Forage sorghum (Sorghum bicolor x S. bicolor) was cultivated for knowing the effect of alum sludge application on its root growth in a mountainous site, Kumsan. And the results obtained are as follows: The available P205 content in the soil seemed to decrease with the advance of level of alum sludge application. And plant P content decreased with the advance of sludge application without phosphate fertilizer. With phosphate fertilizer, root number of the forage was greater than that without the fertilizer. The root growth of forage sorghum was the highest with NPK and the least in control and alum application, and it is not recommended to apply NPK and alum together for the growth of the forage root. (Key words : Root, Sorghum, Mineral, Sludge, Kumsan)