et al., 1956) had shown that this method extracts as much Sr°*’ from the soil as the more heroic method of fusion with sodium carbonate, and it is shown below that measurements of the total Sr®° fatlout by HCl extraction of soil agree well with the estimated cumulative fallout in rain. 100 4000 PN ; ee DECAY T”2 64 HR nh Qo Q om _ > wy z z = = = 10 w — 400 ™ SC wo EK Z 5 8 E 2 3 G Sr | 0 89 % DECAY T’2 Lo 20 40 54 DAYS L 60 80 HOURS FOR Y%—paYs FOR Sr®9 \ 400 . 120 Fig. 2— Y*and Sr®decay. Extraction with ammonium acetate at pH 7 as well as with HC! has also been done ona selection of soils, aliquots of dried, ground and mixed soil being taken for the two methods. The details are given by Bryant et al. (1956) for the 1955 soils and in Appendix 1 of this paper for the 1956 soils. In Fig. 3 the results in S.U. by the two methods are compared. On calcareoussoils, which correspond to low S.U. values, there is considerable divergence between the HCl and NH,Ac results, the latter tending to give higher S.U. ratios. On acid soils there does not seem to be any systematic difference in the S.U. results by the two methods. There is also some variation of practice in the measurement of the Sr*” and Ca content of vegetation ash. Since the stable Sr content is low, Sr carrier is added before extraction with acid and the yield of Sr estimated. The Sr*® result is then corrected for yield, on the assumption that the stable and radioactive isotopes are extracted equally. This procedure is impossible for Ca, and in the method as described by Bryant et al., (1956) and used at Woolwich, an independent extraction of the ash with HF /HC1Q, is made for calcium determination. In the Health Physics Division an alternative method has been used in which both Ca and Sr*° were determined in the HC10,,HNO, extract, but it became clear from intercomparison of results (Appendix 2.1) that this extractant did not remove all the Ca from the ash. A third method is being developed in which both Sr*’ and Ca are determined in the HF HCiO, extract, and details of this method will be published later. 211 *y