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At this stage it would 1)C wortli wliilc to take note of the recent experi- 

 mental work in the laboratory and field aimed at elucidatins^ soine of" the 

 factors which have hitherto been obscure. If we start at the beginning of the 

 chain — the soil — this is an extremely complex factor. Soils vary in their 

 texture and in their constituent clay-minerals. They vary as to pH, content 

 of organic matter and content of inorganic ions. When strontium and 

 calcium are imder discussion, it is possible to analyse the total content in 

 the soil of these elements by completely breaking down the soil by drastic 

 chemical treatment, for instance by fusion with soda and elution of calcium 

 and strontium carbonates. However, this does not give any indication of the 

 amount of calcium or strontium available to the plant. A number of milder 

 extracting agents, such as normal anmionium acetate, have been used in an 

 attempt to assay the so-called 'available' calcium and strontium. However, 

 the correlation between what the chemist sees as available calcium and 

 what the plant sees is poor. 



Russell, Schofield and Newbould^'^ of Oxford reported a new concept to 

 the Second Geneva Conference on the Peaceful Uses of Atomic Energy. 

 They consider that an 'equilibrium soil solution' would be one in which 

 the ionic concentration would remain constant irrespective of the ratio of 

 soil to solution. This should be the solution that the roots of the plants take 

 up. To determine the constitution of this equilibrium soil solution would be 

 a formidable proposition. Instead, they added ^"Sr and *^Ca to soil and 

 determined the equilibrium distribution of the tagged ions between soil and 

 solution to derive a soil /solution factor. They also grew plants in such a soil 

 and determined a plant/soil factor. For eight different soils with a wide 

 range of ' extractable ' calcium, they found a close correlation between the 

 soil/solution factor and the plant/soil factor. This suggests that the concept 

 of the equilibrium soil solution is valid for most conditions. In practice, by 

 determining a soil/solution factor one should be able to predict what would 

 be the plant/soil factor. Under most conditions the plant does not discrimin- 

 ate markedly between calcium and strontium. Previous work by Squii-e and 

 RusselF^ suggests that what a plant is mainly concerned with is the total 

 concentration of calcium and strontium in the nutrient solution. 



Another experiment is in progress at the Agricultural Research Council's 

 Field Station at Compton. Middleton and Squire ^^ have 18 drain pipes 

 filled with five difTerent sorts of soil, well-packed and weathered. The surface 

 of each cylinder has been contaminated with one millicurie of ''"Sr. Some 

 cylinders have been kept bare; others are growing crops such as rye-grass. 

 Cores of soil are periodically taken to determine the rate of leaching through 

 the profile of the soil, and the crops, where present, are periodically harvested. 

 So far in one year from 0-3 to 0-9 per cent, depending on the soil type, of 

 the ^"Sr originally applied has been absorbed by the crop. From each type 

 of soil the uptake of calcium by the rye-grass has been relatively constant, 

 but the uptake of ''"Sr has varied with the soil. Low ratios of ^''Sr to calcium 

 are associated with those soils with a high exchangeable content of calcium. 

 Addition of calcium carbonate to a soil initially low in exchangeable calcium 

 has markedly reduced the uptake of ^"Sr. In the soils themselves, after 19 

 months the major part of the radio-activity has not penetrated more than 

 two inches down the soil profile. 



214 



