Application of fertilisers and amelioran

The Science of the Total Environment. 137 (1993) 173-182
Elsevier Science Publishers B.V.. Amsterdam

173

Application of fertilisers and ameliorants to reduce
soil to plant transfer of radiocaesium and
radiostrontium in the medium to long t e r m - a
summary
A.F. Nisbet a, A.V. Konoplev b, G. Shaw ¢, J.F. Lembrechts d, R. Merckx e,
E. Smolders ~, C.M. Vandecasteele f H. L6nsj6g, F. CarinP and O. Burton ~
aNational Radiological Protection Board. Chilton. Didcot. Oxon. OXl! ORQ. UK.
blnstimte of Experimental Meteorology. Lenina Avenue 82. 249020 Obninsk. Kaluga Region.
Russian Federation.
Centre for Analytical Research in the Environment. Imperial College at Silwood Park. Ascot.
Berks. SL5 7TE. UK.
°Nationai Institute of Public Health and Environmental Protection. PO Box 1. NL-3720 BA
Bilthoven. Netherlands.
eKatholieke Universiteit. Kardinaal Mercierloan 92. 9-3001 Leuven. Belgium.
fCEN/SCK. Boeretang 200. 8-2400 Mol. Belgium.
WSwedish University of Agricultural Sciences. PO Box 7031. S-75007. Upt,sala. Sweden.

hChemistry Institute UCSC. Via Emilia Parmense. 84. 1-29100 Piacenza. Italy.
~Facaltd des Sciences Agronomiques. Avenue de la Facultd 8. 8-5030 Gembloux. Belgium

ABSTRACT
Factors influencing the effectiveness of fertilisers and ameliorants as soil.based
countermeasures for reducing the transfer of radiocaesium and radiostrontium to plants are
described. The applicability of potential treatments and treatment rates for different soil types
is discussed. Data from small scale experiments conducted under controlled conditions, as
well as field investigations carried out in Belarus and Ukraine are summarised in an overall
evaluation of the most common and most effective soil-based agrochemical treatments
available.

Key words: radiocaesium: radiostrontium; countermeasures; soils; g,lants

INTRODUCTION

Agrochemicals may be applied to soils to reduce the transfer of radionuclides from soils to food products and hence to limit ingestion doses.
Such amendments are only relevaat in the medium to long term (i.e., months

! 74

A.F. NISBET ET AL

to years) since direct contamination of crops predominates immediately after
an accident. Over the longer time scale radiocaesium and radiostrontium are
recognised as the most important contributors to the ingestion dose for most
accident scenarios (Wilkins et al., 1993). Therefore, chemical amendments to
the soil should be directed at reducing the transfer of these two radionuclides from soils to plants.
Effective soil-based countermeasures for one radionuclide are not
necessarily effective for any other radionuclides present in the soil and so
may produce undesirable effects when applied to soil containing a mixed radionuclide deposit. Several factors have therefore to be taken into consideration before selecting the most appropriate treatment and treatment rate.
Firstly the radionuctide composition of the deposit should be quantified.
Calculations to estimate potential ingestion doses under existing circumstances from the relative amounts of radiocaesium and radiostrontium
present enable decisions to be made on whether to select countermeasures for
reducing the uptake of one or both of these radioelements by vegetation. It
is also essential to know the geographical distribution of the amount of activity deposited, the land use of the contaminated area and the principal soil
types affected. Furthermore, the behaviour of radiocaesium in soil differs
from that of radiostrontium. Radiocaesium tends to be more available in
soils either poor in their clay mineral content, high in their organic matter
status, and/or acidic in nature with low levels of available potassium. These

soils are characterised by their low immobilisation capacity for radiocaesium
(Livens an~l Lov¢la~d, 1988) and are most commonly found in semi-natural
ecosystems or on low productivity agricultural land. Radiostrontium is available to plants in a wide variety of soil types, as unlike radiocaesium, it is not
immobilised by clay minerals.
Chemical treatment of the soil is a relatively simple and inexpensive
countermeasure technique which, if chosen carefully should cause minimal
damage to the environment. The correct choice of chemical treatment
depends on a thorough understanding of soil-plant processes and factors affecting the distribution of ions between soil, soil solution and the plant root
interface. Implementation of such a technique ~an he done in isolation if it
leads to the desired reduction in foodchain dose. Alternatively, chemical
treatments may be more effective when applied in combination with other
measures. For example, in arable farming systems, ploughing can be done to
invert the concentration profile in the soil, while crops can be selected according to their rooting depth, However, changes in land use may have dramatic
ecological, economic or social consequences (Alexakhin et al., 1993). For example, it might be impractical to plough grassland because of the need to
relocate animals while new pasture becomes established. In grassland systems radionuclide uptake oq:curs from the basal tissues and the surface soil

AGROCHEMICALS TO REDUCE SOIL TO PLANT TRANSFER OF RADIONUCLIDES

! 75

and so a chemical treatment applied directly to the existing pasture may be
tI.~e most appropriate approach.
FACTOP,S INFLUENCING THE EFFECTIVENESS OF SOIL BASED CHEMICAL
TREATMENTS
For a soil based agrochemical t~eatment to be effective, either or both of
the following criteria should be fulfilled:
(i)

The solid-liquid equilibrium of various ions in the soil should be
modified such that the concentration of stable nutrient analogues
(i.e., K or Ca) in soil and soil solution is increased, whilst at the same
time the quotient of radioelement:nutrient analogue concentrations
in soil solution is decreased (i.e., Cs:K, St:Ca);
(ii) The fixation of the radionuclide in a non-available or less available
form on the solid phase should be increased, for example by adsorption onto clay minerals or by co-precipitation with insoluble salts.

The application of any chemical treatment to the soil causes a change in
the distribution of ions between solid and liquid phases. The net effect of
these changes varies from one soil to a~other, according to texture, clay
mineral and organic content, buffer capacity and base saturation. Consequently large differences in the effecti,-eness of chemical treatments may be

observed between soil types which can then greatly influence the uptake and
accumulation of both stable ions and radionuclides by plants.

Radiocaesium
Potassium is the best known, effective soil-based fertiliser treatment for
reducing radiocaesium uptake to plants. It is used because of its analogous
chemical behaviour to caesium which can result in it successfully competing
with radiocaesium absorption by roots. Thus it is applied to increase levels
of exchangeable potassium in soils, to increase potassium concentrations in
the soil solution and as a result to reduce the ratio of Cs+:K ÷ in the soil
solution.
To use potassium effectively as a soil-based countermeasure the following
factors must first be taken into consideration:

(l)

The ability of different soils to supply potassium. Radiocaesium uptake by plants is reduced when potassium is added to soils with a low
exchangeable potassium status. Determinations of potassium release
characteristics (i.e. potassium buffer capacity, levels of exchangeable
and solution phase potassium) are potentially useful in deciding

176

A.F. N , S n ~ ~T AL.

whether soils contaminated with radiocaesium will respond to additional potassium and if so, what the frequency and quantity of application should be (Nisbet, 1993). Potassium supply is dependent on
mineralogy, organic matter status, nutrient status and interactions
with other fertiliser treatments applied to the soils.
(2) The effect of added potassium on radiocaesium desorption and Cs:K ratios in the soil solution. The comparable electrical charge and
hydrated size of caesium and potassium ions means that both ions
can be fixed by binding sites within the array of cation exchange sites
of the soil. Consequently, application of potassium fertiliser to soil
might result in the displacement of radiocaesium from these sites into
the soil solution, with obvious implications for plant uptake. The extent to which radiocaesium is desorbed depends on a range of soil
parameters as well as the rates of potassium treatment applied. These
effects may be evaluated from batch equilibrium studies (Nisbet,
1993).
(3) Potassium concentrations in the soil solution and their relationship with
uptake efficiency of radiocaesium by plants. Caesium uptake by plants
is closely related to potassium uptake, although it depends on

Cs+:K + activity ratios in the soil solutior,. The uptake mechanism
exhibits a discrimination between Cs + and K + which can be expressed as the 'observed ratio'(i.e, the measured ratio of Cs/K in the plant
divided by the ratio of Cs/K in the external solution or root environment). This observed ratio is reported to decrease with increasing
K + concentration (Middleton et al., 1960). i'he result of this is that
radiocaesium uptake by plants is mos~ efficient (> 90%) at very low
potassium concentrations (i.e., < 10-2 - - 10-I ~M). Conversely, uptake efficiency becomes very low (< 2%) once potassium co'ncentrations exceed 100 ~M (Shaw, 1993). Determination of potassium concentratioas in soil solutions from contaminated soils sl',ould therefore
give an indication of the likely reduction in radiocaesium uptake by
the plant following applications of potassium fertilisers. Thus, vegetation growing on organic and/or mineral poor soils with inherently
low concentrations of potassium (< 10 ~M) in the soil solution are
likely to show a larger response to applied potassium than plants
growing on highly fertile agricultural soils.
Potassium is not the only soil-based countermeasure that is eff~tire in reducing radiocaesium uptake by plants. Lime and aluminosilicates may be ,sed to increase the pH and cation exchange capacity
of th~ soil. This shifts the solid-liquid equilibrium towards the solid
phase, thereby removing radiocaesium from the soil solution and
reducing its immediate availability to the plant root (Konoplev et ai.,

AGROCHEMICALS TO REDUCE SOIL TO PLANT TRANSFER OF RADIONUCLIDES

177

1993). However, these treatments are more effective when applied in
conjunction with potassium, as this further decreases Cs÷:K ÷ ratios
in the soil solution.

Radiostrontium
Similarities in the chemistry of calcium and strontium mean that calcium
is a potentially effective soil-based countermeasure for reducing radiostrontium uptake to plants. Thus lime or other calcium containing minerals are
applied to soils to increase exchangeable calcium levels in soils, to increase
calcium concentrations in the soil solution and thereby to decrease the ratio
of Sr2+:Ca 2+ in the s,A~ solution. To use calcium effectively, however,
several factors must first be taken into consideration:
Exclu~ngeable calcium and base saturation of the contaminated soils.
These two parameters determine ~rhether thr~ soils will respond to additional calcium. Soils with a low calcium .~tatus and }ow percentage
base saturation show the greatest benefit from lime applications by
marked reductions in radiostrontium uptake by plants.
(2) St/Ca selectivity in the contaminated soils. If the contaminated soils
adsorb strontium preferentially to calcium, then calcium will be an
effective countermeasure. If, however, soils adsorb calcium preferentially, any additional calcium supplied may displace strontium from
exchange sites and increase radiostrontium levels in soil solution.
Mineral soils generally adsorb strontium preferentially (Juo and

Barber.. 1969), making calcium a useful treatment. This is not always
the case for organic soils, for which further investigation would be
necessary before lime was applied. The effect of different rates of lime
application on radiostrontium desorption should be amenable to
evaluation for different soil types from batch equilibrium studies.
(3) Calcium concentrations in the soil solution and their relationship with
radiostrontium uptake efficiency by plants. Uptake of strontium is
usually related to the ability of plants to take up calcium. The qu~.ntity taken up depends on the ratio of Sr2+:Ca2÷ in the soil solution
bathing plant roots. Various experiments with both nutrient solutions and soils indicate the 'observed ratio' to range between 0.7 and
1.4 for a variety of different species (Arnold, 1990). Gelaerally
though, there is no significant discrimination in the uptake of strontium and calcium by plants over a wide range of Sr2+:Ca :÷ ratios.
Consequently, increasing the available pool of calcium in the soil and
soil solution will decrease the amount of radiostrontium taken up by
plants. Soils with an initially low concentration of calcium in soil solution (-ffi 2 meq (exchangeable) Ca !00 g-i ~oil) will tend to show a

(1)

"]8

A.F. NISBET El" AL

larger response to applied calcium than those with relatively high calcium status (> 10 meq Ca 100 g-~) (Milbourn, 1960).
EVALUATION OF SOIL-BASED CHEMICAL TREATMENTS FOR REDUCING THE
TRANSFER OF RADIOCAESIUM AND RADIOSTRONTIUM TO VEGETATION

The data available for evaluating the overall effectiveness of soil-based
countermeasures come from two sources: (1) small scale experiments conducted under controlled conditions as summarised by Lcmbrechts (1993);
and (2) large scale field investigations principally conducted in Belarus and
Ukraine following the Chernobyl accident, as described by Konoplev et al.,
Prister et al. and Alexakhin (1993). Results from both types of study indicate
that chemical treatment of the soil alone can reduce the transfer of
radiocaesium by factors of up to five and radiostrontium by factors of up to
ten. Such countermeasures were particularly effective in the contaminated
areas around Chernobyl, due to the increased soil fertility and plant productivity that resulted from the fertiliser treatments. For the reasons given
earlier, lower reductions in radionuclide transfer might be expected from
areas where soil fertility is already high. However, even reductions in the activity concentrations of foodstuffs by a factor of two represent a 50% saving
in dose from the appropriate foodchain pathway. The treatments evaluated
include commt, n agricultural chemicals, applied at rates sometimes in excess
of those normally used in agricultural practice, especially for lime. For example, rates of 2-100 t ha "~ of lime were applied in the case of small scale controlled experiments, compared with standard rates of around 3 t ha -~ commonly used by farmers and also by the field investigators. The use of
excessively large quantities of lime clearly did not increase its effectiveness

as a countermeasure, but instead was potentially harmful to the vegetation
as it causes a reduction in the bioavailability of many important
micronutrients. The application of other treatments at unnecessarily high
rates would also be expected to disturb the nutrient balance o f the soil-plant
system and in semi-natural ecosystems, upset the species composition.
Both experimental and field data were used to compile Tables 1 and 2,
which give an evaluation of the most frequently used chemical amendments
for reducing the transfer of radiocaesium and radiostrontium to vegetation.
Many of these treatments arc already used in common agricultural practice
and are therefore relatively inexpensive, available in large quantities, easy to
apply to large areas of land and cause minimum damage to the environment.
The terrain and/or inacessability of many semi-natural ecosystems may prevent the application of these soil-based chemical treatments under some circumstances. However, such areas arc often used only for animal grazing
lather than the production of crops for direct consumption by man and so

TABLE I
g3

Evaluation of soil-based countermeasures for reducing radiocaesium uptake !o vegetation
Comments

Countermeasure

Reduction factor

Objective

Effectiveness

Potassium-based
fertiliser~

up to 5

(i) To increase levels of
exchangeable potsssium
in the soil;
(ii) To decrease Cs+:K"
ratios in soil solution:

High when applied to soils Inexpensive. ~asy to apply.
Maximum benefit from
with low levels of
standard agricultural
exchangeable potassium
apphcations of no more than
and low concentrations of
200 kg ha -t yr -t. Can cause
potassium in soil solution
desorption of radiocaesium
(i.e. < 20 t~M)
frora solid phase in soils with
~ low immobilisation capacity
ior radiocaesium.

Lime

up to 2

High when applied to acidic Inexpensive, easy to apply. Most
(i) To increase soil pH:
effective when applied in
soils
(ii) Tc increase cation
conjunction with potassium.
exchange capacity o r the
No benefit to be gained from
soil and increase ~,.orption
applying
> 3 t ha -~.
of radiocaesium.
High

Sapropell

up to 6

(i) To decrease Cs÷:K ÷
ratios in soil solution.

Aiu~i,~ ~osliicates

up to 2

Limited
~:' To increase calion
exchange capacity of the
soil and increase
sorption of radiocaesium

Inexpensive. easy to apply.
Increases plant productivity.
Reduces radionuclide uptake
in subsequent years. Increases
micro-element content of
treated soil. Application of
large quantities of > 150 t ha -t
not harmful.

m

gt-'

:¢
m

l-

"o
[-.

Z
-I
-I

z
m

O

0
Z
e-

c

Expensive at application rates of
several t ha -I.
,,D

TABLE 2
~valuation of soil-based countermeasures for redv~ng radiostrontium availability to vegetation
Countermeasure

Reduction factor

C)bjective

Effectiveness

Commenls

Lime

Up to 10
(but a factor of
2-3 is m o ~
likely)

(i) To increase soil pH;
High
(ii) To increase cation exchange
capacity of the soil and
increase sorption of
radiostrontium;
(iii) To increase levels of
exchangeable calcium in
the soil;
(iv) to decrease Sr2+:Ca 2÷
ratios in soil solution.

Applicable to mineral soils with
low % base saturation. May
cause desorption of radiostrontium in organic soils
due to preferential adsorption
of calcium

Organic matter
(including sapropeli)

Up to 5

(i) To increase complexion of High
radiostrontium with
soluble organic matter;
(ii) In addition, sapropell
provides an extra source
of calcium to reduce
Sr2+:Ca 2÷ ratios in soil
solution.

Applicable to mineral soils in
place of liming but not in
conjunction with it. If mixed
deposit, organic ma~ter may
increase availability of radiocaesium.

Phosphates,
sulphates
and silicates

Up to !0

(i) To reduce the availability Limited
of radiostrontium through
coprecipitation with
insoluble salts.

May reduce availability of other
essential nutrients when
applied at high rates.

q]
Z
w

t-

AGROCHEMICALS TO REDUCE SOIL TO PLANT TRANSFER OF RADIONUCLIDES

181

the implementation of animal-based c,~antermeasures may be a more effective option. Recommendations on rates and frequency of fertiliser application depend on the soil parameters, as discussed in the previous section,
although generally rates will be only a few times greater than those normally
used. Although other more sophisticated chemicals are available as potentially effective countermeasures, their price and/or undesirable side effects
precludes their use on a wider scale. Sapropell is one ameliorant that was
used with great effectiveness in the CIS (former USSR) following the Chernobyl accident. Its characteristics are fully described in these proceedings by
Prister et ai. Briefly, sapropell is derived from lake sediment and is composed
of organic material with a naturally high mineral content. Although abundant in the CIS, little is known of the availability of sapropell elsewhere.
Soil-based chemical treatments are often designed to increase soil fertility,
which increases plant growth rate and reduces the soil-to-plant transfer factor of radionuclides (Alexakhin, 1c~93~Smolders and Merckx, 1993). Fio~ever, there are some fertiliser treatments that should be avoided ah'.~geth, r
following contamination of land with radiocaesium or radiost~oatium. The
application of acidic or ammonium-based fertilisers Jbr example, have been
found to increase the availability and ~hence transfer of radiocaesium to
plants (Minotti et al., 1965; Evans and Dekker, 1966). To avoid this, fertilisers should be supplied as neutral salts and nitrogen apptied in tlle fol;~,
of nitrate.
Finally, there are two circumstances which reduce the effectiveness of
some of the recommended soil-based countermeasures. These are: (!) when
two potentially beneficial treatments are applied jointly, but their effectiveness together is lower than either treatment individually (e.g., sapropell
plus lime or fertilisers); (2) in the case of a mixed deposit of activity, when
the beneficial effect of a countermeasure for reducing transfer of one radionuclide from soil to plant may be offset by detrimental effects caused by
increasing the availability of another (e.g., treatment of organic soils with
calcium and potassium to reduce radiocaesium availability may v,ell enhance
desorption and availability of radiostrontium).
To date, many sttMies on the effectiveness of soil-based countermeasures
have produced inconclusive or contradictory results. This can be remedied
in the future through a better exploration of the existing literature and a better understanding ,of the processes involved in the transfer of various ions
from soil to plant. Methods can then be developed to enable the effectiveness
of practicM soi!-b,:sed treatments to be predicted accurately for different soil
types and !and !laaoagemeot regimes.
REFER EN('~S
Alexakhin, R.M., ]1993. Countermeasures in agricultural production as an effective means of

182

A.F. NISBET ET AL.

mitigating the radiological consequences of the Chernobyl accident. Sci. Total Environ.,
137: 9-21.
Alexakhin, R.M., M.J. Frissel, E. Schulte, B.S. Prister and B.T. Wilkins, 1993. Changes in
land use and crop selection. Sci. Total Environ., 137: 169-172.
Arnold, L., 1990. Methods to reduce agricultural impact subsequent to a nuclear accident.
ANS report No. 2387-R i, Eastleigh House, Epsom, Surrey.
Evans, D.W. and A.J. Dckker, 1966. Plant uptake of '37Cs from nine Canadian soils. Can.
J. Soil Sci., 46: 167-176.
Juo, A.S.R. and S.A. Barber, 1969. An explanation for the variability in St-Ca exchange selectivity of soils, clays and humic acid. Soil Sci. Soc. Am. Prec., 33: 360-363.
Konoplev, A,V., N.V. Vikrorova, E.P. Virche~)ko, V.E. Popov, A.A. Bulgakov and G.M.
Desmet, 1993. Influence of agricultural countermeasures on the ratio of different chemical
forms of radionuclides in soil and soil solution. Sci. Total Environ., 137: 147-162.
Lembrechts, J.F., 1993. A review of litera~.ure on the effectiveness of chemical amendments
in reducing the soil.to-plant transfer of radiostrontium and radiocaesium. Sci. Total Environ,, 137: 81-98.
Livens, F.R. and P.J. Loveland, 1988. The influence of soil properties on the environmental
mobility of caesium in Cumbria. Soil Use Manage., 4: 69-75.
Middleton, LJ., R. Handley and R. Overstrect, 1960. Relative uptake and translocation of
potassium and caesium in barley. Plant Physiol., 35: 913-918.
M:!bourn, G.M., 1960. The uptake of radioactive strontium by crops under field conditions
m the United Kingdom. J. Agric. Sci,, 55: 273-282.
Minotti, P.L., D. Craig and W.A. Jackson, 1965. High caesium uptake in wheat seedlings culture,:i with ammonium. Soil Sci. Soc. Am. Prec., 29: 220-221.
Nisbet, A.F., 1993, Effect of soil-based countermeasures on solid-liquid equilibria in agricultural soils contaminated with radiocaesium and radiostrontium. Sci. Total Environ., 137:
99-118.
Prister, B.S,, G,P, Perepelyatnikov and L.V, Perepclyatnikova, 1993. Countermeasures used
in the Ukraine to produce forage and animal food products with radionuclide levels below
intervention limits after the Chernobyl accidento Sci. Total Eaviro,, 137: 183-198.
Shaw, G., 1993. Blockade by fertilisers of caesium and strontium uptage into crops: effects
on the root uptake process. Sci, Total Environ. 137: 119-1.~3.
Smolders, E, and R, Merckx, 1993. Some principles behind the selection of crops t ~iniraise
radlonuclide uptake from soil. Sci. Total Environ,, 137: 135-146.
Wilkins, B,T,, B,J. Howard, G,M. Desmet, R.M. Alexakhin and H. Maubert, 1993. Strateg,es
for the deployment of agricultural countermeasures, Sci. Total Environ., 137: 1-8.

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