Substantially extending the life span of peat-and perlite-based growing media is a measure to increase the sustainability of soilless cultivation. The extraction of peat from pristine peatlands threatens these sensitive ecosystems and carbon sinks, meanwhile resulting in increased emissions of greenhouse gasses. Each batch of peat that is reused, results in a clear reduction in CO 2 emissions and a lower impact on the climate. After using growing media for one cultivation, we aim at reusing the spent material as growing medium for another crop. Spent peat and perlite based growing media from strawberry and cucumber cultivation were upcycled after steam treatment. We tested the effectiveness of steaming to reduce phytosanitary risks. The hygienisation efficiency of the steam treatment was confirmed: plant pathogenic fungi, larvae of vine weevils and weed seeds added or already present before the process were killed by the steam treatment. As the upcycled spent growing medium already contained high nutrient levels, the fertilizer application in the reused growing medium should be reduced, especially for P and K. Five indicators for assessing stability of the materials were used: CO 2 flux measurements, oxygen uptake rate (OUR), biodegradation potential, mineral N content and risk for N immobilization. The spent growing media had a low decomposition rate and the release of nutrients in a leaching experiment was lower than for a fertilized peat-based growing medium, being a reference blend for open field cultivated Chrysanthemum. N mineralisation and P uptake were tested in an incubation and pot trial, respectively, and the upcycled spent growing medium was found to be an important source of plant-available K and P. Steam treatment did not severely affect the microbial biomass and diversity of the spent growing media. Blending the steam-treated spent media with other materials or inoculating by a commercially available biocontrol fungus also had a limited effect, indicating that newly introduced microorganisms do not easily establish in steamed-treated spent growing media (SSGM). Acidification of the SSGM was achieved by a low dose of elemental S. The steam-treated growing medium was tested for growing Chrysanthemum cuttings and plantlets. Spent growing media were not able to supply sufficient mineral N, but the stored amounts of P and K in the media were sufficiently plant available for optimal crop growth.

The aim of the current study was to create a high quality growing medium blend that replaces 70 vol% peat with 40 vol% woody green compost and 30 vol% bark compost and organic fertilizers (i.e., blood meal and chitin), all locally sourced. A range of “woody composts”, i.e., green composts based on feedstock selection with mainly woody material from tree prunings, were produced for this purpose at green compost facilities. First, the woody composts were characterized chemically and biologically, including their microbial biomass and net N release. In comparison with regular green composts or vegetable, fruit and garden (VFG) waste composts, woody composts are more suitable for use in growing media due to their lower pH, EC and inorganic C content; however, the woody compost had a low N mineralization rate. Three types of composts supported a higher microbial biomass than wood fiber or bark compost. The additional mineral N release after 100 days for compost mixed with blood meal was tested for different VFG and green composts. A significantly higher additional net N release was measured for composts with higher initial mineral N concentrations (317 mg Nmin/L) as well as VFG composts (417 mg Nmin/L) than for green composts with a lower initial mineral N concentration (148 mg Nmin/L). In a last step, woody compost, bark compost, wood fiber, coir and peat were mixed in different ratios, resulting in six blends ranging from 100% peat-free to100% peat. Two batches of woody compost were compared, and the blends were mixed with blood meal or chitin. A strong effect of both the organic fertilizer and the blend composition on the mineral N release was observed, with a clear link between the microbial biomass and the net N release. There was a significant negative correlation between the net change in microbial biomass and the net N release (R = −0.85), which implies that a larger increase in microbial biomass during incubation with the organic fertilizer resulted in less N mineralization. The blends containing woody compost scored better for nitrification activity, as the NH4-N concentrations were lower in these than for the blends without woody compost. For the peat blends, no effect on the microbial biomass was observed after addition of organic fertilizers; mineral N did increase, however. Woody composts have the potential to be used in high vol% in growing media blends, but the blends still need further optimization when supplemented with organic fertilizers.

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