Results of tests considering the influence of root reinforcement on shear strength of soil are presented in the paper. The tests were performed using two methods. In the first method were used Wu et al. [1979] model, in which the increase of soil shear strength due to presence of roots is calculated using values of tensile strength of roots. In the second method were performed tests in direct shear box apparatus for soil reinforced by roots. In both methods for tests were used roots of Scotch pine. The results of tensile tests showed that the tensile strength of Scotch pine are basically in a range from 5 to 25 MPa. The results of the shear strength tests of soil reinforced by roots showed that the increase of the shear strength of the soil due to presence of roots is generally associated with an increase in the cohesion of soil (about 2,9-8 kPa), while the reinforcement of the soils does not cause significant changes in the value of its internal friction angle. Comparison of the results of the two methods showed that the increase of the shear strength estimated theoretically, based on the results of the tensile tests, were much too high in comparison to the results obtained from direct shear test.

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