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

1 Introduction

For a successful commercial horticultural production, growing media are essential. For a long time, peat was the most used substrate raw material in Europe (Hirschler et al., 2022). However, the harmful effects of carbon dioxide release from drained peatlands on the climate and the lack of sustainable extraction practices have increased the demand for peat substitutes (Joosten and Clarke, 2002; Chapman et al., 2003). New raw materials are needed to cover the demand for peat substitutes and to compensate for the drawbacks of individual materials. To minimize competition for land use and to support the circular economy, there is increased interest in residual materials from various manufacturing processes as substrate components (Hirschler et al., 2022; BMEL, 2024). Additionally, resources that can be cultivated locally and therefore avoid long transportation distances are also attractive, such as materials derived from marginal sites, agroforestry systems, or dual-use crops.

To reach the goals of reducing greenhouse gas emissions by 65% in Germany by 2030 as defined by the Climate Action Programme 2030 (BMU, 2019) and the Climate Action Plan 2050 (BMBF, 2016), the use of peat as a growing medium and soil conditioner has to be reduced as far as possible. The German substrate industry made a self-commitment for peat reduction, which includes reducing peat in growing media for the consumer market to 30% peat and in growing media for commercial horticulture to 70% peat (IVG, 2020). However, this is far behind the required reduction targets for peat-free substrates to achieve the goals of the Climate Action Plan.

Already established peat substitutes are, for example, coconut products, softwood fibers, or green compost. Although coconut products can now be certified to meet ecological, economic, and social sustainability goals (Horticert, 2024), long-distance transport from Asian countries and insecurity of the supply chain are recurring concerns. The supply of softwood fibers is affected by the decline of coniferous forests due to massive bark beetle infections, as well as the competing use of wood in other sectors. The application of green compost in mixtures is often limited due to the low amount of available green compost in growing media quality or due to large heterogeneities among different production batches (Hirschler and Osterburg, 2022; Hirschler et al., 2022).

The joint research project ToPGa (Development and evaluation of peat-reduced production systems in horticulture) aimed to investigate and evaluate the effects of peat-reduced substrates as holistically as possible. Here, the approach was to study new candidates for peat substitutes, such as fermented digestate residues and products and fiber nettle, and to systematically assess their potential as an essential contribution to the replacement of peat in growing media. A focus hereby lies on locally available peat alternatives and those that might be waste products from other economic processes, in particular solid residues from biogas plants and fiber nettle shives.

Some studies have already investigated the suitability of digestate residues from biogas plants in horticulture (Asp et al., 2022; Blok et al., 2024; Jankauskienė et al., 2024), e.g., the utilization of the solid fraction of digestate from anaerobic digestion as container media substrate (Crippa et al., 2013). Advantages of first studies of fermented digestate vs. unfermented digestate were also discussed by Gruda (2021). Fresh residues may contain phytotoxins and high amounts of phosphate, and show an unstable nitrogen balance (Meinken et al., 2009). Therefore, further processing of fermentation residues was recommended (Meinken et al., 2009). An in-depth study of several treated and untreated digestate residues of local origin was carried out within the ToPGa joint project.

Stinging nettle is a common plant found not only in gardens but also along forest edges, on ruderal sites, or in riparian habitats (Vogl and Hartl, 2003). There is evidence that nettle has been harvested for textiles since the Late Bronze Age (Bergfjord et al., 2012). Nettle cultivation, similar to flax and hemp production, peaked in Europe in the 19th century and almost completely disappeared after World War II due to the availability of cheaper cotton fibers and the development of synthetic fibers (Viotti et al., 2022). Fiber nettle (Urtica dioica L. covar. fibra) is a variant of stinging nettle whose fiber content and overall yield have been improved by breeding with different wild types (Bredemann, 1959). The cultivation of these nettle genotypes allows for higher biomass production, which is beneficial for the local production of renewable resources. As a perennial plant, nettle can be cultivated extensively even in colder climates (Di Virgilio et al., 2015; Viotti et al., 2022). Commercial harvesting of the whole plant is estimated to be possible for at least five to ten years (Vogl and Hartl, 2003; TLLLR, 2021). In addition, nettle has low requirements for fertilizers, pesticides, and irrigation, and it grows on different soil types (Viotti et al., 2022). Due to its low environmental requirements and tolerance of low-light habitats, it can be cultivated on otherwise unproductive soils or marginal lands, such as in the transition zone from field to forest in an agroforestry system (Lewerenz et al., 2025). Although fiber nettle cultivation has a high potential for success, the lack of infrastructure for bast fiber processing, such as machinery and available processes, as well as marketing, currently allows only contract farming. In the ToPGa joint research project, we aimed at exploring the cultivation of fiber nettle with a potential dual-purpose use of the fibers and the shives, the woody component of the stem. Based on the concept of a circular economy, fibers can hypothetically be used to produce textiles or composites, and the shives are obtained as a waste product. Within the circular economy concept, shives as a waste product are seen as a by-product and were tested for their potential as a peat substitute. In this way, the long-term local availability of these renewable raw materials for the production of peat alternatives could be investigated, and our obtained results on fiber nettle could be transferred to other fibrous plants.

Therefore, our research question was whether local raw materials, such as digestate residues and products from biogas plants, as well as shives of fiber nettle, are suitable for horticultural use as peat substitutes in growing media.

2 Materials and methods

2.1 Raw materials

Digestates from eleven German biogas plants from the region around Ellerhoop (E), Schleswig-Holstein, Germany, and Bad Zwischenahn (Z), Lower Saxony, Germany, were selected for the systematic evaluation of their potential use as peat substitutes in growing media (Supplementary Table S1). According to the processing of the solid material after separation, digestates were divided into untreated separated digestate residues (DR), which were fresh or matured, and treated separated digestate products (DP), which were fermented or air or mechanically dried. If digestate residues were separated at the biogas facility and then directly sampled and used, they were defined as “fresh”. If digestate residues were separated and stored for approximately six weeks at the biogas facility, they were defined as “matured”. One of these digestates was later selected for further experimental evaluation on the basis of its chemical substrate parameters and its potential use in a large number of horticultural applications (see Section 2.4 below).

The second raw material, fiber nettle, was grown on an experimental field, retted, harvested, dried, and subsequently chopped and sieved to 0–10 mm to separate fibers and shives. To fulfill the goals of a circular economy, fiber nettle shives, a by-product of fiber production from nettles, were used for further analyses and plant experiments. This shives fraction is referred to as “fiber nettle” in the following text. Fiber nettle was used as a substrate component for the cultivation of petunia (Petunia × hybrida), basil (Ocimum basilicum), and strawberry (Fragaria × ananassa). Except for strawberry, fiber nettle was derived from a field near Soltau, Lower Saxony, Germany, and harvested in fall 2021, hereafter referred to as fiber nettle FALL. Due to the large amounts needed for the strawberry cultivation, fiber nettle material was derived from a field located in an agroforestry system of the Julius Kühn Institute, Institute for Crop and Soil Science, in Wendhausen near Braunschweig, Lower Saxony, Germany. This fiber nettle was harvested in February 2022 and is hereafter referred to as fiber nettle WINTER. Both origins of fiber nettle were subjected to base parameter analysis separately (see Section 2.2 below).

Milled white peat originated from Kaliningrad, Russia, and several countries of the European Union. Fine peat (5 to 10 mm; Kekkilä Brill GmbH, Georgsdorf, Germany) was used for the substrates of petunia and basil, whereas coarse peat (15 to 25 mm; KIasmann-Deilmann GmbH, Geeste, Germany) was used for the cultivation of strawberry.

2.2 Analyses of raw materials and substrate mixture treatments

Raw materials and treatments were analyzed according to a test procedure with several consecutive test criteria (TC; Leiber-Sauheitl et al., 2021a). In general, all units containing “L^-1” refer to “per liter of substrate”. Please see Supplementary Table S2 for a concise overview of the test procedure and the included materials and methods at each test criterion.

Basic parameters (TC I and TC II) of processed raw materials and treatments were determined, which included volume weight (g L^-1; VDLUFA, 1997a), pH value in CaCl₂ (VDLUFA, 2016a), salt content in distilled water (g KCl L^-1; VDLUFA, 1991), and nutrient contents (% N, P, K in dry matter; Leiber-Sauheitl et al., 2021a), as well as the C/N ratio. C (% in dry matter, d. m.) and N (% in d. m.) were measured by oxidative burning of the sample at 1080°C with an elemental analyzer (vario EL III, Elementar; DIN ISO, 1995). Dry ashing of the raw materials and treatments was done according to Miller (1998). K (% in d. m.) was measured in a CsCl matrix at 767 nm with an Atomic absorption spectrometer (AAS) (Perkin Elmer, AAnalyst 300). P (% in d. m.) was measured at 470 nm with a UV-VIS spectrometer (SPECTROstar Nano, BMG Labtech) after staining with the molybdenum yellow method (Scheffer and Pajenkamp, 1952).

For extended substrate analyses (TC III), plant available nutrients (mineral nitrogen (N_min), phosphorus (P), and potassium (K) mg L^-1) were extracted with CaCl₂ and diethylenetriaminepentaacetic acid (CAT extract; VDLUFA, 1997b) and measured according to Leiber-Sauheitl et al. (2021a). Maximum water-holding capacity (WHC_max, v/v) was determined according to VDLUFA (2002), and plant available water (% v/v) was calculated as the amount of water between −1 and −10 kPa (DIN EN, 2011). Buffer curves of the treatments were determined in order to determine and adjust the pH value with lime in subsequent substrate and plant experiments. For each mixture, five bottles were filled with 500 mL of moist substrate material. Subsequently, defined amounts of 85% CaCO₃ (0 g, 0.76 g, 2.48 g, 5.76 g, and 11.76 g) were added to each bottle and mixed with the material. Water was added until a water gloss appeared, and bottles were stored at room temperature, loosely closed to allow gas exchange. After 7, 14, and 21 days, pH values were measured according to VDLUFA (2016a). A curve was plotted in order to determine the required amount of CaCO₃ to achieve the desired pH value.

In TC IV, more complex substrate analyses of the potential substrate components followed, including stability tests and a standardized growing test. The nitrogen immobilization capacity according to VDLUFA (2007) was evaluated by adding 1000 mg of mineral N as NH₄NO₃ to 75 g of material of each treatment (80% WHC_max) and measuring the difference in N_min (CAT) between day 0 and day 20. Positive values indicate mobilization of N from the raw material tested, whereas negative values indicate immobilization of N in the microbial biomass. For initial testing of plant compatibility of the potential substrate components, standardized growing tests with Chinese cabbage (Brassica rapa subsp. pekinensis) under controlled temperature and light conditions were performed (VDLUFA, 2016b), modified by Leiber-Sauheitl et al. (2021a). Fertilization of the treatments was adjusted to the amount of immobilized N determined in the N stability test. Three weeks after sowing, plant biomass was harvested, and fresh and dry matter, as well as N, P, and K in the dry matter of the plants, were determined (Miller, 1998; Leiber-Sauheitl et al., 2021a).

Degradation stability of the raw materials and mixtures was evaluated by a CO₂ incubation experiment, and salt-extractable carbon compounds, as well as hemicellulose, cellulose, and lignin contents, were also analyzed. Degradation stability was determined by measuring microbial-derived CO₂ of each treatment at 60% WHC_max (Alef, 1991), modified by Leiber-Sauheitl et al. (2021a). Incubation of the substrate samples was performed at 20°C for 13 days at pH 6, a pH value relevant to horticulture. NaOH was placed in a flask to absorb the released CO₂ from the substrate samples. NaOH was changed at distinct time intervals, titrated, and the amount of released CO₂ was calculated. Cumulative CO₂ respiration over 13 days is reported. Easily degradable compounds, such as carbohydrates, were extracted from processed raw materials and treatments with 0.5 M K₂SO₄ at 20°C (Amha Amde, 2011, modified by Leiber-Sauheitl et al., 2021a). Total contents of hemicellulose, cellulose, and lignin were analyzed by gravimetric determination of the amount of acid-detergent-fiber (ADF; VDLUFA, 2012b), neutral-detergent-fiber (aNDF; VDLUFA, 2012a), and acid-detergent-lignin (ADL; VDLUFA, 2012c), respectively. NDF represents the total cell wall content (cellulose, hemicellulose, and lignin). By subtracting the ADF value from the NDF value, the hemicellulose content was estimated. Similarly, subtracting the ADL value from the ADF value resulted in an estimate of the cellulose content. The biodegradation potential was calculated as the holocellulose/lignin (HC/L) ratio: (%hemicellulose + %cellulose)/%lignin, with %hemicellulose = %aNDF – %ADF, and %cellulose = %ADF – %ADL according to Vandecasteele et al. (2017). The more stable the raw materials are, the closer the values are to 1.

2.3 Experiment treatments for subsequent plant trials in test criterion V

The two new raw materials – fermented digestate product Z III and fiber nettle – showed promising characteristics for use in horticulture and were subsequently tested in plant trials (Supplementary Figures S1, S2). The fermented digestate product was derived from a biogas company in Lower Saxony (Texas Bio-Energie GmbH & Co. KG, Cloppenburg, Germany). For the production of the fermented digestate product, corn silage (70%) and liquid manure (30%) were likely used as starting substances in the biogas plant. After the biogas digestion process, the solid phase of the digestate residue was separated and fermented with 10% to 30% sawdust for four months under a tarp.

Nine different mixtures of fermented digestate product, fiber nettle, and peat were created to systematically evaluate substrate-based risks for plant cultivation, such as high pH values, elevated salt and nutrient contents, high N immobilization, or low water-holding capacity (Table 1). Milled white peat was used for mixing and also as a control (Peat 100). In order to investigate systematic effects of the two new raw materials in the substrate mixtures, the content of the fermented digestate product in treatments with fermented digestate product and peat was increased from 25% to 50% and up to 100%. In fermented digestate product-fiber nettle mixing treatments, the proportion of fermented digestate product was fixed at 25%, and the content of fiber nettle was increased systematically. A mixing treatment with fiber nettle but without fermented digestate product was used to investigate mixing effects and interactions between the raw materials used.

Table 1. Treatments, proportion of peat substitution, and mixing ratios in% v/v (DP, fermented digestate product, FN, fiber nettle).

2.4 Plant growth experiments

In TC V of the test procedure, plant growth trials under practical conditions were performed at experimental stations in Germany. In the plant experiments, the following cultivars were used: seedlings of petunia (Petunia x hybrida; “GO!Tunia Lucky Lilac”, Florensis), seeds of basil (Ocimum basilicum; “Edwina”, Enza Zaden), and seedlings of strawberry (Fragaria × ananassa; “Aurora Karima”, Scandola fruit). Further plant growth trials were conducted with St. John’s wort (Hypericum cv.) and lettuce (Lactuca sativa). For easier readability, these data are not presented in this manuscript, but they are available upon request from the corresponding author. Amounts of base dressings of the fertilizer were adjusted to the amount of immobilized N determined in the N stability test for petunia, basil, and strawberry (Supplementary Table S3). During cultivation, substrate analyses were performed in order to check fertilization, liming, and irrigation. At the intermediate sampling of basil and petunia, three to four pots per treatment were removed which were completely used for substrate analyses. At the end of cultivation, plant biomass, N, P, and K (% in d. m.) in leaf biomass as well as substrate analyses were performed (Miller, 1998; Leiber-Sauheitl et al., 2021a).

In May 2022, the young petunia plants were potted in 11 cm pots at the horticultural training and research center in Hannover-Ahlem, Lower Saxony, Germany. The plants were cultivated either in a substrate mixture containing the fermented digestate product and fiber nettle in three blocks of 20 petunias each, or as control plants cultivated in professional growing media (PGM; Supplementary Table S12) in two blocks of 30 plants each. The nematode Steinernema feltiae was used to protect the plants against fungus gnats. Samples were taken from each substrate mixture treatment at the start, after 12 days for substrate analyses, and after 42 days at the end of the trial for further substrate and plant analyses. After the 12-day rooting phase, during which the plants were irrigated with overhead watering, fertigation was switched to the ebb and flood method with 0.8 g L^-1 Universol Blue (ICL Growing Solutions) with an N-P-K ratio of 18-11–18 as fertilizer. For treatments with fiber nettle, an additional 90 mg L^-1 NH₄NO₃ was added. The petunias were cultivated for six weeks until they were ready for marketing. At the end of the trial, the plants (shoots and roots) were evaluated, and their height and width were measured. Fresh and dry mass of the shoots was determined. Growing media parameters (pH, salt, and nutrient contents) were determined for six pots of each treatment containing one plant each.

In July 2022, basil was sown in 12 cm pots with 30 seeds per pot at the horticultural training and research center in Heidelberg, Baden-Württemberg, Germany. The pots containing the various substrate mixtures were arranged in four experimental units, each containing two rows of 20 pots. Due to different amounts of fertilizer application, two experimental units comprised all treatments containing fiber nettle, whereas the other two contained the mixture treatments and controls without fiber nettle. Steinernema feltiae was used as a preventive measure against fungus gnats immediately after sowing. Samples were taken from each substrate mixture at the start and, after 11 days, for substrate analyses, and at the end of the trial for further substrate and plant analyses. During an 11-day germination and rooting phase, plants were irrigated with overhead water. Afterwards, fertigation was carried out using the ebb and flood method with 1 g L^-1 Peters Excel for hard water (ICL Growing Solutions) with an N-P-K ratio of 18-10-18, as well as an additional 90 mg N L^-1 as NH₄NO₃ for treatments with fiber nettle. The basil pots were cultivated for four weeks until they were ready for marketing. At the end of the trial, the plants were evaluated based on leaf color, rooting intensity, and plant height. Fresh and dry mass of the shoot was determined. Growing media parameters (pH, salt, and nutrient contents) were determined for eight containers of each treatment.

The strawberry trial was set up in a tunnel using the Hochstaedter system at the soft fruit experimental station in Vechta-Langfoerden, Lower Saxony, Germany, at the beginning of June 2022 (Supplementary Figure S4). In a Hochstaedter system, plants are cultivated in an earth dam covered with foil, which has a small recess to accommodate long containers similar to gutters. There were four replicates, each consisting of eight strawberry plants arranged in a block system. The plants were irrigated using drip irrigation. The intensity of irrigation for each variety was controlled by the number of drippers, ranging from two to five per Hochstaedter system (Supplementary Table S9). The treatments of fermented digestate product, fiber nettle, and coarse peat (Table 1) were mixed with 4 mL wetting agent Break-Thru S240 (Alzchem) per 10 L water during the first watering. The substrate mixtures were fertilized with 3 g L^-1 slow-release fertilizer Osmocote Pro 3–4 M (ICL Growing Solutions) with an N-P-K ratio of 19-9-10 (+2MgO+TE) and 100 mg L^-1 trace element fertilizer Micromax Premium (ICL Growing Solutions). Nitrogen fertilization of the treatments containing fiber nettle and 100% fermented digestate product by volume was increased with methylurea Sazolene 39G (Saviola) with an N-P-K ratio of 18-0-0. In accordance with the expected nitrogen immobilization, 0.4 g L^-1 for DP25 FN12.5 and DP100, 0.6 g L^-1 for DP0 FN12.5, 0.9 g L^-1 for DP25 FN25, and 2.1 g L^-1 for DP25 FN50 were applied. Harvesting took place from mid-June to mid-November 2022. Harvestable marketable strawberry quantities, as well as plant and substrate parameters, were recorded.

2.5 Statistical analyses

Variance analyses were performed to analyze differences among treatments. The following data were log-transformed due to deviations from normality and/or heteroscedasticity, as indicated by Levene’s test, boxplots, the Shapiro-Wilk test, and Q-Q plots. Parameters with log transformation included: Table 2 (N-CAT, K-CAT), Table 3 (C/N, K-CAT), Table 4 (N-CAT), Table 5 (N in d. m.), Supplementary Table S4 (K-CAT), Supplementary Table S5 (Fresh mass), Supplementary Table S6 (P-CAT), and Supplementary Table S8 (pH, salt content, P-CAT, K-CAT). When the assumptions for ANOVA were not met, a non-parametric Kruskal-Wallis test was applied, followed by a pairwise Dunn test as a post-hoc analysis. These statistical analyses were performed with R (version 4.4.1; R (2024)) using the R packages “car” (version 3.1-3), “rstatix” (version 0.7.3), and “agricolae” (version 1.3-7).

Table 2. Basic parameters of the investigated digestate residues (DR) or digestate products (DP) with standard deviations.

pH value, salt content, NPK extracted with CAT, WHC_max: n = 4; C/N, NPK in d. m.: n = 1, variance analysis for all parameters with n = 4: ANOVA (except P-CAT Kruskal–Wallis), post hoc test: Tukey HSD (except P-CAT Dunn test), p < 0.05. Different letters indicate significant differences among raw materials. d. m.: dry matter, WHC_max: maximum water-holding capacity.

Table 3. Basic chemical and physical parameters of investigated raw materials with standard deviations.

Volume weight, pH value, salt content, NPK extracted with CAT, WHC_max n = 4; C/N, NPK in d. m. n = 3, variance analysis: ANOVA (except for N_min-CAT and WHC_max Kruskal-Wallis), post-hoc test: Tukey HSD (except N_min-CAT and WHC_max Dunn test), p < 0.05. Different letters indicate significant differences among raw materials. d. m.: dry matter, WHC_max: maximum water-holding capacity.

*Peat = Milled white peat

Table 4. Basic parameters of the mixture treatments consisting of fermented digestate product (DP) and fiber nettle (FN) with standard deviations.

Numbers indicate volume percentages of the components in the mixture with peat fine as the mixing component (see also Table 1).Volume weight dry and moist, pH, salt content, NPK extracted with CAT, WHC_max, air capacity: n = 4, C/N: n = 3; variance analysis: ANOVA, post-hoc test: Tukey HSD, p < 0.05. Different letters indicate significant differences among treatments, WHC_max: maximum water-holding capacity.

Table 5. Fresh and dry matter (g per pot) and NPK in d. m. of the cabbage biomass at the end of the experiment.

n = 4, variance analysis: ANOVA, post-hoc test: Tukey HSD, p < 0.05. d. m.: dry matter. Different letters indicate significant differences among treatments consisting of fermented digestate product (DP) and fiber nettle (FN). Numbers indicate volume percentages of the components in the mixture with peat fine as the mixing component (see also Table 1).

Additionally, a coefficient of determination (R²) was calculated by plotting N immobilization (y) against the content of fiber nettle shives in the substrate mixture (x) and calculating a linear regression curve in Excel (MS Office 2019).

3 Results

3.1 Selection of a digestate residue or product for further analyses derived from basic parameters

In a first step, eight untreated separated digestate residues (DR; n=8) and three treated separated digestate products (DP; n=3) were investigated on their basic parameters (pH, salt, and nutrient contents; Table 2; Supplementary Table S1).

All digestate residues and products showed a range of pH values from neutral to alkaline, with 7.0 for Z III up to 9.4 for Z I (Table 2). Salt contents of all digestate residues and products ranged from 1.80 g KCl L^-1 (Z III) up to 13.73 g KCl L^-1 (Z I). Plant available nutrients such as N_min showed a wide range from 81 mg N_min L^-1 (Z I) and 114 mg N_min L^-1 (Z III) up to 1929 mg N_min L^-1 (E II). C/N ratios of the dry matter were in the range of 15 (Z I) to 31 (Z III). N, P, and K in the dry matter differed among the digestate residues and products: N ranged from 1.38% (Z III) up to 3.07% (E V), P from 0.51% (Z III) up to 1.79% (E V), and K from 0.61% (Z III) up to 4.25% (E II matured; Table 2).

Only the fermented digestate product Z III showed neutral pH values, low salt contents, low N and K concentrations, and moderate P contents whereas all other digestates residues and products were alkaline and contained high amounts of salts and nutrients. There were only two digestate products and one digestate residue showing low N_min values, with 81 mg L^-1 (Z I) due to mechanical drying, 115 mg L^-1 (Z III) due to the fermentation process, and 173 mg L^-1 (E IV fresh). All other digestate residues and products showed higher N_min contents in ranges which can be critical for plant growth, since 200 to 250 mg N L^-1 are often recommended as a base fertilization of growing media (VDLUFA, 1997b; Röber and Schacht, 2008). The mechanically dried digestate product Z I was compact, hydrophobic, and difficult to rewet. It also showed extremely high K and salt contents as well as high pH values. The fresh digestate residue E IV fresh also showed high K and salt contents as well as high pH values.

Due to its neutral pH value, low salt and nutrient contents, and neutral odor compared with the other investigated digestate residues and products, the fermented digestate product (Z III) showed promising properties as a potential peat substitute. Hence, it was selected for further analyses and horticultural experiments.

3.2 Basic parameters of raw materials and mixtures

In a second step, all raw materials (peat fine, peat coarse, fermented digestate product (Z III from Table 2, fiber nettle FALL, and fiber nettle WINTER) were analyzed as single components. The control and mixing component, white peat (fine and coarse), showed significantly the lowest and acidic pH values, salt contents, volume weight, as well as P and K contents compared with all other raw materials (Table 3). The N content in both fiber nettle materials was up to 75% lower than in peat. The maximum water-holding capacity (WHC_max) of peat was 25% to 330% higher compared with all other substrate components. The fermented digestate product showed a high and basic pH value, salt contents of two magnitudes higher, nutrient contents of one (N_min) to two magnitudes (P, K), and up to 2.5 times higher volume weight in comparison to the control and mixing component (Table 3). Both fiber nettle origins had a slightly acidic pH value but differed significantly in salt and nutrient contents as well as in their C/N ratios. Fiber nettle harvested in fall (FALL) showed a 6.5 times higher salt content, 20% more plant-available P, 20 times more plant-available K, and a 35% lower C/N ratio than fiber nettle harvested in winter (WINTER, Table 3). The C/N ratio was highest for fiber nettle WINTER, with 102, and lowest for the fermented digestate product, with 31. The control peat was in a medium range with C/N ratios of 52 and 48 for peat fine and coarse, respectively (Table 3). The WHC_max of the fermented digestate product (45% v/v) was 52% of the control (Peat fine 86% v/v), whereas the values for fiber nettle FALL (28% v/v) and WINTER (26% v/v) were 33% and 30%, respectively.

For further investigations and plant experiments, treatments of fiber nettle FALL, fermented digestate product Z III, and peat fine were mixed systematically in different ratios according to Table 1. Figure 1 shows an example of the mixed treatments used in the plant experiments. The substrate mixtures showed chemical and physical properties correspondingly to the proportions of the raw materials fermented digestate product, fiber nettle and peat fine. Therefore, pH values, salt contents, as well as P and K contents rose with increasing proportions of fiber nettle and fermented digestate product. The pH values of the mixtures ranged from 3.7 to 7.0, and salt contents reached up to 1.8 g KCl L^-1 (Table 4). The treatment with 100% fermented digestate product (DP100) showed the highest pH value and the highest salt content (1.8 g KCl L^-1). The mineral nitrogen content showed a maximum of 115 mg per liter of substrate in DP100, and the plant-available P and K contents of all treatments were significantly higher than those in milled peat (Peat 100). WHC_max decreased with increasing proportions of fiber nettle, accompanied by an increase in air capacity, except for DP25 FN12.5 (Table 4). The WHC_max of DP25 FN75 (39% v/v) was 55% lower than that of the control Peat 100 (86% v/v).

3.3 C and N degradation stability tests of mixture treatments

The mineral nitrogen balance of the mixture treatments was examined using the current VDLUFA method A 13.5.1 (VDLUFA, 2007) to derive information on the amount of fertilizer application for the plant experiments. During the 20-day investigation period of the mineral nitrogen balance, all treatments immobilized nitrogen to differing degrees (Figure 2). The 100% peat treatment and those with 25% and 50% fermented digestate product Z III proved to be stable, with immobilization rates of 44, 16, and 49 mg N L^-1, respectively. Nitrogen immobilization increased proportionally to the fiber nettle content in the mixtures. In the treatment containing 100% peat replacement with fermented digestate product Z III and fiber nettle, almost 100% of the added nitrogen was immobilized (997 mg N L^-1). These results were confirmed by incubation tests to further evaluate degradation stability. CO₂ release increased with increasing proportions of fiber nettle (Figure 3), which may be linked to the higher content of salt-extractable compounds (Figure 4). In treatment DP25 FN75, CO₂ release was seven times higher than in the control Peat 100 (13 mg CO₂ g^-1 d. m. vs. 95 mg CO₂ g^-1 d. m.).

The degradation stability of the mixture treatments was determined by measuring the cumulative CO₂ release of the individual (unfertilized) treatments over 13 days. In order to determine the net N mineralization during the CO₂ experiment, the N_min content was measured at the start and at the end of the incubation period (Figure 5). During the incubation, the N_min content declined similarly to the results observed in the VDLUFA incubation experiment conducted to determine the N_min budget. The treatments with 50% and 75% fiber nettle immobilized the largest amounts of nitrogen, with values of -18 and -42 mg N L^-1, respectively.

3.4 Compound classes as indicators for degradation stability of the raw materials and mixture treatments

Besides the N_min budget and the CO₂ release, the salt-extractable C (SCC) and N (SCN) compounds of the treatments and raw materials also significantly increased with increasing proportions of fiber nettle in the mixtures (Figure 4). SCC and SCN comprise easily degradable compounds such as amino acids, fatty acids, and carbohydrates. Peat, as a very stable material, showed the lowest amounts of SSC (0.8 mg C g^-1 d. m.) and SCN (0.05 mg N g^-1 d. m.). The fermented digestate product showed slightly higher values, with SSC of 3.1 mg C g^-1 d. m. and SCN of 0.45 mg N g^-1 d. m. In contrast, the amounts of easily degradable C and N of fiber nettle were an order of magnitude higher, with SSC of 14.6 mg C g^-1 d. m. and SCN of 1.58 mg C g^-1 d. m.

As another indicator for substrate stability, the biodegradation potential, calculated as the holocellulose/lignin (HC/L) ratio, was determined (Table 6). Peat fine (1.4) and the fermented digestate product Z III (1.6) showed the lowest HC/L ratios, indicating high stability (Vandecasteele et al., 2017), whereas the highest value was found for fiber nettle FALL (4.4). Fiber nettle WINTER showed a lower HC/L ratio than FALL (3.8 versus 4.4, Table 6), indicating higher stability of fiber nettle WINTER in comparison to FALL.

Table 6. Cellulose, hemicellulose, lignin content, and HC/L index of raw materials, n = 1 (according to VDLUFA, 2012a, 2012b, 2012c).

d. m. = dry matter.

3.5 Chinese cabbage plant tolerance tests

To assess plant tolerance to the growing media mixture treatments, trials with Chinese cabbage were carried out. The growing tests with Chinese cabbage were based on VDLUFA A 10.2.1 (VDLUFA, 2016b with modifications according to Leiber-Sauheitl et al. (2021b). The fiber nettle mixture treatments were fertilized with higher amounts of N due to the results of the N budget (Figure 2). Due to pronounced N immobilization (Figure 2), treatments containing fiber nettle DP25 FN25, DP25 FN50, DP25 FN75, and DP0 FN50 received extra N fertilization between 40 and 180 mg N per pot during the plant experiment. Nevertheless, plant growth decreased with increasing fiber nettle content in the mixtures (Table 5; Figure 6).

Fresh and dry matter were highest in treatments DP25 FN12.5 and DP25 FN25, and lowest in the treatment with 100% peat replacement (DP25 FN75). The biomass values of the control (Peat 100) were in the medium range, while DP100 showed a lower biomass than the control.

Treatments containing 25% fermented digestate residue and 25%, 50%, or 75% fiber nettle showed significantly higher leaf N contents in comparison to the peat control (Table 5). The cabbage plants did not take up all the N from the additional fertilization. Between 20 and 70 mg N L^-1 remained in the substrate at the end of the experiment (Supplementary Table S4). P contents in cabbage leaves were generally quite high in comparison to earlier measured values (Leiber-Sauheitl et al., 2021a). The highest P contents were observed in treatments with 100% peat replacement (DP25 FN75 and DP100), which also exhibited the highest pH values in the substrate (8.3 and 7.6, respectively; Table 5; Supplementary Table S4). Leaf K contents were very high in all treatments containing more than 25% peat replacement and reached the highest levels in the 100% peat replacement treatment (DP25 FN75; Table 5).

At the end of the experiment, pH values in treatments DP25 FN50, DP25 FN75, DP0 FN50, and DP100 were very high, at 7.0, 8.3, 6.9, and 7.6, respectively (Supplementary Table S4). These pH values were above the recommended range of 5.0 to 6.5 for the cultivation of Chinese cabbage and may therefore induce plant damage (VDLUFA, 2016b).

Additionally, in treatments with fiber nettle, the growth of saprophytic fungal fruiting bodies was observed in some pots at the end of the experiment. This is a further indication of the relative degradation instability and high microbial activity of the fiber nettle material (Van der Wal et al., 2013; Di Lonardo et al., 2020).

3.6 Plant experiments in experimental stations

Plant trials with ornamentals (petunia), potted herbs (basil), and soft fruits (strawberry) were performed under practical conditions at three German experimental stations (see Section 2.4). In these plant trials, the substrate mixture treatments with fermented digestate product and fiber nettle were investigated as depicted in Table 1. Due to high N immobilization (nearly 100% of the added N; Figure 2) and low performance during the growing experiment with Chinese cabbage (Figure 6), treatment DP25 FN75 was excluded from further plant trials. Depending on the plant tested, established commercially available growing media mixtures from three German growing media producers were used as additional controls (Supplementary Table S12).

In the plant trials with petunia and basil, 11 different substrate mixtures were tested. Six substrate mixtures contained the fermented digestate product and/or fiber nettle shives in combination with peat in varying proportions (Table 1). As control treatments, Peat 100 and three commercially available growing media mixtures from different growing media producers adapted to the respective crop were used (Supplementary Table S12). In plant trials with strawberry, ten different substrate mixtures were tested, as only two commercially available growing media mixtures were used (Supplementary Table S12).

3.6.1 Ornamentals: petunia

Petunia plants in treatments with up to 50% v/v of the fermented digestate product Z III and up to 25% v/v fiber nettle were equally well developed as those in two of the control treatments (Peat 100 and PGM3; Figure 7). Petunias in the commercially available growing media were visually the most developed and had the highest fresh and dry mass. The treatments with 100% v/v fermented digestate product (DP100) and 50% v/v fiber nettle (DP0 FN50) showed the significantly lowest values (Supplementary Table S5).

At the end of the experiment, the substrates of all treatments contained sufficient amounts of plant-available N (141 mg N L^-1 in Peat 100 and 444 mg N L^-1 in DP25 FN50), and some treatments were even oversupplied (340 mg N L^-1 in DP25 FN12.5, 397 mg N L^-1 in DP25 FN25, and 444 mg N L^-1 in DP25 FN50; Table S 6). Since the experimental design did not allow fertilizing each treatment separately, the treatments with a high proportion of fermented digestate product had very high amounts of P and K in the substrate. We observed phosphorus concentrations of 40 mg P L^-1 in Peat 100 and 258 mg P L^-1 in DP100, while potassium ranged from 142 mg K L^-1 in Peat 100 up to 709 mg K L^-1 in DP100 (Supplementary Table S6).

After six weeks, N contents in plant dry matter (leaves) ranged from 3.2% (DP0 FN50, DP100, and PGM2) to 4.4% (DP25 FN50; Supplementary Table S5). The treatment DP25 FN50, which received the highest base fertilization and liquid fertilizer top dressing, had the significantly highest N content. Moreover, P contents in d. m. of all treatments except DP100, which showed 0.38%, were between 0.52% (PGM2) and 0.67% (Peat 100; Supplementary Table S5). The plants in treatments with the highest salt and P contents in the substrate at the same time showed the lowest P content in their leaves. K contents in the plant dry matter ranged from 4.0% in Peat 100 to 6.4% in DP100 (Supplementary Table S5). The higher the proportion of fermented digestate product Z III and fiber nettle in the substrate mixture, the higher the K content tended to be in both the substrate mixtures and the plants.

3.6.2 Herbs: basil

Basil plants developed equally well in the treatments with up to 25% v/v of fermented digestate product and in three of the four control treatments (Peat 100, PGM2, and PGM3; Figure 8). The use of fiber nettle in the mixtures resulted in a reduction in biomass development. Basil plants grown in treatments containing only fermented digestate product and peat, as well as those in the commercially available growing media, were the most developed and had the highest fresh and dry mass (Supplementary Table S7). The treatments with 50% v/v fiber nettle showed the significantly lowest values, 50% lower than those of the controls. Treatments with the lowest dry masses were the two treatments with 50% fiber nettle content, DP25 FN50 and DP0 FN50. Whereas the treatment with the highest K content as well as with the lowest N and P contents in the dry matter of the plant was DP100 (Supplementary Table S7).

At the end of the experiment, the substrates of all treatments showed sufficient amounts of plant-available N (181 mg N L^-1 in PGM3 up to 399 mg N L^-1 in PGM1), while some treatments were even oversupplied (681 mg N L^-1 in DP25 FN12.5, 678 mg N L^-1 in DP25 FN25, and 660 mg N L^-1 in DP25 FN50; Supplementary Table S8). Since the experimental design did not allow fertilizing each treatment separately, the treatments with a high proportion of fermented digestate product had very high amounts of P and K in the substrate. We measured 62 mg P L^-1 in Peat 100, up to 255 mg P L^-1 in DP100, 315 mg K L^-1 in Peat 100, and up to 1141 mg K L^-1 in DP100 (Supplementary Table S8).

After four weeks, the N contents in the dry matter of the plants (leaves) ranged from 5.0% (DP100) to 6.7% (DP25 FN50). All treatments containing fermented digestate product and fiber nettle received increased base fertilization and liquid fertilizer top dressing and therefore had the highest N contents (Supplementary Table S7). No significant differences were observed between these treatments and the Peat 100 control (6.6% N). Higher proportions of fermented digestate product (DP50 FN0, DP100) in the substrate mixture led to lower N contents in plant dry matter, while higher fiber nettle proportions (DP25 FN25, DP25 FN50) led to higher N content in plant dry matter. P contents in the dry matter of the basil leaves ranged from 0.65% (DP100) to 1.05% P (DP25 FN25; Supplementary Table S7). Analysis at the end of the experiments showed that basil plants grown in substrate mixtures had the highest pH values, such as DP0 FN50 with a pH value of 7.0 and DP100 with a pH value of 6.6, as well as had the lowest P content in the leaf dry matter, for example DP0 FN50 with 0.72% and DP100 with 0.65%. The K content in leaf dry matter ranged from 5.5% (DP25 FN50) to 7.1% (DP100; Supplementary Table S7).

3.6.4 Soft fruit: strawberry

Substrate mixture treatments containing fiber nettle and fermented digestate product resulted in comparable or higher yields of marketable strawberries than the control treatments, with 640 g per plant in DP0 FN50, up to 815 g per plant in DP25 FN50 versus 602 g per plant in Peat 100 coarse and 690 g per plant in PGM2 (Supplementary Table S9). Salt contents of substrate mixtures containing fermented digestate product and/or fiber nettle ranged from 1.0 g KCl L^-1 in DP0 FN50 to 3.8 g KCl L^-1 in DP100 (Supplementary Table S10).

With the exception of mixtures containing fermented digestate product and fiber nettles, the pH values of the two commercially available growing media dropped considerably during cultivation and were no longer within the target range of 5.7 (Supplementary Table S10). In June and August of 2022, leaf nitrogen content ranged from 2.21% N in d. m. (PGM1, 08/2022) to 3.17% N in d. m. (PGM1, 06/2022; Supplementary Table S9). By the end of the trial, almost all of the plants showed signs of nitrogen deficiency due to the limited duration of the fertilizer and the nitrogen immobilization in the treatments with fiber nettle (DP25 FN12.5 and DP0 FN50). As expected, the fiber nettle mixtures had significantly less ability to store water than the other mixtures (Table 4; Supplementary Table S11), and therefore the number of drippers was increased (Supplementary Table S9).

4 Discussion

4.1 Basic parameters of raw materials indicated fermented digestate product to be suitable for horticultural use

The basic parameters of all raw materials used for mixing the substrate treatments (Tables 2, 3) could be compared with distinct raw materials already in use for growing media. White peat (fine and coarse), which served both as the control (Peat 100) and mixing component in the peat-reduced treatments (Table 1), showed pH values, salt and nutrient contents, volume weight, and maximum water-holding capacity (Table 3) consistent with values reported in the literature (i.e. Schmilewski, 2008; Leiber-Sauheitl et al., 2021a). The N_min, P, and salt contents, as well as the pH value of the fermented digestate product Z III, were within the range reported for substrate composts with up to 300 mg N L^-1 and 2.5 g KCl L^-1 (BGK, 2025) or composted plant-based digestate residues with a pH value of 6.9, 254 mg N L^-1 and 241 mg P L^-1 (Schmitz and Meinken, 2009). The fermented digestate product Z III contained no NH₄ (data not shown) and exhibited a lower pH value in comparison to other anaerobic digestates after solid-liquid separation. These digestates had 26%-49% of NH₄ share on total N (data not shown) and a pH value of 8.5, which may have a negative effect on plant growth (Möller and Müller, 2012). Both harvest dates of fiber nettle shives showed pH values (Table 3) and maximum water-holding capacities (Table 3) in the range of flax shives (18% v/v), cattail (Typha angustifolia; 15% v/v to 35% v/v), or reed (Phragmites australis; 12% v/v to 15% v/v) (Stucki et al., 2019; Leiber-Sauheitl et al., 2021a; Boutin and Otte, 2024). Lewerenz et al. (2025) reported that the variation in chemical properties between the two batches of fiber nettle was due to the time of harvest. Compared to the fall harvest, fiber nettle harvested in February generally had a higher C/N ratio, since the nitrogen content decreases through microbial degradation processes during winter retting. Additionally, precipitation during the winter period led to the leaching of salts from the retted nettle stalks, which lowered their salinity.

4.2 Basic parameter analysis indicated considerable differences between digestates, with the fermented digestate product Z III showing the highest potential for horticultural use

The pH values, salt and nutrient contents, and water-holding capacities of the investigated digestate residues and products (Table 2) were comparable to values for other types of digestates reported in previous studies (Fischer and Schmitz, 2002; Meinken et al., 2009; Schmitz and Meinken, 2009; Crippa et al., 2013). In our analyses, those parameters differed greatly across the different digestates investigated. This was somewhat expected due to the considerable variance in digestate origins and treatments (Supplementary Table S1). Therefore, it was vital to distinguish between composted and uncomposted digestates when comparing results with literature values. Fischer and Schmitz (2002) reported a mean pH value of 7.5 in composted digestates derived mainly from municipal biowaste and treated by composting. This was comparable with the pH values in digestates Z III, which was derived from corn silage, liquid manure and treated by fermentation with sawdust, E II matured, which was derived from corn silage, grain/maize meal, cattle, and poultry manure, and Z IV, which was derived from solid manure and feed residues and matured (Supplementary Table S1). The salt contents of 75% of the investigated digestates in our study (E I, E II, E V, E II matured, Z I, and Z IV) were much higher than those reported by Fischer and Schmitz (2002), ranging 1.1 g KCl L^-1 to 7.9 g KCl L^-1 (Table 2; Supplementary Table S1). This was due to the high contents of animal by-products and/or the treatment of the digestate after separation (Table 2; Supplementary Table S1). Meinken et al. (2009) also evaluated a digestate with a very high salt content (11.0 g KCl L^-1) as not suitable for use as a growing-media component, which is similar to the high salt contents observed in E II and Z I. The two digestates used in Crippa et al. (2013) consisted of pig slurry, agro-industrial waste, and corn, while the other consisted of pig slurry and cow manure. These digestates showed pH values of 7.5 and 7.9 and electrical conductivity of 1.3 dS m^-1 and 1.1 dS m^-1, respectively, which is much lower than those measured for ten of the eleven digestates investigated in out study (Table 2).

The most critical parameters for further investigation in the plant trials were pH value, salt content, and nutrient contents. Due to its neutral pH value, low salt and nutrient contents, and neutral odor, in comparison to all other investigated digestates, the fermented digestate product Z III was the only one that showed the potential to be used in all sections of horticulture as a growing-media component (Table 2).

4.3 Fermented digestate product and fiber nettle shives had systematic effects on basic parameters of mixtures

Increasing amounts of fermented digestate product Z III and fiber nettle shives showed significant effects on several basic parameters of the mixture treatments. With increasing amounts of fiber nettle in the mixture treatments, we observed that volume weight moist and water-holding capacity decreased by 19% and 37%, respectively, while air capacity increased by 42% (Table 4). Singh (2024) reported a decrease in the volume weight by 20% from 111 g L^-1 to 92 g L^-1 and an increase in air capacity by 33% from 45% v/v to 67% v/v when increasing amounts of straw-like raw material Typha from 10% to 40% in substrate mixtures. Ridout and Tripepi (2011) used composted digestate residues in substrate mixtures with increasing proportions, ranging from 0 to 60% v/v. They similarly observed the trend that digestates lead to an increase in pH values and nutrient contents in mixture treatments (Table 4). In their study, pH values increased by 2.5 levels, from 4.8 (0% digestate) to 7.2 (60% digestate), while plant-available nutrients were several times higher in mixture treatments with 60% digestate than in those without, with N up to 29 times, K 15 times, and P 17 times higher (Ridout and Tripepi, 2011).

4.4 Low chemical stability of fiber nettle and high stability of fermented digestate product in C and N stability tests

All treatments with fiber nettle immobilized N in substantial amounts, ranging from 4% in Peat 100 up to nearly 100% in DP25 FN75 of the added N. Moreover, there was a strong positive correlation between N immobilization and the proportion of fiber nettle added to the fermented digestate product - fiber nettle mixtures (R² = 0.97; Supplementary Figure S5). Other studies also reported high N immobilization of fibrous materials such as flax shives, Miscanthus, or reed straw (Frangi et al., 2012; Stucki et al., 2019; Boutin and Otte, 2024; Singh, 2024). This was especially the case when the raw material was not treated with steam or extrusion techniques (Vandecasteele et al., 2018), which leads to higher stabilization of the material. The trend of N immobilization of fiber nettle is also reflected in a high microbial activity measured as CO₂ release, a trend that can similarly be found for other straw-like materials or plant fibers (Leiber-Sauheitl et al., 2021a; Vandecasteele, 2023).

Composts used in growing media often originate from green waste material such as leaves, twigs, and branches (Eymann et al., 2015; Hirschler and Osterburg, 2025) due to their favorable properties, including low salt content and usability in higher proportions in growing media. The composting process results in a chemical and physical alteration and stabilization of the starting material, which is reflected in decreased microbial activity and N immobilization. Similarly, fermented or composted digestate residues show lower CO₂ release compared with peat when incubated at pH 6, a value favorable for microbial activity (Figure 3). Lower CO₂ release in connection with more stable raw materials and mixtures was also found by Pot et al. (2022); Vandecasteele (2023) and Leiber-Sauheitl et al. (2021a).

Fresh, uncomposted digestate residues showed a relatively unstable N budget and tended to immobilize N, while composted ones released relevant amounts of N during 60 days of incubation. This needs to be considered during plant cultivation (Fischer and Schmitz, 2002; Meinken et al., 2009). During the 20-day incubation period for determining the N budget, the fermented digestate residue used in our study immobilized nearly 14% of the added N (Figure 2) and was categorized as slightly unstable according to VDLUFA A 13.5.1 (2007). Differences in digestion substrate, incubation time, or added components for the fermentation are determining factors when it comes to different N budgets reported in studies with digestate residues.

4.5 HC/L ratio and SSC indicate contrary degradation stability of fiber nettle and fermented digestate product

The results of stability tests such as N budgets or CO₂ release can be explained or interpreted by the chemical composition and occurrence of specific compound classes in substrate components or growing media mixtures, such as holocellulose/lignin (HC/L) ratios or the amount of easily extractable C or N compounds. The amounts of hemicellulose, cellulose, and lignin are directly linked with N immobilization and CO₂ release (Vandecasteele, 2023). Vandecasteele et al. (2018) reported that the risk for N immobilization can be explained by pH, the hemicellulose content, and water-extractable C concentration of the fiber.

A strong parameter for the assessment of the decomposability of the raw materials is the HC/L ratio, with values close to 1 indicating high stability (Vandecasteele et al., 2017). In our study, the HC/L ratio of peat (Table 6) was within the range reported in the literature (Vandecasteele et al., 2017). The HC/L ratio of the fermented digestate product was close to that of fine peat and green waste compost (1.5; Vandecasteele et al., 2017), whereas the values for the fiber nettle shives (Table 6) were higher than those of flax shives (2.6; Vandecasteele et al., 2017) and lower than Miscanthus straw (6.7; Vandecasteele et al., 2017). This reflects the high stability of the fermented digestate product and the low stability of the fiber nettle shives.

Amha Amde (2011) investigated different types of peat and found a correlation between the content of water- or salt-soluble C and N contents and microbial activity. Leiber-Sauheitl et al. (2021a) used salt-soluble carbon at 20°C (SSC 20°C) as a proxy to estimate the degradation stability of processed raw materials. In our study, the amount of SCC 20°C had a significant effect on the level of CO₂ emissions of the mixture treatments adjusted to the pH value of 6 (R² = 0.89). The higher the SSC 20°C content, the higher the degradation potential of the raw material. Fiber nettle in our study contained even three to five times higher contents of SSC 20°C than other cultivated biomass plants such as cattail (Typha; 5.3 mg C per g d. m.) or reed (Phragmites; 2.9 mg C per g d. m.; Leiber-Sauheitl et al., 2021a). The higher the content of fiber nettle in the fermented digestate product – fiber nettle – peat treatments, the higher the amount of extracted SSC 20°C. These findings suggest that the increasing instability of the treatments with decreasing proportion of peat is due to the fiber nettle shives and not the fermented digestate product.

4.6 Chinese cabbage plant tolerance tests reflect results from substrate tests

Fresh and dry matter of Chinese cabbage biomass of the different mixture treatments were in the range of earlier observations (Leiber-Sauheitl et al., 2021b, 2021a). For the control Peat 100, they were in the medium range, which could be due to the high amount of additional nutrients of P and K in the mixture treatments containing the fermented digestate product. With increasing fiber nettle content in the mixtures, plant growth of Chinese cabbage was diminished (Table 5; Figure 6). The coarser structure of the mixtures with fiber nettle had no negative effect on the germination rate. There was no significant difference in the number of germinated seeds among the mixture treatments 7 days after sowing (data not shown). Gaseous pollutants were excluded as a possible cause of damage in the test with Chinese cabbage, since during the previous cress test (data not shown) no gas-derived damage was visible in any mixture treatment.

The additional N input to treatments containing 25% fermented digestate residue and 25%, 50%, or 75% fiber nettle in order to compensate for immobilized N in the substrate resulted in significantly higher leaf N contents compared to the peat control (Table 5). In addition, a substrate-based nutrient deficiency was excluded, since at the end of the experiment still high amounts of P and K were found in the substrate (Supplementary Table S4). On the contrary, in some treatments the plants were rather oversupplied. In the case of high P contents in the substrate mixture, this may have led to immobilization of iron as Fe-III-phosphate and thus to transport inhibition of Fe in the plant (Bergmann, 1983; Yang et al., 2024). Additionally, high pH values as measured at the end of the experiment (Supplementary Table S4) could have led to a nutrient deficit of several elements such as Fe, B, or Mn and therefore cause damage to the plants and growth delays (Bergmann, 1983; VDLUFA, 2016b). The oversupply of macroelements could have resulted in high salt concentrations and led to an imbalance in plant nutrition in Chinese cabbage plants.

4.7 Plant experiments in experimental stations

4.7.1 Ornamentals: petunia

Dubský et al. (2019) showed good results for petunia plants with the use of the dried solid phase of digestate between 20% v/v and 40% v/v in substrate mixtures, which is similar to our results for the fermented digestate product. We observed a decline in petunia biomass in treatments with more than 50% v/v of the fermented digestate product Z III and more than 25% v/v of the fiber nettle shives. This was likely caused by high pH values and salt contents, as well as low water availability due to the lower water-holding capacities of these substrate mixtures. Low shoot dry mass of petunia at high pH values (Supplementary Tables S5, S6) was also found in other studies. For instance, Smith et al. (2004) reported a decrease in dry mass at pH values larger than 6.1. High pH values (>6.2) can also lead to micronutrient deficiencies, which can hamper plant metabolic processes, especially for iron-inefficient species such as petunia (Fisher et al., 2003; Smith et al., 2004). After 14 days, treatment DP100 exhibited symptoms of iron deficiency chlorosis (Supplementary Figure S3), which was remedied by a single application of 2 g L^-1 Fe chelate (Leiber-Sauheitl et al., 2022a). In addition to high pH values, the fertilized DP100 treatment showed very high salt contents. This may have delayed rooting of the young petunia plants (Sonneveld and Voogt, 2009).

All treatments had sufficient available N, and some were even oversupplied (Supplementary Table S6). Since the experiment design did not allow fertilization of each treatment separately, the treatments with a high proportion of fermented digestate product had very high levels of P and K in the substrate (Supplementary Table S6). Petunia is known to show no signs of leaf scorch when the fertilizer concentration is increased; at the same time, their fresh mass increased linearly in a study by Sonneveld and Voogt (2009). Therefore, nutrient oversupply could not visually be detected in our experiment.

After six weeks, N contents in d. m. of the plants (leaves) were in the usual range for petunia (Röber and Schacht, 2008). The treatments with the highest base fertilization and liquid top dressing had the highest N contents, which indicates a sufficient N delivery from the substrate mixture. Moreover, P contents in d. m. of all treatments except DP100 were within the usual range for petunia (Röber and Schacht, 2008). The plants of the treatment DP100 with the highest salt and P content in the substrate at the same time showed the lowest P content in the leaves. High salt contents in the substrate limited the P uptake via the plant roots; additionally, P availability is lowered due to P complexation caused by high pH values (Talbi Zribi et al., 2018; Dey et al., 2021). K contents in d. m. were within the usual range for the crop (Röber and Schacht, 2008). The higher the proportion of fermented digestate product Z III and fiber nettle in the substrate mixture, the higher the K content tended to be in both the substrate mixtures and subsequently in the plants, which is due to the higher mobility of K in comparison to N and P.

Besides the chemical properties, also the physical properties, such as the WHC_max of the mixture treatments, differed in our experiment. The combination of substrate mixtures with different water capacities can induce problems during cultivation, including water and nutrient scarcity for mixtures with lower water capacity (Sonneveld and Voogt, 2009). Due to the varying water capacities among the treatments (Table 4) and the use of a single ebb and flood irrigation system for all treatments, water supply could not be optimally adapted to the demands of every single treatment, which also contributed to growth differences among the treatments.

4.7.2 Herbs: basil

Mixture treatments with low fresh and dry mass of the basil plants (Supplementary Table S7) often had very high pH values and/or salt contents in the substrate during or at the end of the experiment (Supplementary Table S8; Leiber-Sauheitl et al., 2022b). These parameters were identified as causes for the reduced growth in the mixture treatments with the potential peat substitutes. In addition, in other experiments with pot herbs, pH, salt, and NPK contents were higher in mixtures with anaerobic digestate derived from crop residues and plant-based residues from food industries (Asp et al., 2022). Salt stress also reduced basil biomass in other studies (e.g. Caliskan et al., 2017). Although a mineral fertilizer was applied in our experiment, organic nitrogen contained in the fermented digestate product or in the fiber nettle could have been mineralized during the four weeks of culture. At neutral or higher pH values, the formed ammonia could have negative effects on root growth of basil (Frerichs et al., 2020).

After four weeks, the N contents in the dry matter of the leaves were in a culture-typical range (Degen et al., 2017; Asp et al., 2022). Basil leaves of all varieties except DP100 and DP0 FN0 were within a range customary for the crop (Degen et al., 2017; Asp et al., 2022). The K content in leaf dry matter was also within a range customary in cultivation (Degen et al., 2017; Asp et al., 2022).

The large range in water-holding capacity of the mixture treatments used for plant experiments (Table 4) may require different irrigation frequencies to optimize the water supply of the basil plants. This could not be realized in this basil trial due to structural conditions in the greenhouse.

In conclusion, depending on the control used for comparison, 25% (compared to Peat 100) up to 50% (compared to PGM3) of the fermented digestate product used in our study is a suitable proportion in growing media. Asp et al. (2022) found that up to 50% of the used solid digestate could be applied to growing-media mixtures without hampering the growth of basil plants.

4.7.4 Soft fruit: strawberry

Since in strawberry cultivation, fruits play a more significant role than leaf biomass production, the discussion focuses on fruit yields. All substrate mixture treatments containing fermented digestate product and fiber nettle resulted in comparable or higher yields of marketable strawberries than the control treatments (Supplementary Table S9). In comparison to the three-year average yield of marketable strawberries of the variety Aurora Karima determined at the experimental station (2020-2022), the yields of our mixture treatments ranged from 84% up to 107% (Felix Koschnick, personal communication). Quality of strawberries varied but was still similar to that of the control treatments (data not shown).

Salt contents of substrate mixtures that contained fermented digestate product were generally above the recommendations for strawberry culture of 2 g KCl L^-1 (Supplementary Table S10). Surprisingly, the high salt content did not have a negative effect on strawberry yield. In particular, the DP100 treatment, where the plants grew solely in the fermented digestate product, was not significantly different from the others. There are several possible explanations for this observation. First, it could be due to the duration of cultivation. If the plants had been cultivated into the next season, the adverse effects might have become more noticeable. This has also been demonstrated in other trials (Lieten, 2006). Second, the treatments with higher salt content also had higher nutrient content, leading to a sufficient supply for the plants, as seen in the leaf content. Other experiments have demonstrated that calcium (Ca) can mitigate the negative effects of sodium (Na) and/or chloride (Cl) (Kaya et al., 2002b, 2002a). This protection could have been effective in the present case as well, since the varieties with high salt content also contained high levels of Ca (Leiber-Sauheitl et al., 2022c). However, the DP100 treatment with the highest salt content had the highest number of plant failures (3 out of 32). This trend could continue with a longer cultivation period and would not be tolerated by strawberry growers due to economic reasons.

With the exception of mixtures containing the fermented digestate product and/or fiber nettle, the pH values of the two control growing media dropped considerably during cultivation and were no longer within the target range of 5.7 (Supplementary Table S10). This decline was likely due to the relatively high ammonium content of the fertilizer and the low carbonate hardness of the irrigation water used in this experiment.

In June and August 2022, leaf nitrogen content was predominantly within the recommended range of 2.0% to 2.8% (Supplementary Table S9; Pritts, 2015). By the end of the trial, almost all of the plants showed signs of nitrogen deficiency. This was likely due to the limited duration of the fertilization but also to nitrogen immobilization in treatments DP25 FN12.5 and DP0 FN50 despite higher N fertilization.

As expected, the fiber nettle mixtures were significantly less able to store water than the other treatments (Table 4). However, this property, i.e., the physical stability of the fiber nettle, was largely retained; there was relatively little reduction in air-filled pores due to decomposition (Supplementary Table S11). In contrast, substrate mixtures with more than 50% fermented digestate product tended to have insufficient air-filled pore content. However, according to De Boodt et al. (1974) and Ownley et al. (2019), the air capacity of almost all other mixtures was above the recommended levels. A decrease in air capacity during cultivation due to degradation processes was also observed here (Supplementary Table S11). Since strawberries are considered rather sensitive to oxygen deficiency (University of Arizona, 2013), there is no evidence that the formerly excessive air capacity had a negative effect.

4.8 Recommendations for the use of the fermented digestate product and fiber nettle shives as peat substitutes

The test procedure by which all raw materials were characterized allowed us to compare the peat substitute candidates Z III (fermented digestate product) and fiber nettle shives to those already widely used in professional horticulture (Leiber-Sauheitl et al., 2021a). Our results also allowed us to give recommendations on the percentages of those raw materials that could be added to growing media.

The fermented digestate product Z III was the most processed in comparison to all other digestate residues and products we investigated. According to the manufacturer, this product was mixed with 10%-30% sawdust and then submitted to the fermentation process. This likely led to a dilution effect paired with stabilization of the material. Therefore, Z III possessed the best properties for horticultural purposes in comparison to the other digestates tested. Z III was similar to green compost in regard to nutrients, salt content, and pH values. Accordingly, it could be used in the quantities recommended for green waste compost of up to 40% (Neumaier and Meinken, 2015). If air capacity in growing media mixtures with Z III needs to be increased, mixing components such as coconut fiber or a coarse softwood fiber with a predictable N immobilization should be chosen.

Despite their comparably low nutrient contents, all digestates investigated had very high nutrient and salt contents compared with established raw materials used in growing media. Therefore, methods for nutrient reduction, currently under development in Germany, should be considered.

Fiber nettle shives were comparable to Miscanthus shreds in terms of particle structure and level of N immobilization (Stucki et al., 2019). The addition of fiber nettle shives led to an increase in air capacity, which can be useful in container cultures. However, overall only very low proportions of fiber nettle shives can be recommended for use in growing media. One possible application of untreated shives, as used in our experiments, could be as a covering material for tree nursery containers, similar to hemp shreds. Alternatively, the shives could be subjected to more complex processing, such as composting or ensiling, to enhance their feasibility as a peat substitute. These processes lead to a concentration of nutrients and thereby enable the use of fiber nettle shives in quantities similar to green waste compost.

The recommended proportions of fermented digestate product and fiber nettle vary depending on the plant species. For ornamentals, we were able to compensate for the high N immobilization of fiber nettle by increased fertilization. However, as a result, we observed a negative effect on petunia growth due to the resulting high salt contents combined with a lower water capacity in the substrate. Therefore, only low proportions of fiber nettle could be used for petunia. For potted herbs, on the other hand, higher proportions of the fermented digestate product could be tolerated in growing media mixtures. Basil, which was used in our experiments, is also known to grow well in substrates with high proportions of high-quality substrate compost (e.g. Degen et al., 2022, 2024). For soft fruits, good results were achieved with higher proportions of fiber nettle using shives harvested in winter, which had lower salt contents. Since high air capacity is important for soft fruit crops such as strawberry, the fermented digestate product should be used at lower proportions in these systems.

4.9 Challenges in experimental design with mixture treatments varying vastly in substrate parameters

Our experiments followed a systematic experimental design. Contrary to common practice, the same mixtures with fermented digestate product and fiber nettle shives were used for all plant species, which meant that the structure of the mixtures was not adapted for the individual crops. This approach enabled us to better compare the substrate analysis results between the experiments. However, on the other hand, the results of our plant experiments may partly show fewer promising results for fermented digestate product and fiber nettle shives than would have been possible if mixtures had been optimized for each crop. Nevertheless, our approach effectively demonstrated the limitations of each raw material.

An additional challenge was the combination of substrate mixtures with different water capacities (Table 4) in one experiment. We had one system available for all treatments, which meant that water supply could not be optimally adapted to the demands of each single treatment. This limitation may also have contributed to differences in growth between the treatments (Sonneveld and Voogt, 2009). More individualized irrigation frequencies for each mixture treatment to provide optimized water and fertilizer supply could likely have improved plant development and growth in petunia and basil. It is important to note that ebb-flood irrigation systems are widely used in commercial horticulture, and therefore these observations highlight the challenges growers face when working with peat-reduced production systems.

5 Conclusion

In this study, we investigated the potential of 11 digestates and fiber nettle shives from two harvest dates as peat substitutes in substrate mixtures for professional applications. Parameters investigated included physical and chemical properties as well as plant experiments, in which systematic mixtures of the most promising digestate, fiber nettle shives, and peat were applied to three example cultures from different sectors of horticulture.

Physical and chemical parameters varied widely among the digestates investigated. The fermented digestate product Z III showed the greatest potential among the digestate residues investigated due to its low pH, salt and nutrient contents, and neutral odor. Subsequent plant trials revealed that it was suitable as a peat substitute across various horticultural sectors, ranging from young plant cultivation to tree nurseries. Overall, our results suggest that digestates have a high potential as components of growing media if paired with suitable processing to stabilize and unify physical and chemical parameters.

Fiber nettle shives showed very high rates N of immobilization. Shives from the winter harvest had a lower salt content and contained less easily degradable C than those from the fall harvest. When used in substrate mixtures, they showed positive effects on strawberry yield. For all other plants tested (basil and petunia), however, plant growth decreased as the proportion of fiber nettle shives derived from fall-harvested plants increased in the mixtures. Further processing would be necessary to optimize the suitability of fiber nettle as a substrate component, similar to the processing of other fibrous materials, for example through composting or fermenting.

Plant growth could probably be improved by using different mixing ratios or lower proportions of the fermented digestate product and fiber nettle. In combination with additional additives or modified fertilization strategies, these strategies should be tested in further trials.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fhort.2026.1690820/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Jean-Charles Michel, Institut Agro Rennes-Angers, France

Reviewed by: Sonia Cacini, Council for Agricultural and Economics Research (CREA), Italy

Akinson Tumbure, Teagasc, Ashtown Food Research Center, Ireland

Luiz Henrique Natalli, Hochschule für Forstwirtschaft, Germany