Compost and Vermicompost from Vine Pruning and Sewage Sludge as Peat Alternatives in Cucumber Seedling Production

Abstract

The replacement of peat in horticultural substrates is a priority for sustainable plant production. This study evaluated compost and vermicompost, derived from vine pruning and sewage sludge, as partial peat substitutes in cucumber (Cucumis sativus L.) seedling production. Germination, early growth traits, growth efficiency indices, and leaf nutrient contents were assessed, and the relationships among variables were explored using correlation analysis and principal component analysis. Five substrates were tested: peat-perlite alone (control) and mixtures containing 10%, 20%, or 40% compost or vermicompost as peat replacements. Results showed that incorporating 10% vermicompost significantly improved germination, seedling vigor, and biomass accumulation, with performance comparable to, or exceeding, the control. In contrast, higher proportions of compost or vermicompost negatively affected germination and seedling quality. Nutrient analysis revealed that 10% vermicompost enhanced Ca and K accumulation, traits positively correlated with growth, whereas 20% compost and 20% vermicompost were associated with higher P and Mg contents but reduced seedling performance. Overall, these promising findings demonstrate that a low proportion of vermicompost (10%) is sufficient to successfully partially replace peat in cucumber seedling production, benefiting both performance and sustainability, whereas higher compost or vermicompost levels disrupt nutrient balance and limit this species’ growth.

1. Introduction

The intensification of agriculture to meet growing food demands generates large volumes of organic residues [1,2,3] that pose significant environmental and management challenges. Among these residues, vine pruning and sewage sludge represent abundant but underutilized waste streams, with their appropriate valorization providing both environmental and agronomic benefits [1,4]. Composting and vermicomposting offer sustainable pathways to valorize these residues into valuable soil organic amendments, thereby improving soil fertility and reducing reliance on synthetic fertilizers.

Composting is an aerobic biological process in which microorganisms decompose organic material using carbon, nitrogen, oxygen and water as energy sources. This process generates heat, carbon dioxide, and water vapor [5] and results in a stabilized soil-enriching end-product known as compost. Compost is widely recognized as a soil amendment that improves soil fertility and crop yield [6] by enhancing the physical, biological and chemical properties of soils [7]. Vermicomposting is a similar biological process [8] but involves the addition of earthworms that accelerate organic matter fragmentation and digestion [9], creating optimal conditions for microbial activity [10]. The joint activity of decomposing microorganisms and earthworms leads to humification of organic and inorganic residues, generating the final product called vermicompost [9]. Both composts and vermicomposts have been shown to improve soil structure, nutrient availability, plant performance, crop productivity, and resilience against abiotic and biotic stressors [6,11,12,13,14].

Peat has been widely used in nursery production [15,16,17] due to its unique physical, chemical and biological properties [18,19], such as high water retention and cation exchange capacity, absence of phytotoxic substances, and low bulk density [20]. However, peat extraction depletes a non-renewable resource [21,22] and releases stored carbon [18], conflicting with climate and biodiversity goals [13]. Given the slow regeneration of peatlands [17], their substitution is therefore a global priority [20,23]. Organic amendments, including composts and vermicomposts, have been suggested as sustainable alternatives to peat in nursery production [18,24,25].

Previous studies have reported positive effects of peat substitution with organic residues, but outcomes depend on the composition of the raw materials [17,25], the process employed [25], the quality of the end products, and the dosage used [26]. For example, Adamczewska-Sowinska et al. [27] found that replacing peat with 25–50% of chipped willow compost produced healthy and vigorous cucumber transplants, while Abdel-Razzak et al. [23] reported that mixing 5–10% tomato waste compost with peat provided an ideal growing medium for tomato, hot pepper, cucumber, and summer squash seedlings. Similarly, Gavilanes-Terán et al. [21] found that a mixture of 25% tomato waste compost in peat was optimal for growing tomatoes, courgettes, and peppers, whereas excessive compost proportions may reduce seedling growth due to salinity beyond the acceptable limits for growing media. Vermicompost has also shown benefits. Büyükarslan & Demir [28] found improved pepper and cauliflower seedling growth with a mixture of 35% vermicompost and peat. Lazcano et al. [29], in a comparative study using 0–100% of compost and vermicompost obtained from animal manures, found that vermicompost could fully replace peat in tomato, whereas compost proportions above 50% caused plant mortality due to high salt content and bulk density [29], highlighting differences between the two strategies that need to be considered. Similarly, Gong et al. [24] observed that geranium and calendula performed better in vermicompost, which replaced 50–100% of the peat used in containerized production. These findings illustrate both the potential and the species-specific responses to peat substitution with compost or vermicompost [21]. For instance, Calisti et al. [26] found that compost derived from digestate promoted olive seedling growth but had a substantially negative effect on hazelnut seedlings at the same compost proportion. Therefore, determining the optimal proportions for replacing peat without compromising seedling quality is challenging.

In this context, the objective of this study was to evaluate compost and vermicompost produced from vine pruning residues and sewage sludge as partial substitutes for peat in cucumber seedling production. Despite the abundance of these organic residues, their potential as components of growing media remains largely unexplored, particularly when combined. The study focused on their effects on germination, early seedling growth, and leaf nutrient dynamics. We hypothesized that vermicompost would outperform compost in promoting cucumber germination and early growth, but that the magnitude of the effect would depend on the proportion used. The findings are expected to contribute to the development of sustainable strategies for peat substitution that support circular economy principles, reduce the environmental impact of residue disposal and peat extraction, and provide viable solutions for horticultural production.

2. Material and Methods

2.1. Compost and Vermicompost Production

A mixture of vineyard pruning, collected from vineyards in the Douro region (Northern Portugal) and used as the primary lignocellulosic substrate, and sewage sludge, from a municipal wastewater treatment unit, with the solid fraction of cattle slurry as inoculum, was prepared on a dry weight basis. Composting was carried out in insulated reactors (135 L) with regular turning to promote aeration and moisture adjustment during a 140-day thermophilic phase. Following this phase, part of the compost was transferred to wooden boxes (56 L) for vermicomposting, which lasted an additional 100 days, under controlled conditions (20–25 °C, 70–80% moisture). The remaining compost underwent maturation in the same insulated reactor and for the same period (100 days). At the end, both the compost and the vermicompost were sieved at 5 mm and stored for further analyses.

2.2. Substrate Preparation and Treatments

Seedling substrates were prepared by mixing commercial substrate (BIO Siro, Leal & Soares, Lda., Mira, Portugal), composed by Sphagnum blonde peat, pine bark humus, coco peat, and biological organic fertilizer, with compost or vermicompost at different proportions (v/v), based on the physicochemical properties of each substrate (

Table 1. Physicochemical properties of the substrates used in the experiment.

Parameter	Compost	Vermicompost	Commercial Substrate
Moisture (g kg−1)	511	535	415
pH (H2O) #	6.0	5.6	5.8
Organic matter (mg kg−1 DW)	860	849	749
Electrical conductivity (dS m−1) #	1.7	2.7	0.8
N (g kg−1 DW)	35.1	33.0	7.7
P (g kg−1 DW)	7.3	6.2	1.4
K (g kg−1 DW)	16.8	16.9	4.3
Ca (g kg−1 DW)	17.5	18.5	17.3
Mg (g kg−1 DW)	5.0	4.6	1.7
S (g kg−1 DW)	3.7	3.6	1.7
B (mg kg−1 DW)	25.0	30.0	10.7
Fe (mg kg−1 DW)	3429	3841	5051
Cu (mg kg−1 DW)	116	106	14
Zn (mg kg−1 DW)	352	335	43
Mn (mg kg−1 DW)	231	239	164
Ni (mg kg−1 DW)	6.7	6.1	5.5
Cd (mg kg−1 DW)	0.3	0.4	0.1
Pb (mg kg−1 DW)	15.3	15.8	7.4
Cr (mg kg−1 DW)	6.7	8.3	11.4
Hg (μg kg−1 DW)	15.2	97.6	15.4
C/N	14	15	56
NH4+-N/NO3−-N	0.271	0.013	0.055

DW: dry weight; ^# (1:5).

), determined according to standard protocols [30] at UTAD Soil and Plant Analysis Laboratory.

Both the compost and the vermicompost had a higher organic matter content and electrical conductivity values when compared to the commercial substrate. In general, they also exhibited higher contents of macronutrients, especially N, P, K, and micronutrients. The only exception was the content of Fe and Cr, which was higher in the commercial substrate, and the content of Zn, which was higher in both the compost and the vermicompost. The vermicompost presented the lowest ratio of NH₄⁺-N/NO₃⁻-N, indicating a high maturity level. The C/N ratio of the compost and the vermicompost was approximately 15, considered mature and optimal for agronomic applications [31]. The germination index (GI) values of the compost and the vermicompost revealed the absence of phytotoxicity, as evidenced by a germination rate exceeding 85% and a GI greater than 60%, compared to the control treatment with distilled water According to Portuguese legislation (Portaria nº 185/2022 de 21 July 2022), both the compost and the vermicompost fall within Group 5 (organic correctives) and class II (regarding maximum admissible levels of metals), being therefore suitable for agricultural use.

In total, seven different formulations, with varying percentages of compost (C) and vermicompost (VC), were prepared. The treatments consisted of mixtures in which 10%, 20%, or 40% of the commercial substrate was substituted by compost (C10, C20 and C40, respectively) or vermicompost (VC10, VC20 and VC40, respectively), by weight, plus a control treatment (CT) with no substitution. All treatments contained the same proportion of perlite (20%) to ensure adequate aeration. The CT consisted of 80% of the commercial substrate and 20% of perlite. The main chemical properties of the substrates are shown in

Table 2. Main chemical properties of the mixtures used in the experiment.

Substrates	Organic Matter (g kg−1 DW)	pH (H2O) #	Electrical Conductivity # (dS m−1)	Total C (g kg−1 DW)
CT	536	5.78	0.524	311
C10	525	5.54	0.959	304
C20	556	5.57	1.550	323
C40	554	5.69	1.715	321
VC10	553	6.10	0.304	321
VC20	562	5.68	0.794	326
VC40	580	5.62	1.181	364

DW: dry weight; ^# (1:5).

.

2.3. Experimental Setup

The experiment was conducted outdoors, near the University of Trás-os-Montes and Alto Douro, Vila Real, Portugal (41°30′2″ N, 7°38′55″ W; 770 m altitude), during the summer of 2024. The region has a Mediterranean climate, with average minimum and maximum temperatures of 15.2 °C and 25.0 °C, respectively, during the experimental period (from 27 July to 20 August).

Cucumber (Cucumis sativus L. cv. Market; Eurosementes, Riachos, Portugal) was selected for the trial due to its popularity in the Mediterranean region, fast growth cycle [32] and moderate tolerance to salinity, tolerating values of electrical conductivity up to 2.5 dS m⁻¹ [33], which is suitable for the compost and the vermicompost produced (Table 1). Cucumbers can also grow in a wide range of pH values but prefer slightly acidic soils [34].

Two cucumber seeds were placed per cell (30 × 30 × 65 mm) in polystyrene trays. After germination, only one seedling was kept in each cell. Each treatment was replicated five times, with 10 plants per replicate, resulting in 50 plants per treatment. The trays were watered daily with tap water and placed under natural sunlight to provide favorable conditions for germination and growth, with their positions re-randomized weekly. Seedlings were allowed to grow until they had developed two pairs of true leaves, which occurred ca. 3 weeks (22 days) after sowing, the time at which seedlings are usually transplanted. At the end, 15 randomly selected seedlings per treatment were studied. No fertilizer was added throughout the experiment to ensure that any differences in seedling performance were solely due to the substrates tested.

2.4. Data Collection

Seed germination was recorded daily for 12 days after sowing. Seeds were considered germinated when seedlings became visible above the soil (substrate) surface. The germination percentage (G, %), mean germination time (MGT, day), time to 10% of germination (t10, day), time to 50% of germination (t50, day), synchronization index (z), and germination index (GI), were determined at the end of this period, following the equations of Ranal & Santana [35].

At 22 days after sowing, fifteen seedlings per treatments were randomly selected for harvest and measurement of seedling height (H, cm), measured with a 30 cm ruler from the base to the tip of the seedling, root length (RL, cm), stem diameter (SD, mm), measured at 1 cm under the soil surface using a Vernier caliper, fresh shoot and root weight (g), and total number of leaves per plant. Known quantities of plant material were oven dried at 60 °C to a constant weight, to calculate their dry weight. Total fresh and dry weight were obtained by summing shoot and root components. Growth efficiency indices were calculated, including root/shoot ratio (R/S, g g⁻¹) calculated for each seedling, by dividing the dry matter of the shoot by the dry matter of the root, height/shoot dry matter (H/SDM, cm g⁻¹) and shoot dry matter/height (SDM/H, mg cm⁻¹) ratios were also calculated for each seedling by using its height and dry mass. Leaf area (LA, cm²) was determined using the Digimizer image analysis software (Version 5.7.2. available at http://digimizer.com), the specific leaf area (SLA, cm² g⁻¹) was calculated by dividing the leaf area by the leaf dry weight, and the leaf area ratio (LAR, cm² g⁻¹) was calculated by dividing its surface area by its dry weight. Additionally, the Dickson quality Index (DQI, g cm⁻¹ mm⁻¹) was also determined following the equation of Dickson et al. [36].

$$DQI={\displaystyle \frac{Seedling dry weight \left(\mathrm{g}\right)}{\frac{height \left(\mathrm{c}\mathrm{m}\right)}{stem diameter \left(\mathrm{m}\mathrm{m}\right)}+\frac{Shoot dry weight \left(\mathrm{g}\right)}{Root dry weight \left(\mathrm{g}\right)}}}$$

Dried shoot material was ground and analyzed for macronutrient (N, P, K, Ca, Mg, S) and micronutrient (B, Fe, Zn, Mn, Cu) contents at UTAD Soil and Plant Analysis Laboratory, following established procedures [30]. For treatments C40 and VC40, nutrient content was not determined due to insufficient available biomass for analysis.

2.5. Statistical Analysis

One-way analysis of variance (ANOVA) and comparison of means based on the Tukey HSD test at a 5% level of significance, were used to determine significant differences between treatments. All data sets satisfied the ANOVA assumptions of homogeneity of variance (Levene’s test) and normality of errors (Shapiro–Wilk test). In addition, correlations among germination traits, morphological growth traits, and shoot nutrient contents were assessed using Spearman’s rank correlation coefficients. A principal component analysis (PCA), was also performed on the average values of each trait in order to analyze the relationships and to reduce the number of traits to a more significant set. To avoid the influence of the unit of each parameter on the result, all data were standardized. C40 and VC40 were excluded from this analysis because insufficient biomass precluded mineral determinations. All statistical analyses were performed using the GraphPad Prism 10.5.0 software, developed by GraphPad Software Inc. (San Diego, CA, USA).

3. Results

3.1. Germination Performance

Cucumber seed germination was significantly influenced by substrate dose and composition, as shown in Figure 1. Twelve days after sowing, the control (CT) and VC10 achieved the highest germination percentage (G ≅ 90%), with no significant differences compared to C10, VC20 and VC40 (Figure 1a). In contrast, higher compost proportions (C20 and C40) significantly reduced germination with declines of 59% and 50%, relative to CT and VC10, respectively. Mean germination time (MGT) did not differ significantly among treatments (Figure 1b), although seeds from C40 and VC40 tended to germinate later. In agreement, a similar pattern was observed for the time to 10% (t₁₀) and 50% (t₅₀) germination (Figure 1c,d), with both parameters delayed in C40 and VC40 compared to other treatments. The synchrony index (Z) remained unaffected (Figure 1e), but the germination index (GI) was significantly higher in CT and VC10 than in C20, C40, and VC40, which showed reductions of about 56%, 71%, and 58%, respectively (Figure 1f).

Overall, a low vermicompost proportion (VC10) in the substrate enhanced cucumber germination and vigor, whereas higher doses of compost or vermicompost negatively affected germination performance.

3.2. Morphological Traits and Seedling Quality

At 22 days after sowing, substrate composition significantly influenced all morphological traits of cucumber seedlings (Figure 2). Seedlings in VC10 exhibited superior performance, being much taller (H ≈ 8 cm), representing an increase of 22–46% compared to the other treatments (Figure 2a). The lowest values (≈3.5 cm) occurred in the substrates containing higher proportions of compost (C20 and C40). The stem diameter (Figure 2b) value was highest in VC10 cucumbers, with no significant differences from CT, C10 and VC20. In contrast, C40 and VC40 significantly reduced cucumbers diameters (≈2 mm). For root length (Figure 2c), VC10 and CT recorded the longest roots (≈7 cm), while all other treatments (C10, C20, C40, VC20, and VC40) showed significantly reduced root growth, reaching less than 5 cm on average. The number of leaves (Figure 2d) was similar across treatments, except in C40, where seedlings produced only 3 leaves. Seedling dry matter accumulation (Figure 2e) was highest in VC10 (≈0.25 g) with no significant differences from C10, whereas CT, C20, VC20, and especially VC40 had significantly lower biomass, with C40 displaying the minimum value (≈0.04 g). Finally, the leaf area (Figure 2f) was highest in VC10 (≈19 cm²), statistically similar to CT. In contrast, C40 strongly inhibited leaf area development, showing reductions of up to 70% compared to VC10.

Seedling quality indices (Figure 3) further highlighted these differences. The root/shoot ratio (Figure 3a) was highest in VC10 and C10, with no significant difference from C20. Conversely, C40 and VC40 had the lowest values, approximately 70% lower than VC10 and CT. The height/shoot dry matter ratio (H/SDM) was significantly higher in C40 compared to all other treatments (Figure 3b), whereas the shoot dry matter/height ratio (SDM/H) followed the opposite trend, with significantly higher values in C20, VC10, and VC20 (Figure 3c). Seedlings from CT, VC10, and VC20 exhibited the largest specific leaf area (SLA) values (Figure 3d), whereas VC40 showed a reduction of nearly 40% compared to CT. The leaf area ratio (LAR) was highest in CT and similar in C40 and VC40 (Figure 3e). Finally, the Dickson Quality Index (SQI) was highest in C10 and VC10, whereas C40 exhibited a drastic reduction of this index, of nearly 90% compared to VC10, indicating severely compromised seedling quality.

3.3. Shoot Nutrient Content and Integrative Analyses

Macronutrient and micronutrient contents in cucumber shoots varied significantly among substrates (

Table 3. Content of macronutrients (g kg⁻¹ dry weight) in the shoots of cucumber seedlings, 22 days after sowing. Values represent mean ± SD (n = 2). Different lowercase letters denote significant differences among substrates (p < 0.05) according to Tukey’s test. CT—control; C10, C20—10%, and 20% compost substitution; VC10, VC20—10%, and 20% vermicompost substitution. For C20, only one analysis was performed due to limited leaf material, and it was not included in the ANOVA analysis.

Substrates	N	P	K	Ca	Mg	S
CT	44.28 ± 0.71 c	9.54 ± 0.14 b	80.38 ± 0.78 b	22.27 ± 0.02 b	6.45 ± 0.05 c	10.13 ± 0.15 a
C10	49.85 ± 0.46 b	10.52 ± 0.13 a	79.07 ± 0.67 b	22.89 ± 0.39 b	7.76 ± 0.07 b	8.15 ± 0.10 c
C20	56.37 (n = 1)	10.73 (n = 1)	90.40 (n = 1)	18.43 (n = 1)	8.41 (n = 1)	7.50 (n = 1)
VC10	45.92 ± 0.50 c	9.52 ± 0.04 b	91.17 ± 2.07 a	25.69 ± 0.04 a	7.76 ± 0.02 b	8.70 ± 0.10 b
VC20	56.11 ± 0.29 a	9.90 ± 0.04 b	89.10 ± 0.72 a	22.76 ± 0.19 b	8.66 ± 0.05 a	7. 64 ± 0.12 d

and

Table 4. Content of micronutrients (mg kg⁻¹ dry weight) in the shoots of cucumber seedlings, 22 days after sowing. Values represent mean ± SD (n = 2). Different lowercase letters denote significant differences among substrates (p < 0.05) according to Tukey’s test. CT—control; C10, C20—10%, and 20% compost substitution; VC10, VC20—10%, and 20% vermicompost substitution. For C20, only one analysis was performed due to limited leaf material, and it was not included in the ANOVA analysis.

Substrates	B	Fe	Zn	Mn	Cu
CT	47.94 ± 0.22 a	570.36 ± 0.82 b	194.00 ± 0.84 a	69.22 ± 1.08 c	19.52 ± 0.35 b
C10	46.59 ± 0.77 ab	519.63 ± 8.71 c	146.71 ± 0.78 d	72.82 ± 0.31 bc	22.19 ± 0.73 b
C20	43.52 (n = 1)	693.55 (n = 1)	144.15 (n = 1)	70.92 (n = 1)	27.41 (n = 1)
VC10	45.95 ± 0.23 ab	53.56 ± 0.26 c	172.84 ± 2.47 c	74.57 ± 1.36 ab	22.01 ± 0.62 b
VC20	45.07 ± 0.66 b	600.42 ± 2.40 a	187.24 ± 0.87 b	77.67 ± 1.02 a	26.82 ± 1.20 a

). In general, substrates containing compost tended to promote the accumulation of N, P, and Fe, especially in C20. However, this result must be interpreted with caution, as nutrient data for C20 were based on a single analysis, which limits the statistical comparison with the other treatments. In contrast, substrates with vermicompost, especially VC10, increased K and Ca, contents, while VC20 resulted in the highest Mg, Fe, and Cu contents compared with the CT.

Figure 4 shows the correlation matrix among germination traits, seedling growth parameters, and shoot nutrient contents in cucumber seedlings. Overall, growth parameters such as shoot height (H), root length (RL), stem diameter (SD), leaf area (LA), and shoot dry matter (SDM) were positively correlated with each other, as well as with germination traits, particularly, germination percentage (G), germination index (GI), and the Dickson quality index (DQI), indicating that larger seedlings were associated with higher germination performance and overall quality. These traits were also positively correlated with shoot nutrients, particularly Ca, S, and B, but negatively correlated with N, P, Mg, Fe, and Cu.

The principal component analysis (PCA), based on all data, revealed a clear separation among treatments (Figure 5). The first two principal components explained 91.46% of the total variance, with PC1 accounting for 70.42% and PC2 for 21.04%.

Along PC1, treatments were mainly separated by seedling growth performance and nutrient accumulation in shoots. The control (CT) and VC10 were positioned on the negative side of PC1, closely associated with root length (RL), leaf area (LA), and Ca content, indicating better overall growth performance. In contrast, C20 was strongly associated with P content, while C10 and VC20 were located in intermediate positions, showing moderate growth and nutrient accumulation. PC2 further distinguished treatments based on biomass allocation patterns and nutrient content. PC2 further distinguished treatments based on biomass allocation patterns. Traits such as the specific leaf area (SLA), height/shoot dry matter ratio (H/SDM), and P content loaded negatively, while Mg and Ca contents loaded positively, confirming variability in nutrient-use efficiency among treatments.

Overall, the PCA confirms that low vermicompost substitution (VC10) behaves most similarly to the CT, supporting optimal growth performance and balanced nutrient uptake. On the other hand, higher compost (C20) and vermicompost (VC20) doses are associated with distinct nutrient dynamics, particularly higher P and Mg contents, and reduced growth performance.

4. Discussion

This study shows that the composition of the growth substrate strongly influences cucumber seed germination, early seedling growth, and shoot nutrient dynamics. Among the organic amendments tested, vermicompost generally produced a better-performing substrate than compost. Similar results were reported by Gong et al. [24], Atiyeh et al. [37], and Peña et al. [38], who observed that vermicompost improved the overall quality of ornamental and vegetable plants compared with composted materials.

Among the tested formulations, replacing 10% of the commercial substrate with vermicompost (VC10) was the most effective, improving germination percentage, seedling vigor, and overall seedling quality, with performance comparable to, and in some cases exceeding, the control (CT). These findings suggest that, in this study, VC10 is the best alternative for partially replacing peat in the cultivation of cucumber seedlings. In contrast, higher proportions of compost or vermicompost, particularly C40 and VC40, were associated with reduced germination and seedling growth. This highlights the importance of optimizing amendment doses to achieve better seedling establishment [26,27,29]. However, the optimal level of compost or vermicompost will depend on their elemental composition and the plant species. Previous studies have reported favorable effects of 20–40% vermicompost for eggplant and pepper [39] and of 20–50% for lettuce [40], illustrating the species- and substrate-specific nature of these responses.

In this study, replacing peat by low proportions of vermicompost (VC10) or compost (C10) did not impair germination, as evidenced by germination percentage (G) and germination index (GI) values that were comparable to those of the control (CT). This result is consistent with previous findings of Rivera et al. [41], who reported similar emergence rates in watermelon seedlings when peat was replaced with vermicompost. Conversely, germination was significantly reduced in C20, and especially in C40 and VC40, suggesting that excessive doses of organic amendments may cause osmotic stress or nutrient imbalances that negatively affect seed development [42,43]. Similar inhibitory effects of excessive compost or vermicompost on germination have been reported in cucumber [43] and tomato [29]. In the present study, higher electrical conductivity recorded in compost-rich substrates (C20 and C40) may have imposed salinity stress, restricting water uptake during germination. By contrast, the lowest electrical conductivity value in VC10 (0.30 dS m⁻¹) corresponded with improved germination and vigor. Substrate pH values (5.5–6.1) remained within cucumber’s optimal range [34], suggesting that salinity, rather than acidity, was the main limiting factor in high-compost substrates.

Beyond germination, the growth and morphological traits of cucumber seedlings were strongly affected by substrate composition. Seedlings grown in VC10 exhibited greater height (H), root length (RL), leaf area (LA), stem diameter (SD), and biomass accumulation (DM) than those from other treatments, reflecting higher vigor and balanced growth. These results align with those of Tsakaldimi et al. [44], who demonstrated that seedlings with thicker stems and higher biomass typically show better field survival.

The larger LA in VC10 seedlings suggests a higher photosynthetic capacity, which may be linked to bioactive compounds and beneficial microorganisms present in vermicompost that stimulate plant development [45]. The positive correlation between LA and DM in this study further indicates that seedlings with larger leaves also tend to accumulate more biomass, improving their potential for vigorous establishment in the field [46], which reinforces the role of vermicompost in promoting vigorous seedling establishment. Similarly, the Dickson quality index (DQI), a key measure of seedling robustness, was highest in VC10, indicating that these seedlings were more robust and likely to perform well after transplantation [47]. The root/shoot ratio (R/S) was also highest in VC10, suggesting a better balance between water uptake capacity and shoot growth, a critical factor for survival under variable field conditions [44]. Moreover, the shoot dry matter/height ratio (SDM/H) indicated that VC10 seedlings were more compact and robust, traits also associated with greater field success [47]. In contrast, C20 and VC20, and particularly C40 and VC40, produced smaller, weaker seedlings with lower biomass accumulation and less favorable allocation patterns, as indicated by higher H/SDM and lower DQI values. These traits reduce transplant success and long-term growth potential.

Nutrient analysis in cucumber shoots revealed that C20 and VC20 resulted in higher N, K, Mg, Fe and Cu contents. The strong positive correlations between these macro- and micronutrients suggest that increased macronutrient contents may have limited the uptake of other nutrients from the substrate, resulting in the highest micronutrient levels in the root zone [48]. However, the highest nutrient contents were not accompanied by better growth performance, suggesting that excessive nutrient accumulation can negatively affect nutrient-use efficiency [49].

Correlation analysis also revealed that Ca and K contents were positively associated with seedling vigor traits such as RL, LA, DM, and DQI. In contrast, P and Mg showed negative correlations with these variables. These findings suggest possible antagonistic interactions between P and Mg and other essential nutrients, which may explain the poorer growth in C20 and VC20 substrates despite higher nutrient concentrations [50]. This interpretation is consistent with previous reports showing that excessive P or Mg may interfere with the uptake of other essential nutrients for growth [24].

The PCA results synthesized these patterns, confirming the trends observed in univariate and correlation analyses. VC10 clustered closely with CT, associating with traits indicative of vigorous growth and balanced Ca/K nutrition. Conversely, C20 and VC20 were separated along the first principal component, aligned with higher P and Mg contents, respectively, but positioned opposite to growth-related traits, highlighting their lower seedling quality. C10 occupied an intermediate position, reflecting its moderate growth performance and nutrient uptake. Together, these analyses confirm that peat replacement by low vermicompost additions in the substrates may have facilitated a higher capacity for nutrient absorption while minimizing the potential for blockages [51], thereby supporting optimal growth and nutrient balance for cucumber seedlings. In contrast, higher amendment content disrupted nutrient dynamics and compromised seedling performance.

Although all substrates contained sufficient quantities of macro- and micronutrients, this does not guarantee the availability of nutrients to plants, which depends on factors such as dissolved salts, pH, moisture [52], biological stability of the substrate, and plant adsorption capacity [53]. The content of metals in compost and vermicompost was within legal limits for soil application. However, the long-term use of amendments derived from sewage sludge requires monitoring of metal dynamics to ensure environmental safety, especially under repeated application scenarios [54]. Overall, these results emphasize that substrate quality depends not only on adequate nutrient provision but also on physicochemical properties such as pH and electrical conductivity. Organic amendments represent sustainable alternatives to peat, but their benefits are maximized only when applied at optimized rates, highlighting the importance of dose optimization in nursery practices.

5. Conclusions

This study highlights the potential of compost and, particularly, vermicompost derived from vine pruning and sewage sludge as sustainable alternatives to peat in cucumber seedling production. Replacing 10% of peat with vermicompost significantly enhanced cucumber germination, height, root development, and biomass accumulation, while maintaining balanced nutrient profiles and low electrical conductivity. The results showed that VC10 was closely associated with growth-related traits, whereas higher proportions of compost or vermicompost in the substrate were aligned with higher electrical conductivity and altered nutrient dynamics, which reduced seedling quality. These findings demonstrate that substrate performance is determined not solely by nutrient content but by the balance between nutrient supply and plant growth. A small amount of vermicompost can partially replace peat without compromising and even improving seedling quality. This contributes to more sustainable nursery practices and promotes the valorization of agricultural (vine pruning) and urban (sewage sludge) residues in line with circular economy principles.

Future research should explore the physiological and microbial mechanisms underlying these effects, with particular focus on nutrient interactions, bioactive compounds, and osmotic regulation in amended substrates, to further optimize organic amendment strategies for sustainable seedling production.