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Full text of "The behavior of radioactive fallout in soils and plants"

The Behavior of 

RADIOACTIVE FALLOUT 

in SOILS and PLANTS 



National Academy of Sciences- 
National Research Council 

Publication 1092 



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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 



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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. 





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. 





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. 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<potassium 

< ammonium < rubidium < cesium < hydrogen < magnesium < calcium < strontium 

< barium < iron <aluminum< lanthanum (48; 60; 62; 124; 128, pp. 158-181; 134). Var- 
ious exchange equations have been suggested, but none appears universally applicable. 
The effect of anions cannot be neglected, for it was found that nitrate, chloride, and 
sulfate reduce the sorption of strontium in that order, whereas oxalate and phosphate 
tend to increase it (61, pp. 170-190). 



D. pH Effects 

In most studies of pH effects, the pH of the leaching solution has been varied. 
In agriculture, the pH of the soil rather than the contaminating solution is variable. 
The results of some studies (61, pp. 170-190; 91; 100; 101) indicate that the maximum 
adsorption of strontium occurs between pH 7 and 9, cesium at 6 and higher, yttrium 
and cerium above 6, and plutonium from 2. 5 to 9. 0. Since highly acid and alkaline 
conditions result in the decomposition of the soil minerals, it is expected that under 
such conditions there would be less fission-product adsorption as a result of com- 
petition by the products of decomposition, particularly aluminum. 

E. Clays 

Clays differ in their exchange capacity per unit weight and in the energy with 
which adsorbed ions are held. Exchange capacities in terms of milliequivalents of 
strontium per g of some representative clays (38) are vermiculite, 1.36; Utah 
bentonite, 1.28; illite, 0.23; and kaolinite, 0.05. The percentage of water-soluble 
strontium in a bentonite suspension is 4 compared to 30 for kaolinite (82). Less 
strontium is adsorbed on illite than on bentonite and a greater uptake of strontium 
is observed in plants grown in illite (63). 



F. O rganic Matter 

Additions of decomposable organic matter can reduce the uptake of strontium 
(40, 80, 81). A major factor is probably the increased microbial population, although 
adsorption to the organic matter itself is also important (56, 93, 108, 117). 



G. Fixation 

Fixation is a general name for processes occurring in soils that convert ions 
from forms available to plants into those not available. Soil culture experiments, 
such as those conducted with the Neubauer technique, have been used to evaluate the 
amount of applied fertilizer that is available for plant uptake. Neutral normal salt 
solutions are often used to extract the exchangeable cations, which are considered 
to constitute the major source of the available quantities of many nutrients. The 
difference between the applied amount and the available or extractable amount is 
usually considered fixed. Thus, fixation is an arbitrary term which depends upon 
the experimental conditions. 

Proposed mechanisms for fixation include precipitation as slightly soluble 
materials, physical trapping between clay platelets and in other insoluble precipi- 
tates, and diffusion into existing crystals (89, 97, 113, 114). 

Nonexchangeable amounts of strontium (82, 98, 103, 114) and cesium (82, 83, 
113, 129) have been found in some soils. In other soils, no fixed strontium was 
found (43, 47). The magnitude of strontium fixation ranged as high as 20 per cent 
of fallout strontium-90 in North Carolina soil samples taken in 1958 (103). Labora- 
tory studies showed that increasing the temperature from room temperature to 60° C 
tripled the amount fixed in these soils. Increasing the equilibration time from one 
to two weeks doubled the amount fixed at room temperature but had no effect at 60° C 
(98). In other laboratory studies with these soils, fixed strontium appeared to be 
extractable at 80° to 90° C. ^ 



H. Erosion 

Because most fission products are strongly adsorbed to clays, it is expected 
that any redistribution of surface soil will cause a similar redistribution of fallout. 
The strontium-90 concentration in runoff from field plots was 10-30 times the con- 
centration in the soil and was almost entirely associated with the sediment (69). 
The strontium-90 concentration on cultivated watersheds, where the amount of soil 
erosion was known, was one third to two thirds of the concentration on watersheds 
where there had been no erosion (25). Slope and cropping history appeared to be 
related to the amount of loss. 



PLANT RELATIONS 



A. Uptake 

The uptake by plants of ions from solution has been the subject of many in- 
vestigations. There have been short-term experiments with adsorption periods of 
minutes or hours and long-term experiments with adsorption periods of days, weeks, 
and months. Short-term experiments are useful in studying mechanisms of the 
initial steps, whereas longer-term experiments elucidate over-all effects and the 
general distribution within the plant. 

Numerous mechanisms have been proposed for the uptake of ions by plant roots. 
Most hypotheses state that the process of concentrating the ions within the plant root 
is a metabolic function. Essential in most of these mechanisms is a biological com- 
pound that serves as a carrier (20). Evidence has been obtained that calcium, stron- 
tium, and barium compete for an identical carrier, whereas potassium, rubidium, 
and cesium compete for a different carrier (20, 21, 28). Hydrogen appears to compete 
with all ions (27). 

In addition to the accumulation of ions within the root, there is adsorption of 
ions on the root. The CEC of roots can be increased by nitrogen fertilization (121). 
A linear correlation was observed between the CEC of different species and the uptake 
of strontium- 90 (75). The exchange adsorption does not appear to be controlled di- 
rectly by metabolism (19, 45) and has been considered by some investigators (50) to 
be independent of active transport. 

The point of maximum uptake of strontium and iodine appears to be within a 
few mm of the root apex (45), and no enhanced uptake is observed with root hairs. 
Other work (140) indicates that the tips of barley roots absorb various ions readily 
but that the greatest translocation occurs from a region 30 mm above the tip. 

It has been suggested that strontium can partially substitute for calcium (133, 
137) and even that strontium is an essential element (141). Sixty to 70 per cent of the 
strontium in the plant has been found to be water-soluble, whereas 97 per cent of the 
cesium and only 16 per cent of the cerium-144 was water-soluble (83). 

A possible error in short-term experiments is the exchange of the radioactive 
isotope for the stable isotope already in the plant. This is particularly true for ions 
of slow turnover rate, such as calcium. Some workers (73, 133) consider the first 
24 hours of calcium uptake to be largely nonmetabolic exchange. 



B. Translocation 

Most investigators believe that translocation of ions is governed less by me- 
tabolism than by the uptake process (5). The translocation of rubidium from the root 
to the top has been related to the transpiration stream (29). However, no such rela- 
tionship was found for calcium (7). 



10 



The upward translocation of strontium and calcium relative to that of phosphorus, 
sulfur, iodine, and rubidium is limited. The main path appears to be the central zone 
of the vascular tissue (58). The redistribution of strontium, calcium, yttrium, and 
other multivalent cations is much less than that observed for cesium, rubidium, and 
potassium (35, 131). 



C. Aerial Contamination 

A principal pathway of intake of fallout nuclides, immediately following deposi- 
tion, is through contamination of aerial plant parts. A conclusion from studies of 
plant material near the Operation Hurricane test site (108) is that the radioactive 
contaminants were carried by airborne soil, which lodged upon the leaves, and were 
then partially dissolved by nocturnal dew. Most of the fission-product radioactivity 
associated with vegetation near the Nevada tests was in external dust (83). The reten- 
tion of the particles by foliage was enhanced by mechanical trapping by hairs, glands, 
and stomata. Particles of less than 44 n diameter were preferentially retained on 
foliage, whereas particles having diameters over 88 m were rarely retained. Greater 
absorption is generally expected from contaminants in solution than from dry con- 
taminants. 1 

It also appears that aerial contamination of plants from stratospheric fallout 
is important in the entry of nuclides into the food chain. The cesium- 137 to 
strontium-90 ratio in milk in 1959 and 1960 was rather constant, although cesium 
uptake through roots is known to be much less than that of strontium. The conclu- 
sion was reached that cesium and strontium in the forage are largely derived from 
foliar deposition (49). By determining the specific activities (strontium-90/strontium) 
of different parts of wheat plants, other workers concluded that over 90 per cent of 
the strontium-90 in the grain came from current fallout in 1959 (70). About two per 
cent of the strontium-90 fallout during the time the heads were exposed was retained 
in the grain. The whole crop retained about three per cent of the deposited 
strontium-90 and removed only about 0. 2 per cent of the strontium-90 in the soil. 
Ryegrass grown in flats absorbed directly 23 per cent of the current strontium-90 
fallout (74). This accounted for 55 to 80 per cent of the total plant strontium-90. 

Autoradiograms show that strontium enters directly through the intact epidermis 
of the tomato fruit (58). About four per cent of the applied strontium and two per cent 
of applied ruthenium is absorbed by tomatoes (46). The stage of maturity of the fruit 
had some effect on the amount absorbed. 

The species of plant is important in the absorption of foliar- applied elements 
(71), partly because of the degree of waxiness of the leaves and partly because of the 
death of some leaves before maturity. Wheat plants absorbed 85 per cent of the 
applied strontium and 93 per cent of the cesium, but cabbage absorbed only five or 
six per cent of either of these elenaents. 

The time of contajnination in relation to the maturity of the plant is important 
also, especially for the relatively nonmobile elements. Less than 0. 1 per cent of 
the applied strontium is found in wheat grain if the surface deposition occurs before 
head development; up to one per cent is found when the head is contaminated (71). 



11 



Iodine- 131 can occur in the gaseous state, and in this form it is taken up by 
both the mesophyll (35 to 40 per cent) and the epidermal tissue (118). The rate de- 
pends upon the concentration. Stable iodine does not reduce iodine- 131 absorption, 
but it does reduce translocation. 

The washing effect of rain can reduce the foliar intake of strontium by a factor 
as large as six (71). Intake of cesium is reduced to a lesser extent. Contaminated 
dust is satisfactorily removed from leaves by washing with 0. 1 N HCI, but spray 
contamination is much more difficult to remove (4). Over 50 per cent of the iodine 
in foliar contamination is removed by washing, 70 to 85 per cent by different ad- 
hesives, and up to 97 per cent by stripping off the upper and lower epidermis (42). 
Not only can washing remove surface contamination, but it can also remove ions 
from within the plant (55). Sodium, potassiura, and manganese are readily leached; 
calcium, magnesium, sulfur, potassium, and strontium are moderately leached; and 
iron, zinc, phosphorus, and chlorine are leached with difficulty (130), 



D. Plant-Base Absorption 

Some British workers have given attention to the absorption of fission products 
from the root mats of long-established grass pastures (107). The root mat is com- 
posed of roots, basal portions of stems, and organic matter. The strontium- 90 in 
rainfall and that washed off the plants may be held in the mat long enough for con- 
siderable absorption to occur. This pathway bypasses soil reactions and may be 
very important where the root mat is present. 

The proposed plant-base mechanism provides a reasonable explanation for 
the strontium- 90 concentration in pasture vegetation, which appears far too high to 
be accounted for from expected soil uptake and foliar absorption. 



E. Distribution in Plants 

Strontium tends^ to accumulate in the aboveground portions of plants (16, 47, 
67, 78, 106, 123), particularly in the vascular tissues (77), although the root con- 
centration increases with time (58). The greatest concentration of strontium is 
usually found in the older leaves (85, 94), with only about one tenth as much in the 
seeds (78, 94, 120). However, the seeds of a few plants, e.g.. Euphorbia, accumu- 
late strontium to a greater extent (13). 

Cesium and rubidium are similar to potassium, and therefore it is expected 
that they are distributed more uniformly throughout the plant than strontium. About 
10 per cent of the total plant cesium is found in the grain of wheat and oats (47), and 
other work (67) indicates that there is a slight tendency for both cesium and rubidium 
to accumulate in the young leaves and flowers. 

In four plant species, the highest concentration of iodine was found in the roots, 
followed by older leaves and, finally, the younger leaves (119). Other radioactive 
constituents of fallout, which have not been specifically mentioned, concentrated in 
the roots, with little translocation to the top (47, 85, 94, 106), 



12 



F. Species Differences 

The order of fission-product uptake by the different plant families is 
Leguminosae> 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. (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. 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. T he 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. 

Strontium and cesium are retained in the soil partly by ion-exchange bonds on 
clay minerals and organic colloids. A part of the strontium may be synthesized into 
organic compounds by the microbial population. A third means of retention in the 
soil can involve fixation processes. A large fraction of cesium- 137 appears to be 
fixed irreversibly. Exchangeable strontium is leached through soils at the rate of 
about one inch per 100 inches of leaching water. The downward movement of stron- 
tium, and probably cesium, is essentially an exchange reaction and proceeds by 
successive desorption-adsorption sequences. 

The soil that will provide minimum uptake of fission products usually appears 
to be one considered ideal for maximum crop production. These requirements in- 
clude high exchangeable calcium, high exchangeable potassium, high organic-matter 
content, and a slightly alkaline reaction. 



22 



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23 



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24 



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