Potential Use of Paper Mill Sludge in Improving Soil Quality for Plant Growth

Abstract

This study evaluated the impact of soil modifiers on seed germination and soil quality. Mixtures of paper mill sludge (PMS) with camellia oilseed cake (CO) and peat moss (P), as well as with oilseed cake of toothache tree (TO) and peat moss (P), showed promising results as biostimulants for seed germination. The addition of PMS increased the germination index, indicating its beneficial effects on seed performance. The pH levels remained optimal, and the electrical conductivity values stayed within acceptable ranges, suggesting minimal hindrance to plant growth. The cation exchange capacity increased with PMS, enhancing nutrient availability. Composted mixtures exhibited increased phosphorus levels, contributing to soil fertility. While the organic matter content decreased after composting, the carbon-to-nitrogen ratio remained balanced. The bulk density increased after composting, potentially improving soil drainage. Overall, the TO-containing mixture demonstrated superior growth rates compared to the P. These findings support the use of PMS-based soil modifiers for enhancing seed germination and improving soil quality.

1. Introduction

Soil quality enhancement is critical for optimizing plant growth and ensuring sustainable crop production. Organic amendments, such as soil modifiers, play a vital role in improving soil properties and providing essential nutrients for plants. Paper mill sludge (PMS), a waste product generated in significant quantities by pulp and paper mills, is one potential organic amendment that should be explored. PMS is a mixture of organic matter, inorganic matter, and water. It is generated during paper production, with approximately 40–50 kg of dry sludge produced per ton of paper [1,2].

PMS has been shown to improve soil structure, nutrient availability, and water retention [1]. It can also help to reduce soil erosion and improve crop yields. However, there are challenges to using PMS as a soil modifier, such as its high organic content and potential for heavy metal contamination [1]. Despite these challenges, the potential benefits of using PMS as a soil modifier are significant. If PMS can be used safely and effectively, it could be a valuable tool for improving soil quality and promoting sustainable crop production. One way to address the challenges of using PMS is to compost it before applying it to soil. Composting can help to reduce the organic content of PMS and remove some of the heavy metals [3,4,5]. It can also help to improve the nutrient content of PMS, making it more beneficial for plant growth.

Another way to use PMS as a soil modifier is to mix it with other organic materials, such as oilseed cakes. Oilseed cakes contain nutrients that are essential for plant growth, such as nitrogen, phosphorus, and potassium. Mixing PMS with oilseed cakes can help to improve the overall nutrient content of the soil, making it more productive [6,7].

PMS can be disposed of or reused in various ways, depending on the regulations and available options in a particular region. Common methods of PMS disposal or reuse include landfilling, incineration, land reclamation, cement production, and alternative fuel sources [8,9,10,11,12,13,14]. In the past, landfilling and incineration were the most common methods of PMS disposal [8,9,10]. However, landfilling of PMS is considered less sustainable due to limited space and the potential for leachate contamination [11]. Incineration can be a more sustainable way to dispose of PMS, but it requires specialized facilities and careful emission management [12,13]. Moreover, PMS can serve as a source of calcium, silica, and other minerals required in the cement and ceramic manufacturing process [14,15,16,17]. Additionally, due to its organic content, PMS has been explored as a potential alternative fuel source [18,19,20,21]. It can be co-fired with other fuels in power plants or used in the production of biofuels. Some studies have also demonstrated that PMS can be economically used as a raw feedstock for producing solid fuels when appropriately mixed with other woody biomasses in suitable proportions [21,22]. Furthermore, some studies have shown that PMS mixed with old newspaper (ONP) can be used to create biodegradable seedling pots for replacing plastic pots [23,24,25].

While PMS holds potential as an essential resource, an efficient method for its large-scale utilization has not been developed yet. Ensuring the safe and effective use of PMS is crucial in order to mitigate potential risks. Therefore, further research is necessary to optimize the utilization of PMS as a soil amendment and to explore new and improved methods for managing PMS.

Currently, a technology is urgently needed to treat the large amount of PMS that is generated every day. The most effective way to consume PMS in large quantities is to apply it to the soil, but this requires a pre-treatment process that does not burden the soil before direct land spreading of PMS. The primary objective of this study was to investigate the potential of PMS as a soil modifier in conjunction with oilseed cakes and peat moss to enhance soil conditions for seed germination and plant growth. By analyzing the combined effects of these organic materials on soil fertility, structure, and nutrient availability, we aimed to determine the suitability of PMS as a sustainable soil amendment in agriculture and horticulture. The study focused on evaluating the growth performance and germination rates of selected plant species when subjected to various treatment combinations of PMS with other organic substances, such as peat moss and oilseed cakes. The results of this study will provide valuable information on the potential of PMS as a sustainable and environmentally friendly soil amendment. This could also be used to develop new and improved methods for managing PMS and promoting the use of PMS in agriculture and land reclamation.

2. Materials and Methods

2.1. Raw Materials and Composting

To prepare a soil modifier from paper mill sludge (hereafter referred to as PMS), dehydrated PMS was provided by Moorim P&P Co., Ltd. in Jinju, Korea. It has been reported that the PMS does not contain harmful heavy metals in the soil [1]. The auxiliary raw materials used to make the soil modifier were oilseed cake (hereafter referred to as CO) from camellia (Camellia japonica), oilseed cake (hereafter referred to as TO) from the toothache tree (Sapium japonicum), and peat moss (P, NordTorf’s Miniballen, Aknistes novads, Latvia) with an organic nitrogen content of 0.3% and an organic carbon content of 52%. CO was provided by the Korea Camellia Research Institute in Tongyeong, Korea, and TO was supplied by Jirisan Sancho Co. in Hadong, Korea. All the raw materials were ground to a particle size that was passed through a 40-mesh wire before mixing.

Table 1. Types of raw materials used to manufacture soil modifiers.

Paper Mill Sludge (PMS)	Camellia Oilseed Cake (CO)	Oilseed Cake of Toothache Tree (TO)	Peat Moss (P)

shows the types of raw materials used in the study.

CO and TO were used after aging for one month in a sealed state in a thin plastic bag at room temperature. The samples in which CO and TO were mixed with P in ratios of 5:5 and 3:7 were marked as COP55, COP37, TOP55, and TOP37, respectively (refer to

Table 2. Mixing proportions of the raw materials for preparing soil modifiers.

Mixing Ratio (%)	COP55:PMS	COP37:PMS	TOP55:PMS	TOP37:PMS
	7:3	7:3	7:3	7:3
	5:5	5:5	5:5	5:5

). A total of 30% and 50% of PMS was added to the COPs and TOPs, respectively, and used for plant growth experiments.

The mixtures of PMS with camellia oilseed cake (CO) and peat moss (P), as well as with oilseed cake of toothache tree (TO) and peat moss (P), were mixed with water in a ratio of 1:1 and then composted in a sealed container at room temperature for 30 days. Every 15 days, these mixtures were turned upside down to mix, aerate, and accelerate the degradation process.

2.2. Germination Test

To determine the optimal soil modifier for plant growth, a physicochemical test method for fertilizers was used to calculate the germination index. The experiment involved determining the germination percentage of seeds germinated after specified time intervals through repeated observations and/or the calculation of the germination rate according to previous studies [26,27]. A total of 30 romaine lettuce (Lactuca sativa var. longiflora) seeds were used for the germination experiments. The seed-containing dishes were wrapped with parafilm and placed in a growth chamber (SW-96PH, Gaon Science, Gwangju, Korea) at 25 °C under a 25,000 lux light. After five days, the number of germinated seeds and the length of their roots were counted. Using Equation (1), the germination index (GI) was calculated by combining the germination rate (GR) and relative root length (RL). A GI of 70 or more was considered suitable for a soil fertilizer.

$$\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\left(\mathrm{G}\mathrm{R}\right)=\frac{\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o} \mathrm{o}\mathrm{f} \mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h} \mathrm{d}\mathrm{i}\mathrm{s}\mathrm{h}}{\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$$

(1)

$$\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t} \mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\left(\mathrm{R}\mathrm{L}\right)=\frac{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n} \mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t} \mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h} \mathrm{o}\mathrm{f} \mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h} \mathrm{d}\mathrm{i}\mathrm{s}\mathrm{h}}{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n} \mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t} \mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$$

(2)

$$\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}\left(\mathrm{G}\mathrm{I}\right)=\frac{(\mathrm{G}\mathrm{R}\times \mathrm{R}\mathrm{L})}{100}$$

(3)

2.3. Physicochemical Properties of Soil Modifiers

The pH, electrical conductivity (EC), and bulk density of the soil modifier were measured according to ISO standards 10390 [28], 11265 [29], and 11272 [30]. The available phosphorus and organic matter in the soil modifier was measured according to the Lancaster soil test method and ASTM D 2974 [31], respectively. The nitrate–nitrogen content in soil was measured based on ISO standard 14255 [32]. The C/N ratio in the soil was analyzed according to ISO standard 23400 [33]. The cation exchange capacity (CEC) of the soil was measured using the ammonium acetate method according to ISO 22171 [34,35].

2.4. Seedling Test

For the seedling test of romaine lettuce and bok choy (Brassica rapa subsp. Chinensis), two trays with five pots were prepared for each vegetable, and two seeds were sown in each pot containing soil modifiers. The pots were placed in a growth chamber at 18 °C and 75% relative humidity (RH) under a light intensity of 25,000 lux with a regular water supply. After 30 days of growth in the chamber, the length and width of the leaves and the length of the roots were measured to assess the effects of the soil modifiers.

2.5. Statistical Analysis

Statistical analyses were conducted using the statistical package SPSS for Windows, version 16.0 (SPSS, Chicago, IL, USA), which was used to evaluate the bulk of TMP and CTMP prepared using TP A and TP B through an analysis of variance (ANOVA) at a 95% confident level (p ≤ 0.05). The results were analyzed using a one-way ANOVA followed by a Tukey’s test as a post hoc test. The effects of the different soil modifiers were considered to be not statistically significant when the p-value was higher than 0.05 at the 95% confidence level.

3. Results and Discussion

3.1. Germination Index (GI)

Figure 1 and Figure 2 compare the seed germination of romaine lettuce in various soil conditions. The germination index (GI) is a comprehensive parameter that considers the germination rate and relative root length. It provides a more holistic evaluation of seed germination performance. The resulting value reflects the overall vigor and quality of the seeds. A higher germination index indicates better seed performance. If soil modifiers contain toxic substances, such as heavy metals or chemical pollutants, then they can impede the germination of seeds. In Figure 2, all the soil modifiers, except the one prepared by mixing 50% paper mill sludge with a 5:5 mixture of camellia oilseed cake and peat moss (COP55:PMS = 5:5), exhibited a GI over 100. This finding suggests that the mixtures of PMS with camellia oilseed cake (CO) and peat moss (P), as well as with oilseed cake of toothache tree (TO) and peat moss (P), hold promise as potential biostimulants for enhancing seed germination. Importantly, these mixtures were found to be safe for use, as they did not negatively affect seed germination. Moreover, it is worth noting that an increase in the amount of paper mill sludge (PMS) added to the mixtures of COP and TOP resulted in a higher germination index. This observation suggests that PMS contains organic matter and essential nutrients that can enrich the soil. Undurraga et al. reported no acute or subacute toxic effects on seed germination when PMS prepared as pellets was applied to soil [36]. In particular, there is also a concern about contamination from toxic substances in wastewater, as PMS is a solid waste generated in the wastewater treatment process. However, Kumar et al. reported high seed germination rates in paper mill effluents, confirming that PMS does not introduce toxic substances from wastewater [37].

Two soil conditions ticked in red in Figure 2, COP37:PMS = 5:5 and TOP55:PMS = 5:5, exhibited high GI values over 140 and were selected to assess their potential as soil conditioners. Under these two conditions, the soil modifiers were divided into pre- and post-composting for the performance evaluation."

3.2. pH of Soil Modifiers

The ideal soil pH for plant growth varies depending on the type of plant. However, most plants prefer a soil pH between 6 and 7.5. When the soil pH is too low or too high, it can affect the availability of nutrients to plants. Nutrient deficiencies or toxicities can stunt plant growth or even kill the plant. The advice for the UK and most other countries is to maintain soil pH values at optimal values of 6.5 (5.8 in peaty soils) for cropped land and 6.0 (5.3 in peaty soils) for grassland [37,38]. The pH values of the newly prepared soil modifiers were 6–7 regardless of composting, as shown in

Table 3. pH values of different soil modifiers.

	Peat Moss	PMS	COP37:PMS = 5:5		TOP55:PMS = 5:5
			Before Composting	After Composting	Before Composting	After Composting
pH	5.5 (±0.028) *	7.5 (±0.049)	6.10 (±0.099)	6.7 (±0.049)	6.5 (±0.042)	6.5 (±0.014)

* standard deviation.

. The addition of PMS in COP and TOP resulted in a slightly elevated pH compared to peat moss alone. This was likely due to the calcium carbonate in PMS that could neutralize the acidity of the COP and TOP.

In conclusion, PMS normally had a high pH value due to the alkaline additives used in the papermaking process. When applied to acidic soils, it can help neutralize the pH, making the soil more suitable for plant growth.

3.3. Electrical Conductivity of Soil Modifiers

The electrical conductivity (EC) of soil measures the amount of dissolved salts in the soil. It is an important indicator of soil salinity and nutrient availability for plant growth. High soil EC can indicate high levels of soluble salts in the soil, which can be toxic to plants and reduce their growth. In general, the optimal range of soil EC for most plants is between 0.5 and 3 dS/m.

In Figure 3, the initial EC values of the non-composted mixtures were found to be approximately 0.35 dS/m. After composting, the EC of the COP37 and PMS mixture increased to 0.8 dS/m, while the EC of the TO and PMS mixture increased to 0.9 dS/m. These findings indicate a significant rise in EC following composting, suggesting the release of soluble salts into the soil as a result of organic matter decomposition. The ECs of these soil modifiers composted with COP37 and TOP37 mixed with PMS were significantly different from those before composting, with p-values less than 0.05. Additionally, the presence of PMS, initially exhibiting a higher EC of 0.5 dS/m, likely contributed to the overall increase in EC values. Nevertheless, the values obtained for both mixtures remained within the acceptable range of soil EC (0.5–3 dS/m). Therefore, it is reasonable to conclude that the composted soil modifiers, including PMS, would not significantly impede plant growth.

3.4. Cationic Exchange Capacity of Soil Modifiers

Figure 4 shows the cation exchange capacity (CEC) of the prepared soil modifiers, which measures their ability to hold cations such as calcium, magnesium, and potassium. When COP37 and PMS were mixed in a 5:5 ratio, the mixture exhibited a CEC of 4.2 meq./100 g. Similarly, when TOP55 and PMS were mixed in the same proportion, the resulting mixture had a CEM of 3.6 meq./100 g. Following their compost, the CEC of the COP37 and PMS mixture increased by 59%, while the CEC of the TOP55 and PMS mixture increased by 72%. These findings indicate that PMS likely played a role in enhancing the CEC of both mixtures by facilitating the decomposition of organic matter during the composting process.

The increase in CEC after composting holds potential benefits for plant growth. Soils with high CECs can bind more cations, such as Ca²⁺ or K⁺, to the exchange sites located on the surfaces of clay and organic matter particles. Additionally, soils with increased CEC exhibit a higher buffering capacity, enhancing their ability to resist pH fluctuations [39]. Consequently, these findings suggest that PMS might contribute to the observed rise in CEC after composting.

3.5. Available Phosphorus of Soil Modifiers

Figure 5 compares the prepared soil modifiers’ available phosphorus before and after composting. It is crucial to emphasize the significance of phosphorus in promoting plant growth and contributing to soil fertility. When COP37 and PMS were combined in a 5:5 ratio, the P of the resulting mixture exhibited a phosphorus level of 39.21 ppm. Similarly, when the TOP55 and PMS were mixed in a 5:5 ratio, the phosphorus content of the mixture reached 42.35 ppm. Composting led to a significant difference in the available phosphorus of the COP37, PMS, and TOP55 and PMS mixtures, as evidenced by p-values less than 0.05 compared to the non-composted mixtures. It became evident that the addition of PMS to both mixtures increased their phosphorus levels compared to the peat moss alone. Studies have shown that the phosphorus content of PMS is between 0.5% and 1.5%, with an average content of 1% [40]. Furthermore, the phosphorus levels in both mixtures experienced a further increase after the composting process. Composting, as a method of decomposing organic materials, enhances the nutrient content of the resulting soil amendment. These findings indicate that the composting process for the newly prepared soil modifiers with PMS enhanced phosphorus availability, which improved plant growth.

3.6. Organic Matter of Soil Modifiers

Figure 6 shows the changes in the organic matter content of the soil modifiers before and after the composting process. Peat moss had an organic matter content of approximately 80%, whereas PMS contained approximately 50% organic matter. Organic matter in PMS can improve soil fertility by enhancing soil structure, water-holding capacity, nutrient availability, and microbial activity, leading to improved plant growth [1,5,7].

The mixtures of COP37/PMS and TOP55/PMS initially had similar organic matter contents, which decreased to comparable levels following composting. This reduction in organic matter content was likely due to the decomposition of organic matter by microorganisms during the composting process. The decline in organic matter content after composting could have implications for the agricultural use of soil modifiers. While soil modifiers with high organic matter content can enhance soil structure and fertility, their effectiveness may diminish after composting. Composting played a meaningful role in breaking down the soil modifiers into nutrient-rich soil amendments. This process might help to reduce their C/N ratio, making them more available to plants.

3.7. Nitrate and Ammonium Nitrate of Soil Modifiers

Figure 7 shows the nitrate and ammonium nitrogen content changes among different soil modifiers before and after the composting processes. Nitrate nitrogen represents a readily available form of nitrogen for plants, while ammonium nitrogen is less easily accessible compared to nitrate nitrogen. The mixture of TOP55/PMS exhibited higher levels of nitrate nitrogen compared to the COP37/PMS mixture, both before and after composting. This was likely because the TO may have undergone easier breakdown during composting compared to the CO. The decomposition process could release additional nitrate nitrogen into the mixture. The increase in ammonium nitrogen found in the COP37/PMS mixture after composting could be attributed to the breakdown of organic matter during the composting process. As the organic matter decomposed, it appeared to release ammonium nitrogen as a byproduct, resulting in an elevation of the ammonium nitrogen content. Conversely, the ammonium nitrogen content in the TOP55/PMS mixture remained relatively unchanged after composting, likely due to peat moss (P) in the TOP55 already being a material with a relatively high nitrogen content.

Therefore, PMS could be a source of various nutrients, including nitrogen, phosphorus, potassium, and micronutrients. When properly managed and applied, the nutrients in the sludge can replenish the soil, promoting plant growth and productivity.

3.8. C/N Ratio of Soil Modifiers

Organic matter with a high C:N ratio decomposes at a slower rate compared to organic matter with a low C:N ratio. When the C:N ratio of organic matter is too high, microorganisms will consume all the available nitrogen before breaking it down, potentially leading to nitrogen deficiencies in plants. A lower C/N ratio can result in the liberation of soluble basic salts, resulting in unfavorable conditions for plant growth [41]. Figure 8 shows the variation in the C/N ratio of the soil modifiers before and after composting. The initial substrate C/N ratio is regarded as the key factor, with an optimum initial C/N ratio for composting ranging from 25 to 30 [42,43]. This indicates that the mixtures of COP37/PMS and TOP55/PMS were well-balanced in the carbon-to-nitrogen ratio before and after composting. The COP37/PMS mixture initially showed an increased C:N ratio before composting, but this decreased afterward, suggesting an increase in nitrogen availability. Conversely, the C/N ratio of the TOP55/PMS mixture showed minimal differences before composting, and the impact on the ratio remained minor after composting. Therefore, managing the C:N ratio of any soil modifier added to the soil is crucial for optimizing nutrient cycling and preventing nitrogen starvation. Finally, if the prepared soil modifiers have a high carbon-to-nitrogen (C/N) ratio even after composting, mixing them with high-C:N ratio materials such as nitrogen-rich sources like manure or compost can help maintain an optimal C:N ratio. This promotes efficient decomposition and enhances nutrient availability for plant uptake.

3.9. Bulk Density of Soil Modifiers

Figure 9 presents a comparison of the bulk density of the prepared soil modifiers before and after composting. After composting, the bulk densities of the COP37/PMS and TOP55/PMS mixtures increased by approximately 46% and 39%, respectively. However, it should be noted that the bulk densities of both COP37/PMS and TOP55/PMS exceeded the recommended range for soil, which typically falls between 1.3 and 1.5 g/cm³. It has been reported that seedlings face less difficulty emerging through the soil’s sheath layer when the bulk density is within this lower range [44]. Nevertheless, it is crucial to recognize that the appropriate bulk density range applies to soil, not organic matter. Organic matter has a looser structure compared to soil, resulting in a lower bulk density.

Consequently, the increase in bulk density of COP37/PMS and TOP55/PMS after composting may not necessarily be negative. In certain cases, a higher bulk density can actually benefit soil quality. For instance, it may aid in improving soil drainage and reducing erosion [44,45].

4. Seedling Tests in the Prepared Soil Modifiers

Figure 10 displays the images of the seedling tests conducted on the prepared soil modifiers before and after composting. Bok choy and romaine lettuce seeds were planted in these prepared soil modifiers, and their growth rates were observed under specific conditions. The bok choy and romaine lettuce grown in the peat moss exhibited a remarkably faster growth rate than those in the individual mixtures of COP37/PMS and TOP55/PMS without composting. Furthermore, COP37/PMS showed a much slower seed germination and growth rate than TOP55/PMS.

After composting, both vegetables planted in the representative mixtures of COP37/PMS and TOP55/PMS showed faster germination and growth rates than before composting. However, even after composting, COP37/PMS exhibited a slower growth rate than TOP55/PMS, as evidenced by Figure 11, which illustrates the lengths of the roots and leaves of bok choy and romaine lettuce grown in the prepared soil modifiers before and after composting. Regardless of the vegetable types, COP37/PMS and TOP55/PMS showed longer roots and broader leaves after composting, but TOP55/PMS contributed to a better growth rate than COP37/PMS. In terms of soil properties, TOP55/PMS did not show significant differences from COP37/PMS in pH, EC, CEC, organic matter, available phosphorus, ammonia nitrogen, C/N ratio, and bulk density after composting. However, TOP55/PMS contained more nitrate nitrogen than COP37/PMS, which was considered to have greatly contributed to vegetable growth (refer to Figure 12).

Based on these findings, it can be concluded that the mixture of toothache tree oilseed cake (TO) and peat moss (P), including PMS, was more effective in improving soil quality for plant growth when composted. Additionally, it can be inferred that PMS does not have any harmful effects on plant growth, even after composting [45].

5. Conclusions

The mixture of TO and P, including PMS, showed promise as a biostimulant for enhancing seed germination and improving soil quality when composted. The addition of PMS to the mixtures improved the availability of phosphorus and increased the CEC of the soil modifiers. Despite some changes in soil properties after composting, the soil modifiers remained within acceptable ranges for plant growth. Overall, these findings suggest that the prepared soil modifiers, including PMS, are safe and effective for promoting seed germination and plant growth, highlighting their potential as sustainable alternatives for enhancing agricultural practices.