niger and transported to its spores (Au and Beckert, 1974; Au et al., 1975). This suggests that soil fungi, and probably bacteria, could facilitate the transport of radionuclides in soil and enhance their transfer from soil to plants. Therefore, the laboratory became interested in fluctuations of soil microbial populations caused by agricultural use of a desert soil, and in their possible consequences for plutonium translocation in soil and plutonium uptake from soil by other organisms. Changes were assessed of the soil microbial biomass which resulted from controlled plant growth and irrigation under simulated agricultural conditions. These changes were studied as part of a field investigation designed to determine plutonium uptake by radishes and lettuce under controlled conditions from previously uncultivated soil in Area 13 of NTS. METHODS AND MATERIALS Small, transportable greenhouses were placed over two l.2mx1.2m plots in Area 13 of NTS. Trenches were dug for their emplacement so that the greenhouse structures extended approximately 30 cm below the soil surface. The access doors were sealed to prevent surface con- tamination by dust. The air in the greenhouse was recirculated and cooled or heated with portable temperature-modifying units. Each 1.2 mx 1.2 m plot was subdivided into four 0.6 m x 0.6 m subplots. The soil surface of experimental areas was covered with plastic sheets with 2 cm holes arranged in an 8 cm grid pattern. These sheets eliminated surface contamination of the aerial plant parts from soil within the greenhouses. Radish and lettuce seeds were placed through the holes into undisturbed soil. irrigants were used on the crops: Four different distilled water, water containing the disodium salt of diethylenetriaminepentaacetic acid (DTPA), water containing inorganic fertilizer (ammonium nitrate plus phosphoric acid), and water containing both DTPA and the inorganic fertilizer. DTPA and inorganic fertilizer were applied at a rate corresponding to 2,750 kg per hectare for each of these components. 69