The article shows a direct relationship between the maximum frictional force F-st (static friction) and the mean frictional force F-kin (kinetic friction). The tests were carried out for three plant granular materials (peas, maize, triticale). The modified Amontons-Coulomb model of friction was proposed for the F-st and F-kin description. The additional statistical analysis showed a strong linear correlation (R-2 = 0.95) between the F-st and F-kin forces for all combinations of moisture levels (9, 12, 15, 18%) and loads (16, 27,42, 50, 61 N). This makes it possible to simplify the measurement procedure, which in the case of kinetic friction is relatively difficult to implement. (C) 2019 Elsevier B.V. All rights reserved.