Thus the strontium is discriminated against relative to calcium in going from the soil to man by a factor of about 14. This is a factor of protection. It is good to double-check this conclusion and to find out how the calcium to strontium ratio changes step by step in going from the soil to man. One finds a factor of 1.4 in going from the soil to the plant, a factor of 7 in going from the plant to the milk, and a factor of about 2 in going from the milk to man. Actually, if we put all these factors together we should expect that on the way from the soil to man the calcium to strontium ratio increases by a factor 20. This is in reasonable but not in excellent agreement with the ratio 14 given above. Once the factor of protection is established we can get a value of the expected strontium uptake from the way in which the radioactive material is diluted by calcium rather than by normal strontium. This is a less straightforward but, for the time being, a more practical method than the direct Sr⁹⁰—normal strontium comparison. It is particularly important when one compares soils of rather different calcium content. Plants and animals require calcium. When they do not get it, they develop a calcium-hunger. Since strontium is chemically similar to calcium, a lack of calcium in the soil is readily substituted by available strontium. One would expect that plants grown on calcium-poor soil and animals raised on such land would exhibit abnormally high natural strontium content and also a proportionately high Sr⁹⁰ content. The high Sr⁹⁰ content has in fact been verified. Some sheep in Wales, for example, appear to have about ten times the average amount of Sr⁹⁰ in their bodies. Fortunately most people derive their food from many areas widely separated from each other. Soil that is deficient in calcium is not likely to supply more than a small part of an individual’s sustenance. However, the possibility of a large fluctuation cannot be ignored. In this event corrective measures would be needed. One simple measure would be to fertilize deficient soil with additional calcium. That soil can be successfully treated in this way is illustrated by the present situation in Wales. The sheep with the abnormally high Sr⁹⁰ content all come from the steep, poor pastures which are not limed. The sheep from the lower pastures, which are limed (not because of the fallout but for economic reasons), show an activity of only one third the value mentioned above. The point we have tried to make in this chapter is that the present human levels of Sr⁹⁰ can be satisfactorily accounted for by simple arguments based on the chemical similarity of elements and the identity of isotopes. These arguments give us confidence that we correctly understand how Sr⁹⁰ and how much Sr⁹⁰ is getting from the soil to the human body.