Compost and biochar mixtures used to replace mineral N fertilisers in agricultural systems showed contrasting results, especially when applied in combination. We hypothesised that their effect may depend on the mixing ratio between the two components. The aim of our study was to determine if varying ratios of biochar/compost mixtures in presence or absence of chemical nitrogen (N) fertilisation influence ryegrass growth and nutrient uptake. To this end, we set up a pot experiment with an agricultural soil amended with five compost and biochar mixtures differentiated by their mixing ratio. We followed soil fertility parameters and ryegrass growth over three harvests. Results showed that the addition of amendments improved nutrient availability. The highest nutrient availability was observed with pure compost. Despite their high C/N ratios, biochar and compost increased N availability while preventing its microbial immobilisation. Biomass and nutrient uptake by plants were improved by amendments, but were dependent on the biochar/compost ratio. Addition of a mixture containing less biochar than compost (30% biochar/70% compost, on a dry weight basis) showed highest plant growth response and nutrient uptake as compared to the chemical N fertilisation. In addition, the beneficial effects of biochar and compost on plant growth were strengthened by N fertiliser. All in all, we demonstrated the synergistic effects between biochar and compost, which were predominantly driven by the mixing ratio between those two materials, and highlighted their potential use as a sustainable alternative to chemical fertilisers.