Plant accumulation of radioactive strontium with special reference to the strontiumcalcium relationship as influenced by nitrogen

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Plant accumulation of radioactive strontium with special reference to the strontiumcalcium relationship as influenced by nitrogen. (Thesis)

Andersen, Arna Jacob

Publication date: 1973 Document Version Publisher's PDF, also known as Version of record Link to publication

Citation (APA): Andersen, A. J. (1973). Plant accumulation of radioactive strontium with special reference to the strontiumcalcium relationship as influenced by nitrogen. (Thesis). (Denmark. Forskningscenter Risoe. Risoe-R; No. 278).

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Risø Report No. 278

* O

Z o


Danish Atomic Energy Commission Research Establishment Risø

Plant Accumulation of Radioactive Strontium with Special Reference to the Strontium-Calcium Relationship as Influenced by Nitrogen

by Araa J. Andersen

January 1973

Sale* distributors: Jul. Gjellerup, 87f Sølvgade, DK-1307 Copenhagen K, Denmark Arollabk on exchange from: Library, Danish Atomic Energy Commission MM, DK-4O00 Roskilde, Denmark

We regret that some of the pages in the microfiche copy of this report may not be up to the proper legibility standards, even though the best possible copy was used for preparing the master fiche*

U.D.C. 633:546.42.02.90


January 1973

Risø Report No. 278


Denne afhandling er i forbindelse med de i forordet I - XI nævnte tidligere offentliggjorte arbejder af Den kgl. Veterinær- og Landbohøjskoles fagråd for landbrugsvidenskab antaget til offentligt at forsvares for den jordbrugsvidenskabelige doktorgrad. København, den 1. februar 1973 J. Wismer-Pederaen Formand for fagrådet for landbrugsvidenskab

Forsvaret finder sted fredag den 25. maj 1973 kl. 14 præeis i auditorium 3-03, Den kgl. Veterinær- og Landbohøjskole, Thorvaldsensvej 40.

ISBN 87 550 0177 7

- 5 -

CONTENTS Page Preface






Experimental Methods


2 . 1 . Plant Culture 2.2. Analytical Procedures 2 . 3 . Terminology

11 12 13

Factors Affecting the Accumulation of Radioactive Strontium by Plants


3.1. 3.2. 3.3. 3.4. 3.5.

14 15 18 19 22


Influence of Soil Properties Effects of Soil Management Effects of Stable Strontium and Calcium Effects of Fertilization Variation Between Plant Speciel' and Varieties


The Strontium-Calcium Relationship in Plante



The Strontium-Calcium Relationship in Oats as Influenced by Nitrogen


5 . 1 . The Strontium-Calcium Ratio in the Soil Solution 5 . 2 . Transport to the Root Adsorbing Surfaces 5 . 3 . Uptake and Transport in Roots 5.4. Upward Transport and Translocation in the Plant

27 28 30 35

General Discussion and Conclusions






Dansk Resume





_ 7_

PREFACE T h e p r e s e n t r e p o r t s u m m a r i z e s and d i s c u s s e s a s e r i e s of i n v e s t i g a tions on t h e plant uptake of r a d i o a c t i v e s t r o n t i u m c a r r i e d out during the l a s t t e n y e a r s . T h e m a i n r e s u l t s of the e x p e r i m e n t s have been published in v a r i o u s a r t i c l e s and r e p o r t s a s l i s t e d below. T h e s e p a p e r s will be r e f e r r e d t o by Roman n u m e r a l s , while o t h e r l i t e r a t u r e will be given by a u t h o r ' s n a m e and publishing y e a r and l i s t e d at the end of the r e v i e w . I

A n d e r s e n , A. J . , 1963. Influence of l i m i n g and m i n e r a l fertilization on plant uptake of r a d i o s t r o n t i u m from Danish s o i l s . - Soil Science, 95:52-59.


A n d e r s e n , A. J . , 1965. Uptake by p l a n t s of r a d i o s t r o n t i u m from c o n taminated soils. - N a t u r e , 208:195-196.


A n d e r s e n , A. J . , 1966. Reduction of t h e


Sr uptake by b a r l e y through

h i g h - t e m p e r a t u r e t r e a t m e n t and d e e p p l a c e m e n t of t h e contaminated s o i l l a y e r . - p . 421-427 in: Radioecologieal C o n c e n t r a t i o n P r o c e s s e s , P e r g a m o n P r e s s , Oxford and New York, 1966. IV

A n d e r s e n , A. J . , 1967. I n v e s t i g a t i o n s on t h e plant uptake of fission p r o d u c t s from c o n t a m i n a t e d s o i l s . I. Influence of plant s p e c i e s and s o i l types on t h e uptake of r a d i o a c t i v e s t r o n t i u m and c a e s i u m . R i s ø R e p o r t No. 170 p p . 32.


A n d e r s e n , A. J . , 1967. I n v e s t i g a t i o n s on t h e plant uptake of fission p r o d u c t s from c o n t a m i n a t e d s o i l s . II. T h e uptake of r a d i o a c t i v e s t r o n t i u m placed at different depths in t h e s o i l s . - R i s ø Report No. 174. pp. 19.


A n d e r s e n , A. J . , G. Gis s el N i e l s e n and G. Nielsen, 1967. Effects of f e r t i l i z a t i o n on t h e s t r o n t i u m - c a l c i u m and c a e s i u m - p o t a s s i u m r e l a t i o n s h i p s in p l a n t s . I. T h e uptake and distribution of r a d i o a c t i v e s t r o n t i u m and c a l c i u m in o a t s . - Royal V e t e r i n a r y and A g r i c u l t u r a l College, D e n m a r k , Y e a r b o o k ,


VII A n d e r s e n , A. J . , 1971. T h e uptake and d i s t r i b u t i o n of s t r o n t i u m in o a t s a s influenced by the t i m e of supply. - Soil Science 111:379381. VIII A n d e r s e n , A. J . , 1971. Influence of p h o s p h o r u s and n i t r o g e n nutrition on uptake and d i s t r i b u t i o n of s t r o n t i u m and c a l c i u m in oat p l a n t s . Soil Sci. Soc. A m . P r o c . 3 5 : 1 0 8 - 1 1 1 .


IX Andersen« A.J. andW.A. Jackson, 1972. Influence of nitrogen supply on uptake and transport of strontium and calcium in wheat seedlings. Phys. Plantarum 26:175-181. X

Andersen« A. J.« 1972. Influence of nitrogen nutrition on the discrimination between strontium and calcium In oat plants. - Proceedings of FAO/IAEA Symposium on the Use of Isotopes and Radiation in Research on Soil-Plant Relationships including Applications in Forestry, Vienna 13-17 December 1971: 21-40.

XI Andersen, A. J.« 1972. Effects of nitrogen nutrition on the uptake« distribution and chemical binding of strontium and calcium in oat plants. - Rise Report No. 256:177-186.

- 91. INTRODUCTION The occurrence and behaviour of strontium in the biosphere have acquired increasing interest since the advent of its long-lived radioactive isotopes which are products of nuclear fission. Small quantities of native strontium are widely distributed in soils, plants and animals, where its abundance i s usually compared to that of the homologous element calcium. Considerable variation in the atomic ratio Sr/Ca has been found in parent rocks and minerals (Goldschmidt 1958). The ratio in the plant available exchangeable fraction of surface soils varies much l e s s (Mitchell 1945), although appreciable differences between s oils have been reported (Menzel and Heald 1959; Williams and David 1963). An analysis of some Danish soils showed that they contained from 500 to 1000 times as much calcium as strontium (Ref. IV), which corresponds very well with the above-mentioned reports from other parts of the world. Radioactive isotopes of strontium are produced in large amounts by nuclear explosions, and strontium-90 i s considered the most biologically hazardous of the fission products because it i s readily transported through the food chain and, because of its long half-life (28 years), may become a long-term source of radioactivity in food. The radiation hazard from food contaminated with radioactive fall-out i s the internal radiation from fission products entering the bodies of animals and humans. The principal source of internal radiation immediately following a nuclear explosion is the external contamination of edible plants resulting from fresh fission products being deposited on agricultural areas. As time passes and the initially contaminated food has been discarded, the principal sources of internal radiation become indirect. From the contaminated soils the radioactive isotopes are absorbed through plant roots into food- and feeder ops. Radioactive strontium has been shown to be accumulated in above-ground parts of the plant, to a greater extent than any other of the long-lived fission products (Jacobson and Overstreet 1948; Gulyakin and Ydintseva 1957; Nishita, Romney and Larson 1961; Romney, Mshita, Olafson and Larson 1963), and numerous experiments have been carried out in order to evaluate in detail the various steps in the transport of radioactive strontium from the atmosphere through soil, plant and animal to the human organism. A comprehensive description of the problems involved has been edited by Dr. R. Scott Russell in; "Radioactivity and Human Diet", published by J'crgamon Press 1966.

- 10 -

The present treatise summarizes investigations on the strontium accumulation in plants grown in soils which have been artificially contaminated with water-soluble radioactive strontium. Since the strontium-90 in radioactive fall-out has been shown to be largely water-soluble, the r e sults from such experiments are assumed to be closely related to the conditions encountered in actual fall-out situations. The purpose of these investigations was to elucidate the consequences of a heavy contamination of agricultural areas with radioactive strontium and to evaluate the possibilities of influencing the amounts accumulating in crop plants. The plant uptake of strontium i s closely related to that of calcium and depends on the relative concentrations of the two cations adjacent to the root surface. Soil properties, which affect the relative availability of Strontium and calcium, were evaluated for some typical Danish soils. The uptake of strontium can be influenced either by changing its actual concentration in the soil solution adjacent to plant roots or by changing the competition from calcium. Both possibilities have been tested. An effective reduction in strontium uptake was obtained by heating the contaminated soil layer and by deep ploughing. The availability was also effectively r e duced by heavy applications of superphosphate. Whereas application of lime proved to be rather ineffective to reduce the uptake from most of the soils used. The uptakes by different plant species and varieties have been compared in pot experiments and the results revealed considerable variation in the strontium uptake. Many investigations reported in the literature have been based on the assumption that strontium behaves like calcium in the soil-plant system, and it is generally recognized that the plant accumulation of the two e l e ments occurs by quite similar processes. Several studies have, however, indicated that the relative uptake and transport of strontium and calcium may vary among plant species and be influenced by environmental growth conditions. It was shown in pot experiments (Ref. I) that nitrogen simply changed the relative distribution of strontium and calcium in oat plants. Since nitrogen is one of the most important growth factors in modern agriculture this was considered of great practical interest and it was therefore decided to elucidate these effects in more details. Attempts were made to evaluate how nitrogen addition affects the sequence of reactions which determines the strontium-calcium movement from the soil solid phase to the grain of oat plants.

- 11 -

2. EXPERIMENTAL METHODS The uptake by plants of radioactive strontium was studied in different soil types collected from the ploughed layers of typical Danish agricultural areas. The content of stable strontium in these soils was found to be of the order of 1 to 2 micro-equivalent (jieq) per milli-equivalent (meq) of calcium. A similar ratio between stable strontium and calcium was used in the water culture experiments, whereas the radioisotopes were supplied in a 1:1 ratio in the uptake nutrient solutions. Deviations from this 1:1 ratio of radioisotopes in plant material were then taken as a measure of the extent to which the plante discriminated between strontium and calcium when they were grown in double labelled nutrient solutions, 2.1. Plant Culture Three different types of plant culture technique were used. Pot Experiments Comparisons of uptake by different plant species and varieties and the influence of soil type and soil amendment were carried out as pot experiments in the greenhouse during winter or in the open during summer. The radioactive isotopes and plant nutrients were incorporated in the entire rooting zone in these experiments. Field Experiments in MJcroplots A similar technique as described by Frederiksson (1962) was used to investigate the effect of heat treatments« lime, phosphate fertilization and deep placement of the contaminated soil layer (Refs. JJL and V). Thi* technique is convenient for studies of the long-term effects of the treatments. Water Cultures In order to study the accumulation of radioactive strontium in oats as influenced by the time of supply (Ref. VII), a technique was used which allowed the plants to be grown from seedlings to maturity without too frequent renewals of solutions (0.8 - 1.0 litre per plant). Studies on uptake and transport of strontium and calcium in seedlings were carried out as short time experiments and required less solution per plant {Refs. IX andX).

- 12 -

2.2. Analytical Procedures Radioactivity in plant samples as well as in soil and plant extracts was determined by use of standard procedures. The Y-activity was measured with a well-type scintillation crystal and the p-activity was detected either with an end-window flowcounter or by liquid scintillation counting in Triton-Toluene counting solution. Appropriate corrections for self-absorption and activity decay were made. Soil Analyses The reported chemical and physical characteristics of the soils were determined by use of official methods and performed by The Laboratory of the Danish Heath Society. The extent to which added radioactive strontium and calcium equilibrated with stable calcium in the soils was measured in saturation extracts. The extracts were obtained by wetting 200 g of the labelled soil to saturation, allowing 48 hours for equilibration between added radioisotopes and native calcium. The wet soil was then transferred to a buchner funnel and the saturation extract collected by suction. The relationship between the Sr/ Ca ratio in the solid phase to that in the solution phase is referred to as the selectivity coefficient, K , and is defined by Khasawneh, Juo and Barber (1968): K -

8 5 s r 4 & C a to 8o:&d




Sr-solid • 45Ca-eolutlon


45 Sr/ 4 5 Ca in solution Ca-solid • 85Sr-soluUon 85 45 where Sr-solid and Ca-solid are obtained as the difference between the added activity and that remaining in the solution after equilibration. If the soil adsorbs strontium-85 preferentially to calcium-45, the K value the soil adsorbs strontlu] will be greater than one.

Plant Analyses Chemical analyses of plant materials were carried out on ash solutions by use of standard procedures. The chemical binding of strontium and calcium in plants was studied (Ref. XI) by use of a procedure similar to that described by Schilling (1960). One gramme of plant powder was successively extracted with water, 0.5N NaNOj, and 1. ON acetic acid. The p-radioactivity in these fractions was"" determined on evaporated aliquots of the solutions. Corrections for selfabsorption were made.

- 13 -

2.3. Terminology The transport of radioactive strontium in biological systems is usually related to that øf calcium, and th. differential behaviour of the two elements is conveniently described by the strontium-calcium observed ratio (OR) as proposed by Comar, Russell and Wasserman (1957); e. g. the OR So n_nlant is defined as: cm UR

- Sr/Ca in plant s oil-plant ~ Sr/Ca in soil


Sr in plant - Ca in soil Ca in plant • Sr in soTT *

where Sr/Ca in soils is the ratio of quantities available to the plants. To denote intermediary steps or specific physiological processes, the distribution or discrimination factor (DF| has been used, e. g. D

_ Sr/Ca in grain straw-grain Sr/Ca in straw


Sr in grain • Ca in straw Ca in grain- Sr in straw

It may be noted that the selectivity coefficient for strontium and calcium in the soil/soil solution system as mentioned above i s in fact a distribution factor similar to the DF valnes.

3. FACTORS AFFECTING THE ACCUMULATION OF RADIOACTIVE STRONTIUM BY PLANTS Plants absorb strontium as the divalent cation from the soil solution. Direct exchange between soil surface and plant roots has been suggested (C onzales and Jenny 1958) but is presumably not important under normal growing conditions. The quantity of an element accumulated by plants is proportional to the external supply, provided the ionic concentration in the nutrient solution is sufficiently low (Russell 1966). Since radioactive strontium is always present at low ionic concentration, the uptake may be expected to be proportional to its concentration unless it is controlled by the presence of stable strontium or influenced by other cations present in the nutrient medium. It is well known that the chemical and biochemical behaviour of radioactive strontium and stable strontium is identical, and consequently the accumulation by plants of radioactive strontium may be affected by the presence of stable strontium in the soil solution. The accumulation of strontium i s also greatly influenced by the presence of calcium and much experimental work has been concerned with vari-

- 14 -

ous factors affecting the ratio in which radioactive strontium and calcium are accumulated by plants grown in contaminated soil. Since the strontiumcalcium relationship is to be discussed in more detail in subsequent chapters it may suffice to mention here the generally recognized similarity in behaviour of the two elements. 3 . 1 . Influence of Soil Properties The influence of soil properties on the uptake of radioactive strontium by plants has been the subject of many investigations reported during recent years (Menzel and Heald 1959; Evans and Dekker 1962; Scheffer and Ludwieg 1961; Schrdder and Gunther 1967; FrederikBsoh and Eriksson 1966). The uptake from 20 soils collected from different parts of Denmark varied almost by a factor of ten (Refs. I and IV). In general the highest uptake occurs from acid sandy soils with a low content of organic matter and the lowest uptake is found from heavy clay soil with a high degree of base saturation, but several factors may modify this general picture. When water soluble radioactive strontium is added to the soil-solid: soil-solution system, the major part of it will be retained by the soil particles in an exchangeable or water soluble form (Schulz, Overstreet and Babcock 1958), while a minor part may be fixed in a non-exchangeable form. The amount present in either form depends on the length of time the radioactive strontium has been in contact with the soil (Squire 1960; Schulz and Riedel 1961), on the kind of exchange sites on the soil particles and on soil pH (Mshita, Kowalewsky, Steen and Larson 1956; Juo and Barber 1970). The radioactive strontium present in non-exchangeable form can be considered as non-available for uptake by plants (Robert and Menzel 1961). Thus the plant-available strontium in soil consists of the content in the soil s o l ution and the amount held in readily exchangeable form on the surface of soil particles. The relative uptake of radioactive strontium and calcium from contaminated soil depends on the relative concentration of the two elements in the soil solution adjacent to the root surface. This, in turn, i s determined by the properties of the soil solid phase. Russell, Schoffield and Newbould (1958) determined the relative extent to which five different soil types adsorbed radioactive strontium and calcium from CaCL,-solutions and their results showed a preferential adsorption of strontium for calcium. This was only slightly affected by the concentration of the CaCl 2 -solution used, but greatly influenced by the soil type.

_ 15 _ The soil properties affecting the relative distribution of strontium and calcium in the soil-solid:soil-solution system have been extensively elucidated by Khasawneh, Juo and Karber (1968) and the exchange selectivity of soils, clays and humic acid has recently been discussed by Juo and Barber (1969). The selectivity coefficient, K£, of 63 soils varied between 0.61 and 1. 51. The dominating factors influencing K were th" content and kind of clay and organic matter. The mineral fraction adsorbed strontium preferentially to calcium, whereas the K for humate and a muck soil was less than one indicating a preferential adsorption of calcium for strontium on the organic exchange complex. When the Sr/Ca ratio in the equilibrating system was decreased the relative adsorption of strontium for calcium increased, indicating that the affinity for binding strontium varied with the relative concentration of the two ions. Nishita and Taylor (1964) have also reported evidence of variation in relative affinity of strontium and calcium in clay, minerals and soils. The selectivity coefficients of the soils used in the pot experiments reported in Refs. I and IV varied between 1.13 and 1.52 with the lowest values in the muck soil from Lammefjorden, which was also to be expected according to results reported by Barber and co-workers. A close correlation (r • 0. 941) was found between the Sr/Ca ratio in water extracts (soil:water, 1:5) and that in rye-grass in these soils (Fig. 1). A very acid and Cadeficient soil, no. 29, was not included in this comparison because it was found that the Ca content in this soil was too low for accurate determination. The rate of cation exchange between the soil solid phase and the extracting solution is normally fast, and equilibrium between added strontium and soil calcium is attained within few hours. However« slowly exchangeable fractions have been found in some Bails (Taylor 1969) and it is possible that the exchange processes proceed slowly when the system is relatively dry as is normally the case in pot experiments. It was noted (Ref. I) that the Sr/Ca ratio in rye-grass decreased with time. This may be of importance for the interpretation of data on the distribution of strontium arid calcium In other plant species grown in pot experinv.'*;- with soil recently contaminated with radioactive strontium (Refs. VII and Vin). 3.2. Effects of Soil Management Several attempts have been made to develop techniques to be used in emergency situations' for reducing the plant uptake of radioactive strontium.

- 16 -

Sr/Cc IN WATER EXTRACTS, uCl/g Figure 1. TM rdattomhEp crop of rye-fran (Ref. IV), tfaeooila (Milnntw, 1:»).

tt*MSr/C* the ratio in

lath« first of

Different methods of removing the contaminated surface soil have been investigated and almost complete decontamination could be accomplished by removing the upper few • cm soil (Menzel 1962) or by •praying the contaminated surface with an asphalt emulsion, allowing it to harden, and removing the crust that is formed. Heating the contaminated soil to 800-1000°C was found to reduce the extractabi&ty and plant availability of radioactive strontium very effectively {Gregers-Hansen 1964 and Ref. n). Application of root inhibiting chemicals followed by deep placement was found by Menzel* Eck and Champion (1967) to be effective in reducing the radioactive strontium uptake by plants. However, such drastic remedial measures are not

- 17 .

likely to be applicable on a large scale, and more attention should be focused on the possibility of influencing the uptake by normal agricultural practices. The effects of deep ploughing on plant uptake of radioactive strontium have been elucidated in several experiments. The microplot experiment reported in Ref. V showed little change in uptake when the radioactive strontium was placed deeper than about 50 cm (Expt. I). Similar results were obtained by Schulz, Meberg and Overstreet (1959) from a lysimeter experiment with barley. Frederiksson, Haak and Eriksson (1969) simulated different tillage operations in microplot experiments and found that the deeper the radioactive strontium was placed the lower was the uptake by plants, though the effect was modified by a number of soil conditionB and climatic factors. tfyhre, Menzel, Roberts et aL (1964), Frere, Menzel, Roberts, Myhre et aL (1967) and Menzel, Eck, James and Wilkins (1968) reported extensive studies at various localities in the USA on the effects of ploughing to about 50 cm depth compared to conventional ploughing to about 15 cm. Deep ploughing reduced the accumulation of radioactive strontium in corn and soybean by 60% at most and that of oats and wheat-grain by about 20%, but there was much variability among locations and crops. In England, ploughing of contaminated fields to the depth of 30 cm and re-seeding with shallow-rooted crops caused the ratio between radioactive strontium and calcium to be less than 30% of that observed when the radioactivity was left on the surface (MUbourn, Ellis and Russell 1959; Milbourn 1960), The long-term effects of deep ploughing can be evaluated from table 1, which summarizes seven years results from experiment IL Ref. V. In treatments A and B the strontium-90 was mixed into the upper 10 and 25 cm soil, while in treatments C and D the strontium-90 solution was sprayed upon the soil at depths of 25 and 45 cm, respectively. The soils were obtained from the State Experimental Stations at Blangstedgard (II), Ødum (HI), St. Jyndevad (IV) and Borris (V). The results showed considerable variations between soil types and the deep placement seems to reduce the uptake by rye-grass more than mat of barley. The uptake was only slightly reduced when the strontium-90 was mixed into the entire plough-layer (B) as compared to the uptake from the upper 10 cm (A). An effective reduction was obtained when the contaminated soil is placed below the plough-layer and the effects persisted throughout the seven-year experimental period. Analysis of the soil profiles showed that the radioactive strontium could be recovered within few cm from the site of placement seven years before.

- 18 Table 1 The effects of placement on the average concentrations of strontium-90 in barley and rye-grass from experiments with four soils in microplots. Data given as nCi strontium-90 per g t dry matter 2 (0.5 mCi strontium-90 per m added 1964) 1

I t


Rye-grass j i Soil i Treatments I Barley, 1964-1966 r Straw i ,1967;1968 1969 1970 Grain !* Nos. 1) ''


n (s. c. 1.)

m (s. c. 1.)

rv (Lc.s.)

V (LB.)


A, 0-10 cm ] B. 0-25 cm • C. 25 cm i D, 45 cm J



2.68 1.73 0.64| 0.23

3.26 2.06 0.91 0.35

0.15 0.08 0.06

1.52 1.10 0.71 0.48

2.31 1.55 0.57 0.31

A, 0-10 cm j B, 0-25 cm ; C, 25 cm D, 45 cm

0.38 9.31 0.28 0.10

2.44 2.19 1.97 0.92

4 . 2 9 3 . 3 2 4.14 3.67 | 2.08 2.4012.77:2.41 j ' i i 1.12i 1.03I1.60J1.73 i 0.70}' 0.46 f* 1.03J0.62

A, 0-10 cm B, 0-25 cm C, 25 cm D, 45 cm

0.36 0.30 0.18 0.18

2.65 2.27 1.90 1.85

6.19)5.50 3.47(3.90 1.72(2.29 0.67 0.50

A, 0-10 cm B, 0-25 cm C, 25 cm D, 45 cm

0.38 0.28 0.23 0.16

2.70 1.99 1.99 1.25

6.40 4.87 5.76 3.22 3.33 4.04 1.75 1.78 3.01 1.22 0.91 1.38




3.19 2.19 i 1 0.76 , ^.35 !



5.46:5.17 4.00 4.86 2.71 2.83 0.70 0.34 4.99 4.00 2.24 1.28

1) s. c.l. * sandy clay loa.rn; L c . s . * loamy coairse sand; L s , * loamy sai 3.3. Effect* of Stable Str (1959) 116-132. Minotti, P. L . , Doris C. Williams and W. A. Jackson, Nitrate uptake and reduction as affected by calcium and potassium. - Soil Sci. Soc, Amer. Proc. 32 (1968) 692-698. Minotti, P, L . , Doris C. Williams and W, A. Jackson, Nitrate uptake by wheat as influenced by ammonium and other cations. - Crop Sci. 9 (1969)9-14.

-54 -

Mitchell, R. L , , Base exchange equilibria in soil profiles. - J. Agr. Sci. 27 (1 937) 557-568. Myhre. D . L . , R.G. Menzel, H. Roberts j r . , M.H. Frere, M. Amemiya, O.W. Beale, D. R. Timmons and E.H. Wood, Reduction of strontium90 uptake by corn and soybeans with deep placement, irrigation, and soil amendments. - Agron. J. 56 (1964) 463-467. Newbould, P . , Relationship between isotopically exchangeable calcium and absorption by plants. - J. Sci. Food Agr. r4 (1963) 311 -319. Newbould, P. and R. Scott Russell, Isotopic equilibration of calcium-45 with labile soil calcium. - Plant and Soil US (1 963) 239-257. Nishita, H., B.W. Kowalewsky, A.J. Steen and K.H. Larson, Fixation and extractability of fission products contaminating various soils and clay. - Soil Sci. 81_ (1 956) 317-326. Nishita, H., E. M. Romney and K. H. Larson, Uptake of radioactive fission products by crop plants. - J. Agr. Food Chem. j9 (f 961) 101 -105. 90 45 Nishita, H. and P. Taylor, Influence of stable Sr and Ca on Sr and Ca in soils and clay minerals. - Soil Sci. 98 (1964) 181 -186. Oliver, S. and S. A. Barber, An evaluation of the mechanisms governing the supply of Ca, Mg, K, and Na to soybean roots (Glycine max). - Soil Sci. Soc. Amer. Froc. 30(1966)82-86. Queen, W.H., H. W. Fleming and J. C. OKelley, Effects on Zea mays seedlings of a strontium replacement for calcium in nutrient media. - Plant Physiol. 38 (1963) 410-413. Rasmussen, D. C., L. H. Smith and W. M. Myers, Effect of genotype on accumulation of strontium-89 in barley and wheat, - Crop Sci. _3 (1963) 34-37. Reisslg, H. and H. J. Fiedler, Investigations on the effects of calcium and 90 potassium on Sr uptake by red clover and ryegrass. - AlbrechtThaer-Arch. 10_

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