x 3 lad NI * GRASS * = 1000 }— = x © SHEEP BONE — x oO z a oO > _ Oo ° x a Oo x x 0 * o | * oO _| Ou x oO oO 10 x | ‘ oO | 10 100 CALCIUM IN SOIL BY HCI EXTRACTION (G/KG) Fig. 5—Correlation of Sr”? in grass and sheep bone with soil calcium (normalised to 5000 pc /m? in soil). 4.3 Sr® in Grass In the autumn of 1956 a series of measurements was madeof the Sr®/Sr*° ratio in grass at Chilton, near Harwell. Grass samples (accumulated growth) were taken at monthly intervals and rain was also collected during the month previous to each grass sample. The Sr® and sr*° activities were measured, and the Sr®/sr*° ratio at the time of sampling worked out. These re- sults are shown in Table 3. The Sr®/Sr*® ratio in the soil was also estimated, by numericalintegration of the monthly fallout of Sr® and Sr®”. + Table 3— Sr®®/sr®” RATIOS IN GRASS, RAIN, AND SOIL AT CHILTON sr®9/sr** ratio eoVO7O0O——o—eeeeeeeO Date Grass Rain Soil Oct. 5, 1956 Dec. 3, 1956 22 15 40 28 2.4 2.2 Jan. 1, 1957 Feb. 1, 1957 Mar, 1, 1957 1G 13 12 19 17 15 2.3 2.3 2.1 The Sr®/sr* ratio in rain decreased from 40:1 to 15:1 during the period of these measurements but the calculated soil ratio declined only from 2.4:1 to 2.1:1, because of the effect of build-up and radioactive decay. The Sr® /sr*? ratio in grass was on each occasion intermediate between that of rain and of soil. “For this purpose results on the Sr® and Sr™ content of rain at Milford Haven due to Osmond (1957) were used. This was necessary because the Chilton results did not extend far enough back. Over the period during which the Sr®*/sr*° ratio from both stations was available, there was good agreement between them. 216