The present study is concerned with the application of the methods of stereological analysis to the investigation of the structure of agricultural material. The most important issue in the investigation of such three-phase media is to account for their structure, which is continually changing in the deformation process, and more precisely the physical parameters of the structure such as the size of solid phase elements and pores being a complementary phase. The study presents examples of the structures of agricultural materials (soil, root, grains, potato tuber cells) and their changes resulting from the operation of external forces. Common features of the materials being investigated and resulting search for a common method to investigate their structure have been stressed here. Determination of the distribution of grain size, aggregates, pores and forces in the contact of the solid phase elements and the sizes of cells, cell walls and their strength parameters is indispensable while applying a probabilistic equation to determine the changes of medium volume at any time during the deformation process. A probabilistic equation for a three-phase, granular medium (soil) considering for the function of the distribution of structural element sizes has been presented as an example. In order to solve the problem of empirical determination of such functions a stereo-logical analysis has been proposed. Basic assumptions of such approach have been presented stressing their use for the quantitative characteristics of the solid phase element geometry as well as of the geometry of the space created by those elements and porosity being the substructure of the material being investigated. Using stereological analysis one can quantitatively determine physical parameters describing the three-dimensional structure of the object being investigated on the basis of the analysis of a flat image. As it is easy to obtain microscopic images of agricultural materials, it seems futuristic to leam such approach. Stereological analysis has not, so far, been applied in the investigations of agricultural materials (except for own investigations of the authors). That is why the present study includes the basic information and references.