The increasing demand for sustainable substrates in agriculture and urban greening calls for alternatives to peat, whose extraction poses significant environmental risks. This study assesses the potential of olive pomace biochar (OB), wood biochar (WB), and green compost (GC), alone or in combination, to partially replace peat in growing media and improve substrate properties and plant development. Ten different substrates were formulated by substituting 10–20% of a commercial peat-based substrate with these organic amendments, using the commercial substrate alone as a control. The effects of such replacements were evaluated in the following experiments: a germination test conducted in Petri dishes using four forage species (Medicago polymorpha, Lolium perenne, Festuca arundinacea, and Lolium rigidum); and two parallel pot experiments lasting 100 days each (one with M. polymorpha and L. perenne, and another with young Olea Europaea var. Arbequina saplings). This study evaluated the impact on plant development, as well as the physical properties and composition of the substrates during the incubation process. Germination and survival of forage species were comparable or improved in most treatments, except those including 20% OB, which consistently reduced germination—likely due to high electrical conductivity (>10dS/m). In the pot experiments, substrate pH and total carbon content increased significantly with biochar addition, particularly with 20% WB, which doubled total C relative to control. Both forage species (Medicago polymorpha and Lolium perenne) and the olive saplings (Olea Europaea) exhibited normal growth, with no significant differences in biomass, water content, or physiological stress indicators when compared to the control group. Nutrient uptake was found to be stable across treatments, although magnesium levels were below sufficiency thresholds without triggering visible deficiency symptoms. Overall, combining compost and biochar—particularly WB and GC—proved to be a viable strategy to reduce peat use while maintaining substrate quality and supporting robust plant growth. This approach proved effective across the different plant varieties tested, including Medicago polymorpha, Lolium perenne, and young olive plants, which together encompass a wide spectrum of agronomic and horticultural applications as well as contrasting growth and nutrient requirements. Adverse effects on early plant development can be avoided by carefully selecting and characterizing biochars, with specific attention to salinity and C/N ratio. This finding is crucial for the successful large-scale implementation of sustainable alternatives to peat.

SUMMARY: Despite being an essential micronutrient and its recent classification as a beneficial macronutrient, chloride (Cl⁻) has traditionally been considered of limited agricultural relevance and a potentially toxic saline ion. This study provides the first comprehensive demonstration of the quantitative and qualitative importance of Cl⁻ during early vegetative development (EVD) of tobacco and Arabidopsis thaliana plants. During this developmental stage, these and other species (including celery, lettuce, Swiss chard, spinach, squash, tomato, chili pepper, eggplant, and perennial ryegrass) exhibit the highest demand and transport rate of this non‐assimilable mineral nutrient to maximise growth of these herbaceous and also woody (such as citrus and olive) species. While Cl⁻ promotes cell expansion across all growth stages, its particularly pronounced stimulation of plant growth during EVD is associated with enhanced photosynthetic performance and PSII activity. This enhancement is in turn linked to a reduction in non‐regulated energy dissipation in PSII and an increase in the electron transport rate, along with ultrastructural changes in chloroplasts, underscoring that Cl⁻ is specifically required during EVD to drive the maturation of the photosynthetic apparatus. Unlike adult plants, the growth deficiencies caused by sub‐macronutrient Cl⁻ levels during EVD cannot be mitigated by equivalent nitrate (NO₃⁻) supplementation. As EVD concludes, plant demand for Cl⁻ gradually decreases, accompanied by a reduced growth response to Cl⁻ and an increased reliance on NO₃⁻, emphasising stage‐specific nutrient needs. The relevance of Cl⁻ as a morphogenic driver during a critical stage of development has significant implications for optimizing agronomic practices, particularly by reducing dependence on nitrogen fertilisers.