Sustainable Horticulture: Advancements and Challenges in Organic Fertilizer Applications

1. Introduction

Modern horticulture is increasingly focused on enhancing crop quality while prioritizing environmental sustainability. Beyond traditional quality attributes, consumers are demanding fruits and vegetables with higher nutritional value and minimal pesticide residues. This shift necessitates sustainable management practices that optimize plant–environment interactions and reduce reliance on external inputs. While intensive horticulture still depends heavily on synthetic fertilizers to ensure rapid nutrient replenishment and high yields [1,2], their prolonged and excessive use has led to significant environmental challenges, including soil degradation, microbial imbalance, erosion, acidification, and groundwater pollution [3]. Transitioning to organic horticulture offers a promising solution, as organic fertilizers have demonstrated the potential to improve yields, enhance soil health, and promote sustainable agricultural practices [4].

Organic fertilizers provide a balanced mix of macronutrients and essential micronutrients such as iron and zinc. They facilitate the gradual release of nutrients into the soil, stimulating microbial growth and activity, which, in turn, enhances nutrient and water availability. These processes contribute to improved soil structure, supporting healthy root development in vegetable crops [5]. Additionally, organic fertilizers help reduce soil acidity, mitigate heavy metal contamination [6], suppress pests and diseases, and minimize nutrient leaching. Their slow decomposition ensures a steady supply of nutrients over an extended period, making them a sustainable alternative to synthetic inputs [7]. Innovations in horticulture, such as the integration of cover crops and diversified crop rotations with organic fertilizers, further enhance soil properties, microbial diversity, and nutrient availability, thereby reducing the environmental impact of chemical fertilizers.

This integrated approach not only enhances crop productivity but also mitigates the environmental impact of chemical fertilizers. The primary challenge today lies in developing innovative organic fertilizers capable of transforming intensive vegetable cultivation practices. The scientific community is increasingly focused on exploring a diverse range of products and waste-derived materials. Recent advancements include the creation of bioformulations that integrate organic matter with mineral particles, nanomaterials, and plant-growth-promoting microorganisms. These “smart” fertilizers are designed to improve nutrient use efficiency and facilitate the transition to more sustainable agricultural systems [8]. Among these innovations, algae-based formulations have garnered significant attention due to their environmental friendliness and cost-effectiveness. Their application has demonstrated productivity increases in various horticultural crops, including tomatoes [9].

However, several knowledge gaps persist regarding the use of new organic fertilizers. A major challenge is the variability in nutrient content across different organic materials, which complicates the precise fulfillment of the specific nutritional needs of vegetable crops. Additionally, the slow-release nature of these fertilizers may not align with the immediate nutrient demands of certain horticultural crops, potentially leading to reduced yields. There is also a critical need for long-term, field-scale studies conducted under diverse climatic conditions to thoroughly evaluate the effects of organic inputs. Furthermore, the improper management of high-nitrogen organic fertilizers can result in increased nitrous oxide emissions, a potent greenhouse gas, under specific conditions. To address these challenges, further research is essential to optimize organic fertilizer formulations and application methods. This will ensure that these fertilizers meet crop nutrient requirements effectively while minimizing negative environmental impacts. By overcoming these barriers, the horticultural sector can fully realize the potential of organic fertilizers in promoting sustainable and productive agricultural systems.

2. The Importance of Organic Fertilizers in Horticulture

In light of these considerations, the use of organic fertilizers in horticulture is a timely and important topic in the context of environmental sustainability, and it has been explored in different ways in this Special Issue (SI). Therefore, the goal of this SI is to highlight innovative research on organic fertilizers in horticulture to further disseminate foundational knowledge in this field and to promote its practical application on a larger scale. This issue presents a series of papers covering a range of topics related to the use of different matrices as fertilizers for horticultural crops. In this editorial, we briefly describe ten research articles (contributions 1–10). Readers will be able to find information about the potential use of unconventional solid matrices, such as cereal flours and by-products from the cold pressing of fruits and seeds (contribution 1–3), sewage sludge and by-products from insect farming (contribution 4,5), biostimulants (contribution 6,7), and liquid matrices like vermicompost extracts (contribution 8,9), as well as companion cropping systems (contribution 10) to improve crop yields and soil quality.

3. Unconventional Solid Matrices

Parecido et al. (contribution 1) assessed the impact of pure and mixed castor meal with ground hydrothermalized phonolite rock (CM+HP mixture) on nutrient supply, particularly nitrogen (N) and potassium (K), as well as on the optimization of sweet potato yield and quality. The field experiment, which involved treatments with and without synthetic fertilizers combined with varying percentages of organic fertilizers, revealed that the CM+HP mixture helped maintain adequate N and K levels in the plant leaves. Additionally, organic fertilizers increased both the number of reserve roots per plant and the sweetness of those roots, while synthetic fertilizers enhanced the average weight of the reserve roots. Castor meal, when combined with synthetic fertilizers, also contributed to improved soil health. The combined application of synthetic fertilizers with 2.4 Mg ha⁻¹ of castor meal or 4.5 Mg ha⁻¹ of the CM+HP mixture yielded the greatest benefits for storage root production, with an average increase of 128% in marketable storage root yield and nutrient removal compared to the use of organic fertilizers alone.

Butterfield et al. (contribution 2) also explored the potential use of corn gluten meal (CGM) and soybean meal (SBM) as organic fertilizers. The aim of this study was to evaluate the feasibility of integrating weed and nitrogen management by examining the effects of varying seed meal application rates within plastic-mulched film planting holes on weed density, soil nitrogen availability, and crop yield in tomato (Solanum lycopersicum) and broccoli (Brassica oleracea). Increasing seed meal rates led to a reduction in weed density, regardless of the type used. However, while soil nitrogen availability increased with the application rate, ammonium mineralized from seed meals applied at the highest rates was likely phytotoxic to both weeds and crops. In terms of crop yield, the seed meal did not have a positive effect. For tomatoes, yields were reduced by 39% to 64% in 2018 compared to the no-weed control. These findings suggest that using the seed meal in plastic-mulched planting holes for weed and nitrogen management is not a viable option, as the application rates required for effective weed suppression also proved to be toxic to the crops.

As an alternative to seed flour, by-products from the oil extraction process of oilseeds can also serve as organic fertilizers. This is exemplified by the experimental trial conducted by Tong et al. (contribution 3), which aimed to uncover the mechanisms underlying the effects of various fertilization treatments—such as chemical fertilizer alone, chemical fertilizer combined with cake fertilizer, chemical fertilizer combined with manure fertilizer, and a combination of all three—on soil fertility, soil enzyme activities, and pecan fruit quality. The results showed that combined fertilization could enhance both the yield and quality of pecan nuts. Among the treatments, the combination of chemical fertilizer with both organic fertilizers yielded the most promising results, with the pecan kernel oil content and unsaturated fatty acid levels reaching 72% and 98%, respectively. While the combined fertilization treatments had no significant impact on soil trace elements, they did significantly increase the activities of available phosphorus, total nitrogen, soil organic matter, and S-ACP (soil acid phosphatase).

4. Sewage Sludge and Insect Farming By-Products

Another valuable by-product that can be repurposed as an organic fertilizer is sewage sludge. Globally, nearly 75–100 million tons of dry matter (DM) are produced annually, with projections estimating an increase to 127.5 million tons by 2030 [10]. Its reuse in agriculture presents a sustainable solution that reduces waste while enhancing soil fertility and horticultural productivity. AL-Huqail et al. (contribution 4) investigated the effects of sewage sludge (DM) amendments on the growth, yield, and biochemical properties of marigolds (Tagetes erecta L. var. Pusa Basanti Gainda). Their findings revealed a significant improvement in plant growth, yield, and biochemical attributes as the DM concentration increased from 0% to 10%. The optimal treatment was 10% DM, which resulted in the highest flower yield (318 g per plant). However, the bioaccumulation factor (BAF) values (>1) calculated by the authors indicated that marigold plants absorbed substantial amounts of six heavy metals, following the order Cd < Cr < Cu < Zn < Mn < Fe. Additionally, predictive models based on multiple linear regression (MLR) effectively estimated heavy metal uptake in marigold plants.

Over the past few years, byproducts derived from rearing Black Soldier Fly Larvae (BSFL) have emerged as some of the most extensively studied organic fertilizers. Their growing popularity worldwide stems from their significant potential to enhance environmental sustainability and support the circular economy. BSFL excreta contribute to improving soil health, enhancing plant immunity, and ultimately boosting crop quality. Romano et al. (contribution 5) examined the use of BSFL frass as a nutrient supplement in an aquaponic system for cultivating sweet potato seedlings. In their study, BSFL eggs were placed on two different substrates: spoiled fish feed formulated for catfish (Rangen; 32% protein) and a mix of fruit (orange peels, banana peels, apple cores, and strawberries) and vegetables (sweet potato and peas). The findings suggest that the initial substrate used to produce BSFL frass did not significantly impact sweet potato slip production. Despite variations in mineral composition among the frass types, neither water quality nor slip production and sugar content were affected. This indicates that a broad range of substrates could be effectively utilized to produce BSFL frass as a fertilizer in aquaponic systems.

5. Biostimulants

In recent years, considerable efforts have been made to develop new fertilizers and fertilization systems for organic horticulture that reduce chemical fertilizer inputs. The goal is to enhance nutrient uptake, promote vegetable growth and development, and improve quality, productivity, and environmental sustainability. For this purpose, biostimulant products have emerged on the market. These products are derived from plant extracts, algae, fungi, bacteria, or animal hydrolysates and contain components such as oligosaccharides, vitamins, humic substances (e.g., mixtures of humic and fulvic acids), micronutrients, and protein hydrolysates [11]. The use of biostimulants to promote plant growth has been extensively studied. For instance, Maková et al. (contribution 6) evaluated the combined effects of experimental (PGPB) and commercial (G) microbial biostimulants, along with a humic substance product (A), in combination with a mineral nitrogen fertilizer (N), on soil microbial communities and strawberry yield over a two-year field trial. The results demonstrated that enzymatic activity (FDA hydrolysis and phosphatase activity) was positively influenced by the N+G, N+G+A, and N+PGPB+A treatments. Additionally, plant biostimulants increased basal- and substrate-induced respiration but did not significantly affect the culturable bacteria population. On the other hand, although the N+PGPB and N+PGPB+A treatments negatively affected the number of culturable fungi, the latter resulted in a 95% higher strawberry yield in the second year compared to the control. Therefore, combining microbial biostimulants with humic substances may offer an effective solution to enhance the production of vegetables, such as strawberries.

Also, in the study by Ollio et al. (contribution 7), the effects of biofertilizers and inorganic fertilizers on a vegetable crop were compared, focusing on their influence on soil microbial abundance, microbial community structure, functional genetic diversity, broccoli yield, and greenhouse gas emissions. In contrast to the previous study, the results revealed that reduced fertilization, along with the application of both biofertilizer products, had no significant impact on soil nutrients, microbial populations, microbial activity, or yield when compared to conventional inorganic fertilization.

6. Liquid Matrices

It is well established that recent regulations have restricted the use of mineral and synthetic fertilizers to mitigate the environmental impact of horticultural systems [12]. As an alternative, fertilizers derived from composted solid organic waste and vermicomposting processes have gained attention. While the beneficial effects of solid vermicompost on soil and plant health are well documented, research on the potential of aqueous extracts and percolates derived from vermicompost remains limited. One such product, known as vermiliquer, is a nutrient-rich liquid that percolates through worm beds containing vermicomposted waste, bedding materials, and worm populations. Vermiliquer has been reported to be abundant in essential plant nutrients, enhancing plant growth and mineral uptake. In addition to dissolved organic and inorganic materials, it contains complex microbiota, plant growth regulators, and humic acids [13]. Furthermore, this organic fertilizer has demonstrated a broad range of benefits, including improving plant resistance to abiotic and biotic stresses, such as controlling insect pests and diseases, alleviating soil salinity, and mitigating drought stress [14]. In this context, Kosem et al. (contribution 8) investigated the effects of liquid vermicompost applications (25%, 50%, 75%, and 100%) on the agronomic traits, phenolic composition, and essential oil content of basil plants subjected to drought stress. Their study revealed that vermicompost applications significantly influenced nearly all measured parameters, except for leaf length in well-watered plants. Regarding essential oil compounds, estragole was identified as the dominant component (85–90%), with the highest levels observed in the 25% vermicompost + water stress, water stress, and control groups. Among the main phenolic compounds, caffeic acid levels declined under drought stress but increased with vermicompost treatments. Meanwhile, rosmarinic acid content rose under water stress conditions, reaching its peak at 25% vermicompost application. Overall, vermicompost applications at 25% and 50% enhanced the phenolic compound content in basil plants, regardless of the irrigation conditions, suggesting its potential as a natural stress mitigator.

For vegetable cultivation, aqueous extracts of vermicompost derived from horticultural crop residues can also serve as a viable option for hydroponic fertilization. For this purpose, Salas-Sanjuán et al. (contribution 9) assessed the effectiveness of both aerated and non-aerated aqueous extracts when used as recirculating nutrient solutions in a hydroponic nutrient film technique (NFT) system for lettuce. Their performance was compared to a conventional nutrient solution containing mineral or synthetic fertilizers. The results demonstrated that lettuce plants fertilized with the organic-aerated nutrient solution achieved not only good yields but also enhanced quality, with nitrate (N-NO₃⁻) levels in edible leaves reduced by sixfold compared to those treated with mineral fertilizers. These findings highlight the potential of aqueous extracts as a sustainable nutrient source, contributing to circular agriculture and improving efficiency in intensive production systems.

7. Companion Cropping

To reduce reliance on chemical fertilizers, in parallel with the study of innovative organic fertilizers, researchers are increasingly focused on developing environmentally friendly agroecological strategies. In this context, agronomic management practices such as companion cropping can play a crucial role in maximizing yields while preserving soil health and horticultural biodiversity [15]. In the study by Moran-Chamorro et al. (contribution 10), interactions between fruit trees and bean plants were analyzed under three fertilization levels. The combination of cape gooseberry and blackberry yielded particularly positive results, with increased leaf production and reduced pest incidence, highlighting the benefits of companion planting. Additionally, the UF system (Physalis peruviana and Phaseolus vulgaris) exhibited the greatest plant height, while the TF system (tomato and bean) demonstrated the best stem perimeter development. These findings also suggest a growing trend toward integrating chemical and organic fertilizers, which the authors identified as a promising approach to reducing costs while enhancing crop growth.

8. Conclusions and Future Directions

The research presented in this SI underscores the potential of organic fertilizers to enhance crop yields, improve vegetable quality, and promote soil health while reducing environmental impacts. However, challenges such as nutrient variability, slow-release dynamics, and potential greenhouse gas emissions require further investigation. Future research should focus on optimizing organic fertilizer formulations, developing precision application methods, and conducting long-term field studies under diverse climatic conditions. Standardized guidelines for organic fertilizer use will be essential to maximize benefits and minimize risks. By integrating innovative organic amendments, biostimulants, and agroecological practices, modern horticulture can achieve a balance between productivity and sustainability, addressing global food security and environmental challenges.