Rapid available potassium (RAK) in black soil (Mollisol) of 0–60 cm depth was measured to analyze its dynamics systematically in horizontally and vertically. The results indicated: RAK ranged from 97.5 to 395.5 mg/kg, decreased from horizon 0–20 cm to 20–40 cm, and then increased to 50–60 cm. Generally, variable coefficient (CV) and its range decreased from surface to deep. RAK decreased from top slope to down slope, and the dynamics were similar when horizons were closer. CV in horizon of 0–60 cm was higher in top slope (CV=23%–28%) and lower in shoulder slope (CV=12%–14%). RAK decreased at beginning, then increased and was lowest at horizon 20–40 cm.