RAni(^-A(;ri\i: fission-I'rodicis in iiii: hlm.w food chain 



was greater when the soil was low in available calcium. In liic areas sown 

 with cereal crops the uptake was notable only f)n one sf)il with little readily 

 extractable calcium. On all scjils the uptake into the plant was almost 

 exclusively into the straw or chafT, or both; extremely little entered the grain. 

 The I'oot or fodder crops varied according to the conditions; kale and sugar 

 beet took up significant amounts of strontium from several soils. The effect 

 of emplacement by the various forms of cultivation was very small in the 

 deeper rooted species. It is notable that when stabilized pasture had been 

 contaminated the level of '^■'Sr in the tissues formed subsequently, was up to 

 five times higher than in grass sown on contaminated ground. 



This brings us back to predictions about the future. RusselP^ has just 

 published his appreciation of this problem. He has taken the results of the 

 field experiments and noted the uptake b)' rye-grass according to the calcium 

 status of the soil. Figures are available for the fraction of total cultivated 

 agricultural soils in Britain, according to the extractable calcium in the soil. 

 Britain is fortunately placed and half the cultivated area has more than 

 15 mequiv. of calcium per 100 g of soil. Only a few per cent have less than 

 5 mequiv. Combining the data obtained from the field experiment with 

 appropriate weights for the fraction of total cultivated area corresponding to 

 each particular soil type, he thus derived a national mean for ^"Sr in [i[iC/g 

 calcium. This calculation leads to the conclusion that there should now be 

 1 • 1 S.U. in plants per mC/km"^ of '•'"Sr deposited on agricultural land. 

 However, the observed results in pasture grasses and correspondingly in milk 

 are 8 to 14 times greater than would be expected on this basis. On the other 

 hand, annual crops have values approximately of this amovmt. Russell notes 

 that established pastures, particularly uncultivated hill grazings, have not 

 only a root system penetrating the true soil and an aerial leaf system, but 

 each plant has a crown which can mechanically trap particulate matter. 

 In this area between the true aerial leaves and the roots there is a mat con- 

 sisting of organic matter and decayed leaves infiltrated with surface roots. 

 Thus fall-out which is not directly trapped by the aerial leaves is deposited 

 in this mat and material which is first deposited on the leaves can be later 

 washed off into the mat. There is thus an intermediate reservoir between the 

 aerial parts and the true soil. The capacity of the plant to absorb micro- 

 nutrients, such as strontium, from this mat is likely to be high, and under 

 natural conditions the strontium will not be diluted here with calcium as 

 in soil, however calcium deficient. While this reservoir is being filled the 

 plant will absorb ''"Sr according to the total deposited ; but when it is filled 

 there should be an equilibrium between the intake from the atmosphere 

 and aerial parts of the plants and the output to the true soil below ; thus the 

 plant will then absorb from it according to the rate of deposition. It is 

 likely that this reservoir will take several years to attain this equilibrium, 

 especially where the layer of mat is substantial. If this hypothesis should be 

 proved correct, some of the calculations of others based on levels obtained now 

 or in the immediate past and on the past trends, ma)- be unduly pessimistic. 



If one regards the uptake at equilibrium from the mat and from the aerial 

 parts of the plant as both dependent on rate of fall-out and only the uptake 

 from the true soil to be dependent on accumulated fall-out, then given a 

 constant rate of fall-out as over the last few years continuing in the future 



216 



