Table 2—STRONTIUM-90 IN SOIL, GRASS, AND SHEEP BONE Soil (HCl extraction) Station pH Al A2 A3 B Cc 4.3 4.5 4.9 5.4 6.2 0.14 0.17 1.1 1.8 2.7 D 5.6 5.8 E F 3.6 7.5 0.37 4.7 G H 6.8 6.0 I 6.6 J 7.1 K 8.0 Grass gCa/kg puc/m? puc/gCa Bone pyc/m?> puc/kg puc/gCa uyc/g Ca 4900 5706 9600 6600 3300 800 760 130 59 14 60 90 15 145 2550 2500 230 1000 2100 1400 116 123 160 10000 28 50 2100 125 53 5000 2900 220 5.2 64 20 790 450 625 64 71 13 14.6 1.6 3400 2600 2.6 19 15 a 24 250 150 41 68 3.0 2500 8.0 39 1900 0.66 156 2200 0.15 b1.3 68 48 a 24 b 2,3 alé 100 89 100 33 26 32 b 2,5 a 27 b 4.8 72 190 210 26 33 28 41 24 8.7 This agreement shows that Sr*® remains substantially in the top soil and available to HCl extraction for periods of the order of years. This lack of penetration has previously been re- ported from the U.S. (Libby, 1956), and is shown also in the results from the 4 to 8 in. and 8 to 12 in. horizons from stations H to K, given in Appendix 1 and previously reported (Bookeretal., 1957). The soil results are also expressed as the specific activity of Sr*° with respect to Ca (S.U.) in the extractant. The S.U. content of the soil is a function of the extractant used as well as the soil, especially for calcareous soils. On acid soils different methods of extraction appear to give approximately the same S.U. results (Fig. 3 and Bryant et al., 1956). A more serious objection to the S.U. as a unit of Sr*® in soil is that it takes no account of the vertical distribution of Sr®? which may be very nonuniform even within the top 4 in. This is shown in the results from the two stations (D and E) where there was a sufficiently discrete layer of matt between vegetation and soil for a separate sample to be taken. The matt was found to contain about half the total activity in micromicrocurie per square meter, and the S.U. ratio in it was 41/ times that of the soil beneath. Equally sharp variation with depth may occur on other soils. 4.2 Sr® in Grass The detailed results from the sheep stations are given in Appendix 2.1, and from the other stations in Appendices 2.2 to 2.6. A summary is given in Table 2, in which the figures quoted for stations A3 and H to K are the averages of the results on the sequential samples taken between May and September, 1956. At Stations H to K samples of accumulated growth and of new growth were taken separately, and the results on the two types are given in the rows labelled (a) and (b) respectively for each station. The results are expressed in activity per square meter of ground, per kilogram dry weight and per gram Ca in the vegetation. The Sr*° in micromicrocurie per square meter in vegetation may be compared with the corresponding figure for soil in the fourth column of Table 2, and the vegetation activity expressed as a percentage of that in the soil. This percentage varies from 2.2 at station B to 0.05 at station H (new growth). The Sr*° expressed in micromicrocurie per square meter in the new growth samples at stations H to K was only about one-sixth of that in the nearby samples of accumulated growth. This was largely due to the difference in the weight of vegetation. Per unit dry weight the ac- cumulated growth had 17 per cent more Sr®® than the new growth and per gram calcium 28 per cent more. Only the latter difference is significant on a test, and both are much smaller than 214