Volcanic ash soil in Tengchong is derived from eruptive pyroclastic materials of Quaternary volcano. Generally, the soil is acid in reaction, and has pumice and other kinds of blocky ejects in its solum. The A horizon is characterized by a very dark colour, a very high content of organic matter, a very large water holding capacity, and a particularly low bulk density. There are considerable quantity of allophane in the clay minerals. A large amount. of fine hmnus particles and vitric debris are present in soil matrix. The soil is fairly fertile and productive. The formation process of the soil is dominated by accumulation of organic matter which belongs to lithomorphic meadow type, and ferrallitization accompanied by the biological silicatization in A horizon. Many of the genetic characteristics of these soils are similar to those in other regions of the world.