Disentangling pH and Salinity Effects in Biochars Used as Peat Substitutes: Insights from Water Washing and Tomato Plant Growth Responses

This study evaluated the effects of water washing on the properties of biochars produced from tomato greens (TB) and vineyard pruning (VB), and examined how these changes influence plant performance in biochar–peat substrates. A 31-day pot experiment was conducted using tomato plants (Solanum lycopersicum L.) grown on biochar/peat mixtures at a 60:40 (v/v) ratio. Fresh biochars (TB 0, VB 0) and biochars subjected to one (TB 1, VB 1) or two (TB 2, VB 2) washings were assessed. Washing substantially reduced pH from 10 to 9 in both materials. The electrical conductivity (EC) and salt content of TB 0 (9147 ± 96 μS cm−1) were markedly higher than those of VB 0 (1539 ± 33 μS cm−1). Successive washing effectively lowered EC to 1019 ± 18 µS cm−1 and 75.4 ± 4 μS cm−1, respectively. Plant performance improved significantly as salinity decreased. Statistical analyses indicated a stronger influence of EC than pH on plant growth. Reduced germination was primarily associated with osmotic stress caused by elevated concentrations of soluble salts, particularly Na, K, Ca, Cl, and S. Biomass production was negatively correlated with these ions. Only Ca and K exhibited significant negative relationships, suggesting that nutrient antagonism may also have contributed to growth inhibition. These findings demonstrate that water washing is an effective strategy for enhancing the horticultural suitability of salt-rich biochars. Salinity, rather than alkalinity alone, appears to be the principal constraint limiting their use as peat substitutes. Washing therefore broadens the potential application of salt-rich green waste-derived biochars in horticultural substrates, supporting peat conservation and circular-economy approaches to organic waste valorization.