This work includes an introduction and literature review of mycorrhizal enhancement of the mineral nutrition of host plants. Particular attention was paid to the role of mycorrhizae in acquiring P from inorganic phosphates. Two experiments were designed to study a mechanism by which the ectomycorrhizal fungus Hebeloma westraliensis can enhance the availability and absorption of P from Ca phosphate by <u>Eucalyptus diversicolor</u> in calcareous soil. Chapter 2 reports on a study on the growth and oxalic acid production of <u>H. westraliensis</u> in calcareous and noncalcareous liquid media. Oxalic acid is thought to chelate cations from phosphate precipitates allowing the release of phosphate into solution. L-threonine was added as an additional treatment to asses the ability of this amino acid to inhibit oxalic acid production by the fungus. Growth and oxalic acid production of <u>H. westraliensis</u> were increased in the calcareous systems. L-threonine had little effect on dry weight of mycelium but substantially reduced oxalic acid production in the calcareous media. Chapter 3 reports on a study of E. diversicolor X <u>H. westraliensis</u> mycorrhizal synthesis in model calcareous and noncalcareous systems. Oxalic acid production was measured in these systems as well as various measurements of solution and host plant nutrition. Solution P concentration decreased and solution Ca increased in the calcareous systems. Mycorrhization decreased the solution concentrations of both of these nutrient elements, due partly to sequestration of these elements in fungal and plant tissues. Plant tissue P concentration was decreased and tissue Ca concentration increased in the calcareous systems. Mycorrhization increased the concentrations of both of these elements in plant tissues. Mycorrhization also increased the height and dry weight of seedlings at harvest and ameliorated symptoms of nutrient deficiencies seen in nonmycorrhizal plants in the calcareous system. Oxalic acid production by the fungus and by the plant were increased in the calcareous system. Attempts to correlate oxalic acid production with solution and plant nutrition were unsuccessful. The relationships of these results to the mycorrhizal enhancement of plant nutrition in calcareous soil is discussed. / Master of Science