Use of raw and acidified biochars as constituents of growth media for forest seedling production

In plant nurseries devoted to the propagation of shrubs and trees for landscaping, gardening or forestry it is first concern to produce robust seedlings which resist the stress of transplanting to soil. The selection of appropriate growth media is crucial. Biochar, the product of pyrolysis of organic matter, has been suggested as a new organic amendment for soil or for soilless growth media. Biochar is usually strongly alkaline. We studied the possibility of acidifying biochar with nitric and phosphoric acids. The effects of raw and acidified biochars in peat-based substrates on rooting and growth of cuttings of Rosmarinus officinalis and in sandy soil-based substrates on growth of Phillyrea angustifolia seedlings were compared. The physical and chemical characteristics of the growth media, and the growth and nutrient content of seedlings were analysed. Results showed that biochar acidification with nitric and phosphoric acids improves the pH and enriches the biochar with N and P without excessively increasing electrical conductivity. However, a column experiment showed that nitrate was readily leached whilst phosphate was tightly retained by biochar, which questioned the practical availability of these nutrients to plants. The agronomical assays showed that both raw biochar and acidified biochar improved rooting and growth of Rosmarinus cuttings. In Phillyrea, however, the acidified biochar did not affect plant growth whilst the raw biochar gave satisfactory results both for shoot and root growth. Results led to the conclusion that biochar without further treatment might be successfully used as growth medium constituent, even at large proportions, both in organic and in mineral substrates.