A radioactive isotope progressing through a biological or physical system can be followed by several tracking procedures. For example, fertilizer containing radioactive phosphorus can be added to soil. Plants grown in this soil then take up the radioactive phosphorus just as they do non-radioactive phosphorus. If one of these plants is placed on a photographic plate, radiation from the radioactive phosphorus exposes the plate. The plant "takes its own picture," showing where the phosphorus concentrates in the plant. Radiotracers are a highly desirable research technique as they do not require the destruction of an organism for its study.