Removal of Salts from Biochar Used in Growing Substrates Improves Germination and Growth of Tomato Seedlings

Biochar is a carbon-rich material commonly used as a soil amendment but also has been considered as a potential peat substitute in plant growing substrate in horticulture. However, its impact on soil or substrate quality largely depends on its chemical composition and physical properties that are defined by the production conditions and the feedstock. Commonly, pyrolysis of green waste leads to an accumulation of salt which can lead to an increase of the alkalinity and salinity of the biochar and thus of amended soils and gardening substrate. Often, this is associated to negative impacts on seed germination and plant growth. In order to overcome this problem and assuming that the high alkalinity is caused by high cation contents, we hoped that plant performance can be improved by salt removal Therefore, an experiment was designed to test whether consecutive water washing of two types of highly alkaline biochars enhanced their quality as a plant growing substrate. Whereas the first biochar derived from derived from tomato greens (TB) rich in Sodium, the second was produced from vineyard (VB) pruning residues which were rich in Ca. The pH, elemental composition and ionic contents of the biochars were monitored as a function of washing time. Subsequently, the impact of washing on plant performance was tested performing a greenhouse pot experiment during which tomato plants were grown on a mixture of 40% biochar and 60% gardening substrate. The results show a notable reduction of the pH of the substrate after the washing treatments, leading to improved germination, growth, and biomass weight of the tomato plants. We observed further that high sodium concentrations in the biochar have a stronger negative impact on plant growth than calcium. This work demonstrates that the feasibility of peat substation in growing substrates by some biochars previously considered practically unusable, can be enhanced through simple and relatively inexpensive water washing: This research opens up the possibility of recycling not only of pruning residues but also green residues produced during tomato cultivation as valuable substitute for peat in plant growing substrates in horticulture or gardening, promoting the circular economy and closing the carbon cycle.