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.
At the same time we have seen how many factors influence the eventual uptake into the human body: geographical latitude, frequency of rainfall, the chemical form in which strontium is found, the calcium content of the soil, the method of agriculture. Even though the United States has pushed this investigation vigorously since 1952 the bulk of the work is still ahead of us.
For instance, in the United States, dairy products provide most of the calcium and strontium in our diets. In Japan, however, the situation is somewhat different. There the main source of calcium and strontium is rice. As a result, the ratio of strontium to calcium may be passing differently from the soil to man. Also the fallout strontium might be washed deeper into the soil and the soluble to non-soluble ratio might be different.
Considering the complex nature of the Sr⁹⁰ uptake into man, it is important to keep close track of the actual Sr⁹⁰ levels in the soil, in our food, and in our own bodies. The following graphs show how these levels have risen in the last several years due to the bomb tests:
Sr⁹⁰ in the soil—measured in thousandths of a gram per square mile.
