The slow-neutron flux as a junction of distance from a 90μg252Cf neutron source buried several feet below the surface of the ground was measured with BF3detectors in damp sand and clay strata. From 1 to 7ft from the source, the expected decrease in flux was observed, However, when similar measurements were made in shallow holes near the air-soil interface, the decrease with distance was less than exponential and a peak was noted at a distance of 8ft from the source. A qualitative explanation of the peak is given indicating that the density of thermal neutrons in the high-density thermal strata located several inches below the surface is a junction of the incident neutron energy and that the optimum incident energy is ~2 MeV. For this reason252Cf is a good neutron source for the purpose of in situ capture gamma analysis of soil constituents when used for mineral exploration purposes.