Study on Preparation and Performance of Biomass–Polyurethane Light Planting Substrate

Abstract

A biodegradable, lightweight substrate for facility-based stereoscopic planting was developed via a one-step polyurethane foaming process. The substrate was synthesized by incorporating a biomass mixture of bamboo charcoal and cassava flour into a polyurethane foam matrix. This study investigated the effects of varying the content ratios of polyether polyol, isocyanate, bamboo charcoal powder, and cassava flour on the structural and functional properties of the composite foam. Results indicated that the biomass blend significantly influenced the foam’s physicochemical properties, water retention capacity, hardness, and elasticity. Specifically, bamboo charcoal powder enhanced the porosity and degradation rate of the foam, whereas the swelling of cassava flour upon water absorption improved the matrix’s resilience and cohesion. A polyether polyol/isocyanate ratio of 4:1 yielded a substrate with superior physicochemical properties, water retention capacity, germination rate, seedling index, and plant dry weight. Subsequently, the optimal overall performance was achieved at a biomass/polyol–isocyanate ratio of 1:3. This optimal formulation exhibited a degradation rate of 6.24 ± 0.94%, porosity of 66.07 ± 1.10%, and water retention capacity of 86.03 ± 1.59%. Consequently, it also produced the highest seed germination rate (84 ± 5.16%), seedling index (12.49 ± 1.94), and mature plant dry weight (4.00 ± 0.51 g). Microscopic analysis confirmed that the biomass addition refined the substrate’s pore structure, leading to greater uniformity and stability of the internal pores. This enhancement reduced the foam’s susceptibility to collapse and improved its elasticity and cohesion, thereby making it more amenable to mechanized handling and planting operations.

1. Introduction:

Based on data from the United Nations (UN), the average cultivated area per person is experiencing a continuous decline. As of 2022, the average cultivated area per person in the world is 0.1973 hectares, in the United States is 0.4574 hectares, and in China is 0.088 hectares. The Food and Agriculture Organization of the United Nations (FAO) [1] reports that, compared to 1961, these figures have decreased by 54%, 53.5%, and 43%, respectively (Figure 1). Meanwhile, with the acceleration of the global urbanization process, more than half of the world’s population now resides in cities. Furthermore, it is expected that the global urban population will increase to 9.6 billion by 2030 [2]. In addition, frequent extreme weather events in the future that are led by climate change, coupled with water resource shortages, have placed enormous pressure on safe agricultural production. In response, advancing sustainable agriculture on a global scale increasingly relies on soilless three-dimensional cultivation technology as a viable and effective solution [3,4].

In the context of soil-free cultivation in facilities, solid substrates provide plants with a pathogen-free environment that ensures sufficient air, water, and nutrients for the roots, playing a crucial role in soil-free cultivation [2,5]. A variety of solid substrates are employed in soil-free cultivation, mainly including inorganic and organic substrates. Among these types of substrates, inorganic rock wool and organic peat are more widely used in practice currently [6,7]. Rock wool is easy to apply but has high costs and is not biodegradable; peat is cost-effective but represents a non-renewable resource (

Table 1. The cost of already available products in the market.

	Rock Wool	Rock Wool	Rock Wool	Rock Wool	Peat Substrate Blocks	Peat Substrate Blocks	Peat Substrate Blocks	Foam Sponge
Units	Blocks	Blocks	Blocks	Blocks	Blocks	Blocks	Blocks	Blocks
Model (cm)	7.5 × 7.5 × 6.5	3.6 × 3.6 × 4	2.5 × 2.5 × 4	7.5 × 7.5 × 6.5	10.5 × 6.5 × 10	3.3 × 3.6 × 2.8	2 × 6.2 × 1	Φ32 × 30
Price (CNY)	28.99	0.73	0.6	4.1	7.49	1.32	0.7	0.14
Manufacturer	The Netherlands (Grodan)	The Netherlands (Grodan)	The Netherlands (Grodan)	China (Le Pao)	Germany (OIMG)	Germany (OIMG)	Germany (OIMG)	China (QWC)

) [8,9,10,11,12,13,14,15]. Therefore, there is a pressing need to identify an environmentally friendly alternative to peat-based substrates for controlled-environment agriculture. Polyurethane foam has emerged as a promising candidate, attracting considerable interest within the industry. This is attributed to its advantageous properties, including high porosity, excellent water retention, significant elasticity, a lightweight structure, and superior energy absorption through deformation [16,17,18,19]. Liu used agricultural waste to prepare biomass-based soft polyurethane materials with excellent performance, improving the utilization rate of biomass resources [20]. Silva et al. effectively improved the rigidity and stability of the finished product by adding rice ash in the polyurethane foam formula [18,21]. Qi et al. prepared a polyurethane-peat lightweight foam cultivation substrate with a three-dimensional network structure through a one-shot method, which overcame the drawback of traditional substrates being easily dispersed, obtaining a kind of green cultivation substrate that had strong water absorption, high porosity, and high nutritional value [22]. Basso et al. synthesized formaldehyde-free, non-toxic flower peat foam material based on the mixing of tannins with inorganic components such as vermiculite, providing a good root environment for hydroponic cultivation [6]. Du et al. prepared a lightweight cultivation substrate using polyurethane as the substrate base and soil composite, observing that the increase in soil content led to a decrease in cell diameter uniformity, water absorption rate, and foaming ratio [23]. Ma and Wang found that different hydroxyl values of polyether have a significant impact on the water absorption and water-holding capacity of soil-free cultivation polyurethane foam [24]. With the increase in hydroxyl value, the porosity of polyurethane foam increases, and both water absorption and water-holding capacity are significantly improved. Lightweight and environmentally friendly cultivation substrate has become an inevitable trend in soil-free cultivation. However, there are still some problems regarding soil-free lightweight polyurethane substrates in terms of environmental protection, cultivation, and stability [25].

In this study, bamboo charcoal powder and cassava flour were used as additives to improve the stability, degradation, and cultivation performance of polyurethane foam. Bamboo charcoal powder and cassava flour are low-cost and renewable, which are expected to improve the performance of lightweight polyurethane foam. This article aimed to produce an eco-friendly, lightweight foaming substrate suitable for factory mechanized operations by combining polyurethane foam with biochar and cassava flour. A systematic investigation into how different formulations affect substrate properties—including water absorption, air permeability, and degradation—led to the development of a soilless planting substrate characterized by high porosity, structural stability, and excellent water absorption and retention capabilities. This lightweight foamed substrate demonstrates robust mechanical properties and superior cultivation performance, positioning it as an ideal medium for soilless cultivation in controlled environments and a promising solution for large-scale, three-dimensional factory farming.

2. Materials and Methods

2.1. Experimental Materials

Polyether polyol and isocyanate, both of industrial grade, were purchased from Beijing Hagibis Technology Co., Ltd. (Beijing, China). The average molecular weight of polyol (DL-4000D) is 4410, and the hydroxyl value is 27–29 mgKOH·g⁻¹; the isocyanate is TDI 180 (toluene diisobyanate). The biochar used in this study was bamboo charcoal, purchased from Longyou Shengtangshan Bamboo Charcoal Co., Ltd. (Quzhou, China), with a particle size ranging from 80 to 100 mesh. Additionally, cassava flour, with a particle size of 3000 mesh, was purchased from Hangzhou Starpro Starch Co., Ltd. (Hangzhou, China).

2.2. Test Instruments

The following instruments were used in this study: a texture analyzer(TMS-PILOT, FTC Company, Washington, DC, USA); an electric mixer (90 W, Shanghai Specimen Model Factory, Shanghai, China); an electric blast drying oven (DHG-9420A, Shanghai Yiheng Technology Instrument Co., Ltd., Shanghai, China); an electronic balance (AUY220, Shimadzu Corporation, Kyoto, Japan); and a binocular digital microscope (Optec BK 5000TR, Chongqing Optec Instrument Co., Ltd., Chongqing, China).

2.3. Methods

2.3.1. Preparation of Bio-Polyurethane Light Foam Matrix

In this study, a one-step foaming process was used to produce lightweight biomass and polyurethane planting substrate, where the biomass components consisted of bamboo charcoal powder and cassava flour. The preparation procedure was as follows: first, bamboo char powder and cassava flour were mixed in a container of polyether polyol at a predetermined ratio, then stirred at 800 rpm for 10–15 s to ensure thorough blending, yielding “Material A”. Next, a specified amount of isocyanate (referred to as “Material B”) was added to Material A; the mixture was stirred at 800 rpm speed for another 10–15 s to achieve full homogenization, after which it was rapidly injected into a mold. The mixture was allowed to rest at room temperature for 2 h, and the mold was subsequently demolded to obtain the formed lightweight biomass and polyurethane planting substrate. The detailed preparation process is illustrated in Figure 2.

For characterization and performance testing, the fabricated substrates were cut into cylindrical specimens. A total of 10 specimens, each measuring 22 mm in diameter and 35 mm in height, were prepared for each experimental condition. These specimens were then subjected to a series of observations, characterizations, and performance measurements.

2.3.2. Effects of Biomass Composition Ratio on the Performance of Lightweight Foam Substrate (LWFS)

To establish the lightweight biomass and polyurethane substrate, the masses of polyether polyol, isocyanate, and the total biomass component were held constant at 40 g, 10 g, and 17 g, respectively. The experimental variable was the composition of the biomass component, wherein the ratio of bamboo charcoal to cassava flour within the biomass was systematically adjusted to 6:1, 4:1, 2:1, 1:1, 1:2, 1:4, and 1:6. A total of seven distinct formulations were prepared and comparatively analyzed to determine the optimal substrate composition (

Table 2. The components of different substrates.

Treatments	Biomass Charcoal (g)	Cassava Flour (g)	Polyacid Polyol (g)	Isocyanate (g)
6:1	14.57	2.43	40	10
4:1	13.60	3.40
2:1	11.33	5.67
1:1	8.50	8.50
1:2	5.67	11.33
1:4	3.40	13.60
1:6	2.43	14.57

).

The mechanical performance of the substrates was assessed using a texture analyzer (TMS-PILOT, FTC, Washington, DC, USA) equipped with Texture Lab Pro software (v.1.88-408). Key parameters—hardness, resilience, and cohesion—were measured under standardized conditions: a test speed of 60 mm/min, an initial force of 0.75 N, and a compression rate of 50%. Porosity was quantified using the methods established by Ma [24] and Lian [26]. The degradation rate was determined according to the procedure detailed by Chen [27], calculated using the following equation:

$$\mathrm{P}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} {\eta }_w = {\displaystyle \frac{w_1-w}{V}} (\mathrm{g}/{\mathrm{cm}}^3)$$

where w is the dry mass of the substrate(g); w₁ is the water-saturated mass of the substrate (g); and V is the volume of the water-saturated substrate (cm³).

$${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{r}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y} W}_{HR}={\displaystyle \frac{w_2}{w_1}}$$

where water retention capacity (W_HR) was determined by the funding agency’s specification. Inside the plant factory (24 °C day/20 °C night, 70–80% RH), substrate blocks were fully saturated by submersion (w₁) then allowed to drain for 6 h on a draining tray before re-weighting.

$${\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} D}_w={\displaystyle \frac{w-w_3}{w}}$$

where w₃ is the post-degradation dry mass of the substrate, measured following a standardized soil burial method [27]. In this procedure, pre-massed substrate samples were buried in soil at a specified depth (40 cm) and marked for later retrieval. After a burial period of 30 days, the samples were excavated, gently rinsed with deionized water to remove any adhering soil particles, and then oven-dried to a constant mass. The final weight of each sample was recorded as w₃.

2.3.3. Effects of Polyol/Isocyanate Ratio on the Performance of LWFS

Building upon the optimal biomass composition identified in Section 2.3.2, a series of LWFS samples was prepared. For all samples, the total mass of the biomass component was fixed at 17 g, while the combined mass of the polyo/isocyanate was kept constant at 50 g. The key variable investigated was the polyol/isocyanate ratio, which was systematically varied to 6:1, 5:1, 4:1, 3:1, and 2:1. The effects of these ratios on the substrate’s apparent structure, porosity, water retention capacity, water absorption, hardness, rebound rate, and cultivation performance were then investigated. The apparent structure of the samples was examined using a binocular digital microscope. Hardness and rebound rate were assessed as described in Section 2.3.2. Furthermore, water retention capacity and water absorption were measured to the methods of He et al. [28] and Ma et al. [24], respectively.

$$\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(WS\right)={\displaystyle \frac{w_1-w}{w_1}} (\mathrm{g}/{\mathrm{cm}}^3)$$

where w is the dry mass of the substrate (g); w₁ is the water-saturated mass of the substrate (g).

To evaluate the cultivation performance of lightweight biomass and polyurethane substrate, a germination and seedling growth experiment was conducted. The experiment assessed the impact of different substrate formulations on plant growth, as determined by seed germination rate, seedling index, and fresh weight. Each treatment group consisted of three replicates, with each replicate containing ten samples (n = 30 per group). Into each substrate sample, one seed of “Guangfu” oilseed rape (Brassica napus L.), sourced from the Guangdong Provincial Improved Variety Introduce Service Corp, was sown. The substrates were placed into a tray (280 mm × 560 mm) with 72 pots, and the trays were placed in an environment-controlled greenhouse. Germination rate was calculated on the 7th day after sowing, seedling index on the 15th day, and plant fresh weight was measured around the 35th day. The germination rate was calculated using the following equation:

$$\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\left(GR\right) ={\displaystyle \frac{N_2}{N_1}}\times 100\%$$

where GR is germination rate (%), N₁ is the number of planted seeds, and N₂ is the number of germinated seeds.

$$\mathrm{S}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}\left(SVI\right)=({\displaystyle \frac{S}{H}}+{\displaystyle \frac{M}{N}})\times (M+N)$$

where SVI is the seedling index [29], S is the seedling stem thickness (mm), H is the seedling height (mm), M is the dry weight of the underground part of the seedling (mg), and N is the dry weight of the above-ground parts of the seedling (mg).

2.3.4. Effects of Biomass/Polyol–Isocyanate Ratio on the Performance of LWFS

To determine the optimal composition of the biomass-polyurethane substrate, a series of samples was fabricated with a constant total mass of 66.7 g. This investigation builds upon the preliminary component ratios established in Section 2.3.1 and Section 2.3.2, focused on optimizing the macroscopic ratio between the biomass component and the polyol–isocyanate mixture. Specifically, biomass/polyol–isocyanate ratios of 1:1, 1:2, 1:3, 1:4, and 1:5 were investigated. The influence of these varying ratios on key performance indicators—including degradation rate, water retention capacity, water absorption, hardness, rebound rate, and cultivation performance—was systematically evaluated to identify the most suitable substrate formulation.

3. Results and Discussion

3.1. Effects of Biomass Composition Ratio on the Performance of LWFS

The composition of the biomass significantly influenced the performance of LWFSs, including their porosity, degradation rate, and hardness (p < 0.05,

Table 3. Effects of different biomass component ratios on the properties of lightweight foams.

Bamboo Charcoal/Cassava Flour	Porosity (g/cm3)	Degradation Rate DW (%)	Water-Holding (%)	Natural State Sample			Water-Saturated Sample
				Hardness (N)	Rebound Rate (%)	Cohesion (J/m3)	Hardness (N)	Rebound Rate (%)	Cohesion (J/m3)
1:6	51.73 ± 6.25 e	5.06 ± 0.81 d	90.13 ± 0.83 a	48.06 ± 4.80 de	94.78 ± 0.53 a	0.87 ± 0.01 a	47.95 ± 5.87 a	95.64 ± 0.82 abc	0.87 ± 0.05 a
1:4	64.58 ± 3.72 d	4.73 ± 0.26 d	87.33 ± 0.54 b	56.98 ± 11.16 abc	94.99 ± 0.66 a	0.87 ± 0.01 a	48.43 ± 6.18 a	95.67 ± 0.63 a	0.89 ± 0.01 a
1:2	66.07 ± 3.24 cd	4.84 ± 3.13 d	86.03 ± 0.50 bc	61.13 ± 8.66 ab	93.96 ± 0.58 a	0.84 ± 0.01 b	42.96 ± 3.74 a	95.48 ± 0.65 ab	0.88 ± 0.02 b
1:1	67.95 ± 2.79 cd	7.64 ± 3.24 c	84.52 ± 0.43 cd	63.79 ± 6.84 a	92.66 ± 0.62 b	0.81 ± 0.01 c	40.39 ± 3.95 b	94.45 ± 0.57 abc	0.88 ± 0.01 b
2:1	66.48 ± 3.52 bc	10.75 ± 3.14 b	83.78 ± 0.76 d	53.49 ± 10.76 bcd	91.02 ± 1.65 c	0.79 ± 0.01 d	32.44 ± 5.59 bc	93.77 ± 1.14 bc	0.87 ± 0.02 c
4:1	70.72 ± 3.55 bc	11.06 ± 1.83 b	81.36 ± 0.47 e	48.91 ± 7.12 cde	90.28 ± 1.34 c	0.78 ± 0.01 de	30.61 ± 4.71 c	93.04 ± 0.84 c	0.87 ± 0.01 c
6:1	73.14 ± 2.82 a	12.08 ± 0.69 a	81.19 ± 0.86 e	42.11 ± 10.08 e	88.01 ± 2.35 d	0.78 ± 0.01 e	31.38 ± 4.98 d	91.89 ± 1.76 d	0.85 ± 0.01 c
F	31.63	21.154	24.612	7.74	41.019	88.319	23.133	21.418	2.708
p	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.021 *

* p < 0.05, *** p < 0.001. The different little capton letters after SE indicate the difference between treatment is significant.

). As the proportion of bamboo charcoal in the mixture increased, porosity and degradation rate of the LWFSs also increased. This is attributed to the inherent properties of bamboo charcoal, a biochar derivative with a highly porous structure and a large specific surface area (500~1500 m²/g), which facilitates both structural porosity and biodegradation [28,30]. Conversely, the water retention capacity of LWFS decreased as the bamboo charcoal content rose. This inverse relationship is explained by the corresponding reduction in cassava flour, a component rich in starch (>75%) that is known for its excellent water retention. Therefore, the observed trade-off between enhanced porosity and reduced water retention capacity is a direct result of the shifting ratio between bamboo charcoal and cassava flour within the biomass composition.

Upon reaching water saturation, the LWFS exhibits decreased hardness alongside increased resilience and cohesion (Table 3). This phenomenon is attributed to starch and fibers within the cassava flour, which absorb water and swell. This swelling action softens the substrate’s overall structure, thereby reducing its hardness. The influence of the bamboo charcoal to cassava flour ratio on the LWFS’s hardness, however, is more complex and follows a non-linear trend. Initially, as the bamboo charcoal content increases, the hardness of the substrate rises, peaking at ratios of 1:1 and 1:2. This is because bamboo charcoal particles act as a reinforcing filler, strengthening the cell walls of the foam structure generated during the foaming process. Beyond this optimal point, a further increase in bamboo charcoal content leads to a decline in hardness. This reversal occurs for two primary reasons: First, the three-dimensional polymer network becomes structured and can no longer effectively integrate the excess biocarbon, leading to poor interfacial adhesion and structural defects. Second, the corresponding reduction in cassava flour content diminishes the substrate’s resilience and cohesion, further compromising its mechanical integrity. Consequently, the relationship between the bamboo charcoal to cassava flour ratio and hardness is characterized by an initial positive correlation that shifts to a negative correlation after a certain threshold is reached (Table 3).

The correlation matrix visually depicts the relationship among the multiple properties of the LWFS as a function of its bamboo charcoal to cassava flour composition (Figure 3). The bamboo charcoal/cassava flour ratio shows a notable positive correlation with porosity (r = 0.876) and degradation rate (r = 0.722). Conversely, a very strong negative correlation is observed between the bamboo charcoal ratio and the substrate’s mechanical integrity. Natural cohesion exhibits a significant (p < 0.05) negative correlation with both porosity (r = −0.684) and degradation rate (r = −0.886). Furthermore, the analysis reveals a clear divergence in material behavior between its natural and water-saturated states. The properties measured in the natural state show stronger intercorrelations among themselves compared to their water-saturated counterparts. Notably, water-saturated hardness demonstrates a significant negative correlation with the bamboo charcoal ratio (r = −0.718), which water absorption softens the charcoal-rich, porous structure.

In summary, the correlation analysis provides a comprehensive, data-driven visualization of the fundamental design trade-offs in the LWFS formulation. Optimizing the substrate for high porosity and rapid degradation (by increasing bamboo charcoal) inherently compromises its mechanical strength and cohesion. Conversely, formulations rich in cassava flour, while offering superior mechanical integrity and resilience, exhibit lower porosity and slower degradation.

3.2. Effects of Polyol/Isocyanate Ratio on the Performance of LWFS

Figure 4 illustrates the evolution of the apparent morphology in the biomass–polyurethane LWFS as the polyol/isocyanate ratio is varied, while the biocomponent ratio is maintained at 1:2. A low polyol–isocyanate ratio results in a foam with a heterogeneous, open-cell structure, featuring large, coalesced pores and thin walls (Figure 4a,b). This morphology is indicative of a rapid, less-controlled polymerization process where gas bubble expansion outpaces the stabilization of the matrix. Conversely, increasing the polyol–isocyanate ratio fundamentally alters the reaction kinetics and the rheology of the system. Higher polyol content leads to a more controlled reaction, producing a finer and more uniform cell structure (Figure 4c). At the extreme ratio of 6:1, the foam morphology transitions to a dense, closed-cell-like structure with significantly smaller pores and substantially thickened walls. This demonstrates that an excess of polyol relative to isocyanate promotes the formation of a more viscous reacting mixture, which better stabilizes the forming bubbles and allows for the development of a thicker, more cohesive polymer scaffold.

The polyol/isocyanate ratio exerts a significantly (p < 0.05) and multifaceted influence on the LWFS porosity (Figure 5). The physical properties display a distinct non-linear response. Porosity and water absorption both exhibit an inverted U-shaped relationship with the polyol/isocyanate ratio, maximizing at 4:1 (Figure 5a,b). Conversely, water retention capacity increases monotonically with the polyol ratio, likely due to enhanced hydrophilicity and more uniform cell structure at higher polyol contents (Figure 5c). These trends align with established foaming principles, where reduced isocyanate content leads to lower expansion ratios and finer cellular structures [17,23]. The mechanical properties reveal a complex interplay between formulation, hydration state, and performance. Hydration consistently softens the substrate, decreasing hardness while increasing resilience and cohesion across all formulations (Figure 5d–f). The polyol/isocyanate ratio also critically modulates these mechanical responses. The difference in hardness between the natural and water-saturated states diminishes as the polyol content increases, with the minimum absolute hardness observed at the 4:1 ratio (Figure 5d). Both resilience and cohesion follow a similar parabolic pattern to porosity, peaking at a 4:1 ratio before declining (Figure 5e,f). In summary, a polyol/isocyanate ratio of 4:1 emerges as the optimal formulation. This ratio produces a substrate with a superior combination of characteristics: high porosity and water absorption capacity, excellent water retention, and balanced mechanical properties, including minimized hardness and maximized resilience and cohesion.

Figure 6 illustrates significant effects of the polyol/isocyanate ratio on the cultivation performance of Guangfu oilseed rape in LWFSs (p < 0.001). The germination rate exhibited a parabolic trend with respect to polyol/isocyanate ratio, peaking at the 4:1 formulation, where it reached a high and consistent range of 80–88%. This optimal ratio was also consistently beneficial for subsequent growth states. Both the seedling index and above-ground fresh weight followed a similar trend, reaching their maximum values at a 4:1 ratio. At harvest, the plant growth in the 4:1 substrate not only achieved the highest dry weight but also displayed a relatively uniform distribution (70–88 g), indicating consistent and reliable growth performance.

Gray correlation theory was employed to analyze the relationship between various performance indicators and the polyol/isocyanate ratio (

Table 4. Effects of different polyol/isocyanate ratios on the properties of LWFSs.

Evaluation Indicators		Correlation Coefficient	Gray Relational Grade	Ranking
Porosity (%)		0.662 **	0.848	1
Water absorption (%)		−0.095	0.784	9
Water-holding (%)		0.792 **	0.829	2
Natural state sample	Hardness (N)	−0.821 **	0.664	13
	Rebound rate (%)	−0.689 **	0.806	6
	Cohesion (J/m3)	0.443 **	0.815	4
Water-saturated sample	Hardness (N)	−0.663 **	0.75	10
	Rebound rate (%)	−0.798 **	0.807	5
	Cohesion (J/m3)	−0.328 *	0.806	7
Germination rate (%)		−0.722 **	0.691	12
Fresh weight (mg)		0.144	0.788	8
Seedling rate (%)		0.029	0.7	11
Dry weight of plant (g)		0.458 **	0.826	3

* p < 0.05; ** p < 0.01.

). The analysis revealed that porosity had the highest gray relation grade of 0.848, indicating it is the most responsive parameter to changes in the ratio. This was followed by water retention capacity (0.829), plant dry weight (0.826), cohesion (0.815), and rebound rate (0.807). These findings are corroborated by a standard correlation analysis, which confirmed significant positive relationships (p < 0.01) between the ratio and porosity (r = 0.662), water retention capacity (0.792), plant dry weight (0.458), and cohesion (0.433). Conversely, resilience exhibited a significant negative correlation with the ratio (r = −0.798, p < 0.01).

3.3. Effects of Biomass/Polyol–Isocyanate Ratio on the Performance of LWFS

With the biomass component ratio fixed at 1:2 and the polyol/isocyanate ratio at 4:1, this study investigated the effects of varying the overall biomass/polyol–isocyanate ratio on LWFS performance (Figure 7). The ratio had a highly significant effect on performance parameters (p < 0.001). A decrease in biomass content led to reduced degradation rate and hardness but increased porosity, water absorption, rebound rate, and cohesion. Notably, these trends plateaued when the biomass/polyol–isocyanate ratio exceeded 1:3, with no significant changes. This behavior is attributed to the evolving polyurethane microstructure. The increasing proportion of the polyol–isocyanate mixture promotes the formation of a more extensive three-dimensional network via chain extension and cross-linking reactions [31]. As shown in Figure 8, this results in a microstructure with larger, smoother cells, which directly governs the observed changes in the structure’s physical and mechanical properties.

The biomass/polyol–isocyanate ratio significantly affected the cultivation performance of the LWFSs (Figure 9). A high biomass content of 50% resulted in the lowest germination rate and complete yield failure. As the polyol–isocyanate content increased, key performance indicators—including germination rate, seedling index, fresh weight, and plant dry weight—initially increased before subsequently decreasing. The optimal performance was consistently observed at a 1:3 ratio, where the germination rate peaked at 84%, the seedling index reached a maximum of approximately 12.49, and the plant dry weight was highest and most uniformly distributed (Figure 10). These performance trends are closely linked to the substrate’s physical properties, which are governed by its composition. At high biomass contents, the foaming system’s increased viscosity resulted in a dense structure with small pores and high hardness. Conversely, as the polyol–isocyanate content increased, the foam developed larger, more uniform pores with reduced hardness and greater elasticity. This improved microstructure facilitated better root penetration and seed germination, a finding consistent with previous research [32]. This observation also aligns with studies on other bio-based polyurethane foams, which have shown that a relatively dense structure can be conducive to seed germination [33,34]. These insights suggest that incorporating liquefied biomass into the polyol component could be a promising strategy for further work to enhance the applicability of biomass/polyurethane form as growth media.

4. Conclusions

In this study, a biodegradable biomass-based lightweight planting foam substrate (LWFS) was fabricated via a one-step polyurethane foaming process. The incorporation of biodegradable biomass mixtures during the initial foaming stage yields a substrate with a three-dimensional network structure. Composed of biological organic matter, this substrate addresses key drawbacks of traditional media like rock wool and peat, namely their tendency to disperse and their non-biodegradable nature. It possesses excellent properties, including light weight, high porosity, strong water-holding capacity, and high elasticity, making it a promising new carrier for hydroponic cultivation. The specific research outcomes are as follows:

i.

The proportion of bamboo charcoal to cassava flour within the biomass blend significantly influences the LWFS characteristics. Increasing the bamboo charcoal content effectively enhances porosity and degradation rate. Concurrently, cassava flour, upon absorbing water and expanding, contributes to improving substrate resilience and cohesion. Based on these effects, the optimal formulation was determined to be a 1:2 ratio of bamboo charcoal/cassava flour.

ii.

An increase in the polyol content enhanced LWFS’s water retention capacity. The optimal polyol/isocyanate ratio was identified as 4:1, combined with a 1:2 bamboo charcoal/cassava flour ratio. At this specific formulation, the substrate demonstrated significantly superior overall performance, including key metrics such as water retention capacity, germination rate, seedling index, and plant biomass.

iii.

The LWFS exhibited optimal overall performance at a biomass/polyol–isocyanate ratio of 1:3, with a 4:1 ratio of polyol/isocyanate and a 1:2 ratio of bamboo charcoal/cassava flour. At this ratio, the degradation rate was 6.24 ± 0.94%, while porosity, water absorption, and water retention capacity reached 66.07 ± 1.10%, 389.18 ± 16.42%, and 86.03 ± 1.59%, respectively. Critically, the germination rate (84 ± 5.16%), seedling index (12.49 ± 1.94), and plant dry weight (4 ± 0.51 g) all reached their maximum values. The resulting structure is stable and resistant to collapse, highlighting its significant potential for applications in controlled-environment agriculture and three-dimensional cultivation systems within plant factories.