Developing plant bioassays to evaluate the performance of sustainable growing media

Environmental and ecological concerns related to the use of peat in horticulture and agriculture are pressuring growers and growing medium providers to find sustainable alternatives. Identification and screening of substrates from renewable resources suitable for a circular economy are required. As part of the on-going “BioSubstrate” project, we aimed to test the performance of newly developed materials from different biomasses and residual products and different processing. The suitability of 25 new materials, based on a range of raw materials from extruded willow to degassed digestate fibers, were tested as: 1) standalone substrates and 2) partial peat replacements. Two independent plant bioassays were conducted: a lab germination test using lettuce seeds and a greenhouse pot test using three different leaf vegetable species (chinese cabbage, lettuce and cress). The lab germination test found six growing media (originating from different processing methods of willow, wood chips and miscanthus) to be non-phytotoxic as indicated by a germination index >80% and a germination speed similar to double-distilled water. The direct sowing test found that all growing media tested performed better with partial peat replacement than as standalone. The pH and electrical conductivity (EC) of the 25 substrates measured in the lab assay were significantly correlated (0.576** and 0.761**, respectively) with the extracted solution from the pots in the greenhouse assay. Fresh weight (FW), dry weight (DW), germination percentage at day 2 and at day 5 were positively correlated with each other. The EC, pH and germination percentage on day 2 accounted for 93.43% of variance in principal component analysis. Four mixtures of peat and willow composted with a N-source clustered together with the control when a cluster analysis was performed, showing that some types of processed willow-based substrates have the potential to replace or partially replace peat in the future.