Reduction in Peat Usage in Container Production of Cherry Laurel (Prunus laurocerasus): Effects of Biochar and Compost Amendments on Substrate Quality and Plant Growth

Abstract

With increasing emphasis on sustainable horticulture, optimizing substrate composition is essential to reduce peat usage in container production. This study evaluated the effects of biochar and compost amendments on the growth and nutrient status of cherry laurel (Prunus laurocerasus) in two separate experiments conducted over five months. Experiment I assessed growth in pure peat and in peat–compost blends at volume ratios of 100:0, 70:30, 50:50, 30:70 and 0:100. Experiment II investigated the effect of adding biochar to a pure peat substrate at rates of 3 g·dm⁻³ and 5 g·dm⁻³. Key parameters were monitored, including the above and below-ground biomass, leaf and shoot counts, chlorophyll content, and the chemical composition of plant tissue and substrate. Compost addition increased the substrate pH from ~4.6 to ~6.4, while electrical conductivity increased with a higher compost content, reaching values approximately 2–3 times greater than in pure peat. Nutrient levels (Ca, K, Mg, P, NO₃⁻) also rose consistently with an increasing compost share. While a higher compost content generally reduced the biomass, leaf and shoot number, the greatest plant height and relatively favorable biomass were observed at 30% and 50% compost mixtures. Biochar addition slightly increased plant height, while the total biomass, root mass, and shoot number tended to decrease compared to pure peat, particularly at the lower biochar dose (3 g·dm⁻³). The substrate pH remained relatively stable, whereas electrical conductivity (EC) showed a slight upward trend with increasing biochar levels. Biochar also slightly increased the substrate nutrient content (Ca, K, Mg, P, NO₃⁻).

1. Introduction

The growing emphasis on environmentally sustainable horticultural practices has prompted the search for alternatives to peat, which, despite its favorable physicochemical properties, is a non-renewable resource whose extraction is associated with significant environmental degradation [1]. In Europe, regulatory and market pressures are accelerating efforts to reduce peat use in horticultural production systems [2]. In container-grown ornamentals such as Prunus laurocerasus, traditional practices are being re-evaluated in favor of more environmentally friendly approaches.

Cherry laurel (Prunus laurocerasus L.) is an evergreen shrub widely used in ornamental horticulture, especially in urban greenery, hedging, and landscape design, due to its aesthetic foliage and adaptability to varied environmental conditions. Its characteristics make it a practical model species for substrate evaluation in nursery-grown broadleaf evergreens.

In recent studies, attention has also turned to alternative raw materials such as wood fiber, coconut coir and compost, of which compost, owing to its high production volume, frequently constitutes the principal component of growing media formulations [3]. Moreover, investigations are being conducted into the valorization of agricultural by-products, such as rice straw [4] and miscanthus [5,6], which demonstrate considerable promise as renewable substitutes for peat in horticultural substrates. Simultaneously, researchers are investigating the incorporation of organic amendments like biochar as potential substitutes or supplements to peat. Many studies suggest that biochar and compost amendments can enhance water retention and nutrient availability [7,8]. Recent studies have highlighted the multiple agronomic benefits of biochar application, including improved soil structure, pH regulation, enhanced nutrient retention, and increased plant growth and yield. Miroshnichenko et al. [9] emphasized that these advantages are particularly evident when biochar is combined with compost or fertilizers, contributing to the long-term sustainability of agricultural systems. Although the use of composted materials in substrate formulations has been extensively studied, further investigation is warranted into the properties and performance of municipal green-waste compost, which is derived from pruning residues, lawn clippings, and other biomass from urban park and roadside maintenance. This abundant and locally available resource remains relatively underrepresented in container cultivation research. Exploring its potential as a partial peat substitute may yield valuable insights and foster more sustainable, circular horticultural practices by recycling urban biomass back into nursery production.

Recent studies have explored the effects of alternative substrate components on the growth of Prunus laurocerasus. For instance, Tozzi et al. [10] investigated the use of phytoremediated dredged sediments as partial substitutes for peat and coconut fiber in container-grown cherry laurel. Their findings indicated that incorporating 25–50% treated sediment into the substrate did not adversely affect vegetative growth or the physiological parameters, suggesting its viability as a sustainable alternative. Similarly, Frangi et al. [11] assessed the use of wood fiber as a peat substitute in growing media for Prunus laurocerasus ‘Rotundifolia’. The study demonstrated that incorporating 25–50% wood fiber, alongside appropriate adjustments in clay content to maintain water retention, supported satisfactory plant growth, indicating its suitability in reducing peat dependence.

The purpose of this study is to evaluate the impact of different substrate compositions on the growth of cherry laurel shrubs over a five-month period. Two separate experiments were conducted to assess the effects of compost and biochar as peat amendments. Promising results were observed in substrates containing 30% compost, which increased the average plant height while only slightly reducing the biomass, shoot number, and leaf count compared to pure peat. The addition of biochar also contributed to increased plant height and biomass, but the effects were not statistically significant. Furthermore, both compost and biochar amendments influenced the substrate’s properties, particularly its nutrient availability, pH, and electrical conductivity. These findings highlight the potential of compost and biochar to partially replace peat in container-grown nursery production, offering a more sustainable approach to ornamental horticulture.

2. Materials and Methods

2.1. Experimental Site and Climatic Data

The experiments, conducted in a greenhouse tunnel in Biała-Kopiec in Poland (51°16′22.0″ N, 18°26′10.0″ E) over a five-month period, started in the last decade of May and were completed by the end of October. The average monthly air temperatures at the experimental site during the trial period (May to October 2022) were as follows: May—14.4 °C, June—19.1 °C, July—19.4 °C, August—20.7 °C, September—12.8 °C and October—12.0 °C [12].

2.2. Materials Used

Uniform Prunus laurocerasus ‘Caucasica’ shrubs were planted in 3 L containers, with all plant material sourced from commercially available nurseries.

The compost used in Experiment I was a municipal green-waste compost derived from urban park and roadside maintenance residues (mown grass, fallen leaves and pruning branches). It underwent a rapid, thermophilic composting process (≈70 °C) lasting approximately three weeks. The resulting material had a C/N ratio of 12.7 and contained 1.144% organic carbon.

The biochar applied in Experiment II was produced by the torrefaction of orchard pruning residues at 300 °C for 20 min, yielding particles sieved to 1–5 mm. The biochar had a measured pH of 7.2. Prior to being incorporated into the substrate, it was pre-soaked in deionized water at 20 °C for 2 h to ensure full saturation.

2.3. Experimental Design and Setup

The experiments were set up using the randomised block method; a single combination consisted of 12 plants from three replicates, with four plants in each replicate.

Two separate experiments were established: Experiment I assessed the effect of substrate composition by comparing pure peat with blends of peat and compost (100:0, 70:30, 50:50, 30:70 and 0:100 by volume) and Experiment II examined the influence of biochar addition to pure peat substrate by applying two different (3 g·dm⁻³ and 5 g·dm⁻³) biochar dosages.

Substrate mixtures were prepared by thoroughly homogenizing the component materials in defined volume ratios. Each substrate combination was used to fill 12 containers, with each replicate consisting of four plants. In both experiments, 14.1 g of Osmocote 5 fertilizer was applied to each container.

2.4. Sample Collection and Analyses

Plant growth was monitored by measuring key parameters: above-ground and below-ground biomass, leaf and shoot counts, and chlorophyll content. Biomass was determined by harvesting the respective plant parts, removing residual substrate, and weighing immediately after harvest. Leaf and shoot counts were conducted manually. The plant material and substrate used for laboratory analyses were collected at two time points: in the last ten days of July and at the end of each experiment, at the end of October.

For each individual combination, a composite leaf sample was prepared by pooling equal leaves from the five collection points out of all plants in combination. From this composite sample, three independent replicates (each weighing 0.4 g of fresh shoot mass) were processed for chlorophyll determination. Each replicate was ground in a mortar with a mixture of calcined sand and calcium carbonate along with 80% acetone. The resulting mixture was then filtered through a Schott filter using a vacuum pump and topped up to a volume of 50 mL. The absorbance for each sample was then read on a spectrophotometer at wavelengths of 645 nm and 663 nm. The chlorophyll a, b and a + b content was subsequently calculated using the following formulas [13]:

$${\mathrm{C}}_{\mathrm{a}}={\displaystyle \frac{(12.7·{\mathrm{A}}_{663}-2.69·{\mathrm{A}}_{645})·\mathrm{V}}{\mathrm{m}}}$$

(1)

$${\mathrm{C}}_{\mathrm{b}}={\displaystyle \frac{(22.9·{\mathrm{A}}_{645}-4.68·{\mathrm{A}}_{663})·\mathrm{V}}{\mathrm{m}}}$$

(2)

$${\mathrm{C}}_{\mathrm{a}+\mathrm{b}}={\displaystyle \frac{(20.2·{\mathrm{A}}_{645}+8.02·{\mathrm{A}}_{663})·\mathrm{V}}{\mathrm{m}}}$$

(3)

where:

C_a—chlorophyll a content in the sample, expressed in milligrams of chlorophyll per gram of plant material (mg g⁻¹);

C_b—chlorophyll b content in the sample, expressed in milligrams of chlorophyll per gram of plant material (mg g⁻¹);

C_a+b—total chlorophyll (a + b) content in the sample, expressed in milligrams of chlorophyll per gram of plant material (mg g⁻¹);

A₆₆₃—absorbance measured at 663 nm wavelength;

A₆₄₅—absorbance measured at 645 nm wavelength;

V—volume of 80% acetone (extraction solvent) used in cubic decimeters (dm³);

m—mass of the plant material used for extraction, in grams (g).

The leaves were also analyzed for their phosphorus, calcium, potassium, and magnesium content using Nowosielski’s universal method in 2% acetic acid. For each combination, a composite leaf sample (as described above) was used to prepare three independent replicates. In each replicate, 0.4 g of dry plant material was weighed, transferred to a flask, and combined with 100 mL of 2% acetic acid and half a spoon of activated carbon. The flasks were then placed on a shaker for 30 min at 150 rpm. The resulting solutions were filtered into 200 mL conical flasks using paper filters, after which the analyses were performed. Magnesium and phosphorus were determined by the colorimetric method, and calcium and potassium by flame photometry. Additionally, the precision of these methods was confirmed by using control samples with known concentrations.

The substrate was analyzed for phosphorus, calcium, potassium, and magnesium using Nowosielski’s universal method in 0.03 M acetic acid. First, 20 mL of soil in its natural moisture state was collected, then 200 mL of 0.03 M acetic acid was added, and the flasks with the contents were placed on a shaker for 30 min at 150 rpm. The solutions were filtered into 200 mL conical flasks, and analyses were performed. Magnesium and phosphorus were determined by the colorimetric method, while calcium and potassium were determined by flame photometry. As with the analysis of leaves, control samples were used to validate the accuracy and precision of the measurements.

The nitrate content in the leaves and substrate was also analyzed. The filtered solutions previously prepared for phosphorus, calcium, potassium, and magnesium determination were used, then an ionometer was prepared for measurements, and after proper calibration, measurements for each sample were taken.

The electrical conductivity (EC) of the substrate was analyzed at 22 °C using an Orion model 142 conductometer (Thermo Scientific Orion, Waltham, MA, USA), and the soil pH was analyzed using an Elmetron CPI-501 pH meter (Elmetron, Zabrze, Poland) in a soil and distilled water solution in a 1:2 ratio.

To determine the dry mass, a representative sample comprising 30 leaves was collected from each replicate and weighed to obtain the fresh mass. The leaves were subsequently placed in a drying oven at 70 °C until completely dehydrated. Once dried, the material was weighed again to obtain the dry leaf mass, and the percentage of dry mass relative to the fresh mass was calculated. The same procedure was applied to the roots: after carefully removing any substrate residues, the roots were initially weighed, then dried in the oven at 70 °C, weighed again, and the dry mass content was calculated.

The substrate was analyzed before being mixed with fertilizer.

Table 1. The chemical parameters of the substrate analyzed in May at the beginning of the Experiment I before the fertilizer was added.

No.	Substrate Combination 1	pH	EC [μS·cm−1]	Ca Content [mg·dm−3]	K Content [mg·dm−3]	P Content [mg·dm−3]	Mg Content [mg·dm−3]	NO3− Content [mg·dm−3]
1.	100% P	4.52	82.13	484.86	524.66	23.00	30.00	2.50
2.	70% P + 30% C	5.67	1160.00	803.78	998.84	118.50	74.00	3.40
3.	50% P + 50% C	5.83	1650.00	1122.70	1337.53	173.00	116.00	3.70
4.	30% P + 70% C	6.33	2130.00	1441.62	1743.97	237.00	192.00	4.40
5.	100% C	7.36	3570.00	1696.75	2285.89	319.50	240.00	4.10

¹ P—peat, C—compost.

and

Table 2. The chemical parameters of the substrate analyzed in May at the beginning of the Experiment II before the fertilizer was added.

No.	Substrate Combination 1	pH	EC [μS·cm−1]	Ca Content [mg·dm−3]	K Content [mg·dm−3]	P Content [mg·dm−3]	Mg Content [mg·dm−3]	NO3− Content [mg·dm−3]
1.	100% P	4.52	82.13	484.86	524.66	23.00	30.00	2.50
2.	100% P + biochar [3 g·dm−3]	4.49	62.40	644.32	619.49	20.50	16.00	1.30
3.	100% P + biochar [5 g·dm−3]	5.17	97.60	803.78	822.71	40.00	20.00	1.40

¹ P—peat.

show the parameters of the investigated substrates.

2.5. Statistical Analysis

The collected data were analyzed using one-way analysis of variance (ANOVA) to compare treatment effects and determine homogeneous groups. Differences were considered statistically significant at a level of p ≤ 0.05. Homogeneous groups were identified by Tukey’s post hoc test. The statistical analysis were performed in Statistica software version 14.0 (TIBCO Software Inc., Palo Alto, CA, USA) [14].

3. Results

3.1. Effect of Compost Amendment in Peat-Based Substrate (Experiment I)

Experiment I involved studying the effect of the compost content in the substrate on the growth of eastern laurel cherry. Detailed results are presented in

Table 3. Selected biometric characteristics in Prunus laurocerasus cultivated in media supplemented with municipal compost: height (cm); number of leaves (pcs); number of shoots (pcs); plant mass (g); above-ground mass (g); below-ground mass (g); leaf dry mass (%); root dry mass (%) ¹.

No.	Substrate Combination 2	Height [cm]	Number of Leaves [pcs]	Number of Shoots [pcs]	Plant Mass [g]	Above-Ground Mass [g]	Below-Ground Mass [g]	Dry Mass of Leaves [%]	Dry Mass of Roots [%]
1	100% P	65.75 bc	103.75 c	11.83 c	588.69 b	182.98 c	405.71 b	36.27 b	27.35 a
2	70% P + 30% C	70.25 bc	94.17 bc	8.25 b	376.22 ab	180.68 c	195.54 a	37.05 b	39.88 abc
3	50% P + 50% C	72.33 c	81.50 ab	7.75 b	342.77 a	157.78 bc	184.99 a	35.43 ab	51.20 c
4	30% P + 70% C	58.67 ab	81.17 ab	7.58 b	296.03 a	118.79 ab	177.24 a	35.76 ab	46.32 bc
5	100% C	46.58 a	63.08 a	6.17 a	357.32 a	78.53 a	278.79 ab	33.50 a	33.52 ab

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat, C—compost.

,

Table 4. Chlorophyll content (mg·100 g⁻¹ fresh weight) in leaves of plants grown in substrates with the addition of municipal compost ¹.

No.	Substrate Combination 2	Chlorophyll a [mg·100 g−1]	Chlorophyll b [mg·100 g−1]	Chlorophyll a + b [mg·100 g−1]
1	100% P	32.69 a	313.29 a	345.98 a
2	70% P + 30% C	29.38 a	305.61 a	334.99 a
3	50% P + 50% C	28.01 a	315.39 a	343.40 a
4	30% P + 70% C	27.54 a	320.42 a	347.96 a
5	100% C	27.88 a	293.62 a	321.50 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat, C—compost.

,

Table 5. Macroelements (Ca, K, Mg, P) content (mg·100 g⁻¹ dry weight) and nitrate (NO₃⁻) content (mg·100 g⁻¹ fresh weight) in the plant material of eastern laurel cherry cultivated in a substrate with the addition of municipal compost ¹.

No.	Substrate Combination 2	Ca Content-July [mg·100 g−1]	Ca Content-October [mg·100 g−1]	K Content-July [mg·100 g−1]	K Content-October [mg·100 g−1]	Mg Content-July [mg·100 g−1]	Mg Content-October [mg·100 g−1]	P Content-July [mg·100 g−1]	P Content-October [mg·100 g−1]	NO3− Content-July [mg·100 g−1]	NO3− Content-October [mg·100 g−1]
1	100% P	423.53 a	256.76 a	693.27 b	615.37 c	128.33 a	118.33 ab	333.92 c	226.25 bc	304.104 a	368.510 a
2	70% P + 30% C	481.13 a	264.73 a	685.00 b	696.61 d	133.33 a	138.33 b	304.17 bc	200.42 b	378.980 ab	385.214 a
3	50% P + 50% C	481.13 a	243.47 a	667.78 b	555.22 bc	111.67 a	140.00 b	256.67 b	200.83 bc	369.166 ab	351.990 a
4	30% P + 70% C	431.76 a	240.81 a	598.91 a	495.04 ab	110.00 a	98.33 a	164.17 a	155.42 a	563.043 c	536.121 b
5	100% C	456.44 a	227.52 a	676.39 b	431.03 a	120.00 a	96.67 a	118.33 a	141.67 a	425.158 b	433.057 ab

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat, C—compost.

,

Table 6. pH and electrical conductivity (EC, µS·cm⁻¹ at 22 °C) of substrates with the addition of municipal compost in the cultivation of eastern laurel cherry ¹.

No.	Substrate Combination 2	pH in July	pH in October	EC (22 °C) in July [µS·cm−1]	EC (22 °C) in October [µS·cm−1]
1	100% P	4.64 a	4.54 a	1780.67 a	830.33 a
2	70% P + 30% C	4.95 a	5.56 b	2306.67 ab	1364.33 a
3	50% P + 50% C	5.34 b	5.83 b	2129.33 a	1765.33 ab
4	30% P + 70% C	5.89 c	6.40 c	3026.67 ab	1783.00 ab
5	100% C	6.39 d	6.43 cd	4080.00 b	2695.00 b

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat, C—compost.

and

Table 7. Macroelements (Ca, K, Mg, P) and nitrate (NO₃⁻) content (mg·dm⁻³) in the substrate with the addition of municipal compost in container cultivation of eastern laurel cherry ¹.

No.	Substrate Combination 2	Ca Content-July [mg·dm−3]	Ca Content-October [mg·dm−3]	K Content-July [mg dm−3]	K Content-October [mg dm−3]	Mg Content-July [mg dm−3]	Mg Content-October [mg dm−3]	P Content-July [mg dm−3]	P Content-October [mg·dm−3]	NO3− Content-July [mg·dm−3]	NO3− Content-October [mg·dm−3]
1	100% P	617.00 a	632.22 ab	359.24 a	145.37 a	52.00 a	41.33 a	107.00 a	16.50 a	110.00 a	49.67 a
2	70% P + 30% C	1264.34 b	695.07 b	971.77 ab	280.66 ab	120.67 b	119.33 b	145.67 ab	136.33 bc	203.00 abc	35.33 a
3	50% P + 50% C	1357.45 b	488.50 a	976.34 ab	498.63 bc	147.33 b	103.33 b	179.00 b	91.83 b	159.67 ab	78.00 ab
4	30% P + 70% C	1517.06 b	609.75 ab	1616.09 b	701.56 cd	210.00 c	135.33 bc	276.83 c	124.83 bc	251.00 bc	117.00 b
5	100% C	2328.45 c	677.11 ab	2749.37 c	904.49 d	246.00 c	169.33 c	353.67 d	177.67 c	286.33 c	249.00 c

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat, C—compost.

.

The greatest growth was observed in plants grown in substrate no. 3, where peat and compost were mixed in a volumetric ratio of 1:1. In these plants, the height was approximately 26 cm greater than that of the eastern laurel cherry grown in substrate no. 5. For the number of leaves, the highest value was obtained from the control substrate, in which the eastern laurel cherry was cultivated in pure peat without any compost addition, while the lowest value was recorded in plants grown in pure compost.

Regarding the number of shoots, plants growing in the control substrate exhibited the highest number, which was almost 90% higher compared to the lowest number of shoots obtained in the compost substrate. Meanwhile, the peat–compost mixtures showed intermediate values in terms of shoot count. The total plant mass was greatest in peat, and the lowest was in substrate no. 4, which was dominated by compost.

Analyzing the mass of the above-ground parts, plants grown in compost had the lowest value, with the highest value found in plants from substrate no. 1. The mass of the underground parts was also highest for substrate no. 1 and lowest for substrate no. 4. Additionally, the eastern laurel cherry cultivated in peat consistently demonstrated the highest values in both cases. When considering the sum of both components, plants grown in substrate no. 2 had an intermediate mass, although it was over 200 g lower compared to the highest value. The lowest dry leaf mass was found in plants grown in compost, while the highest was in those grown in peat and in substrates with a peat-dominant composition. The dry root mass was highest for combination no. 3, being 88% higher than the lowest value observed in plants grown in peat.

The chlorophyll a content varied from 27.54 mg·100 g⁻¹ in Substrate 4 (30% P + 70% C) to 32.69 mg·100 g⁻¹ in Substrate 1 (100% P), while chlorophyll b ranged between 293.62 mg·100 g⁻¹ (Substrate 5, 100% C) and 320.42 mg·100 g⁻¹ (Substrate 4). Total chlorophyll (a + b) spanned 321.50–347.96 mg·100 g⁻¹ across all treatments. None of these differences reached statistical significance.

The calcium content in the leaves was clearly lower in October compared to the study conducted in July. However, individual combinations did not differ significantly. The calcium content was found to be the lowest in July for combination no. 4, while the remaining combinations did not show significant differences. In October, substrate no. 5 recorded the lowest value, whereas the highest content was observed in substrate 2 in October. For magnesium, no significant differences were observed in July; in October, however, the highest content was found in substrates 2 and 3 and the lowest was in substrates dominated by compost—numbers 4 and 5. The phosphorus content in July was highest for pure peat and decreased with an increasing amount of compost in the substrate, reaching the lowest value for pure compost. A similar pattern was seen in October, although there was a clearly noticeable decline in content for every substrate combination, except the pure compost substrate. In the case of nitrates, the lowest value in July was recorded for pure peat, while the highest was for combination no. 4. In October, the highest value was also found in substrate no. 4, whereas the lowest was in substrate no. 3, although it did not differ statistically significantly from the substrates with a predominance of peat (1 and 2).

The substrate pH increased from July to October in all treatments except Substrate 1 (100% peat). In July, the pH ranged from 4.64 (Substrate 1) to 6.39 (Substrate 5, 100% compost), whereas in October it spanned 4.54–6.43 (Table 6). The largest seasonal rise occurred in Substrate 2 (70% P + 30% C), from 4.95 to 5.56, and Substrate 4 (30% P + 70% C), from 5.89 to 6.40. Electrical conductivity (EC) followed a similar pattern: in July, the EC varied between 1780.67 µS·cm⁻¹ (Substrate 1) and 4080.00 µS·cm⁻¹ (Substrate 5), while in October it ranged from 830.33 to 2695.00 µS·cm⁻¹ across the same treatments. Pure compost exhibited over twofold higher EC in July and more than threefold higher EC in October compared to pure peat.

In July, the highest calcium content was recorded for plants grown in compost—it was over three times higher compared to the lowest value noted in peat. In October, however, the calcium content in the substrate was highest for combination no. 2 and lowest for the equal mixture of peat and compost. The peat substrate exhibited the lowest potassium content in both July and October, while in both months the highest potassium content was found in compost. As the proportion of compost increased, so did the potassium content in the substrate during both study periods. The situation was similar for magnesium, as the lowest values in both months were recorded for peat and the highest for compost; here too, the values increased with a higher share of compost in the mix. The lowest phosphorus content in both July and October, as well as the lowest nitrates in July, were observed in the peat substrate, whereas the highest values were recorded for compost. Nitrates were present at the lowest concentration in substrate number 2 in the October analysis and at the highest concentration in the compost substrate. In the October readings, a decrease in the nutrient content was observed for all the substrates used compared to the values in July.

3.2. Effect of Biochar Amendment in Peat-Based Substrate (Experiment II)

Experiment II involved studying the effect of different biochar contents on the growth of eastern laurel cherry. Detailed results are presented in

Table 8. Selected biometric characteristics of eastern laurel cherry cultivated in a substrate with biochar addition: height (cm); number of leaves (pcs); number of shoots (pcs); plant mass (g); above-ground mass (g); below-ground mass (g); leaf dry mass (%); root dry mass (%) ¹.

No.	Substrate Combination 2	Height [cm]	Number of Leaves [pcs]	Number of Shoots [pcs]	Plant Mass [g]	Above-Ground Mass [g]	Below-Ground Mass [g]	Dry Mass of Leaves [%]	Dry Mass of Roots [%]
1	100% P	65.75 a	103.75 a	11.83 b	588.69 a	182.98 a	405.71 b	32.67 a	27.35 a
2	100% P + biochar [3 g·dm−3]	70.83 a	102.92 a	10.00 a	318.61 a	170.43 a	148.18 a	36.38 a	32.97 a
3	100% P + biochar [5 g·dm−3]	73.83 a	113.75 a	8.92 a	451.45 a	180.03 a	271.42 ab	36.40 a	34.50 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat.

,

Table 9. Chlorophyll content (mg·100 g⁻¹ fresh weight) in leaves of plants grown in the substrate with the addition of biochar in the cultivation of eastern laurel cherry ¹.

No.	Substrate Combination 2	Chlorophyll a [mg·100 g−1]	Chlorophyll b [mg·100 g−1]	Chlorophyll a + b [mg·100 g−1]
1	100% P	32.69 a	313.29 a	345.98 a
2	100% P + biochar [3 g·dm−3]	32.84 a	325.57 a	358.40 a
3	100% P + biochar [5 g·dm−3]	31.02 a	317.29 a	348.31 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat.

,

Table 10. pH and electrical conductivity (EC, µS·cm⁻¹ at 22 °C) of the substrate with the addition of biochar in the cultivation of eastern laurel cherry ¹.

No.	Substrate Combination 2	pH in July	pH in October	EC (22 °C) in July [µS·cm−1]	EC (22 °C) in October [µS·cm−1]
1	100% P	4.64 a	4.54 a	1780.67 a	830.33 a
2	100% P + biochar [3 g·dm−3]	4.77 a	4.29 a	1292.33 a	1051.33 a
3	100% P + biochar [5 g·dm−3]	4.61 a	4.65 a	2016.67 a	1291.00 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat.

,

Table 11. Macroelements (Ca, K, Mg, P) content (mg·100 g⁻¹ dry weight) and nitrate (NO₃⁻) content (mg·100 g⁻¹ fresh weight) in the plant material of eastern laurel cherry cultivated in a substrate with the addition of biochar ¹.

No.	Substrate Combination 2	Ca Content-July [mg·100 g−1]	Ca Content-October [mg·100 g−1]	K Content-July [mg·100 g−1]	K Content-October [mg·100 g−1]	Mg Content-July [mg·100 g−1]	Mg Content-October [mg·100 g−1]	P Content-July [mg·100 g−1]	P Content-October [mg·100 g−1]	NO3− Content-July [mg·100 g−1]	NO3− Content-October [mg·100 g−1]
1	100% P	423.53 a	256.76 a	693.27 a	615.37 b	128.33 a	118.33 a	333.92 a	226.25 b	304.104 a	368.510 b
2	100% P + biochar [3 g·dm−3]	472.90 a	243.47 a	693.61 a	498.77 a	156.67 ab	130.00 a	259.17 a	177.08 a	348.446 a	275.685 a
3	100% P + biochar [5 g·dm−3]	481.13 a	235.50 a	685.00 a	495.98 a	161.67 b	143.33 a	250.83 a	207.50 b	349.076 a	236.530 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat.

and

Table 12. Macroelements (Ca, K, Mg, P) and nitrate (NO₃⁻) content (mg·dm⁻³) in the substrate with the addition of biochar in container cultivation of eastern laurel cherry ¹.

No.	Substrate Combination 2	Ca Content-July [mg·dm−3]	Ca Content-October [mg dm−3]	K Content-July [mg dm−3]	K Content-October [mg·dm−3]	Mg Content-July [mg·dm−3]	Mg Content-October [mg dm−3]	P Content-July [mg·dm−3]	P Content-October [mg dm−3]	NO3− Content-July [mg·dm−3]	NO3− Content-October [mg·dm−3]
1	100% P	617.00 a	583.22 b	359.24 a	145.37 a	52.00 a	41.33 a	107.00 a	16.50 a	110.00 ab	49.67 a
2	100% P + biochar [3 g·dm−3]	838.69 a	281.93 a	400.57 a	141.61 a	60.00 a	46.00 a	82.83 a	45.50 b	66.00 a	45.33 a
3	100% P + biochar [5 g·dm−3]	1339.71 b	452.57 ab	578.78 a	149.13 a	96.00 b	55.33 a	149.67 a	61.50 b	162.33 b	69.00 a

¹ Values in the same column followed by the same letter indicate no significant differences at the p ≤ 0.05. ² P—peat.

.

In terms of height, the number of leaves and shoots, plant mass, mass of the above-ground parts, and dry mass of the leaves and roots, no statistically significant differences were observed. However, the plants did differ in the mass of the underground parts—it was highest for pure peat and lowest for the substrate with the addition of 3 g·dm⁻³ of biochar.

The chlorophyll a content ranged from 31.02 mg·100 g⁻¹ in Substrate 3 (100% peat + 5 g·dm⁻³ biochar) to 32.84 mg·100 g⁻¹ in Substrate 2 (100% peat + 3 g·dm⁻³ biochar). The chlorophyll b values varied between 317.29 mg·100 g⁻¹ (Substrate 3) and 325.57 mg·100 g⁻¹ (Substrate 2), while the total chlorophyll (a + b) spanned 348.31–358.40 mg·100 g⁻¹. None of these differences were statistically significant.

The substrate pH in July ranged from 4.61 in Substrate 3 (100% peat + 5 g·dm⁻³ biochar) to 4.77 in Substrate 2 (100% peat + 3 g·dm⁻³ biochar), and in October from 4.29 (Substrate 2) to 4.65 (Substrate 3). The electrical conductivity in July varied between 1292.33 µS·cm⁻¹ (Substrate 2) and 2016.67 µS·cm⁻¹ (Substrate 3), while in October it spanned 830.33–1291.00 µS·cm⁻¹. None of these differences were statistically significant.

The calcium content in July and October remained at a similar level among the different substrates, although there was a decrease in content in October in all substrate combinations. In October, however, the calcium content was clearly higher in pure peat than in both combinations with the addition of biochar. The potassium levels in July did not show statistically significant differences between the samples, but in October the level of this element was significantly higher in the control medium. The magnesium content was highest in July in the substrate with the addition of 5 g·dm⁻³ of biochar and lowest in substrate no. 1. In October, the magnesium content in the individual combinations was not statistically significantly different, although it can be seen that the level of this element increases as the biochar content increases. Phosphorus also did not show any significant differences between the combinations in July, while in October its level was lowest for substrate 2. As for the nitrate content, no statistically significant differences were observed between the combinations in July, whereas in October their content was highest in the leaves of plants growing in peat without the addition of biochar.

In the substrate, differences in the calcium content were observed—in July, it was highest in the substrate with the addition of 5 g·dm⁻³ of biochar, whereas in October it was highest in the substrate without any addition and lowest in substrate no. 2. The potassium content in both July and October did not show statistically significant differences, but its levels were noticeably lower relative to those in July. Magnesium had the highest concentration in substrate no. 3 in July, while in October the content was similar across the combinations. Phosphorus was present in similar concentrations in all mixtures in July, but in October it was noticeably lower in the substrate without the addition of biochar. The nitrate levels in July were lowest in substrate no. 2 and highest in substrate no. 3. In October, their level did not exhibit significant differences.

4. Discussion

According to Mendoza Hernández et al. [15], compost is not as good a substrate for plant growth and quality as peat. The main negative properties of this medium are its overly alkaline pH and high salinity, which can lead to phytotoxicity. To mitigate the adverse effects of compost, it is recommended that it is blended with another substrate in order to improve attributes such as its porosity, organic matter content, pH and salinity level.

It was found that the substrate pH and salinity increased in direct proportion to the compost fraction. Although pH rose with a higher compost content, it remained within the optimal range for Prunus laurocerasus in all mixtures except in pure compost, which exceeded recommended values. Compost proved particularly rich in essential nutrients (Ca, K, P, Mg and N), and their concentrations increased progressively as more compost was added. These observations corroborate the findings of Grigatti et al. [16], who reported a general rise in pH in peat–compost blends containing 50%, 75% and 100% compost, with only the 25% treatment showing a slight pH decrease. They also observed that salinity peaked in the 100% compost treatment, and that both macro and micronutrient levels scaled proportionally with compost proportion. Similarly, Mendoza Hernández et al. [15] demonstrated that substrates amended with 25% compost produced the greatest plant height and the highest dry biomass of both aerial and root systems. In that study, the post-harvest pH declined even as salinity remained highest in the pure compost treatment and in the 50:50 peat and compost mixture.

In the experiment of De Lucia et al. [17], rosemary plants cultivated on compost-based substrates (olive mill compost, horticultural compost and organic waste compost), with the exception of olive waste compost, exhibited biomass production equal to or greater than that achieved in the peat control, and their ornamental quality did not differ significantly. Although both pH and electrical conductivity increased in the compost treatments, no phytotoxic effects were observed. In contrast, Alexander et al. [18] reported that fuchsia plants grown in peat based media outperformed those in green compost in terms of growth, flower number and overall quality; yet, in a subsequent study, Alexander et al. [19] found that petunias grown in green compost were of higher quality than those grown in peat and produced 26% more biomass.

Grigatti et al. [16] showed that Begonia cultivated in a 75% peat–25% compost blend achieved higher dry mass and flower counts, values comparable to pure peat, while Salvia and Impatiens reached their maximum dry mass in substrates containing 25 to 50% compost (Salvia registering a 16% increase) and, together with Tagetes, produced the most flowers in a 50% compost mix. Across all species tested, the greatest overall growth was observed in substrates with 25% compost. Frangi et al. [20] demonstrated that compost proportions above 33% adversely affected plant growth and shoot number, including in cherry laurel. According to the present findings, cherry laurel achieved its best growth in substrates containing 30 to 50% compost, whereas higher compost levels led to reduced growth; leaf and shoot counts declined as the compost content rose (with the highest counts in pure peat), the above-ground and leaf dry mass in the 30% compost treatment matched the highest peat values, and the root mass peaked in the peat substrate, indicating stronger root anchorage.

It was shown that the highest calcium concentration in Prunus laurocerasus leaves occurred in the 30% compost substrate in both July and October, although these values did not differ significantly from those in the other treatments. For potassium and magnesium, the greatest leaf concentrations in July and October were likewise found in the 30% compost mix. The phosphorus levels in leaves for both months declined as the compost proportion increased. The highest nitrate content was observed in plants grown in the 70% compost substrate, while the lowest was recorded in the peat control.

Mendoza-Hernández et al. [15] reported that the phosphorus and potassium levels in rosemary leaves rose with an increasing content of compost, whereas nitrogen, magnesium, calcium and sodium decreased as the compost proportion grew. Frangi et al. [20] found that leaf phosphorus increased when the compost share fell, an effect attributed to peat’s acidifying effect on substrate pH. Grigatti et al. [16] observed that in Salvia, Tagetes and Impatiens, leaf phosphorus declined with higher compost ratios (due both to a dilution effect at 25 to 50% and deficiency at 50 to 100%), while the magnesium content rose in line with the compost share. Marschner et al. [21] explain that phosphorus deficiency in leaves stems from its accumulation in root cells, where it is sequestered for meristematic use. Cavagnaro demonstrated in studies on Solanum lycopersicum [22] that low compost application rates do not significantly affect the degree of colonization by arbuscular mycorrhizal fungi (AMF), whereas high compost doses may reduce it. Other studies (Etesami et al.) [23] have shown that the combined application of compost and AMF inoculation enhances both plant biometric parameters and total phosphorus uptake compared to the separate use of these treatments. Similarly, in their research on sugarcane, Abreu et al. [24] found that the combination of compost and AMF increased the phosphorus uptake efficiency in tropical soils with a high phosphorus fixation capacity. In the present study, substrate phosphorus was lower in October than in July, yet the leaf phosphorus levels remained statistically unchanged and stayed below those of peat-grown plants. It was concluded that compost addition limited phosphorus uptake.

It has been observed that the plants absorbed nutrients from the substrate throughout the entire growing season. The lower element contents in leaves in October compared with July resulted from plant growth between July and October and the subsequent redistribution of nutrients within the plants. Similar declines in element concentrations were reported by Clark and Smith [25] for Diospyros kaki. Comparative analyses showed decreases in leaf nitrogen, phosphorus and magnesium concentrations, alongside increases in calcium and potassium. In the case of Prunus laurocerasus, the calcium content decreased in all treatment groups. The phosphorus concentration declined in groups grown in peat-dominant substrates, whereas it increased in the group cultivated in pure compost over the course of the season. Potassium levels remained relatively stable, although a more pronounced decrease was noted in plants grown in compost-rich mixes. The nitrogen concentration stayed largely unchanged except in the group grown in pure peat, where an approximately 20% increase was recorded in October. Magnesium also remained at a similar level overall; however, the greatest differences were observed in the 50% peat/50% compost group (approximately 25% increase) and the 100% compost group (approximately 20% decrease).

According to Hou et al. [26], biochar can serve as an excellent alternative source of nutrients for commercially important crops. Like synthetic fertilizers, it can supply plants with macronutrients, micronutrients, and other trace elements. It enhances nutrient retention and uptake by plants through a range of significant physicochemical processes in the substrate as well as biological mechanisms. Depending on the feedstock, pyrolysis conditions, plant species, climatic factors, and substrate characteristics, biochar may influence nutrient uptake to varying degrees. Biochar is used as a substrate amendment to improve its properties [27,28]. It has been determined that the addition of biochar at 3 g·dm⁻³ of substrate reduces salinity during cultivation. Di Lonardo et al. [28] report that biochar application enables the use of saline water for irrigation in nurseries and mitigates salt-induced damage in salt-sensitive species such as Prunus laurocerasus. This effect is attributed to the lower retention of sodium ions in biochar-amended substrates, which increases plant salt tolerance. At the same time, only minor differences in salinity have been observed, which can be ascribed to the use of peat as the primary sterile substrate with only trace nutrient contents and to the use of filtered water for irrigation.

It has been reported that adding biochar to the substrate, in conjunction with nitrogen fertilization, increases the dry biomass of radish relative to the non-amended substrate. The dry mass gains ranged from 95% in the unfired control to 266% in the substrate amended with 100 t·ha⁻¹ of biochar [29]. Under combined biochar amendment and nitrogen fertilization, the leaf nitrate concentrations were found to rise in proportion to the applied biochar doses.

An 11% increase in leaf dry mass and elevated leaf nitrate levels have also been observed following biochar incorporation, although these differences did not reach statistical significance. Yu et al. reported that biochar application has been shown to enhance nitrogen uptake, thereby improving nitrogen use efficiency in cropping systems [30]. Elemental analysis of the biochar revealed high concentrations of calcium and potassium, with lower amounts of magnesium, nitrogen, and phosphorus.

In peat amended with biochar, Prunus laurocerasus exhibited higher leaf magnesium and calcium concentrations than when grown in pure peat, while other nutrient levels remained comparable. In container trials with a 75% peat: 25% biochar mix, increases in tissue calcium and magnesium were likewise recorded, whereas the tissue potassium and phosphorus levels decreased despite elevated potassium availability in the rhizosphere [31]. Similar effects were noted in potted lavender: leaf phosphorus and potassium decreased, and magnesium and calcium increased with a rising biochar concentration [32]. In contrast, the complete substitution of peat with biochar in tomato production yielded higher tissue potassium and phosphorus levels without significant changes in the nitrogen, calcium, or magnesium content [33].

In Experiment II, although biochar amendment led to favorable shifts in substrate chemistry, these changes did not translate into immediate gains in plant biomass. Cultivation in biochar-amended substrate resulted in a greater plant height and leaf number, although these increases were not statistically significant relative to peat-grown controls. The above-ground biomass remained comparable, whereas the overall biomass was reduced, which is an effect attributed to more extensive root anchorage in the control peat substrate. One possible explanation is that the newly added biochar initially acted as a strong sorbent for nutrients and microbial metabolites, reducing their immediate availability to plants. Over time, as soil microorganisms colonize and then senesce on biochar surfaces, bound nutrients and organic compounds may be gradually released, potentially producing more pronounced growth benefits in a subsequent season. To test this hypothesis, future work should include a longer-term trial in which biochar-enriched substrates are monitored over an extended period, ideally spanning more than one full growth cycle with an overwintering phase, to determine whether nutrient release following microbial turnover enhances plant performance in later stages.

It was demonstrated by Bachmann et al. [34] that biochar addition did not produce statistically significant effects on plant growth, leaf number, or shoot count. An increase in substrate pH was induced by the incorporation of biochar into peat. Similar findings were reported by Manickam et al. [35] for the amendment of acidic soil with alkaline biochar. This rise in pH has been attributed to the high calcium cation content of the biochar. However, it was shown by Tian et al. [36] that Calathea plants grown in a 50% peat: 50% biochar mixture exhibited a 23% greater total biomass and an increased leaf mass compared to those cultivated in pure biochar or pure peat. Enhanced growth was ascribed to the ability of biochar to retain nutrients in the substrate via adsorption onto its porous matrix, thereby preventing leaching; these adsorbed nutrients then became available to the plants.

It should be noted that this work focused solely on one plant species, Prunus laurocerasus ‘Caucasica’, which limits the extrapolation of the findings to other ornamental or crop species with different nutritional requirements and growth physiologies. Additionally, the compost used in the Experiment I was sourced from a single origin (municipal compost); future studies should include composts from diverse sources, which may vary in chemical composition, pH, and salinity. Another limitation is that the effect of biochar was investigated only in a peat-based substrate (Experiment II), without evaluating combined compost + biochar mixtures. Testing various compost–biochar ratios could reveal potential synergistic effects (such as enhanced water retention and nutrient availability) or antagonistic interactions, which are highly relevant for optimizing container-grown production. Incorporating these factors represents an important direction for future research on sustainable substrate formulation. Moreover, as the study was conducted under controlled greenhouse conditions, the results may not fully reflect field-level responses. The role of microbial activity in the substrate, which was not assessed in this study, may also have influenced nutrient dynamics and plant performance.

Although this study focused primarily on the horticultural effects of peat substitution, preliminary cost observations further support the practical relevance of using municipal compost. Current market prices for horticultural peat in Poland generally exceed 100 PLN per cubic meter, with some offers starting from around 90 PLN/m³ depending on the quality and supplier. In contrast, compost produced from municipal green waste is significantly less expensive, with bulk prices typically ranging between 40 and 70 PLN per ton and occasionally falling as low as 30 PLN/t depending on the provider and volume of the order. Based on these figures, replacing 30 to 50% of the peat in a substrate mix with compost may lead to a reduction in material costs by approximately 5 to 35%, depending on the price ratio and substitution rate. While this estimate excludes transport and processing costs, it nevertheless highlights the potential for meaningful cost savings. These findings provide further justification for integrating economic criteria into future studies on sustainable growing media.

5. Conclusions

This study demonstrates that municipal green waste compost can partially replace peat in the container production of Prunus laurocerasus. Substrates containing 30 to 50% compost offered a promising compromise between peat reduction and acceptable plant performance. These mixtures resulted in an increased plant height and improved substrate nutrient content and salinity levels, although they were accompanied by slight reductions in the number of shoots and leaves. At compost contents of 70% and above, these negative effects became more pronounced, leading to a significant decline in total biomass and overall plant quality.

Biochar application slightly increased the plant height, with a visible upward trend corresponding to the higher biochar content; however, this effect was not statistically significant. At the same time, the total biomass, root mass, and shoot number tended to decrease compared to pure peat, particularly at the lower dose (3 g·dm⁻³). The substrate pH remained relatively stable, while the electrical conductivity showed a minor upward trend with increasing biochar levels. Biochar also led to small increases in the substrate concentrations of calcium, potassium, magnesium, phosphorus, and nitrate. Although these changes indicate an improvement in the substrate nutrient status, they did not translate into significant gains in plant biometric traits. This suggests that biochar may function more effectively as a nutrient management tool than as a direct growth enhancer, particularly in the short term, possibly due to delayed nutrient release associated with microbial colonization dynamics.

Overall, compost appears to be a promising partial substitute for peat in ornamental plant production, while the role of biochar may be more context-dependent and time-sensitive. Importantly, preliminary cost estimates also indicate that replacing 30 to 50% of peat with municipal compost may reduce material costs by approximately 5 to 35%, depending on market conditions and substitution rates. Although these figures exclude transportation and processing expenses, they highlight the economic potential of compost use in substrate production. Future research should therefore investigate the combined application of compost and biochar to assess possible synergistic or antagonistic effects on plant growth and substrate chemistry. In addition, long-term trials under field conditions are needed to evaluate the performance of these materials over time. Finally, testing composts from different sources will be essential to verify the generalizability of these findings across diverse production systems and environmental contexts.