The use of straw as a soil amendment is a common practice for providing nutrients to crops and conserving soil and water. However, it can lead to negative effects on the soil environment and crop yield. The objective of this research was to determine the effect of straw and biochar amendment on the root system morphology and above-ground biomass of a red clover/grass mixture. Straw was collected from miscanthus and winter wheat and biochar was produced from the biomass of those species. The following treatments were applied: wheat straw at a rate of 5 t ha-1, miscanthus straw at a rate of 5 t ha-1, wheat biochar at a rate of 5 t ha-1 (WBH), wheat biochar at a rate of 2.25 t ha-1 , miscanthus biochar at a rate of 5 t ha-1 (MBH), and miscanthus biochar at a rate of 2.25 t ha-1 . A treatment with mineral fertilizer but without organic amendments was used, and a control treatment without amendments and mineral was also tested. The botanical composition, root morphology parameters and the aboveground yields were determined. The soil amendments resulted in increased root lengths, surface areas, volumes and mean root diameters. There were no differences between the treatments with different feedstock types (miscanthus vs. wheat), materials (straw vs. biochar) or amendment rates (5 vs. 2.25 t ha-1). The resulting root system characteristics were reflected in the aboveground biomass productivity. However, these differences were noticed only during the first and second cuts.