For plants to grow and develop, the growth pressure of plant roots in soil must exceed the counter pressure generated from the soil. This counter pressure is measured not only by soil hardness but is influenced also by pore deformability and by the water content of the soil. In this study, the root growth pressure of soybean (Glycine max Merr.) taproots at the initial growth stage was measured using packed soil cores having different values of dry bulk density, soil water content and hardness in a temperature and light intensity controlled room. The results indicated that the root growing into loose soil with sufficient water showed low growth pressure, whereas the root growing into soil whose physical conditions began to suppress root elongation showed the highest value of growth pressure. Growth pressure decreased when the soil had high hardness with insufficient air porosity ; and more branch roots were observed under such conditions. It was concluded that root growth pressure was not constant but varied by the counter pressure, which was determined by pore deformability, soil water content and soil hardness.
