1. Introduction
The intensification of agriculture has led to a dependency on synthetic fertilizers, significantly contributing to CO
2 and non-CO
2 greenhouse gas emissions [
1,
2,
3]. Conversely, carbon-based nutrients from agricultural and food wastes offer a promising alternative, potentially increasing crop yield through the slow release of nutrients [
4,
5,
6]. This approach also helps in reducing environmental harm associated with conventional fertilizer use [
7]. The efficiency of these organic amendments, however, is contingent upon their mineralization by soil microorganisms, a process that can vary widely [
8,
9,
10]. Aerobic composting and anaerobic processing, such as bokashi (Bok) fermentation, stand out as two principal methods for processing agricultural waste, each with distinct benefits and drawbacks [
11,
12,
13]. Composting, which supports soil health and waste reduction, is a relatively slow process and can result in greenhouse gas emissions [
14]. In contrast, Bok fermentation, which utilizes microbial inoculants, provides a quicker, low-emission alternative, albeit with specific technical needs. Biochar (BC) represents another alternative. BC, derived from organic waste through pyrolysis, improves soil fertility by enhancing nutrient retention, water-holding capacity, and microbial diversity. It acts as a long-term carbon sink, mitigating greenhouse gas emissions and promoting sustainable agriculture [
15,
16]. BC reduces reliance on synthetic fertilizers, potentially decreasing nutrient runoff and environmental contamination [
17]. Integrated strategies with other soil amendments optimize pyrolysis processes, effectively revitalizing low-fertility soils and enhancing crop yields [
18]. Economic analyses suggest that BC can reduce carbon emissions and enhance farm profitability, particularly when the market price of BC is low and a C offset trading system is in place [
19]. The synergistic use of Bok and BC could potentially provide the immediate nutrient supply and microbial advantages of Bok with the long-term soil structure and carbon sequestration benefits of BC.
Various studies consistently demonstrate the positive impacts of Bok on plant growth, survival, and soil health. For example, Bok significantly enhances the survival and growth rates of reforestation species, while also improving overall crop productivity through enhanced soil nutrient levels [
20,
21]. Bok applications enhance microbial activity, soil fertility, and root growth via lactic acid bacteria, further improving chlorophyll levels, biomass, and crop yield compared to chemical fertilizers [
22,
23,
24]. Compared to chemical fertilizers, Bok increases the panicle number, ripened grain percentage, and grain yield in rice under specific planting conditions [
25]. Additionally, Bok compost promotes robust vegetative growth in tomato plants [
26] and sustains beneficial microbes within citrus plant systems [
27]. Studies also demonstrate the effectiveness of Bok in increasing essential soil nutrients, such as nitrogen, phosphorus, and potassium, which are crucial for healthy plant development [
28,
29,
30]. Pagliaccia et al. (2024) revealed that the combined use of Bok and BC can significantly substitute for synthetic fertilizers, improving soil levels of nitrogen, phosphorus, and calcium, as well as improving the soil’s carbon content and potassium levels, especially when applied in specific doses [
31]. These research findings underline the role of organic-waste-derived amendments in promoting the sustainability of indoor agriculture, offering valuable insights for integrating these practices into sustainable farming strategies.
Following the positive impacts of organic amendments like Bok and BC on plant growth and soil health, it is equally important to consider their economic viability. Studies examining the costs and returns of such amendments have begun to shed light on their financial benefits. A comprehensive cost–benefit analysis on recycling agricultural wastes demonstrated that such recycling not only mitigates waste pollution but also generates economic benefits for the agricultural sector [
32]. Various composting models for recycling agricultural waste have been analyzed using cost–benefit analyses, showcasing the economic viability of organic waste composting systems through the lens of time-driven activity-based costing [
33]. A comparative cost–benefit analysis of conventional and organic fertilizers highlights the higher unitary prices of organic fertilizers but also their long-term benefits in sustainable cultivation practices [
34]. A cost–benefit analysis for the agricultural use of coal ash as a soil amendment outlines a methodology for estimating costs and benefits to farmers, emphasizing the broader economic considerations required in the adoption of alternative soil amendments [
35]. These findings suggest that while upfront costs may exist for the transition to organic amendments, the long-term returns through improved yield, soil health, and potential market benefits for organic waste could outweigh initial investments. Therefore, integrating organic waste byproducts into farming strategies not only aligns with sustainable agricultural practices but also offers a promising avenue for greater economic viability.
This paper uses a partial budget approach to evaluate the economic viability of Bok and BC as sustainable soil amendments in greenhouse nursery production, focusing on their role as alternatives to synthetic fertilizers. Leveraging experimental data, insights from agricultural supply retailers, and a survey of California nurseries, this study assesses the financial and agronomic impacts of integrating these organically derived nutrient resources in comparison with traditional soil mixes that are predominantly composed of chemical fertilizers. Our analysis covers operational costs, such as seeds, containers, trays, fertilizers, water, labor, electricity, and soil, along with expenses specific to Bok and BC treatments, like citrus waste, bokashi bran, anaerobic fermentation bins, and BC. Emphasizing the potential cost savings and environmental benefits, we highlight their role in reducing chemical input dependency, improving nutrient efficiency, and conserving water. This paper not only reaffirms the agronomic advantages of Bok and BC amendments but also stresses their significant contributions to improving soil health, boosting plant vitality, and facilitating the diversion of organic waste from landfills. Through this extensive evaluation, our research contributes to the evolving conversation on sustainable soil management practices, advocating for approaches that are both environmentally friendly and economically viable in greenhouse agriculture. This shift toward sustainable amendments like Bok and BC presents a compelling case for rethinking traditional agricultural inputs, offering a pathway to more sustainable and financially sound greenhouse nursery production.
4. Discussion
Despite their undeniable contribution to global food production increases, synthetic fertilizers pose substantial environmental risks, including leaching, greenhouse gas emissions, and the depletion of postrenewable resources. The discussion of this research sheds light on the economic and environmental viability of utilizing Bok and BC as sustainable soil amendments in citrus nursery production, juxtaposed with traditional soil mixes and synthetic fertilizers. The cost–benefit analysis, while drawing on a review of the existing literature, a survey of nurseries, and primary experimental data, underscores the potential for these organic amendments to significantly alter nutrient management practices within greenhouse agriculture. The observed increases in essential nutrients—nitrogen (N), phosphorus (P), and potassium (K)—through the application of Bok, BC, and their combined use not only bolster soil quality and plant health but also translate into marked cost savings. These savings are particularly crucial against the backdrop of the escalating need to transition away from synthetic fertilizers.
This study reveals that Bok and BC amendments significantly enhance the levels of key nutrients—N, P, and K—in greenhouse soils, yielding considerable cost savings and underscoring their potential as sustainable alternatives to synthetic fertilizers. For example, applying BC at a 1400 fertilizer dosage markedly increased N by 27%, P by 34%, and K by 16%, resulting in daily savings of up to USD 32.85, USD 7.74, and USD 18.84 per acre (See
Supplementary Table S16 for comparison of nursery sizes in terms of acreage), respectively. The combination Bok_BC was particularly effective, with a 1400 dosage boosting N by 64%, P by 106%, and showing significant effects on K relative to the control, leading to daily savings of USD 77.47, USD 30.41, and USD 19.17 per acre, respectively. The increases in the N, P, and K levels demonstrate the efficacy of BC amendments in optimizing nutrient management and suggest a synergistic effect that could significantly contribute to sustainable agricultural practices.
This study also explores the impacts of Bok and BC treatments on carbon (C) sequestration and water content management in greenhouse conditions, providing valuable insights into sustainable agricultural practices. The findings demonstrate that the Bok and BC treatments significantly enhanced C retention in the soil, with a notable 41% increase in the C content for the BC1400 treatment, suggesting a potential for reducing atmospheric C release. The price of C sequestration was reported at USD 29.84 per ton [
40]. However, because of the unknown timeframe for C storage, integrating this potential revenue into the cost–benefit analysis alongside the daily savings from the water content and N, P, and K benefits is challenging. These savings were calculated on a per day basis, and without a precise estimate of daily C sequestration, the corresponding potential revenue cannot be determined. Additionally, stringent procedures are involved in C storage, which may incur costs exceeding the potential benefits. Consequently, the ultimate results of the cost–benefit analysis do not account for the potential revenue from C sequestration. Similarly to C, the water content management results indicate that BC treatments can significantly improve water efficiency, leading to substantial cost savings in water expenses and irrigation labor. For instance, the BC1400 treatment consistently outperformed the control treatments in increasing water content, resulting in an average daily saving of USD 21.99 per acre in water and irrigation labor costs (See
Supplementary Table S16 for a comparison of the nursery sizes in terms of acreage).
The analysis of the daily savings across various treatments in nurseries of different sizes, when compared to the CK1400 control, reveals significant financial benefits from Bok and BC amendments. The data demonstrate that small to large nurseries can achieve considerable cost savings on nutrients (N, P, and K), water content, and water irrigation labor by implementing these sustainable soil amendments. The comparison between the total benefits and costs further underscores the economic efficiency of the BC treatments, with BC700 and BC1400 showing the most substantial savings across all nursery sizes. This economic efficiency not only highlights the direct financial benefits of adopting BC amendments but also suggests a broader implication for sustainable agricultural practices by reducing operational costs and enhancing environmental sustainability.
We did not account for electricity costs in our evaluation of the treatments. However, factoring in these costs would reinforce our findings, as faster growth rates in plants, as we observed, imply a shorter duration for each plant’s growth cycle. Consequently, this would lead to a decrease in electricity usage, thus lowering the associated costs. This connection between accelerated growth rates and reduced electricity costs could result in substantial cost savings.
Our findings align with the growing body of research emphasizing the importance of organic amendments in enhancing soil health and sustainability in agriculture [
41,
42,
43,
44]. Our study contributes to existing knowledge by demonstrating substantial nutrient management improvements and cost efficiencies not previously reported with such magnitudes in citrus nursery settings. While some studies report modest improvements in nutrient levels with organic amendments [
45], our results show significant enhancements, especially in the nitrogen content, which suggests a potentiated effect of combining Bok with BC. The ability of BC treatments and organic amendments, such as Bok byproducts, supports more sustainable agricultural practices by enhancing carbon sequestration and water efficiency [
46,
47]. In addition, BC-compost is also effective in improving soil conditions, mitigating nutrient deficiencies, promoting sustainable soil management, enhancing soil health, and increasing crop productivity by improving nutrient availability and reducing contaminants [
48,
49].
In evaluating the economic viability of Bok and BC as organic fertilizers, it is essential to consider a range of factors beyond their agronomic benefits. The timing and conditions of storage, transportation costs, application methods, and labor expenses play crucial roles in determining their overall feasibility in agricultural settings [
50,
51]. While our study has primarily focused on demonstrating the agronomic advantages within controlled greenhouse nursery environments, where variables such as application methods, crop prices, substrate types, climate conditions, and irrigation practices are standardized, we recognize the significance of these logistical factors in broader agricultural applications. To address these considerations, future research efforts should incorporate comprehensive cost analyses that encompass these logistical challenges.
Moreover, organic fertilizers like Bok and BC are particularly cost-effective in intensive farming systems, such as greenhouse cultivation, hydroponics, aquaponics, and irrigated agriculture. These controlled environments optimize the application and benefits of organic amendments, reducing input costs and supporting sustainable soil management. In contrast, open farming without irrigation faces challenges such as fluctuating environmental conditions and higher operational costs, which can limit the economic benefits of these fertilizers [
20,
52]. Therefore, while Bok and BC can improve soil fertility and reduce pollution in these settings, their application must be carefully managed [
16,
21]. Our study emphasizes the strategic use of organic fertilizers in settings where they provide the most economic and environmental benefits, highlighting the importance of tailored management practices based on farming system characteristics. By focusing on intensive farming systems with optimized irrigation practices, organic fertilizers not only reduce input costs but also support sustainable soil management and environmental conservation.
Furthermore, it is essential that long-term studies over multiple growing seasons are conducted to thoroughly understand the sustained impact of Bok and BC on soil health and crop yields. Such studies should focus on evaluating the long-term benefits of these amendments on soil structure, microbial activity, nutrient availability, and crop productivity. This will provide deeper insight into the lasting advantages and potential limitations of these soil amendments and on how these amendments contribute to carbon sequestration and other environmental benefits over time.
Additionally, investigating how different waste sources used in producing Bok influence nutrient release and carbon sequestration capabilities is also crucial, offering insights into optimizing Bok production and identifying the most beneficial raw materials to enhance its positive effects. While the research outlines opportunities for enhancing the application of these organic amendments, it also sheds light on the importance of broadening our understanding across diverse agricultural settings. The study’s insights into greenhouse environments pave the way for future investigations in varied agricultural contexts (indoor and field studies), ensuring the applicability and generalizability of our findings. However, the variability in outcomes, especially concerning the carbon sequestration and nutrient release capabilities of Bok from specific waste types like citrus waste with suboptimal C/N ratios underscores the complexity of these amendments’ effectiveness and the need to test different raw materials. Indeed, Bok byproduct can be produced from a variety of organic substrates, including but not limited to wheat, oats, and other crop residues and brewery, juice processing facility, and preconsumer grocery wastes [
20]. Although the specific microflora formed during the Bok process depends significantly on the microorganisms present in the Bok inoculum (primary microorganisms include Lactobacillus species, Saccharomyces cerevisiae, Rhodopseudomonas palustris, and Actinomycetes) that collectively contribute to the effective anaerobic fermentation of organic materials, each of these substrates supports the cultivation of distinct microbial taxonomic groups that characterize the Bok byproducts [
28]. These variations can lead to differences in nutrient release rates and the ability of the Bok to enhance soil fertility and structure [
21]. Additionally, the presence of particular microbes with specific ecological functions can influence the carbon sequestration capabilities of Bok, further affecting its long-term benefits for soil health and its economic viability [
31]. Therefore, to ensure the broadest benefits and to cater to varying agricultural needs, it is crucial to explore the impact of these microbial communities in detail. This means conducting controlled experiments to compare the effects of Bok produced from different raw materials. Such research should aim to determine optimal compositions and taxonomic microbial profiles that maximize nutrient availability, improve soil health, and increase crop yields, thereby enhancing the economic returns from Bok application. Understanding these dynamics will enable farmers and agricultural producers to make more informed decisions about which type of Bok is best suited for their specific soil types and crop requirements. This tailored approach to Bok production not only informs farmers about the optimal use of locally available safe waste materials but also supports the broader adoption of this practice. By elucidating the specific impacts of various waste byproduct materials and their optimal dosages (%
v/
v), we can underscore the importance of customizing organic amendments based on available resources, thereby maximizing their environmental and economic advantages. On the other hand, BC derived from a single type of waste, such as almond waste, offers a more uniform byproduct, potentially simplifying its use in downstream applications. This uniformity can make it easier for farmers to predict and rely on the amendment’s effects on soil health and crop growth.
In addition, assessing the carbon footprint and promoting decarbonization in agricultural production are critical considerations for advancing the adoption of organic fertilizers such as Bok and BC. These fertilizers offer the potential to sequester carbon during production and application, contributing to carbon accumulation in soils over time [
19,
31]. The concept of establishing a “carbon fund” through prolonged carbon storage in granular dry fertilizers further underscores their environmental benefits. While our study has highlighted the immediate agronomic advantages of Bok and BC in greenhouse nursery environments, the long-term implications of carbon sequestration deserve more detailed examination. Future research should comprehensively assess the carbon footprint of organic fertilizers throughout their lifecycle, integrating carbon sequestration metrics into economic analyses to better evaluate their sustainability and promote global adoption. Another important area for exploration is the integration of potential revenue from carbon credits into cost–benefit analyses. Although our study highlights the difficulties in estimating the duration of carbon storage, future research should address these issues and develop methods to accurately quantify the economic gains from carbon credits.
This study emphasizes the importance of tailored approaches in applying these amendments, considering the unique conditions and requirements of each nursery operation. Future research directions should focus on optimizing the application of Bok and BC to enhance both environmental sustainability and economic efficiency by exploring their long-term effects and potential scalability across various agricultural systems and climates. Scaling the use of organic fertilizers presents challenges that must be addressed, including standardization issues, variability in storage conditions and durations, application methods, and the calculation of profitability considering crop types, growing substrates, climate variations, and irrigation practices. Successful implementation in closed production cycles may not always translate to specialized enterprises purchasing organic fertilizers from the market, where profitability can vary significantly. Moreover, forecasting should account for the increased costs associated with water resources, irrigation, and potential restrictions on electricity use amidst global climate change and diminishing arable land. These factors underscore the importance of comprehensive studies that evaluate the broader economic and environmental impacts of organic fertilizers. By addressing these complexities, future research can provide valuable insights into optimizing the scalability and sustainability of bokashi and biochar applications in diverse agricultural contexts.