Field experiments were carried out at Republic of Benin cities of Cotonou and Abomey-Calavi (at IITA-Benin station), for two cropping seasons in 2022 and 2024. Cotonou (6°20’N, 2°20’E) is bordered by Nokoué Lake in the north, the Atlantic Ocean in south, the municipality of Sèmè-Kpodji (in the Ouémé department) in east side, and the municipality of Abomey-Calavi in West. Annual cumulated rainfall ranges were 870 mm and 969 mm in 2022, for Cotonou and Abomey-Calavi, respectively (INSAE, 2016). Mean temperature varied between 27 °C and 35 °C and relative humidity between 65% and 85% for both cities (INSAE, 2016). The International Institute of Tropical Agriculture (IITA) is located in Abomey Calavi (6°25’N, 2°19’E). Typically, rainfall during the wet season (from June to October) is 748.9 mm, mean temperatures is 26 to 28 °C and relative humidity is 82% to 85% (Santos et al., 2015). The climate of two cities is subequatorial and receives bimodal rainfall. Soil type is sandy to sandy-loamy characterized by poor organic matter (OM > 2), total nitrogen (N > 0.08%), Phosphorus (P_(ppm) > 20), Potassium (K _{(meq/100 g soil)} > 0.4) (Nacoulma and Guigma, 2015; Santos et al., 2015).

Three fertilizers were used in the experiments: two organic fertilizers (BSF frass and poultry manure) and mineral fertilizer (NPK 15:15:15). The BSF frass was obtained from AgroEcoHealth platform of IITA-Benin (https://www.iita.org/iita-countries/benin), and consisted of residues from fruit waste conversion by BSF larvae. The fruits were ground and sieved for two days in a basket to reduce water content. The resulting mash was then mixed with bovine blood purchased from a slaughterhouse. Eight thousand (8000) five-day old BSF larvae, were then placed in a mixture of 3 kg of fruits waste and blood. Two weeks later, the residues consisting of excrement, exuviae, and transformed substrate were collected, dried for about a month, and used for the experiment. Poultry manure was obtained from poultry raisers and mineral fertilizers (NPK and Urea) purchased from agrochemical shops. Characteristics of the different sources of organic manure included in the present study and unfertilized soil before the experiments for lettuce and the African eggplant were given in Table 1.

For L. sativa, Chicken manure gave the highest N (6.7 mg/kg), N0₃^- (29.48 mg/kg), NH4⁺ (8.71 mg/kg), K (81 mg/kg), Mg (24 mg/kg), and Ca (290 mg/kg) contents, making it a rich source of essential nutrients. The BSF frass, while lower in nitrogen (5.35 mg/kg), contained the highest phosphorus concentration (9.16 mg/kg), making it particularly beneficial for phosphorus-deficient soils. Unfertilized soil, with significantly lower nutrient levels, shows poor fertility, particularly in nitrogen (0.24 mg/kg) and phosphorus (0.55 mg/kg). The electrical conductivity (EC) was highest in unfertilized soil (40.5), suggesting potential salinity issues, whereas chicken manure (5.92) and BSF frass (7.04) presented more moderate EC values. In terms of pH, chicken manure was slightly alkaline (7.75), while BSF frass was mildly acidic (6.35), and unfertilized soil is the most acidic (5.30). These results indicate that both organic amendments significantly improve soil fertility, with chicken manure providing a more balanced nutrient supply and BSF frass being a better phosphorus source, allowing for targeted soil enrichment based on specific agricultural inputs.

In S. macrocarpon, the physico-chemical analysis of the soil and amendments shows significant variations in nutrient composition, which directly influence soil fertility. Chicken manure has the highest nitrogen content (1.72 mg/kg), followed by BSF frass (1.04 mg/kg), while unfertilized soil contains the lowest amount one (0.15 mg/kg). Similarly, nitrate (NO₃^-) and ammonium (NH₄⁺) concentrations were highest in chicken manure (7.568 mg/kg and 2.236 mg/kg, respectively), indicating a greater potential for immediate nitrogen availability. The BSF frass also contained notable amounts of these nitrogen forms (4.576 mg/kg NO₃^- and 1.352 mg/kg NH₄⁺), whereas unfertilized soil shows minimal levels (0.64 mg/kg NO₃^- and 0.19 mg/kg NH₄⁺) (Table 1). Regarding mineral nutrients, chicken manure and BSF frass had the highest magnesium respectively (23.67 mg/kg; 17 mg/kg), and potassium concentrations (8.13 mg/kg; 12 mg/kg). Both amendments significantly improved phosphorus levels (≈29 mg/kg) compared to unfertilized soil (5.79 mg/kg), which was an essential element for plant growth. In addition, calcium levels were substantially higher in amended soils, with chicken manure (containing 300 mg/kg) and BSF frass (266.67 mg/kg), compared to that (29.33 mg/kg) of unfertilized soil. Electrical conductivity (EC) is highest in BSF frass (15.85), indicating increased dissolved salts, whereas chicken manure exhibits the lowest EC (6.31). The pH values of all treatments remain within a favorable range, with chicken manure slightly alkaline (7.72) and BSF frass slightly acidic (6.38), ensuring nutrient availability (Table 1). Overall, both organic amendments significantly enhanced specific soil nutrients content, improving potential soil fertility.

The soil and organic fertilizers macronutrients were analyzed at the chemical laboratory of the AgroEcoHealth platform at IITA-Benin. Before planting and after harvesting, soils were sampled at 0–20 cm depth in each experimental plot for soil nutrients analysis, pH and conductivity. To assess the nitrogen (N), phosphorus (P), potassium (K⁺), magnesium (Mg²⁺), calcium (Ca²⁺) content and conductivity of the soil samples, we first extracted them using the protocol described by Mehlich 3 as reported by Wuenscher et al. (2015). It involved weighing 3 g of previously sorted and ground sample, introducing it into a 50 mL tube and adding 30 mL of a solvent solution containing 0.01 M CaCl₂, 0.25 M NaHCO₃ and 0.013 M HNO₃. The mixture was mechanically stirred for 10 min and prior to mineral analysis. 10 mL of the extract was taken and put in a photometer (Wagtech 7100 photometer, UK) at the chemical laboratory of the AgroEcoHealth platform, IITA Benin, to estimate the mineral content and conductivity of the samples.

Soil mineral content was determined before planting and after harvesting of each plant species. The growth parameters considered were the number and size of leaves per plant, the height of the plants, the diameter of the clump, and recorded once a week after transplanting on four central plants per experimental plot. At harvest, plants were counted and weighed to estimate yield in kg/m2. A tape measure was used to take height and clump diameter. Yield was estimated just after harvesting.

Data on leaf number, plant height, clump diameter, and yield were processed by performing the Analysis of variance (ANOVA) with 95% confidence interval (CI) with the software SAS 9.2 (SAS Institute Inc, 2009). Statistically significant differences between the mean values in different treatments were determined using the Student Newman Keuls method with 95% CI (P<0.05). Principal component analysis (PCA) was performed with R software. Model residuals were verified for normality using Shapiro-Wilk normality test. The test t of Student was applied to compare soil nutrients content before planting and after planting of L. sativa and S. macrocrpon.

Highly significant differences (p < 0.001) were observed between treatments for lettuce growth parameters. Table 2 gives the effects of different treatments (T0, T01, T1, T2, T3 and T4) on plant height and stem diameter over two seasons at three observation time points (Week 3, Week 4, and Week 5). In Season 1, T2 (poultry manure 20t/ha) and T3 (BSF Frass 20t/ha + 100 kg/ha Urea) gave the highest plant heights at Week 5 (14.93 ± 0.33 cm and 14.31 ± 1.02 cm, respectively), followed by T1 (BSF Frass 20t/ha) and T4 (BSF Frass 10t/ha + 100 kg/ha Urea) (13.87 ± 0.53 cm and 13.45 ± 0.54 cm, respectively), while T01 (200 kg/ha NPK + 100 kg/ha Urea) and T0 (untreated plot) presented the lowest values across all weeks. A similar trend was observed in Season 2, where T3 and T4 showed the greatest heights. For stem diameter, Season 1 recorded higher values, with T1, T2, T3, and T4 significantly outperforming the control (T0: 19.06 ± 0.92 cm vs. treated plants >25 cm). However, in Season 2, diameter values were lower, suggesting possible environmental influences. Overall, the results indicated that treatments significantly enhanced plant growth, particularly T2 and T3, while seasonal variations likely affected stem development (Table 2).

For lettuce production, the results indicated that organic and combined organic-inorganic amendments significantly enhanced the growth performance compared to control treatments. Specifically, poultry manure (T2) and BSF frass combined with urea (T3) led to the highest plant heights and stem diameters in both seasons, with notable seasonal variations. These findings align with previous studies highlighting the beneficial effects of organic amendments on crop growth. For instance, Adekiya et al. (2019) reported that poultry manure significantly improved lettuce height and biomass accumulation due to its high nutrient content and slow-release properties. Similarly, Liu et al. (2024) demonstrated that organic fertilizers enhance soil structure and microbial activity, thereby improving nutrient uptake efficiency in leafy vegetables.

A key observation in our study is the effect of seasonal variations on plant growth, particularly for stem diameter, which was lower in Season 2. This is consistent with findings by Sun et al. (2022), who noted that environmental factors such as temperature and precipitation affect plant morphological traits. The lower stem diameter in Season 2 observed, even with the soil amendments, may be attributed to fluctuations in temperature or moisture availability, impacting nutrient uptake and allocation.

The graph (Figure 7) comparing total yield per 3m² across different treatments over two seasons revealed highly significant differences (p < 0.001). BSF frass applied at 20 t/ha (T1) produced the highest yield (5.35 and 5.12 kg/3m²), followed by poultry droppings at 20 t/ha (T2) (4.75 and 4.91 kg/3m²). These results demonstrated that at sufficient doses organics amendments could be used singly (without inorganic fertilizers) for lettuce production. This result confirmed those from Sawinska et al. (2024) and Houben et al. (2020) who supported that frass can be as effective in fertilization as classic NPK mineral fertilizers because of its quick mineralization and easily available nutrients and can supplement or even completely replace mineral fertilization.

The higher performance of BSF frass (T1) aligns with findings by Chavez et al. (2023) on lettuce, arugula and tomatoes. Van Huis et al. (2021) reported that insect frass improves nutrient availability and microbial activity, thereby promoting plant productivity. According to Watson et al. (2021) and Klammsteiner et al. (2020), treating frass may also be good to increase nitrogen availability, through anaerobic digestion or composting. Frass can also have a stimulating effect on the number of soil microorganisms: the number of bacteria, archaea and fungi, carbon mineralization and nitrification, which may ultimately translate into the availability of post-food nutrients for the plant. The lower performance of T3 (BSF frass at 20 t/ha + 100 kg/ha Urea) and T4 (BSF frass at 10 t/ha + 100 kg/ha Urea) compared to BSF frass alone suggests potential nutrient imbalances, as previously noted by Elissen et al. (2023), who highlighted the importance of balancing N sources in organic-inorganic fertilization systems. In addition, BSF frass doses (10 t/ha and 20t/ha) already contained enough mineral N to meet the nutrient needs of the lettuce plant. Therefore, the addition of Urea created a N surplus that disrupted the chemical properties of the soil resulting in inhibition of plant growth and reduction of plant yields. This could be due to the toxicity of NH₄⁺. This effect may also be allelopathic (Gärttling et al., 2020). According to Alromian et al. (2020), frass had various effects on the yield and mineral composition of lettuce leaves. Also, no increase in the yield was observed even at higher doses.

In terms of post-harvest soil nutrient concentrations, P levels were significantly higher in poultry manure-treated soils (T2) followed by T1 (BSF frass), while inorganic fertilizer (T01) gave the lowest P availability. This result supports the work of Mensah and Frimpong (2018), who reported that organic amendments, particularly poultry manure, enhance P solubility through microbial mineralization, whereas synthetic fertilizers can lead to phosphorus fixation, reducing plant availability. Likewise, Chavez et al. (2024) reported lower soluble P concentrations in the frass treatments.

Similarly, BSF frass combined with urea (T3: BSF frass 20t/ha+ 100 kg/ha of urea) significantly increased K, Mg, and Ca concentrations, suggesting its potential to improve soil nutrients balance. These results agree with studies by Van Huis et al. (2021), who found that insect frass contributes to soil fertility by enhancing cation exchange capacity and providing essential macronutrients. Likewise, Fernández-Romero et al. (2016) observed the impacts of BSFL frass on lettuce production and found increases in soil organic matter and residual nutrient content. In the case of fertilizing the Gongronema latifolium Benth (Apocynaceae) plant with inorganic NPK fertilizer, this led to an increase in the concentration of P, sodium (Na), Ca, Mg, P and N in the leaves; the concentration of nutrients increases with increasing fertilization (Osuagwu et al., 2010). However, electrical conductivity (EC) was highest in T3, indicating potential risks of salinity buildup leading decrease lettuce yield observed in this treatment. Watson et al. (2022) mentioned nitrite (N02) build-up, emissions of substantial quantities of CO2 and N2O, salinity and ammonia content of the frass. Consequently, decreases in inorganic fertilizer application can reduce subsequent environmental problems, such as soil degradation and eutrophication.

Soil pH remained stable but acidic across treatments probably due to overuse of chemical fertilizers promote soil acidification by releasing H⁺; and intensive farming and monoculture. The soil of IITA-Benin had been used in rice production with a higher application of chemical fertilizers leading to reduced soil fertility and crop yields and increased toxicity of metals such as aluminum (Al) and manganese (Mn), which are harmful to crops (Mensah and Frimpong, 2018).

Moreover, Mg accumulation depends on many factors. These include genetic factors, but also environmental factors such as soil and climate. The plant absorbs nutrients from the soil solution through various mechanisms. Potassium, calcium and magnesium ions have antagonistic effects. The uptake of P by a plant depends on its concentration in the soil solution, and the concentration also affects the uptake mechanism. The role of P in the plant growth was multiple and it mainly, as a support for photosynthesis. It also increases the plant's resistance to unfavorable environmental conditions (water shortage) by regulating the opening of stomata. Calcium in the plant is responsible for the construction of cell walls. The results of PCA supported these conclusions on effects of cations exchanges on lettuce growth and yield.

In the conducted experiment, frass application had influenced the fresh mass of lettuce and did not confirm those from Sawinska et al. (2024). More research is needed on different frass types (and related to the larval feedstock and possible thermal pre-treatment) and in long-term field experiments with a variety of crops with special emphasis on C and N mineralization. Lopes et al. (2022) have compiled data on 17 BSF frass samples and concluded that frass composition is highly variable and dependent on substrate, especially P, K and micronutrient concentrations. Palma et al. (2020) also concluded that the frass needs stabilization before application as soil amendment. Seasonal variations in plant growth were observed, particularly for stem diameter, but total yield remained stable across seasons, reinforcing the long-term viability of organic amendments as part of sustainable food systems (FAO, 2017).

In African eggplant, our results demonstrated that fertilizer treatments significantly affected the growth parameters and yield particularly in the first harvest. The plant height was higher with organic amendments including BSF frass treatment 10 t/ha and 15 t/ha compared to inorganic fertilizers and controls. This finding supports previous studies indicating that organic fertilizers improve plant growth by enhancing soil nutrients availability and microbial activity (Adesina et al., 2020). The greater plant height observed in the first harvest compared to that of the second one may be attributed to initial nutrient availability, which declined over time due to plant uptake and potential leaching losses (Xu et al., 2020). No significant difference was observed between poultry manure and inorganic fertilizers for African eggplant height in first season. This result confirmed those of Olanipon et al., 2020 who reported similar plant height on African eggplant when using poultry manure and inorganic fertilizer. The reduced plant height in the second harvest across all treatments suggests nutrient depletion over time and was consistent with the findings of Agele et al. (2018) and Vanlauwe et al. (2015), who observed that repeated harvesting without adequate nutrient replenishment led to a decline in plant vigor and productivity. Furthermore, the lowest plant height recorded under the control treatment underscores the necessity of fertilizer application for optimal African eggplant growth, as observed by Olanipon et al. (2020). Significant reductions in growth parameters were obtained when Solanum crops were grown without nutrient supplementation (Olanipon et al., 2020).

The number of leaves and leaf dimensions followed a similar trend to plant height, reinforcing the role of nutrient-rich amendments in promoting plant growth. The BSF frass (T2) consistently produced the highest leaf number and size, suggesting its high potential in supporting robust foliage development. The correlation with soil nutrient status is evident, as T2 presented the highest total N, NO₃-, and NH₄+ content. Nitrogen is essential for plant growth, and its higher levels in BSF frass-treated plots may explain the higher leaf and stem development observed (Sarker et al., 2020). The treatment T1 (BSF frass at 15 t/ha) and T4 (poultry manure at 10 t/ha) also promoted substantial leaf development, highlighting their efficacy in enhancing foliar growth. These findings align with previous research indicating that organic matter-rich amendments enhance photosynthetic area, thereby supporting overall plant growth (Sawinska et al., 2024; Vanlauwe et al., 2015). Chicken manure (T4) also maintained a relatively high leaf number in the second harvest, indicating its sustained nutrient release compared to mineral fertilizers, which primarily provide an immediate nutrient boost but lack long-term effects (Chen et al., 2018). This suggests that chicken manure provided a sustained effect on leaf production, consistent with the findings of Nathaniel et al. (2019), who reported that poultry manure improved leaf retention and overall canopy development in vegetable crops. The unfertilized plots gave the lowest number and size of leaves regardless of the harvest as reported by Olanipon et al. (2020).

Leaf length significantly varied between treatments, with T2 consistently producing the longest leaves (31.77 ± 0.87 cm in Harvest 1 and 19.63 ± 0.76 cm in Harvest 2). The results support studies by Ravindran et al. (2019), who found that organic fertilizers promote leaf expansion, contributing to higher biomass accumulation. The wider leaves observed in T2 (22.42 ± 0.81 cm in Harvest 1 and 11.81 ± 0.53 cm in Harvest 2) further reinforce the beneficial effects of BSF frass at 10 t/ha, as previously demonstrated by Beesigamukama et al. (2021) and Ndor et al. (2010).

Yield data further supports the positive impact of fertilizer application on African eggplant productivity. Highly significant differences in yield were observed during the first harvest, with BSF frass at 10 t/ha and 15 t/ha producing the highest yields. This aligns with the findings of Beesigamukama et al. (2021), who found that BSF frass enhances biomass accumulation due to its high nutrient content and slow-release properties. Furthermore, studies reported by Ayeni et al. (2014) and Ndor et al. (2010) confirm that the use of organic amendments contributes significantly to yield improvements in Solanum species. Lower application rates (5 t/ha) resulted in yields comparable to those obtained with chicken manure (10 t/ha) and NPK + urea, suggesting that a threshold exists for optimal nutrient supplementation, as observed by Akanbi et al. (2005) and Nathaniel et al. (2019). Similar findings were reported by Adeniyan et al. (2011), who found that organic fertilizers outperformed mineral fertilizers in promoting higher yields due to their positive effects on soil structure and microbial activity. The unfertilized plots produced the lowest yields, further supporting the necessity of fertilizer application for maintaining productivity. Interestingly, the combination of 200 kg/ha of NPK and 100 kg/ha of urea exhibited stable performance across both harvests, which aligns with previous studies indicating that synthetic fertilizers provide consistent, however they are less sustainable compared to organic amendments (Mahuku et al., 2019).

The higher performance of BSF frass treatments can be linked to increased soil nutrient availability, particularly N, P, and K, which were highest in these treatments. The results align with the findings of Beesigamukama et al. (2021), who demonstrated that black soldier fly frass significantly improved plant growth because of its high nutrient content and slow-release properties. Nurfikari (2022) observed rapid mineralization and release of plant available N due to the chitin content of the frass and easily degradable components with high N content, increasing numbers of chitin-degrading bacteria (e.g. some Gammaproteobacteria) and somewhat increasing numbers of fast-growing high N containing fungi (e.g. Mortierellomycota) were found in frass amended soils. She concludes that insect waste streams have potential as soil health-promoting amendments. Similar benefits of organic fertilizers have been reported by Mahuku et al. (2019), who highlighted that organic amendments enhance soil microbial activity and structure, leading to better nutrient uptake and overall plant health. Additionally, the positive effect of organic fertilizers compared to synthetic fertilizers corroborates the findings of Adeniyan et al. (2011), who found that Solanum crops cultivated with organic amendments exhibited superior growth and resilience against soil degradation. However, during the second harvest, no significant differences were observed among treatments, likely due to nutrient depletion over time and possible nutrient immobilization by soil microorganisms reinforcing the findings of Agele et al. (2018) and Vanlauwe et al. (2015).

Additionally, soil chemical analysis further supports the observed plant growth and yield trends. The highest total N, NO₃-, and NH₄+ levels in T2 indicate that BSF frass amendments contribute significantly to N enrichment. This is consistent with the findings of Gajalakshmi and Abbasi (2004), who reported that organic fertilizers, particularly insect-based frass, improve N mineralization and microbial activity. Additionally, the highest K and P levels in T3 and T2 suggest that BSF frass enhances the availability of these essential macronutrients, which play a key role in root development and reproductive growth (Roy et al., 2006). The stability of soil pH across treatments (pH < 7.5), with a slight decrease in BSF frass-treated plots, indicates that microbial decomposition processes may contribute to mild soil acidification, a common phenomenon in organic matter-rich soils (Zhao et al., 2021). This result support those from Pietri and Brookes (2009) who demonstrated that populations of bacteria and fungi are affected by both soil pH and inputs of C as substrates. These effects could possibly be manifested by the succession of different microorganisms or activities during the decomposition of the added organic matter.

Principal component analysis of S. macrocarpon parameters also showed positive correlation between EC, plant growth and yield in BSF treatments (T2, T1), compared to control and poultry manure reflecting increased ion availability and nutrient dissolution. The relatively lower EC values in control and chicken manure-treated plots (T0, T4) suggest lower nutrient solubility and availability in these treatments. This is in line with principal component analysis of African eggplant parameters and the study by Alam et al. (2020) and Mokolobate and Haynes (2002), which highlighted that organic fertilizers with higher nutrient content led to increased soil conductivity, positively correlating with improved plant performance. Naser El Deen et al. (2023) indicated that OM content in BSF frass is also higher than all other manure and compost types. N content and C/N ratio are closest to the values of cow slurry, while P content is most comparable to that of pig slurry and K content is most comparable to that of poultry manure.

Overall, our findings highlight the agronomic benefits of organic amendments, particularly BSF frass and poultry manure, in enhancing L. sativa and S. macrocarpon growth and maintaining soil fertility. The results suggest that integrating organic amendments, notably BSF frass, into fertilization strategies can optimize nutrient availability while mitigating the environmental drawbacks associated with synthetic fertilizers. This approach directly contributes to achieving key sustainability goals in Sub-Saharan Africa (SSA), including the promotion of climate-resilient agriculture, the preservation of soil health, and the reduction of dependence on imported chemical inputs (Sachs et al., 2024). Furthermore, such practices would contribute to food security goals by improving smallholder productivity, enhancing soil fertility for sustained production, and supporting diversification through the use of locally available organic resources (Van Huis et al., 2021).

These results also reinforce regional and continental commitments, such as the Malabo Declaration and CAADP, which emphasize sustainable intensification, reduction of land degradation, and resilience building for food and nutrition security (NEPAD, 2014). Future studies should focus on long-term effects of these amendments on soil microbial dynamics and crop productivity across different agro-ecological conditions, thereby supporting broader efforts to achieve environmental sustainability and food security in Benin.