Abstract: Using a root-boxing and root scanning measurement system, the leguminous forage grass, Stylosanthes and the Graminaceous forage grass black seed Paspalum, were used to study the effect of phosphorus concentration on root distribution. Root biomasses of grasses significantly decreased with soil depth. The roots of Stylosanthes were concentrated in the 0-10 cm layer and those of the black seed Paspalum in the 0-20 cm layer. The black seed Paspalum had a higher demand for phosphorus and responded more to added phosphorus. For Stylosanthes, a low concentration of phosphorus (0.48 g P₂O₅/kg soil) is more suitable for the root and aboveground growth than a high concentration (1.92 g P₂O₅/kg soil). For the black seed Paspalum, an appropriate increase in the amount of phosphorus into the soil, can increase the root and aboveground dry weight, total root length, root surface area, volume and root average diameter.