The root interaction between neighboring plants is neither obvious nor well studied because there are no easy and convenient methods to distinguish the root system among individual plants. Previously, we developed a method of pressure-injection of dye into the roots of pot-grown plants in dry soil condition. Here, we modified the method for high-pressure injection in wet soil or outdoor fields to counter root pressure. In our preliminary pot experiment, the roots of tomato plants (Lycopersicon esculentum "Momotaro") were stained well in wet soil by dye injection at 0.5 MPa. In the field test, tomato plants were grown in a 2m x 2m concrete frame field. Four successive plant shoots were cut 159 and 170 days after sowing, and dye solutions (Fantasy) of different colors were pressure-injected into the roots from the stumps from 0.05 to 0.5 MPa. The volume of the dye and the time required for staining were respectively in the ranges of 120 to 150 mL and 9 to 96 h. Soil blocks or cores were sampled and the roots were collected by hand picking and the water floating method, and distinguished by color. The recognition and collection of roots were easy because most of them were stained dark. In the 0-15 cm soil layer, most of the roots in the four blocks surrounding the hills came from the hills. In blocks found at the intermediate position between two adjacent plants, the roots of both plants were distributed. In the 15-30 cm soil layer, most of the roots in the four blocks surrounding the hills came from the hills, although roots of other plants also existed. The roots of each plant extended to adjacent plants, even reaching distant plants. The roots of a plant did not grow toward another plant in the opposite row. Findings of this study are useful in relation to effective fertilizer application. The proposed injection method is expected to facilitate the study of the interactions between root systems in the field.