Biochar application to peat based growing media for nursery production of broccoli seedlings

Peatlands are crucial sinks for carbon in the terrestrial ecosystem, but the massive use of peat as the most common and frequently the sole ingredient in plant growth media is jeopardizing their conservation. Therefore, the present study investigated the feasibility of biochar as a partial or total peat substitute in the growing media used for nursery production of broccoli (Brassica oleracea var. italica) seedlings. Biochar was obtained from residues of fruit-tree pruning. Four growing media were prepared to detect the optimum substrate for broccoli plants: peat 100% weight (P), peat 60% weight + biochar 40% weight (B40), peat 40% weight + biochar 60% weight (B60) and biochar 100% (B100). Substituting biochar at rates of 40 and 60% (w/w) for peat increased substrates pH, electrical conductivity, total porosity, water filled porosity, air space, water retention capacity, while having no effect on bulk density. The growth of roots of broccoli seedling, in terms of length and diameter, was similar in all the four growing media. Dry biomass, root volume, stem height and area of leaf surface of broccoli seedlings were lower when biochar was the only substrate component and higher when biochar was mixed to peat. However, the value of these parameters reached the highest values in peat. These was likely due to the lack of nutrients of biochar in comparison to peat and the difficulties of filling the alveolar trays because the fine and dusty biochar texture. The obtained results suggest the need for an optimization of biochar application in nursery seedling production, such as biochar enrichment with mineral nutrient formulations for the specific seedlings requirements, selection of biochar particle size and/or biochar aggregation into more easily handling grains.