The escalating challenges posed by global soil degradation and the increasing unpredictability of weather patterns due to climate change are placing significant pressure on traditional agricultural practices (FAO, 2015). Alternative production systems, such as hydroponics or soilless cultivation, a controlled environment agriculture (CEA) technology, has emerged as a promising method for cultivating crops without soil whilst controlling the crop environment and circumventing many of the limitations associated with traditional farming, especially soil degradation and unpredictable changing weather patterns (Cowan et al., 2022). In addition to challenges like soil degradation and climate change, a significant driver for the adoption of soilless cultivation has been the need to manage the build-up of soil-borne pathogens in intensive monoculture systems. The global phase-out of effective soil fumigants, most notably methyl bromide, accelerated this transition, compelling growers to seek alternatives like hydroponics to ensure crop health and consistent yields (Backstrom, 2002; Shwin et al., 2003).

In hydroponic and soilless cultivation, the soilless growing media serves as the foundational matrix that directly influences plant health and productivity by providing essential support, facilitating aeration, retaining crucial moisture, and enabling the delivery of nutrients to the plant roots (Gruda, 2019). Key physical attributes such as water holding capacity, aeration, porosity, bulk density, and drainage collectively determine the suitability of a media for optimal plant yield (Fields and Gruda, 2021). Water holding is crucial for maintaining adequate hydration of the root system without leading to waterlogged conditions. A media with good water retention ensures that plants can endure periods of reduced irrigation or fluctuating environmental conditions. Aeration, the provision of sufficient airflow around the roots, is equally vital for preventing root rot and promoting healthy plant growth. Inadequate aeration can lead to the buildup of carbon dioxide, which can be toxic to roots, and create an environment conducive to anaerobic pathogens that cause detrimental root diseases (Gliński and Stępniewski, 2018). Porosity, referring to the empty spaces within the growing media, is fundamental for both air circulation and water movement. Total porosity comprises both air porosity and water holding porosity, and an ideal medium typically exhibits a total porosity exceeding 60-70%, with an aeration porosity of at least 20-30% on a volume basis (Caron and Michel, 2021). The distribution of pore sizes is also critical, with macro pores (> 75 µm) facilitating aeration and drainage, while micro pores (<30 µm) are responsible for water retention with available water held in pores between 0.2 µm and 30 µm (Cameron and Buchan, 2017). A balanced distribution ensures both adequate oxygen supply and moisture availability. In loose and packed media, particle size, particle size distribution and particle shape also have an impact on hydrodynamics, water holding capacity, available water and air filled pore space (Durand et al., 2024). Bulk density of a porous material, the measure of a growing medium's mass relative to its volume, influences the media's ability to stabilise container plants and can impact aeration. Lower bulk density is associated with higher total porosity, which is beneficial for root health (Bartley et al., 2022). However, sufficient bulk density is necessary to provide adequate physical support for larger plants, preventing them from toppling over. In addition to physical properties, chemical and biological properties are also important when designing a growing media. However, for this work, we examine chemically and biologically inert media and therefore focus on physical properties.

It is well established that various plant species exhibit distinct preferences for the physical properties of their growing media, particularly concerning water retention and aeration (Gruda, 2019). Furthermore, the requirements for the growing medium can also vary depending on the specific type of hydroponic system employed (Fussy and Papenbrock, 2022). The ability to precisely adjust the physical characteristics of a growing media for specific plant species and hydroponic systems holds significant potential for optimising root zone conditions, enhancing nutrient uptake efficiency, improving water utilisation, and ultimately achieving higher crop yields and superior quality (Fields and Gruda, 2021). For instance, leafy greens might thrive in lightweight, well aerated media, while fruiting plants may require media with higher water retention. Similarly, an ebb and flow system might benefit from a media with good water retention, whereas a deep-water culture system might perform better with a lightweight media that promotes oxygenation. A model growing media platform, based on a single type of material, that allows for the manipulation of physical properties would offer an invaluable tool for researchers and growers to systematically investigate and optimise growing conditions for a diverse range of plants and systems, potentially streamlining both experimentation and commercial production.

Polyurethane (PU) foams, a versatile solid polymeric foam have long found application in horticulture, with the first patent for its use as a synthetic soil issued in 1976 (Bardsley, 1976). Early investigations often utilised PU foam sourced from other industries, which were not specifically designed or optimised for horticultural purposes. However, more recent research endeavours, often in collaboration with polyurethane manufacturers, have focused on developing tailored foam formulations that can match or even surpass the performance of established synthetic media like mineral wool. Notably, PU foam media have demonstrated the potential for reuse over extended periods, lasting up to 10 years (Benoit and Ceustermans, 1995) and can even exhibit improved water holding capacity over successive cropping cycles due to the retention of organic matter from roots (Hardgrave, 1995). Even PUF that have not been optimised for plant growth have been shown to be useable for soilless horticulture, with recycled mattresses being used in NFT systems (Al Meselmani et al., 2020). Polyurethane foam plugs are now commonly employed as collars or supports in solution culture systems, indicating a specific and established role for this material in hydroponics. Polyurethane foams (PUF) possess many of the inherent characteristics required for successful soilless cultivation, including the mechanical strength necessary to anchor plants and a highly porous structure that facilitates the essential transfer of liquids and gases. The promising results observed with early, non-optimised foams spurred further scientific inquiry and development, leading to the creation of specialised formulations designed to enhance plant growth. These specialised formulations now underpin a successful commercial market for PUF plugs, developed through close collaboration between foam manufacturers and growing media experts.

The structural versatility of PUF offers a platform for modelling plant media interactions due to the nature of their formulation chemistry which offer the ability to synthesise foams with a wide array of physical properties based on the same materials chemistry. By carefully adjusting the quantities of polyols, isocyanates, catalysts, surfactants, and blowing agents during the manufacturing process, the cell size and overall pore architecture of flexible polyurethane foam can be controlled. Polyols and isocyanates make up the backbone of foams, and their chemical backbones can affect hydrophobicity of the polymer. Catalysts and surfactants affect the rates of reactions, varying the rate of bubble formation and surfactants stabilise these bubbles and with the catalysts impact the resulting cell size and open cell fraction. Blowing agents influence the amount of gas formation and also impact resulting cell morphology (Szycher, 2013). Design of Experiment (DoE) techniques have been successfully employed to generate polyurethane foams with a broad spectrum of physical properties by systematically varying the ratios of catalysts, surfactants and additives (Wright et al., 2021a, Wright et al., 2022b). These experimental approaches have also enabled researchers to model the complex relationships between the physical properties of the foam and plant growth (Wright et al., 2021a, Wright et al., 2021b). Of these physical properties the open-cell content was an important parameter for controlling hydrodynamic properties. Commercially, an open-cell structure is often achieved via post-synthesis mechanical crushing, a process that ruptures the cell windows. However, this method is largely binary in nature, resulting in a foam that is either closed-cell or almost fully open-cell, offering little control over intermediate states. In contrast, the chemical synthesis approach utilised in these studies provided precise control over open cell fraction allowing the open-cell fraction to be tuned across a continuous range from nearly closed to fully open. This chemical tunability of polyurethane foam makes it an ideal candidate for a model growing media, allowing researchers to systematically vary specific physical properties, such as water holding capacity and aeration, while maintaining other factors relatively constant. This controlled manipulation enables the isolation and study of the specific impact of these individual properties on plant growth and physiology, providing valuable insights into plant media interactions.

Beyond providing insights into plant media interactions this study provides an opportunity to investigate another critical factor in hydroponic success: disease management. To mitigate the disease risk in soilless systems, preventing spread should always be the primary defence, however, ensuring a disease suppressive root environment will also contribute to a healthy crop (Alsanius and Wohanka, 2019). Damping off, caused by oomycete pathogens of the genus Pythium, is a problematic disease in hydroponic horticulture, particularly for young plants (Amrhein et al., 2025), and is responsible for significant losses in a wide range of crops. In tomato, for example, Pythium spp. can cause seed decay along with pre- and post-emergence damping off (Gravel et al., 2005). Physical properties have been shown to influence Pythium disease severity with media that have high water holding capacity favouring the survival and spread of motile Pythium zoospores (Van Der Gaag and Wever, 2005). Conversely, a well-aerated matrix may physically impede pathogen spread and promote a healthier root system more capable of resisting Pythium infection. The PUF system described previously is therefore the ideal platform to test this hypothesis by isolating the effects of properties like water content and porosity on the progression of Pythium disease.

The aim of this study was to determine whether, by manipulating PUF select physical properties (density, cell size, open cell fraction, water uptake), PUF growing media can be used to demonstrate the importance of optimising the physical properties of soilless growing media, depending on the crop that is being grown and the hydroponic system used.

To investigate the potential of polyurethane foams (PUF) as customisable hydroponic growing media, a set of ten foams exhibiting a range of physical properties was synthesised. The objective was to determine if optimising these properties could benefit specific crops or hydroponic systems compared to a 'one-size-fits-all' growing media such as mineral wool. Formulation and synthesis were successful in generating variation among the ten foams regarding key target properties: open cell fraction, cell size, water uptake rate (W_UR), and maximum water uptake (H_max). PUF media all had significantly lower maximum capillary rise (H_max) and water uptake rates (W_UR) than the MW control. This agrees with previous works investigating PUF as alternative growing media where water holding capacity was lower in PUF than MW (Huber, 2006). These foams subsequently underwent two germination and two vegetative growth trials to evaluate their performance against a standard mineral wool (MW) control.

Following the initial germination trial, four foam formulations were excluded from further testing due to significantly lower germination and/or germination rates compared to the MW control. Examination of the normalised physical properties of these four unsuccessful formulations provides insight into potential reasons for their poor performance. Analysis revealed that these four foams had properties at the extremes of the range. They included the two formulations with the largest and the two with the smallest cell sizes as well as those with the highest and lowest effective open cell fractions ( Table 2 ). As previously demonstrated (Wright et al., 2022b) cell size and open cell fraction strongly influence water uptake characteristics. Correspondingly, these four foams exhibited the lowest H_max values among all ten formulations. Furthermore, PCA analysis ( Figure 8 ) revealed that this group was made up of two subgroups, each consisting of two foams ( Figure 8 ; pink dots). These two groups differed along the axes that correlated with the vectors W_ur and p_eff. Two had very high-water uptake rates, and two had low water uptake rates. These findings suggest two distinct failure modes related to inadequate water management within the root zone. Firstly, the two foams characterised by small, predominantly closed cells likely impeded capillary action, resulting in both slow water uptake rates and low maximum capillary rise (H_max). Secondly, the two foams with large, mostly open cells, while potentially allowing rapid initial water uptake, likely also exhibited excessive drainage, ultimately leading to similarly low H_max values. In both scenarios, the resulting suboptimal water availability (either too little retention or too rapid drainage) likely contributed directly to the observed poor germination rates.

The poor performance of these four formulations provides important insights into the boundary conditions for designing effective PUF based growing media. Notably, these results suggest that successful PUF media require a sufficient minimum water retention (H_max). Achieving this appears dependent on attaining a balanced pore structure, likely characterised by partially interconnected pores (avoiding extremes of fully open or closed structures) and moderate cell sizes.

In the initial vegetative growth trial (VGT1), tomato and lettuce yields were comparable across the tested media. Based on performance in the initial germination (GT1) and vegetative growth (VGT1) trials, the three most promising foam formulations (F04, F07, and F08) were selected for further evaluation. In subsequent germination tests (GT2), these three selected foams demonstrated germination and rates comparable to MW for all three tested crops: tomato, lettuce, and pak choi.

A second vegetative growth trial (VGT2) directly compared these three selected foams against MW using two distinct hydroponic systems tailored to different crops: a dripper fed system for tomatoes and a nutrient film technique (NFT) system for lettuce and pak choi. The experimental design for VGT2 was refined to include fewer media types and a greater number of replicates. This modification enhanced the study's statistical power, allowing for the detection of statistically significant differences in growth performance between media in both the tomato and lettuce trials. F04 and F08 outperformed MW in tomato trials and F08 underperformed MW in lettuce trials. Yield in the pak choi trial was similar between the three-foam media and the MW control, with all three foams performing slightly worse than the MW control, likely due to crops germinating below the surface of the foam, hampering early growth. Further studies into optimal planting depth in foam, as well as the option of germinating seedlings in the dark, should be explored to mitigate this effect.

Analysis of the relationship between hydroponic performance and material properties provides insights into optimisation for specific systems. Examination of the hydrodynamic properties associated with the highest yielding foams in the tomato/dripper system (F04 and F08; Figure 8 ; purple triangles and asterisks in Table 2 ) reveals relatively slow water uptake rates coupled with high maximum capillary rise. These characteristics are advantageous for an intermittent dripper system, as slower water uptake and potentially slower release (implied by high capillary retention) help maintain media moisture and nutrient availability between irrigation events. Structurally, these foams (F04, F08) possessed moderate cell sizes and effective open cell contents relative to the full range synthesised.

In contrast, the best performing foam for lettuce in the NFT system (F07) exhibited faster water uptake rates compared to F04/F08, slightly higher maximum capillary rise, a more open cell structure (higher open cell content), but similarly moderate cell sizes ( Figure 8 ; yellow inverted triangle and asterisks in Table 2 ). This more open structure is likely advantageous in an NFT system, where the media's role in water retention is less critical due to continuous nutrient solution flow. Furthermore, open cell structure of F07 may facilitate easier root anchorage and subsequent penetration into the NFT channel. This high porosity enhances airflow through the substrate, improving gas exchange within the root zone. Considering inadequate aeration is a known cause of yield reduction in NFT systems, with inorganic media with higher aeration have been shown to improve crop performance (Jackson et al., 1984), this may aid in explaining why F07 was best performing in NFT lettuce trials.

These initial trials indicate that a single growing media approach is unlikely to be crop and system optimised and furthermore that PUF media can be used as a platform to determine growing media physical property recipes that are both crop and system optimised.

We acknowledge that testing multiple substrates under a single irrigation strategy can limit the applicability of the results, as optimal irrigation is media-specific (Raviv et al., 2019). However, our methodology intentionally uses an irrigation regime optimised for MW. This approach was chosen because growers are often reluctant to modify established infrastructure for novel media, making it critical to demonstrate that foam can perform effectively as a direct replacement within existing systems.

PUF appeared to reduce the effect of Pythium, a common hydroponic pathogen (Zlnnen, 1988), on germination of tomato plants. Compared to the MW medium, the reduced early mortality observed in two of the foam media (F07 and F08) under Pythium challenge was an unexpected finding. This superior germination may be due to the large differences in capillarity between the MW and PUF samples. High water uptake by MW has been shown to be a major contributor to root rot proliferation in hydroponic systems (Juneau et al., 2006) and increasing the height of MW block in these systems, and therefore the reducing water content in the root zone decreases the chance of yield loss due to the disease (Van Der Gaag and Wever, 2005). In addition to the large differences in hydrodynamics, the lack of disease symptoms in PUF could also be explained by the surface properties of the foam which may be less conducive to Pythium proliferation compared to MW; or a chemical component or property of the foam might possess antimicrobial activity effective against the oomycete pathogen. Trials with phenolic foams, oasis cubes, examining root rot in different soilless growing media, also found that their foams had the lowest disease incidence, hypothesising that this was due to different physicochemical parameters, changes in microbiomes or due to chemical components in the foams (DeGenring and Poieatewich, 2023).

Further investigation of the mechanism of this disease suppression is required to understand how microbes interact with the foam media before utilising these foams as model growing platforms in experiments involving biological factors, such as pathogen screening or the application of beneficial plant health or growth promoting microorganisms.

In conclusion, this study demonstrates that a single growing media formulation is not optimal for different crops and hydroponic systems. Furthermore, it demonstrates the potential of PUFs as a versatile platform for developing tailored growing media recipes, allowing for the identification of physical property attributes optimal for a given crop and system requirements. It is important to acknowledge the broader market context and sustainability challenges around growing media development. The PUFs synthesised here were petroleum-based, which may face headwinds from retailers and consumers who are increasingly favouring bio-based or biodegradable materials. Therefore, a next step for this research is to explore the development of bio-based and biodegradable alternatives; by replacing fossil-based reagents and adding specific sites for degradation This would combine the customisable physical properties demonstrated here with a significantly improved environmental profile, aligning the technology with the future sustainability goals of the horticultural industry.