Abstract
Laboratory experiments were conducted to evaluate soil physical properties on the elongation of soybean (Glycine max Merr.) root germinated in a seed bed and grown to the soil core. Soil cores having variable soil-water contents and dry bulk densities were prepared for the test of root elongation. Soil mechanical impedance strongly influenced root elongation at the initial stage of root penetration into the core. The elongation rate of a root penetrated into the soil core for 1 d was influenced by both the dry bulk density and soil mechanical impedance. The soil hardness of a volcanic ash top-layered soil was measured to be in the range of 0.01-8.07 N, and there was no difference between the 5 and 10 mm penetration depth measurements. The difference in soil hardness was measured in 5 mm graduations, ascertained using a guide for the measurement. After these experiments measuring soil hardness with the pinpenetrometer, it was shown that this procedure is suitable for evaluating subtle differences in soil unevenness. The pinpenetrometer, which can measure soil hardness in 2 cm-thick layers, is incompatible with Yamanaka's penetrometer.
