Reduction in Peat Usage in Container Production of Cherry Laurel (Prunus laurocerasus): Effects of Biochar and Compost Amendments on Substrate Quality and Plant Growth

With increasing emphasis on sustainable horticulture, optimizing substrate composition is essential to reduce peat usage in container production. This study evaluated the effects of biochar and compost amendments on the growth and nutrient status of cherry laurel (Prunus laurocerasus) in two separate experiments conducted over five months. Experiment I assessed growth in pure peat and in peat–compost blends at volume ratios of 100:0, 70:30, 50:50, 30:70 and 0:100. Experiment II investigated the effect of adding biochar to a pure peat substrate at rates of 3 g·dm−3 and 5 g·dm−3. Key parameters were monitored, including the above and below-ground biomass, leaf and shoot counts, chlorophyll content, and the chemical composition of plant tissue and substrate. Compost addition increased the substrate pH from ~4.6 to ~6.4, while electrical conductivity increased with a higher compost content, reaching values approximately 2–3 times greater than in pure peat. Nutrient levels (Ca, K, Mg, P, NO3−) also rose consistently with an increasing compost share. While a higher compost content generally reduced the biomass, leaf and shoot number, the greatest plant height and relatively favorable biomass were observed at 30% and 50% compost mixtures. Biochar addition slightly increased plant height, while the total biomass, root mass, and shoot number tended to decrease compared to pure peat, particularly at the lower biochar dose (3 g·dm−3). The substrate pH remained relatively stable, whereas electrical conductivity (EC) showed a slight upward trend with increasing biochar levels. Biochar also slightly increased the substrate nutrient content (Ca, K, Mg, P, NO3−).