The importance of phosphorus in soil-plant relationships is well recognized, as evidenced by the magnitude of the phosphate fertilizer industry in the United States. Knowledge is incomplete, however, regarding the behavior of phosphorus in the soil, its availability for plant uptake, and the extent to which it is affected by the numerous ever-present variables. The isotopie tracer technique, in addition to sensitivity, simplicity, and rapidity, possesses a fundamental advantage over the standard chemical methods in that phosphorus added to a system can be distinguished from that already present. In 1940 Ballard and Dean (3), who reported on the use of radioactive phosphorus in soil studies, used an electroscope for detection purposes and made measurements on soil, plant material ash, and on intact plants. In 1941 Ballard and Dean (4) described the application of these procedures in biological measurements of the retention of applied phosphorus by soils ; they obtained close agreement between phosphorus fixation when determined by the standard water equilibrium system method, and when determined by the absorption by 19-day-old tomato plants of radioactive phosphorus. They were unable to obtain correlation between the standard method and the use of radioactive phosphorus in the chemical estimation of phosphorus fixation. No data as to the sensitivity of the detection procedures used or the specific activities of the original solutions were presented. The use of radioactive phosphorus in short-term plant physiological studies has been reported by several workers (1, 5, 7, 8). This technique has not yet been extensively used, principally because of an inadequate source of radioactive materials and the lack of detection equipment during the war years. With cyclotron output now available for peacetime research, however, and with the reasonable expectation that atomic energy plants will soon be used for the production of radioactive isotopes, the main obstacle will be removed (see Science, June 14, 1946). Experience has indicated that field studies are economically feasible in addition to the laboratory and greenhouse scale of experimentation. It is the purpose of this paper to describe advantageous procedures for the use of radioactive phosphorus in soil and plant studies and to present results with a series of Florida soils. Data are given on the sensitivity of the method, since this factor in consideration with an estimate of the expected ? Published with the permission of the Director of the Florida Agricultural Experiment Station.