Nitrogen applied as fertilizer for crop production is partly absorbed by plant, and the remaining nitrogen in soil might be leached out through complicated processes to the subsoil layer. Especially, NO₃　N in leachate causes environmental pollution. The purpose of this study was focused on understanding of uptake of nutrients by plants, the behaviors of nutrients in soil and the possibility of leaching loss when nitrogen fertilizer and completely decomposed compost were applied. Lysimeters(Volume 0.15㎥, Diameter 62㎝, Height 62.8㎝) were installed for collecting leachate in the Jeju volcanic ash soils. Lysimeter study consisted of thirteen treatments：fallow, fallow with weeding, cropping without fertilizer and compost, three N fertilizer soil surface applications(16, 32, 64㎏/10a), three N fertilizer and compost soil surface applications(16+800, 32+1600, 64+3200㎏/10a), two water dissolved N fertilizer applications(16, 32㎏/10a), and low and high plant densities. N fertilizer was applied as urea. The growth of corn(preceding crop) and potatoes(succeeding crop) and leaching loss were determined during the experimental period. The results obtained were summarized as follows； With increased N, pH of leachate tended to decrease and NO₃　N concentration of leachate increased. NO₃　N leaching loss was remarkably greater in soil from the bare plot without fertilization and the weed control than from plots with medium N rate and was least in the cropping plot without fertilization. NO₃　N concentration in leachates from the water dissolved N fertilizer application plots was 64% of that from the soil surface application plots. The concentration of Ca and K ions and the leaching loss of these ions were least from the cropping plot without fertilization and were greatest from bare plots(T1 and T2) without fertilization. The proportion of leaching and residual N in soil increased as N rate increased indicting that higher N rates increase the possibility of N leaching to subsoil layer. The proportion of N leaching losses was lower at the low N rate and the high plant density. In future, fertilization prescription which can maximize fertilizer use efficiency and minimize the pollution of ground water will be needed for conserving the environments.