Abstract
Since 1961 through 1971, the replaceable 137Cs content in the same surface soil of the same area under clean, mulch and sod systems of soil management at a citrus orchard with that described in the previous paper1) was determined in order to compare its annual change with that of replaceable 90Sr.
The relation of humus content in surface soils classified as clean < sod < mulch closely related to half-time for an exponential decrease of replaceable 137Cs from 1963 through 1971 and with distribution coefficient1) for the downward distribution of replaceable 137Cs in, surface soil. Both half-times of approximately 8.5, 45.2 and 30.7 years and distribution coefficient of 0.094—0.111, 0.142—0.165 and 0.129—0.140 for clean, mulch and sod, respectively, are larger than those of 90Sr. The ratios of replaceable 137Cs to 90Sr of 0.92—1.13 in surface soil found for the year from 1961 through 1967 followed by a slight increase afterwards and reached 1.24—1.36 in 1971. It was found that 137Cs is fixed much more strongly than 90Sr in the surface soil of a citrus orchard.
