Abstract Scientific literature reports mixed results in terms of crop yields grown in soils amended with biochar and compost. These discrepancies are mainly attributed to the heterogeneity of waste materials used, the production processes, and the application rates. To address the challenge of reducing synthetic fertilizer inputs in the Mediterranean area, three experiments were conducted to evaluate the effect of biochar, compost, and their combination on the yield and nutrient uptake of lettuce (Experiment 1), spinach (Experiment 2), and arugula (Experiment 3) grown under three fertilization regimes (none, suboptimal, and optimal). Regardless of the fertilization level, the combination of the two soil conditioners generally exerted a positive synergistic effect on crop yield, although it increased nitrate concentration in spinach leaves (+ 31.44%) and in arugula leaves (+ 84.55% in the first cut and + 112.79% in the second cut) compared to the control conditions. While all three experiments showed a promising reduction of the production gap between plants grown under optimal conditions and those grown under no or sub-optimal fertilization, the best results were not always achieved with the combined application of compost and biochar. In experiment 2, for instance, the combination of compost and biochar exhibited an antagonistic effect on yield under sub-optimal fertilization conditions, whereas in experiment 3, a clear synergistic action was observed, with arugula yield in the first cut increasing by 16.44%. The heterogeneous results obtained across the three different experiments, both in terms of yield and nutrient uptake, highlight the need for tailored agronomic strategies that take into account the multiple interacting factors involved, including crop species.
Campana et al. (2026) studied this question.