Effect of four acid soils on root growth of white clover seedlings using a soil-on-agar procedure

White clover (Trifolium repens L.) is widely distributed in the Appalachian region, except on highly acid soils. We used a procedure where a thin layer of soil is placed on top of solidified water agar to characterize effects of acid soil on seedling root growth. Our objectives were to evaluate the soil-on-agar technique by using four soils (non-limed and limed) with diverse chemical characteristics and to relate root emergence to the chemical properties of the soils. We used three white clover cultivars, Grasslands Huia, Grasslands Tahora and Sacramento. Daily counts of root emergence from soil into agar were made for 12 d. Liming hastened white clover root emergence in three of the four soils. Days to 40% emergence were closely related (P < 0.01) to soil pH and to species of soil solution Al that are associated with Al toxicity in dicotyledonous plants. The r2 values for the regression of days to 40% root emergence on were 0.95, 0.96, 0.94 and 0.96, respectively. Apparently, the primary factor responsible for delayed root emergence in the soil-on-agar procedure was Al toxicity. Because of the close relationship between root emergence and activity of toxic species of soil solution Al, we propose that the soil-on-agar technique should be useful for characterizing the response of many small-seeded species to Al.