roy Overstreet, University of California, Berkeley, California Kenneth B. Raper, University of Wisconsin, Madison, Wisconsin H. A. Rodenhiser, U. S. Department of Agriculture, Washington, D. C. W. Ralph Singleton, University of Virginia, Charlottesville, Virginia Ralph G. H. Siu, Office of the Quartermaster General, Washington, D. C. G Fred Somers, University of Delaware, Newark, Delaware George F. Stewart, University of California, Davis, California Library of Congress Catsilog Card Number 63-60065 FOREWORD Although man has always been exposed to some radiation from naturally- occurring radionuclides in his environment, and although the food he consumes has always carried some small burden of radioactivity, the coming of the "Atomic Age" has already brought with it a rise in the level of radiation to which man is exposed and the appearance of some new sources of radiation that did not in the past consti- tute a part of the natural background. The latter has been tacitly accepted as being of no great concern, even though the level of exposure may vary widely from place to place. The testing of nuclear weapons has resulted in the appearance in man's environment the world over of radionuclides not formerly present. Man's exposure to radiation is in part external— from the materials aroimd him — but it is also in part internal, by reason of the ingestion of food and water having some radioactive components and the inhalation of radioactive particulates or gases in the atmosphere. The effects, if any, on the well-being of the individual "depend upon the radiation dose (and dose rate) delivered to various tissues and upon the radiosensitivity of the tissues. " This report deals with some of the early steps in the sequence of events that transfers radionuclides in the environment to the tissues of man in what has come to be referred to as the "food chain. " The food chain of man is not inherently more complicated than those of other organisms that are herbivorous or carnivorous. There is, however, the difference that man has considerable freedom of selection and, at least in industrialized countries, subsists on foods, fresh or processed, derived from diverse and often remote locations. The dietary exposure of man to radionuclides is therefore a most complex question that can be approached realisti- cally only by examining the principles involved in the various steps of the food chain. Recognizing the fact that the human diet is derived from the soil directly or indirectly through animals, the Committee sought to have prepared a comprehensive review of the fate of fallout radionuclides in cultivated soils and their transfer to or incorporation in crop plants growing thereon. This report is essentially a dis- cussion of the principles involved and makes no attempt at evaluation of hazards to man, which have been discussed elsewhere in reports prepared by the related Com- mittees on Pathological Effects and Genetic Effects of Atomic Radiation. A. G. Norman, Chairman Committee on Effects of Atomic Radiation on Agriculture and Food Supplies CONTENTS Page I. INTRODUCTION 1 II. GENERAL 2 lU. RELATIVE AVAILABILITY OF FALLOUT CONSTITUENTS 5 IV. SOIL REACTIONS 6 A. Adsorption 6 B. Desorption 6 C. Effects of Other Ions ^ D. pH Effects 7 E. Clays 7 F. Organic Matter 8 G. Fixation 8 H. Erosion 8 V. PLANT RELATIONS 9 A. Uptake 9 B. Translocation 9 C. Aerial Contamination 10 D. Plant-Base Absorption 11 E. Distribution in Plants 11 F. Species Differences 12 VI. RADIATION EFFECTS. 13 A. External Radiation 13 B. Internal Radiation 13 VII. SOIL-PLANT RELATIONS 15 A. Basic Aspects 15 B. Competing and Carrier Cations 15 C. Distribution Factors 16 D. Effects of Clays and Anions 17 VIII. MANAGEMENT 18 A. The Effect of Liming 18 B. Fertilizers 18 C. Cultivation 18 D. Moisture 19 E. Prolonged Cropping 19 IX. SUMMARY 20 BIBLIOGRAPHY 22 I. INTRODUCTION Severe radioactive contamination of land might result from the deposition of fallout originating in the detonation of nuclear weapons or nuclear reactor accidents. The deposition of fallout on soil or plants would introduce radioactive isotopes into the food chains of animals and man. The Committee on Effects of Atomic Radiation on Agriculture and Food Sup- plies, National Academy of Sciences— National Research Council, requested the compilation of current informiation on the reactions of radioactive constituents of fallout with soils and crops, in order to determine whether knowledge is available for the formulation of agronomic recommendations. This report is an attempt to summarize the existing information on this subject, especially with respect to origi- nal sources of experimental results. II. GENERAL The nature and magnitude of the fallout hazard to agriculture depends upon the chemical and physical properties of fallout, characteristics of the soil and land sur- face, and the type and density of vegetation, as well as upon the amount of fallout. Thus, the choice of reclamation and decontamination measures would also be influ- enced by these factors. The radioactivity in fallout is derived principally from fission products, and therefore depends on the fission yield of a nuclear explosion. If the fission yield gives energy equivalent to the explosion of one million tons of TNT, the gamma radiation activity of the fission products would be as shown in Table 1 (31). The beta radiation activity would be 2-20 times as great as the gamma activity (51), but unless it is in direct contact with the body, it is of less physiological significance. Alpha activity from unfissioned materials and the radioactivity from neutron- activated products is usually negligible by comparison. TABLE 1 Total Gamma Radiation Activity of Fission Products from a 1-Megaton Explosion (31) Time After Activities Explosion (Megacuries) 1 hour 300,000 1 day 6,600 1 week 640 1 month 110 1 year 5. 5 The rate of decay of fission products is rapid at first and becomes progres- sively slower with increasing time after the explosion (Table 1). This change in rate of decay is caused by the presence of a mixture of short- and long-lived nuclides in fresh fission products. The known fission products include 170 isotopes of 35 elements, ranging from zinc-72 to terbium- 161 (9). Some short-lived nuclides of importance in agricultural products are iodine-131, barium-140, and strontium-89. Many fission products of interest have radioactive daughters by decay. The chemical and biological properties of these daughter nuclides are different from those of their parents. If the half-life of the daughter is sufficiently great, its dis- tribution in the soil or plant depends upon its characteristics, not those of its par- ents. Some possible effects of this phenomenon are discussed in detail'elsewhere (47). The half-lives of 13 parent nuclides and 8 daughters are listed in Table 2. TABLE 2 Half-Lives of Fission Products of Possible Significance in Food Chains and of Some Radioactive Daughter Nuclides Atomic Mass Parent Nuclide Daughter Nuclide Number Element Half- Life Element Half- Life 91 Strontium 9. 7 hours Yttrium 58 days 131 Iodine 8. 0 days 140 Barium 12. 8 days Lanthanum 40 hours 86 Rubidium 18. 6 days 141 Cerium 28 days 103 Ruthenium 41 days Rhodium 5. 4 minutes 89 Strontium 54 days 95 Zirconium 65 days Niobium 35 days 144 Cerium 275 days Praseodymium 17 minutes 106 Ruthenium 1. 0 year Rhodium 2 hours 147 Promethium 2. 3 years 137 Cesium 26. 6 years Barium 2. 6 minutes 90 Strontium 27. 7 years Yttrium 64 hours The discussion of possible fallout patterns is beyond the scope of this report, but it should be stated that fallout distribution depends on many parameters. These include meteorological conditions, yield of the explosion, elevation of the burst, and the nature of the terrain. The fallout from a particular surface nuclear explosion may be classified in four categories — dropout, close-in, tropospheric, and stratospheric. These cate- gories differ in distance and time from the point of detonation. Dropout occurs at or very near ground zero, where the prompt effects of the burst are greatest. Close-in fallout consists of solid particles settling to earth under gravity within a few hours after the explosion. It may extend several hundred miles downwind from the site of a large nuclear explosion. Tropospheric and stratospheric fallout con- sists of very small particles which may remain suspended in air for a long time. The scavenging action of precipitation is important in bringing these particles to earth. High concentrations of radioactive materials are found in areas receiving close-in fallout, and their subsequent distribution in soils and crops is therefore of special significance. Yet, it may be possible to take remedial actions in these areas, whereas such actions might be precluded in areas affected by dropout be- cause of the vast physical destruction. Less than one fourth to more than one half of the fission products formed in a nuclear explosion at or near the ground surface may return to earth as close-in fall- out (51; 135, pp. 105-106). If early rain is associated with the fallout cloud, the amount of close-in fallout increases. Explosions that are so high that the fireball does not touch the ground may produce little close-in fallout. The fate of the radioactive isotopes in deposited fallout depends on the physical properties of the fallout and the chemical behavior of the nuclides. Surface bursts in the kiloton range, over continental soils, yield predominantly siliceous radioactive particles (3, 83, 92). Particles from tower bursts in the same energy range reflect the incorporation of tower materials (83). Megaton bursts over coral islands have produced primarily calcareous particles (92). It has been reported that the dust from the Castle Bravo burst of 1954 was mainly calcite. Presumably, aragonite was evap- orated, re crystallized as calcite, and precipitated as aggregates (44, 126). This wide range in gross chemical composition, considered along with the observed range of particle sizes, leads to the conclusion that the biological availa- bility of the constituent radioactive isotopes cannot be predicted for a particular ma- terial without some knowledge of its characteristics. The solubility in distilled water of selected particles from a continental detonation ranged from 0. 28 to 1.2 per cent of the total radioactivity. One to 74 per cent was dissolved in 0. 1 N HCl (83, 51). In another study (8), it was found that the solubility in 0. 1 N HCl of deposited particles from four tower shots ranged from 20 to 30 per cent and that of airborne particles from 65 to 85 per cent. Some of the nuclides of agricultural importance, notably strontium- 90 and cesium-137, may be partially depleted in the local and close-in fallout. This frac- tionation results from the fact that precursors of these nuclides are noble gases early in the condensation of fallout particles (136, p. 72). A major part of the biological experimentation with fallout constituents has been conducted with soluble sources of the respective isotopes. Consequently, the observed effects exceed those that would be obtained from the same amount of the isotope in the less soluble fallout. It is presumed that the use of soluble sources generally provides maximal effects. In addition to the variability in the composition and solubility of fallout, the soil and plant aspects of the food chain contaxnination are complicated by variations in soil properties and differences in the structure and physiology of plant species. This will be the subject of discussion in the following sections. III. RELATIVE AVAILABILITY OF FALLOUT CONSTITUENTS Single crops of plants may absorb about two per cent of the total radioactivity in a soil contaminated by a nuclear explosion, but usually they absorb less than 0. 1 per cent (85, 120). Strontium-89 and strontium-90 are the major nuclides absorbed (52) and may account for as much as 70 per cent of the absorbed activity from one- year-old, mixed-fission products (99). It is generally accepted that about one per cent of the applied strontium and less than 0. 1 per cent of the other elements are taken up by single crops of plants (47, 78, 94, 96, 104, 105). Higher amounts of strontium uptake, 4 to 8 per cent, have been observed in pot experiments (86, 105). The uptake of an element depends on its concentration in the external medium (17, 30, 67, 76). The ratio of plant-tissue concentration to the external-medium concentration, called a concentration factor, is used to indicate the relative uptake of the different elements. Results from solution culture studies have been based on the fresh tissue weight; oven-dry weight has been used for soil culture studies. There is relative agreement in the order of concentration factors for different isotopes in pot experiments using the Neubauer technique or other techniques and in field experiments (95). Some reported concentration factors for fallout constituents in soil culture are 0. 05 for the alkaline earth group, 0. 009 for the rare earths, 0. 05 for total beta activity in barley, and 0. 02 for total beta activity in beans (117, 120). Using soluble forms of isotopes in nutrient solutions (94), concentration factors from 0. 05 to 1. 0 have been found for strontium, cesium, iodine, and barium. The range was 0. 0001 to 0. 001 for ruthenium, yttrium, and cerium. IV. SOIL REACTIONS A. Adsorption The adsorption of cations by soil particle surfaces from solution can occur by several processes; ion exchange is one of the most important. It was found that ion exchange increases the sorption of calcium and strontium by a volume of soil 10 times greater than that held in solution in the pore space (88, pp. 191-211). The adsorption of plutonium, cesium, strontium, yttrium, and cerium ions from solution was found to be nearly complete up to amounts equal to 0. 01 times the saturation capacity of the soil (61, pp. 170-190; 62). Strontium has a slightly higher adsorption energy than calcium (38, 48, 59). Leaching and uptake experiments indicate sites of differential adsorption (15, 38). The rate of exchange from solution to surface is rapid. -^ For soils of high "cation exchange capacity" (CEC), the reaction is essentially complete in one minute, whereas for soils of lower CEC there is a significant rise in adsorption over a longer period of tLme. The equilibration of strontium-89 and calcium-45 with labile soil calcium is complex, and the differential behavior of strontium and calcium increases up to 70 days (59). Leaching soil columns with mixed-fission product solutions resulted in 80- to 85-per cent adsorption of the total radioactivity in the first few centimeters of the soil (47). This accumulation in the top few centimeters agrees with analyses of soils from test sites (108, 117, 120). Much work has been done on the adsorption of fission products from solution in relation to the disposal of waste products. In such experi- ments, the concentrations of radioisotopes and salts are usually in excess of those expected in agricultural soils, but some of the results at lower concentrations may be applicable. B. Desorption Rare earth isotopes contribute one half to three fourths of the activity in some soils contaminated by fallout (117, 120). In one soil, 50 volumes of water, corre- sponding to 320 inches of rain, were required to leach 10 per cent of the beta activity from one soil volume. The rate of leaching was nearly constant after the first 20 volumes. About four per cent of the radioactivity in fallout from Operation Hurricane was leached through 20 cm of soil in a 12-week field experiment (108). The leached radioactivity was mainly ruthenium- 106 and rhodium-106. The activity of an equilib- rium mixture of the soil, 405 days after the blast, was due mainly to ruthenium- 106, rhodium-106, cerium-144, and praseodymium- 144. ■•^ Unpublished results. Soil and Water Conservation Research Division, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, Maryland. strontium is leached slowly through the soil at a rate related inversely to the CEC. Under cropping and fertilizer treatments in soil columns, calcium-45 moved about four inches downward (11), but no detectable movement three inches laterally or four inches downward was observed after 14. 5 inches of rain in 89 days of field ex- periments (12). Strontium-90 from worldwide fallout was located primarily in the upper two inches of uncultivated soil during 1954 and 1955. In 1957, as much as one half of the strontium-90 was found in the two- to six-inch layer of some soils (1, 2). The desorption of cesium is less than that of strontium, possibly because of fixation by micaceous minerals (113). The rate and depth of leaching increases with increments in salt concentration, acidity, and complexing agents, and with a de- crease in base saturation and buffer capacity of the soil. Lime and organic matter also reduce the desorption of strontium and cesium (47). C. Effects of Other Ions The complementary ion exerts a strong effect on the adsorption of a cation. All cations tend to reduce strontium and cesiunn adsorption if used in large amounts. The order of replacement on soil materials is usually lithium< sodium Solanaceae>Compositae>Gramineae for the tops and Leguminosae >Gramineae >Conipositae>Solanaceae for the roots (142). Other workers report no consistent differences between the lower and higher orders of the plant kingdom (79). The calcium and strontium content of eight legumes was about three times that found in eight grasses (138). The absorptive power of a given species for strontium is con- sidered to be proportional to its absorptive power for calcium (17, 66). Characteristics of the root system may be very imiportant in determining the uptake of radioisotopes from soil. Russian thistle can absorb strontium from a soil depth greater than 3-1/2 feet (116). Since the plants of the grass family have rela- tively shallow root systems, they will preferentially absorb nuclides occurring near the surface rather than those placed at a greater depth. With a grass-clover mixture, it was found that both the strontium content and the strontium-to-calcium ratio were reduced 70 per cent by plowing under the surface contamination (72). More deeply rooted crops showed only small effects from this deeper placement. Bicarbonate has differential effects on plant species, with beans taking up lesser amounts of cations than barley in the presence of bicarbonate (32). Additional interactions of plant species with rate of uptake, distribution, temperature, and other factors are probably of minor importance when considering broad differences. 13 VI. RADIATION EFFECTS The severity of associated heat and blast effects from nuclear test detonations have tended to obscure radiation effects on plants. However, radiation effects may be a significant force in modifying the ecological systems after a nuclear attack. At present, the effects of ionizing radiation have been observed for only a few hundred of the more than a million and a half different kinds of organisms. Most of these data were obtained under experimental conditions of minimum environmental stress. A. External Radiation The median lethal dose for flowering plants ranges from about 1, 000 to 150,000 roentgen units, and the sensitivity of a particular plant may vary widely according to the particular stage in its life cycle (90). The variation in sensitivity between plants has been correlated with characteristics of the cell nucleus. Plants with low chromosome number and high nuclear volumes are the most sensitive (122). Pine trees appear to be relatively more sensitive than other trees. At an un- shielded reactor site, pines died after receiving 2,000 or more rads in an initial burst, but pines at greater distances died after accumulating about 8,000 rads; hardwood trees in the area showed little effect (90). With gamma radiation from cobalt-60, pines showed detectable effects from two roentgens per day for an average of 240 days per year over a period of nine years (122). Several other observations have been made on irradiated trees (90). The winter dormancy is prolonged by an amount proportional to the dose received during the preceding summer — one to two weeks' delay for several hundred rads. The terminal buds are more sensitive than the lateral buds and, of the lateral buds, those farthest from the trunk are most sensitive. Two years after a nuclear explosion at the Marshall Islands, the number of different plant species showing pathological effects and abnormalities increased with an increase in fallout (23). However, differences in edaphic factors such as soil fertility may confound these observations (39). B. Internal Radiation The radiation emitted by the absorbed radionuclides may also cause damage. In greenhouse experiments, at concentrations of 5 mc of strontium-90 or 13 mc of cesium- 137 per g of wheat leaves, the protein levels decreased and the carbohy- drate levels increased (34). A 30- to 50-per cent decrease in yield of grain was ob- served at those concentrations of radioactivity. Resistance to radiation daunage in- creased with age of the plant. 14 In young barley plants, phosphorus-32 radiation damage was confined to cells in zones of active division (10). The lowest specific activity level at which damage was produced corresponded to 3. 2 mc of phosphorus-32 per g of phosphorus, or about 170jLic of phosphorus-32 per g of dry plant tissue. A more complete treatment of this subject is found in the Proceedings of the First National Symposium on Radioecology (115). 15 VII. SOIL-PLANT RELATIONS A. Basic Aspects The soil and plant components of the soil-plant system are individually com- plex, as is evident from the preceding sections. The combination of these two com- ponents increases the difficulties of understanding and generalization. Both systems, independently and together, are dynamic. Plant growth requires the continuous net removal of ions from the soil into the plant. On the other hand, changes in mois- ture and the removal of ions by the plants continually change the quantity of the ions available to the plant. B. Competing and Carrier Cations The kinds and amounts of the complementary ions affect the availability of a given ion (48, 87). Two types of processes can be distinguished: the exchange reac- tions governing the distribution of ions between clay and solution (37, 48) and the competitive effects during the course of ion absorption by plants (17, 20, 21, 27, 28). Since several cations compete for the same carrier site, increasing the concentration of one should decrease the uptake of others in the same group. Examination of this hypothesis in greenhouse and field experiments has shown this to be true within certain ranges. Increasing the calcium concentration in nutrient solution from zero to two milliequivalents per liter reduces the uptake of strontium (43). Further increases in calcium reduce strontium uptake only slightly. A fourfold reduction of strontium uptake in field experiments appears to be the maximum that can be achieved by the addition of calcium to acid, low-calcium soils. Even smaller reduction occurs in soils richer in calcium. The addition of stable strontium has little effect on radioactive strontium up- take because of the similarity of strontium to calcium and the thousandfold greater abundance of calcium in soils (68, 127). In one experiment, no effect of stable stron- tium was observed (43) and a slight increase in strontium-90 uptaike was found in another experiment (104). It was postulated that small increments of strontium dis- placed some of the strontium-90 from the exchange complex into solution. It is esti- mated that five tons of strontium amendments per acre would be needed to reduce the strontium-90 uptake appreciably (104). A depressing effect of potassium on plant uptake of calcium, magnesium, and strontium has been observed (47, 54, 65). Potassium treatments decreased stron- tium uptake 20 per cent in wheat plants (47) and 40 per cent in radish plants (54). In a field comparison of plant concentrations of different elements with the cor- responding soil concentrations (57), it was found that varying levels of calcium and 16 magnesium brought about only slight changes in the strontium content of four pasture species. However, potassium and sodium reduced the strontium content of blue- grass as much as 34 per cent and that of redtop 51 per cent, whereas sodium addi- tions increased strontium in Korean lespedeza. Similar observations have been made on the uptake of cesium- 137. When soil potassium is low, additions of potassium reduce the cesium uptake, but additions of stable cesium often increase cesium- 137 uptake, presumably by displacement of ex- changeable cesium- 137 into solution (84). Rubidium, ammonium, and calcium in- creased cesium uptake 8, 3, and 1-1/2 times, respectively, but when carrier cesium- 137 was used, practically no effects of these ions were observed (128). C. Distribution Factors Because of the similarity in chemical behavior between certain fission products and certain essential elements, fission-product uptake is often reported relative to the uptake of the chemically similar essential element. The "Observed Ratio" (OR) (18), or the "Distribution Factor" (DF) (66), for strontium is the ratio of stron- tium to calcium in plant or plant part divided by the ratio of strontium to calcium in the nutrient medium. The term "discrimination factor" is expressed in the same manner but usually applies to a single step in the various successive processes that determine the over-all relative distribution of the two elements between substrate and tissue. In nutrient solution experiments, when only the plant discrimination processes are measured, the strontium/calcium DF is close to 1. 0 (67, 105). This indicates little discrimination between strontium and calcium and is true for most of the plant parts except the roots, where DF values as high as 6. 0 were observed for low- solution concentrations of strontium. The average DF values of rubidium/potassium and cesium /potassium for millet, oat, buckwheat, sweetclover, and sunflower plants were 0.85 and 0.20, respectively (67). This indicates some discrimination by the plant against rubidium and more against cesium. Alfalfa and wheat grown on eight soils (98), wild plants and corn grown on soil in a radioactive waste disposal area (6), and beans grown on a Sassafras sandy loam with added calcium (105) had strontium /calcium DF values close to 1. 0, indicating little discrimination in soil reactions. However, the DF values can vary within a given plant, ranging from 2. 6 for corn flowers to 0. 5 for corn grain (6). The calculated DF will vary to some extent, depending upon the method of extracting the cations from the soil. Based on the amounts of strontium-89 and cal- cium-45 added to Cinebar soil, the DF for beans ranged from 0. 64 to 1.2 (43). Another experiment with strontium-89 and calcium-45, using a dilute calcium chlo- ride extract of the soil, gave an average DF of about 0. 7 (111). Discrimination factors from 0. 8 to 1.6 were found for strontium /calcium in eight grasses and eight legumes grown in three soils, using ammonium acetate for extraction (138). In a study of soils and vegetation in a disposal area (33), the best soil index of strontium -90 uptcike by plants appeared to be concentrations of strontium-90 in the saturation ex- tract. Others (112) have also suggested that a water extract may provide a better measure of the availability of strontium and calcium in the soil than the exchangeable fraction. 17 Barley, buckwheat, and cowpeas grown on an Evesboro sand gave a cesium/ potassium DF of 0. 02, based on the amount of radioactive cesium added to the soil and the acid-soluble potassium (66). The range was from 0. 06 to 0. 77 for wild plants and corn grown in a radioactive disposal area (6). Upland rice, wheat, and beans grown on a Japanese soil gave much lower DF values — 0. 002 to 0. 003 (41). Discrimination factors of 0. 02 for barium/calcium and 0.4 for rubidium/potassium were also found for the barley, buckwheat, and cowpeas grown on the Evesboro sand (66). Discrimination in plant uptake of strontium and calcium is usually slight in pot experiments, except for roots, but apparent discrimination against either stron- tium or calcium can occur in the field. The strontium- 90 is normally concentrated near the soil surface, or in the plow layer, the distribution of exchangeable calcium in the profile usually is nonuniform, and the root zone varies with the plant species and with soil conditions. D. Effects of Clays and Anions Twice as much strontium is taken up from illite clay suspension as from bentonite clay (63), which indicates that bentonite holds strontium more strongly than illite. Clays have more of an effect than just as an anion, for the aluminum concen- tration affected calcium uptake from calcium sulfate but not from calcium clay (64). Anions have differential effects on various species. In tobacco, calcium uptake is nearly the same from the carbonate, sulfate, or phosphate salt, but alfalfa seems to prefer carbonate to sulfate or phosphate as a calcium source (102). Strontium up- take by bean plants in one soil was reduced 40 per cent by calcium carbonate but only 15 per cent by calcium sulfate (105). Bicarbonate in nutrient solutions (32) reduced strontium uptake by 70 per cent, rubidium by 43 per cent, ruthenium by 24 per cent, and cerium by 19 per cent. Cesium was the only ion studied in which the uptake was not adversely affected. Thus, the reduced strontium uptake from alkaline calcareous soils may be due to a bicarbonate ion effect as well as to a calcium effect. Strontium added to soils as the sulfate, oxalate, hydroxide, fluoride, carbonate, or phosphate was one tenth as available to plants as strontium added as the chloride or nitrate (132). Also, calcium sulfate and calcium carbonate were more effective than calcium chloride in reducing strontium uptake. This indicates an effect of solu- bility, since all unavailable strontium salts are of low solubility. Massive doses of phosphate have reduced strontium uptake 50 per cent on alkaline soils but have given no reduction on acid soils (132). Other studies (14, 53) report conflicting results from the addition of phosphate. Hydroxyapatite and fluoroapatite are insoluble calcium phosphates that exist under alkaline conditions. The strontium analogs of these compounds are expected to be similarly insoluble (26). 18 VIII. MANAGEMENT A. The Effect of Liming The effect of calcium on strontium uptake has been discussed to some extent in two previous sections. A practical application of this is in the liming of acid soils. The uptake of strontium from different soils increased in proportion to the reciprocal of the exchangeable calcium (24, 66). No relation has been found between the total calcium or calcium carbonate content and strontium uptake (22, 30). Since soils have finite exchange capacities, overliming has little additional effect on strontium uptake. This is demonstrated by the application of lime to neutral or alkaline calcareous soils, which reduces the strontium uptake only slightly (30, 98, 105, 139). A fourfold re- duction of strontium uptake by liming acid soils is generally the most to be expected (109, pp. 18-49). The ratio of radioactive strontium to calcium in the plant is as important as the radioactive strontium concentration because deposition of radioactive strontium in animal skeletons depends on the calcium content of the diet (18). Liming tends to decrease this ratio until the exchange capacity of the soil becomes saturated with bases. Then, when the soil is no longer acid, additional lime remains undissolved and thus unavailable to the plant. Therefore, although liming serves a twofold purpose of reducing both the strontium concentration and the strontium-to-calcium ratio, the lowest values for both are usually achieved at a point that coincides with the amount of lime needed for maximum crop yields. B. Fertilizers In some experiments, fertilizer has increased the uptake of fission products, but in others no increase occurred (14, 47, 117, 120). Fertilizers and manures may affect the availability of the fission products, as discussed in previous sections, but it is possible that better plant growth will obscure any other effect. C. Cultivation Cultivation tends to increase strontium uptake by some crops (14), possibly by providing more root contact. Greater uptake of calcium results from a completely mixed calcium application than from a banded application (12). Increased uptake of strontium in the second year of a field experiment has been attributed to a more uniform distribution of strontium (14). In the case of shallow -rooted grasses, as dis- cussed under species differences, it appears that plowing reduces the strontium- 90 to calcium ratio of the new plamting (72). 19 Plowing surface -contaminated fields to a depth of seven inches resulted in 50 per cent of the activity in the zero- to four-inch level. Both rotary tillage to four inches and two plowings resulted in>70 per cent of the activity in the top four inches (110). The uptake of fission products placed at a 60-cni depth, compared to a 30-cm depth, was as low as one thirtieth in the tops and one tenth in the seeds of oats and peas (36). D. Moisture Variations in the soil moisture tension were found to affect cesium uptaJce and the cesium-to-potassium ratio, but no effect was noted on calcium and strontium (125). Several observations (128, 129) of a tenfold greater uptake of cesium by low- land rice than by upland rice have been made. Evidence indicates that increased amounts of ammonium ion under the reduced conditions caused the greater uptake. E. Prolonged Cropping Little variation in strontium availability is observed over a period of years (14, 43), and the uptake of strontium per unit weight is fairly constant (86). The effect of fixation has not been studied sufficiently in the field to determine whether it is important under prolonged cropping. The uptake of cesium increased under prolonged cropping (86), probably because the potassium became depleted by crop removal. 20 IX. SUMMARY Radioisotopes in faillout enter plants by three principal pathways: (1) direct absorption by the aboveground parts; (2) absorption by the stems and roots from the root mat of grass; and (3) absorption by the roots from the soil. Contaminated soil adhering to the aboveground parts of the plants may contribute to the observed up- take of fission products. Foliar deposition and absorption depend on the surface area of the aboveground portion of the plant and the characteristics of the surface. The greater the surface area, the greater the interception per plant. Pubescence increases the retention of the fallout dust against washing and therefore the period of absorption. Many ele- ments seem to be absorbed, some to a greater extent by this method than through the roots. Fallout particles can be washed from the leaf surface, and even small amounts of absorbed elements can be leached from the leaf. Plant-base absorption is a relatively recent concept and its general contribu- tion has not been adequately evaluated. No single crop of plants has been reported to absorb from the soil as much as 10 per cent of the applied dose of fission products. There are two main reasons: (1) the soil has an affinity for the fallout nuclides because most of them are cations; and (2) the plant itself discriminates against them to a certain extent. The uptake of short-lived isotopes, such as barium- 140 and iodine- 131, through the roots is relatively unimportant as most of the isotope decays during the period required for it to reach the roots. The uptake of cations by roots is probably by a carrier mechanism. Stron- tium and calcium compete for the same binding sites on this carrier, whereas cesium and potassium compete for another common site. With the exception of strontium, and possibly cesium, the longer-lived fission products are taken up in relatively small amounts and therefore are not as important as strontium and cesium with respect to uptake from soils. Because strontium is chemically similar to calcium, the strontium content of plants is often reported as a strontium-to-calcium ratio as well as an absolute amount of strontium. Both values have some importance in assessing hazards in the subsequent links of the food chain. The usual maximum uptake of strontium ap- pears to be about one per cent of the applied dose per crop. The average DF for strontium to calcium between the soil and plant tops appears to be close to unity. This factor varies ajnong plants and even among different parts of the same plant and by different soil extractants, but the range of variation is considered to be of little practical importance. Variations in the root zone and differences in the vertical distribution of faillout strontium-90 and csdcium in the field can have greater effects. 21 The average maximum uptake of cesium appears to be about one tenth of one per cent of the applied dose. The cesium-to-potassium DF is small— about 0. 2 for uptake in nutrient solutions and 0. 02 for additions to the soil. In general, it appears that grasses accumulate less strontium than legumes. The fruit and seeds contain less strontium than the leaves or stems because stron- tium tends to accumulate in the vascular tissues of the plants. In contrast with strontium, which only moves readily upward, cesium is easily translocated through- out the plant, with perhaps slightly higher accumulation in young leaves and flowers. 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