[en] Soil injection of ³²P was used to determine the root activity pattern of rainfed Cassava (Manihot esculenta, Crantz) in Thailand. Root activity decreased with depth so that 57, 38 and 5 percent of the total activity was noted at the 15, 30 and 50 cm depths, respectively. The root activity increased with distance from the plant. Twenty two, 28 and 50 percent of the root activity was obtained at the 10, 20 and 30 cm distances, respectively. It was thus concluded that phosphorus fertilizer should be mixed with the surface 15 cm soil 30 cm from Cassava rows to maximize fertilizer uptake