There is an increasing global demand for renewable growing media in horticulture, and the industry is under significant pressure to reduce its peat use. Finding suitable renewable growing media is challenging due to its diverse applications—ranging from short-term plugs to long-term substrates and soil conditioners—which each require specific material properties. In contrast to current (usually peat-based) mixtures, renewable growing media have more complex compositions, often consisting of five or more different component materials. Testing the properties of all possible combinations and mixing ratios of these component materials would be costly and time consuming.Our research addresses this challenge by developing a predictive mixing model in collaboration with a consortium of companies and research institutions. This model aims to predict the key material properties of mixtures based on measurements on the individual component materials, including volume loss due to interstitial filling after mixing. The approach leverages existing data and previous research on materials, and combines it with new processing techniques based on QICPIC particle measurements. The QICPIC device analyze particle size, shape, and distribution using high-speed imaging and laser technology.The involvement of a consortium including growers, substrate suppliers and a testing lab allowed us to design crop trials to validate the mixing model, and also use the data collected to develop and train a volume loss model, ensuring practical applicability and relevance. Our poster presentation will discuss how this modeling approach could serve as guidelines for achieving sustainable, scalable solutions for peat replacement, with broad implications for the future of growing media.

The growing field of peat substitutes has introduced many promising materials, each with unique properties, such as enhanced drainage, nitrogen immobilization, or high salt content, which can limit their universal application in high-volume ratios. However, these new materials have the potential to significantly improve growing media when used in mixtures, balancing out other undesirable properties. At the IGZ (Leibniz Institute of Vegetable and Ornamental Crops), a method has been developed based on a specific mixture design that enables the rapid and effective integration of new peat substitutes into highly peat-reduced mixtures. Additionally, we have developed a framework that effectively represents mixture effects, allowing for a more accurate prediction of how different components interact within these complex blends.

In October and November 2023, an experiment was conducted to explore the integration of depleted biogas residues and nettle fibers into potting mixtures containing 25% v/v peat. Using the R package „mixexp“ alongside XVERT (Smith, 2005) for optimized experimental design, 11 mixtures were formulated, with proportions of green compost (10%-45% v/v), soft wood fibers (15%-40% v/v), biogas residues (0%-20% v/v), and nettle fibers (0%-15% v/v). The total compost and depleted biogas residues were capped at 45% v/v, as was the sum of nettle fibers and soft wood fibers (45% v/v). The cultivation period of Chinese cabbage (Brassica rapa subsp. pekinensis, cultivar: ’Granaat’) in 4 cm x 4 cm press pots extended to approximately 30 days due to weather conditions.

Results indicated that mixtures containing nettle fibers impaired seedling growth, with excessive nitrogen immobilization persisting despite the addition of 125 mg N L^-1. This suggests that nettle fibers, as processed, are unsuitable for vegetable seedlings without additional fertilization. In contrast, depleted biogas residues were effective when combined with lower softwood fiber content, provided that compost made up at least 30% of the mix.

Overall, the framework developed at IGZ proves to be a valuable tool in the quest to reduce peat usage while exploring and optimizing alternative growing media. This approach not only supports peat reduction efforts (Sradnick et al., 2023) but also enables the systematic assessment of peat substitutes, ultimately contributing to more sustainable horticultural practices.

Smith, W. F. (2005). „Experimental design for formulation,“ SIAM.

Sradnick, A., Werner, M., and Korner, O. (2023). Make a choice: A rapid strategy for minimizing peat in horticultural press pots substrates using a constrained mixture design and surface response approach. PLoS One 18, e0289320.

Nitrogen immobilization is an issue for many peat substitutes in growing media. In previous decades, several incubation approaches have been developed to predict nitrogen immobilization. However, these approaches are often poor indicators of the actual nitrogen availability in the root zone of soilless cultivated plants. On the other hand, measuring plant growth and nitrogen uptake to determine the actual immobilization in the substrate is time consuming and requires extensive laboratory investigations. In addition, plant performance depends on several growth factors other than nitrogen, making it difficult to standardize growth tests. A less time-consuming and easily standardized option could be to use nitrous oxide emissions by denitrifying bacteria as an indicator of nitrogen immobilization. Both microbial processes, denitrification and immobilization, are based on the availability of mineral nitrogen and easily decomposable carbon sources. Therefore, a correlation between both microbial processes has been hypothesized. In order to test this assumption, the nitrogen immobilization in diverse wood fiber-based substrates was determined under both anaerobic and aerobic conditions. According to the German VDLUFA method the initial N content was set to 1,000 mg L (substrate)^-1 as well as the water capacity was set to 80 % in these incubation studies. Over a 20-day incubation period, the concentration of mineral nitrogen (nitrate and ammonium) in the substrate, as well as the gaseous emissions (nitrous oxide and dinitrogen) were determined and examined for correlation. The accuracy of the nitrogen immobilization observed in the incubation experiments was verified by a nitrogen balance sheet based on a pot experiment conducted in the greenhouse.

Currently wood fiber is by far the most important peat substitute for professional growing media in Germany. However, due to easy degradability and wide C:N ratio nitrogen immobilization is a major problem and limits the proportion of wood fiber to about 30 % by volume. To counteract nitrogen immobilization more detailed information about height of nitrogen immobilization is needed. Nowadays the incubation test according to the Association of German Agricultural Inspections and Research Institutes (VDLUFA) is the standard method to assess nitrogen immobilization in Germany. However, the test has some shortcomings: Firstly, it is quite time- and labor-intensive and secondly, the results are of limited significance. Thus, new test methods are urgently needed.

In the current research, two approaches were investigated: The first approach is somehow similar to the VDLUFA method. The moistend wood fiber is also incubated at 25 °. In contrast to the standard method, the test runs for 5 days only and instead of the amount of mineral N at the beginning and the end of the test, microbial activity is continuously measured respirometrically. Additionally, soluble organic N is measured as pool of the easily mineralizable nitrogen, often added by growing media manufactures urea-aldehyde condensation products. The second approach is near-infrared spectroscopy (NIRS): Dried wood fibers were placed in a petri-dish and gently compacted. NIR spectra were taken with a ZEISS Corona 1.7 NIR spectrometer. As reference method, a nitrogen balance calculated from a modified plant response test with chinese cabbage was used.

The results confirm the limited significance of the current VDLUFA incubation test. In contrast, both approaches – respometric incubation test as well as NIRS – were closely correlated to the nitrogen balance of the plant response test. Further prospects of the new approaches for stability assessment of wood fiber are discussed.

A crucial success factor for renewable growing media is the ability of materials like compost or biochar to increase the pH of the substrate. This would allow to reduce the need for liming since most biochars or composts are alkaline. The acid-buffering capacity and lime replacement value of materials is thus an important criterion for selecting materials for growing media and for optimizing the lime addition in blends with these materials. Especially for biochar and compost there are important differences in acid-buffering capacity between specific batches. Lab determination of acid-buffering capacity is time-consuming and thus costly. We tested two shorter methods to quantify the acid-buffering capacity of biochar and compost. In the first method pH and inorganic carbon content of the material are measured, with higher pH and inorganic carbon being positively correlated with a higher acid-buffering capacity. In the second method the pH of a 1:5 (volume ratio) biochar:peat or compost:peat mixture is measured after a 10-day incubation at 15°C. The higher the difference of the mixture with peat versus the pH of the pure peat, the higher the acid-buffering capacity of the material. Both methods were successful in forecasting the acid-buffering capacity. Practical aspects are discussed for each protocol in relation to specific characteristics of compost and biochar.

In organic horticulture, nitrogen must undergo mineralization via bacterial and archaeal enzymes through ammonification and nitrification to become available to plants. While these processes are influenced by various environmental factors, the impact of physical properties of growing media (GM) on mineralization and their direct effects on microorganisms remains underexplored. This study evaluated the influence of GM water availability on the processes of nitrogen ammonification and nitrification. Four contrasting GM with various proportions of peat, perlite, wood fiber and compost were examined under three different matric potentials (-1.5, -3.5, or -5 kPa at mid-container height) in 60 L containers without plants over a period of 5 weeks. An organic fertilizer blend consisting of blood meal and pelletized poultry manure was applied at a rate of 400 mg N L-1 to each container located in a greenhouse (Lat. 47° 49' 35.0„ N; Long. 69° 26' 07.5“ W), with weekly nitrogen analysis (N in the GM solution and N₂O efflux) conducted to assess temporal availability.

Our results showed significant differences in nitrogen dynamics among matric potential treatments, but the different GM had little effect on N availability over time, underscoring the critical role of the air/water ratio in nitrification and denitrification processes. Nitrogen availability was highest in the -5 kPa treatment, followed by -3.5 kPa, and lowest in the -1.5 kPa treatment. These observations were corroborated by qPCR quantification of amoA gene copies, suggesting a higher nitrification potential in drier GM (e.g. -5 kPa) relative to wetter ones (e.g. -1.5 kPa). Furthermore, measurements of N₂O efflux indicated increased denitrification in wetter GM compared to drier ones. Despite these findings, additional experiments with actively growing plants are essential to validate the influence of horticultural crops on the observed relationship between the physical characteristics kept in GM and the nitrification process within a realistic production system.

Container-based production systems are receiving more attention as a result of the growing demand for sustainable, soilless cultivation techniques. Vegetables, fruits, and ornamental plants are grown using these systems, which have the potential to provide benefits like less logistical work, better plant health, and resource conservation. To prevent excessive water use and nutrient leaching, however, good growing media management in these systems is crucial. Specifically, to enhance their physical qualities, peat-based substrates are frequently combined with coarser materials like wood fibre. The hydraulic properties of the substrate are changed by this mixing, and these properties are essential for simulating solute transport and water flow. It takes a lot of time and labour to measure these hydraulic parameters using traditional methods. Although earlier research has used tension infiltrometers to examine the hydraulic characteristics of peat-based media, little is known about the hydraulic behaviour of substrates that have been amended with wood fibre.

This study aims to address this gap by estimating the hydraulic properties of wood fibre-amended substrates using data obtained from a MiniDisc Infiltrometer and subsequent numerical inversion with the Hydrus 2D/3D model. The objective is to calibrate the hydraulic parameters of the growing media mixes, providing a more efficient method for characterising these materials. The coefficient of determination (R²) values from the numerical inversion showed a strong fit, with R² > 0.9, indicating the reliability and accuracy of the estimated hydraulic parameters. These findings offer valuable insights for optimising water retention and drainage in soilless growing media, contributing to more sustainable agricultural practices.

The standard method to determine the water retention parameters of growing media according to DIN EN 13041 (double rings, sand bed, determination of bulk density, pore volume and at least water capacity at pF 1.0) takes around 10 days in the laboratory. Using a centrifuge to dewater materials under defined pressure is an established method for mineral materials but has rarely been used for growing media. The objectives of this study were i) to test the practical applicability of the centrifuge method as a quick test for a wide range of growing media, and ii) to compare a simple substitution method to describe the bulk density of growing media with results of the standard method.

Substrate packing method largely determines the resultant bulk density (Db), which subsequently impacts substrate physical properties. Our objectives were to utilize the newly developed disc and “tamp” method to determine if it can be used to consistently pack sample rings to the desired Db and demonstrate how static physical properties and parameters derived from moisture characteristic curves (MCC) are affected by achieving higher Db  observed in a commercial operation. We used an espresso tamp and discs for 100% pine bark, coir, and peat, and blends of each. Db and the resulting MCC produced by packing the sample ring using the tamp method were very consistent with standard deviations ≤0.006 regardless of substrate. Packing method nominally impacted easily available water (EAW; -1 to -5 kPa) and water buffering capacity (WBC; -5 to -10 kPa). To our knowledge this is the first report of utilizing the new “tamp” method to pack sample rings for testing dynamic physical properties.

Green waste compost is already widely used as a peat substitute in horticulture, but often has a pH of 7.0 – 8.5, which is too high for many plants. The increase in pH during composting is accompanied by considerable nitrogen losses due to the emission of environmentally harmful gases (NH₃ and N₂O). The aim of this study is to develop an approach that maintains the pH during composting and thus in the final compost product at 5.0 – 6.0 to avoid gaseous nitrogen losses and produce a high-quality peat substitute for growing media. To achieve this, elemental sulfur (S0) is used, which can be oxidized to sulfuric acid by bacteria of the genus Thiobacillus, and thereby neutralize carbonates.

For this purpose, three composting systems were constructed to investigate the effect of the S⁰ dose, formulation, date of application (heating, thermophilic or maturation stage), and the green waste raw material itself (e.g. C/N-ratio) on its pH-development and its impact on NH₃ and N₂O emissions. The first system simulated the composting process on a small scale (2 L-vessels) in a climate chamber and allowed the examination of a large number of treatments to determine several chemical and physical changes due to S⁰-treatment. In order to additionally record gaseous emissions a second system with aerated reactors (70 L) was installed. A constant ambient temperature and a high insulation of the reactors allowed the biomass to heat up to the required 60 °C during the heating stage. Based on the results of those two systems the most promising treatments will be repeated in compost boxes with a volume of 1.8 m³ in subsequent investigations. Finally, the general substrate characteristics of the supplemented compost product will be determined by pot experiments.

Local and sustainable alternatives to sphagnum peat moss are often an economical replacement in horticultural growing media. But they may have more variable properties than sphagnum. Compost, which is used extensively in growing media substrates, is one of the most highly variable alternatives. Previous research has demonstrated the importance of chemical properties of compost like pH and salinity, as well as stability, in predicting plant performance. Physical properties of composts have been less studied,  but these are also important when making compost inclusion decisions. In this study, 27 different composts from across the United States were analyzed for physical properties and suitability for use in growing media. Overall, composts tended to have greater air space and less container capacity than sphagnum. Regional differences were apparent, with higher air space in composts from the south and higher container capacity in those from the north. Dry bulk density was highly correlated with both total porosity and air space, and negatively correlated with nitrogen drawdown. Container capacity was on average 20% less than sphagnum, ranging from 42-55% v/v. Container capacity was positively correlated with dry bulk density,  pH, and ammonium acetate extractable cations, and negatively correlated with C/N ratio. Particle size had little impact on media porosity. The ability to predict physical properties and stability impacts of composts will help growers screen for composts well suited for growing media, and make informed cultural practice decisions about watering and fertilization.

Agro-industrial compost derived from organic residues presents a sustainable alternative to peat in horticulture with a reduced environmental footprint with interesting sources of fertilization and biostimulation. In spite of these advantages, limited attention has been paid to its physical properties. Its long-term suitability and physical performance in horticultural applications. This study attempts to bridge this gap by examining the physical properties of recycled agro-industrial compost and contrasting them with traditional peat-based media, using Salicornia as a model crop. Compost exhibits higher bulk density and lower water-holding capacity than peat; however, it showed a superior available water content within the critical range of water potential at -1 kPa to -5 kPa, key for optimal plant growth. Moreover, compost displays enhanced air-filled porosity. Salicornia grown in agro-industrial compost resulted in a significantly higher aerial fresh biomass (by 24%) compared to peat. Furthermore, reused agro-industrial compost showed similar aerial fresh biomass compared to new one, while cultivation in reused peat the aerial fresh biomass was significantly reduced (by 32%) compared to the new peat. Similar findings were observed in root fresh biomass and in the dry biomass of the stems and roots. Compost physical properties have been found fascinating for mid-term or long-term cultivating Salicornia crop, since it has higher available water content and higher air-filled porosity than peat, despite the lower water holding capacity in comparison to the peat. We are confident that these good physical properties result in a higher sustainability of the agro-industrial compost versus peat.

Compost is widely used as a sustainable component of growing media. It can improve nutrient retention and add an organic source of macro and micro nutrients. Unfortunately, some composts are contaminated with phytotoxic concentrations of persistent herbicides like clopyralid,  aminopyralid, aminocyclopyrachlor, and picloram. Some carbon based sorbents like wood ash, activated carbon, and biochar have been found to bind persistent herbicides and make them unavailable for plant uptake. However their effectiveness has been found to be inconsistent and unpredictable. The hypothesis of this study was that the sorbent capacity of biochars could be used to predict the ability of biochars to bind persistent herbicides and reduce or eliminate phytotoxicity caused by them. A gravimetric adsorption capacity scan assay with an organic vapor challenge gas was used to measure sorbent capacity of biochars from across the U.S. Then they were added at 5% v/v to compost containing 80 ppb clopyralid. Phytotoxicity of these mixes was measured using a sensitive plant bioassay. The sorbent capacity was found to be highly correlated with the ability of the biochars to mitigate persistent herbicide phytotoxicity. In future studies, the biochars will be tested with composts contaminated with other persistent herbicides. This test will allow both compost producers and home gardeners with contaminated compost, to select biochar amendments best able to mitigate this problem. 

Clay amendments to growing media focus on a balanced nutrient supply for plants and an improved rewettability. For composted green wastes a new requirement for the clay admixture is the immobilization of organic growth inhibitors, having a negative effect on crop safety during the germination phase. Questions arise if surfaces of different clays have a distinct sorption capacity for dissolved organic matter (DOM).

Bentonite, saprolitic and relocated clays from the Westerwald area were studied. Tannic acid and DOM extracted from composted green waste were introduced in batch sorption and desorption experiments. The amount of dissolved organic carbon was quantified in liquiTOC measurements and aromaticity determined by specific UV absorbance.
Clays determined were undersaturated for the binding of DOM. At low additions high shares of tannic acid are adsorbed by the clays. Sorption maximum, determined by the Langmuir isotherm reveals the highest value for the bentonite (1.3 g DOM-C kg-1 clay), closely followed by the relocated clay. Saprolitic clays clearly have the lowest sorption maximum. For growing media it is likely that some of these clays can sorb and immobilize marked amounts of DOM. Uptake of DOM by clays differs greatly with different types of DOM added and depends on the type of clay, the latter strongly related with the specific surface area as higher shares of external surfaces offer more sorption sites and thus a higher sorption capacity. Tannic acid can be sorbed significantly better than DOM separated from composted green waste. At low concentrations the binding capacity for aromatic compounds is particularly high, at higher concentrations only a small part is sorbed. Amendment of especially bentonites can help improve the cultivation safety of plants in the germination phase in growing media from composted green wastes through their sorption capability of organic plant growth inhibitors like tannin.

Conventional horticultural practices often rely on the use of peat as a primary component in growing media. However, due to the environmental repercussions associated with peat extraction and its limited availability, a critical need has been prompted to explore sustainable alternatives. This study focuses on addressing challenges related to nitrogen (N) immobilization, substrate compaction and optimizing plant growth in cultivation systems that minimize or eliminate peat usage by using microorganisms.

First, the impact of various substrates (steamed/ unsteamed), comprising peat, green compost, wood fiber, nettle fiber, digestate, loess/loam, and perlite, on growth of Petunia hybrida cv. 'Mitchell' were examined. The plants' phenotypic characteristics were observed, and the substrates were analyzed for their chemical properties. After six weeks, the plants exhibited superior growth in a sterile, peat-free substrate composed of 50% green compost and 35% wood fiber, as well as in a substrate mixture of 50% peat, 25% green compost, and 25% digestate, irrespective of sterility. In the subsequent phase, diverse microorganisms will be introduced to different substrate mixtures to augment substrate quality and assess their effects on plant growth development. Various plant growth-promoting rhizobacteria (PGPR), Schizophyllum commune, arbuscular mycorrhizal fungi (AMF) – adapted to high phosphate concentrations, endophytic entomopathogenic fungi (EEMPF) and dark septate endophytes (DSEs) will be employed, leveraging their unique qualities for the improvement of peat-reduced/ peat-free substrates. DSEs are tolerant to abiotic stress and can mobilise N from organic resources. It could be proven that Petunia plants inoculated with Leptodontidium sp. showed significantly improvement plant growth under sterilized and peat free conditions. Potential representatives of EEMPF, colonizing plant tissues and protecting the plants against insect attacks, were investigated in terms of their impact against the plant pest Bradysia difformis (fungus gnat) which poses a serious problem in pot cultures under glass and especially highly attracted to peat free substrates. Furthermore, Azotobacter and Azospirillum genera are selected as PGPR due to their ability to fix atmospheric N, enhance nutrient uptake, and stimulate plant growth through hormone production and root colonization. Such mycorrhizal helper bacteria are able to form biofilms on AMF surfaces protecting them from mycophagous bacteria and enhances their vitality. For the final compilation of an effective microbial consortium additional studies are intended to further improve the microbial impact in peat-free substrates focusing on the understanding of interspecific interactions between different microbial groups.

Fungus gnats (Bradysia spp.) are a significant problem for producers of organic potted plants: Especially shortly after germination, the feeding activity of larvae on roots and young stems can lead to considerable losses, if the fungus gnats multiply en masse. The use of peat substitutes such as bark products, wood fibres and, particularly, green waste composts, as well as organic fertilisers, increases the amount of easily degradable organic material in the growing media, which promotes microbial activity and, among others, the colonisation with fungi, making the substrates highly attractive to fungus gnats. However, it has not yet been clarified in detail which materials are especially attractive to fungus gnats.

Thus, the most important peat substitutes (green waste compost, composted bark, wood fibre) and organic solid and liquid fertilisers of different origins (either animal or plant derived) and processing levels (either raw or processed) were tested for their attractiveness for fungus gnats in a standardized laboratory trial. In addition, N-mineralization of organic fertilisers was measured and microbial degradability of different materials was analysed by measuring microbial respiration.

The results show clear differences in the attractiveness between and, in some cases, within the individual product groups. While bark and wood products are not attractive to fungus gnats at all, the attractiveness of composts differs significantly: While some products proved to be completely unattractive, others have a very high attractiveness. For fertilisers a clear grouping was observed: While liquid organic fertilisers do not attract fungus gnats, almost all solid ones are highly attractive. Moreover, this high attractiveness could no be reduced by the addition of non-attractive compost.

Based on the results, possible quality parameters for composts as well as the relationship between the N-mineralisation of fertilisers, the microbial respiration of growing media, and the attractiveness for fungus gnats are discussed.

In the ToPGa project, we aimed to assess peat reduction in growing media for horticultural production systems from a plant protection perspective. Substrate effects on pests were investigated using the example of the most important substrate-associated pest, the fungus gnat (FG) Bradysia impatiens (Johannsen 1912). The trials built on each other consecutively, starting with the investigation of peat substitutes (PS) and mixing effects, through cultivation methods for basil and petunia, to the use of nematodes for biological control. Pre-Infestations of growing media were detected using various extraction methods such as photoextraction, heat extraction and wet extraction. In the studies on choice behaviour of FG, no PS showed a lower attractiveness than peat. Therfore, no repellent properties were found. The reproductive potential of FG was also very low in peat; However, wood fibre, coir pith and Sphagnum were at the same level and hence represent alternatives with comparable good suppression. However, in the studies on mixing effects, a clear superimposition of these suppressive properties by PS with high attractiveness and high reproduction potential was observed. Among these substances, PS that were either subject to microbial processes such as composting and fermentation (green waste compost, fermented Miscanthus fibre, fibre nettle compost, bark humus) or promoted decomposition processes (fibre nettle shives), which appears to contribute significantly to an increase in the risk of infestation, should be emphasised. In the cultivation trials, it became clear that the infestation with fungus gnats had no effect on the above-ground biomass, but did have an effect on root growth, regardless of the substrate. The studies showed that PS cannot be used specifically to prevent FG infestation and are therefore not suitable building blocks for integrated pest management per se. The control of FG therefore remains essential. In our performance tests, entomopathogenic nematodes were not negatively affected by PS, in fact certain products possibly allow for lower doses. The key to better regulation of TM and other substrate-associated pests may lie in the microbiome of growing media. Further studies on the composition and activity of microbial communities are needed to enable targeted regulation of pests. In addition, detailed investigations into the nutritional behaviour of FG are necessary in order to clarify the causes of herbivory and direct plant damage. The project was funded by the Federal Ministry of Food and Agriculture via the Agency for Renewable Resources (FNR) (FKZ2220MT006A).