Field data on water-table depths, plant parameters, and corn yields were collected for 3 years (1984 through 1986) on the Clarion-Nicollet-Webster Soil Association from 50 plots under naturally fluctuating water-table conditions. The stress-day factor (SEW30) and stress-day index (SDI) concepts were used to provide a quantitative means of determining drainage needs of soils. Corn yield and SDI were used to assess the crop production losses due to excessive wetness in the soils. The stress-day factor and index were found to be useful tools in evaluation of drainage needs of poorly drained soils. The highest values of SEW30 and SDI were obtained for naturally very poorly drained soils, while naturally well-drained soils gave about zero values of SDI. Significant differences were found in SEW30 and SDI between all natural drainage classes of soils. This study also showed a strong relationship between relative yield and SDI for corn. This relationship was compared with published data from three other sources -India, North Carolina, and Ohio and found to be in agreement. The information in this paper on transient waterlogging's effect on crop growth under different natural soil drainage regimes should provide a basis for improving the design of field drainage systems.