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Legume establishment is difficult on many moderately acid, infertile soils in the humid northeastern United States. Legume seedling vigor as influence by P, S, and Mo fertilization was studied in order to determine fertilizer needs for improved establishment. A Gilpin silt loam was fertilized with 0, 22, 67, and 201 mg P kg⁻¹ in combination with 0, 22, 67, and 201 μg Mo kg⁻¹ in a greenhouse experiment in 1983. Legumes studied in this experiment were red clover (<i>Trifolium pratense</i>), white clover (<i>T. repens</i>), and birdsfoot trefoil (<i>Lotus corniculatus</i>). Field experiments with red clover, birdsfoot trefoil, and flatpea (<i>Lathyrus sylvestris</i>) were initiated in the spring of 1983 and 1984 using 0, 50, 150, and 450 kg P ha⁻¹, 0 and 60 kg S ha⁻¹, and 0 and 874 g Mo ha⁻¹. Seedling vigor as measured by plant height, trifoliate leaf count, dry weight, and trifoliate leaf area increased the greatest in the greenhouse using 22 mg P kg⁻¹ relative to the higher rates of P fertilization. Seedling vigor without applied P was poor, which clearly indicated the essential need for P in the early stages of legume growth. Molybdenum applied at 201 μg kg⁻¹ increased the growth of the greenhouse grown legumes the greatest above the 0 μg Mo kg⁻¹ rate within each added P treatment. Seedling vigor however, was not enhanced with Mo fertilization until the P deficiency was corrected. Field established legumes increased in height and dry matter yield the most using 50 kg P ha⁻¹ with respect to the additional increments of applied P, but the increase was not as great as that obtained in the greenhouse using 22 mg P kg⁻¹, This was attributed to a higher extractable P level before fertilization within the field experiments. Sulfur fertilization generally did not enhance seedling vigor in the field studies; although, red clover yield was increased using 60 kg S ha⁻¹ in the 1984 field experiment, Molybdenum applied at 874 g ha⁻¹ increased seedling vigor as measured by plant height and yield in the field experiments with or without P or S fertilization. Phosphorus uptake and Mo concentrations were increased in the plant tissues with either P or Mo fertilization. / Master of Science / incomplete_metadata