The author found liquid contrast media suitable for x-ray radiography on soil pore structure. A method of infiltrating a contrast medium into a soil sample was also investigated. As a result, it became possible to observe the macropores in a 4-cm cube of a soil sample stereoscopically. The procedure for this new method is introduced in this paper, and the stereoradiographs of macropores in several Japanese soils obtained by this method are shown. The following conclusions are derived from the stereoradiographs of the 4-cm cube samples: (1) the minimum pore diameter recognizable in the radiographs is around 50 μm; (2) because this method reveals macropores by infiltrating liquid contrast media into the soil sample, the results of radiography mainly show images of continuous macropores. These images show the important pores that contribute to water percolation and aeration and somewhat to water retention in the soils; (3) the structure of continuous macropores in a soil where plants have grown is very similar to the configuration of the root system of the plants. That is, the macropore structure is characterized by channel networks composed of tubular pores.
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