The particle size composition of soil affects the physical and chemical properties of soil. In this paper, the particle size of several rock soils (lime soil, lime soil and pozzolanic soil) in southwest China is measured and analyzed by hydrometer method. Draw the following conclusions:

(1) Purple soil, limestone soil and pozzolanic soil particles account for a large proportion, while sand, clay particles and silt particles account for a small proportion.

⑵ Due to sedimentation and leaching, there are some differences in particle size between different soil layers of the same soil, and the contents of sand and clay in black limestone soil increase with the increase of depth. The content of sand and clay in pozzolanic soil increases with depth.

⑶ Due to the different influences of weathering, leaching and human activities, the particle sizes of the same soil in different places are different.

(4) In the same soil layer, due to different soil-forming rocks, sandy soil is developed on sandstone, limestone and shale, with more rocky clay.

5. The soil of different subgroups of the same soil type has different particle composition, but the trend is similar.