7 and 8 in the surface horizon but occasional values from 4.6 to 9.5 were found. The pH increases with depth and decreasing organic content. The amounts of exchangeable cations in the different soil types are given in Table I . The calcium content is so high that more calcium is brought into solution with repeated extractions. Consequently, strontium units have little or no significance relative to atoll soils and, when given, are based on total calcium rather than on exchangeable calcium. METHODS OF COLLECTION AND MEASUREMENT The vertical distribution of radioactivity in soil and litter was studied by analyzing samples taken mostly by l-inch s0il increments, in a few cases by 1/4- and 1/8-inch increments and by radioautographs of sections of soil cores prepared by the method of Held et al. (1965). The increment collections were made during both the wet and dry seasons in 1958, 1961 and 1963. ‘The cores were collected only in 1963. 1959, Because there is considerable horizontal variation in the levels of radioactivity (Table II) each set of increments was collected to insure the sampling of a single vertical column. The large amount of horizontal variability also made it more profitable to compare the relative amounts of radionuclides at different depths from many profiles than to make precise TUES