The potential of fiber nettle and fermented digestate product as peat substitutes in horticulture

Growing media are the basis for successful commercial horticulture. Peat has long been the most used substrate raw material. However, due to sustainability requirements, the demand for peat substitutes has increased, especially in Western Europe. We tested solid biogas residues and fiber nettle for their suitability as peat substitutes in growing media. Solid residues from biogas plants locally available in Germany contain high nutrient concentrations and are therefore of interest as a peat substitute in terms of the circular economy. For fiber nettle as a peat substitute, we investigated the shives fraction. These are the woody components of the stem and a by-product of fiber production from nettles. Fiber nettles can be grown on relatively poor soils or in agroforestry systems, which makes them an interesting crop to investigate. Both raw materials were submitted to a multi-step test procedure with chemical, physical, and biological substrate analyses for evaluation as a peat substitute, followed by plant growth tests. Fiber nettle shives (FN), digestate residues (DR), and products (DP) were used in high proportions, with up to 75% v/v FN or 100% v/v DP in the mixtures, to assess the maximum percentages that can be used as peat substitutes in a systematic raw material study. The results showed that fiber nettle shives have the potential to increase pore volume but had severe drawbacks in N immobilization. Further processing by fermentation and similar methods could be investigated. The range of tested solid digestate residues and products showed great variability across most chemical parameters and high salt contents, both of which are undesirable for potential substrate components. However, a fermented digestate product showed promising properties due to its relatively low salt content, neutral pH value, and homogeneous structure. It was composed of the solid phase of digestate residues from corn silage and liquid manure, which were subsequently fermented with sawdust. This product also showed promising results in applied plant tests with three different horticultural crop plant species: petunia, basil, and strawberry.