POSSIBLE INFLUENCE OF DESERT SOIL MICROBIAL CHANGES F. H. F. Au, V. D. Leavitt, and W. F. Beckert National Environmental Research Center, Las Vegas* ABSTRACT The soil microbial population was determined for soil depth segments from 0-3 cm, 3-6 cm, and 6-9 cm in a previously uncultivated highdesert area. Plots in this area were covered by miniature greenhouses and planted with vegetables. After harvesting the vegetable crops, another assessment was made of the microbial population. The fungal population nearly doubled during the growing season, and the bacterial population increased by a factor of approximately 12. The fungal and bacterial live weights per hectare of the investigated desert soil were calculated and compared with those of agricultural soils. Possible consequences of the increase in microbial biomass are discussed for plutonium availability to plants, and for plutonium migration in soil. INTRODUCTION Soil fertility can be defined as the ability of the soil to supply nutrients to plants (Bollen, 1959). This ability depends largely on the activities of soil microorganisms. It is well known that soil microorganisms are responsible for the conversion of atmospheric and mineral constituents into forms which can be assimilated by plants. *See footnote, p. 35. 67