The Field experiments were conducted at the International Centre of Insect Physiology and Ecology (icipe), situated in Nairobi (1° 13’ 18.5” S 36° 53’ 50.7’’ E; 1616 m above sea level) over two cropping seasons (June – October 2023 and November – March 2024). The site experiences an average monthly temperature range of 12 – 29°C and receives an average annual rainfall of 787 mm characterized by a bimodal distribution. The short rains typically occur from October to December, while the long rains span from March to June. The study area has well-drained, mostly sandy clay soils, classified as humic Nitisols. There is 2400 mm of evapotranspiration each year (Jaetzold and Schmidt, 1982).

From each plot, the soil was collected from 0–30 cm depth before the application of fertilizers using a soil auger and following the zig-zag pattern. The representative soil samples were collected by quarter sampling. The collected soil was air-dried for 5 days at 25°C, and ground using a mortar and pestle before sieving through a 2 mm sieve to remove foreign objects and bigger particles. The soil was analyzed for physiochemical properties following standard methods described in Okalebo et al. (2002). The results of soil analysis are presented in Table 1 . The land was prepared by hand hoeing to remove all the weeds and create a fine tilth for broccoli cultivation.

The trial comprised of five fertilizers: solid BSFFF, solid BSFFF fortified with 3% BSF chitin, liquid BSFFF, and liquid BSFFF fortified with 3% BSF chitin, and commercial organic fertilizer (Safi). The liquid and solid BSFFF used in the field experiment were prepared at icipe following procedures described by Anedo et al. (2025) and Kisaakye et al. (2024). The solid BSF frass fertilizer was a product obtained from the feeding of BSF larvae on a substrate made of potato peels at icipe, Nairobi, Kenya. The BSF larvae were reared according to Shumo et al. (2019). After harvesting of larvae at 2 weeks, the frass was composted for 4 weeks using the heap method to obtain a mature and stable frass product, which was used in this study as solid BSF frass fertilizer. The chitin-fortified liquid fertilizer was formulated using black soldier fly frass, BSF exuviae effective microorganisms (EMs), biochar and molasses. The mixed solid materials were placed in airtight fermentation tanks as described by Kisaakye et al. (2024). Each tank was tightly covered with a lid fitted with a 5-mm-diameter tube to allow for gaseous exchange. The contents were fermented for 6 weeks with weekly stirring to obtain mature and stable chitin-fortified fertilizer. The maturity and stability of frass fertilizer extracts were determined following previously described procedures (Beesigamukama et al., 2021a). After fermentation, the chitin-fortified fertilizer was sieved through a 150-µm sieve (Endecotts Ltd., London, UK), put into 20-L non-transparent jerrycans, and stored in a cool place pending field experiment. The liquid BSF frass fertilizer was prepared using the same procedure, excluding addition of chitin. The chitin-fortified solid fertilizer was produced by mixing the composted BSF frass fertilizer with milled BSF exuviae following procedures described by Anedo et al. (2025). The commercial organic fertilizer (Safi) was obtained from Safi Organics Limited located in Mwea town, Kirinyaga county, Kenya. The organic fertilizers were analyzed for nutrient levels and other properties following standard laboratory methods described in Okalebo et al. (2002), and the results presented in Table 1 . Broccoli seeds (Green sprouting variety) [Brassica oleracea] were obtained from SimLaw seeds, Nairobi, Kenya. The seeds were sown in 4 seedling trays of 160 wells. The trays were filled with topsoil obtained from icipe farm and then planted with one seed per well. The trays were placed in a greenhouse watered twice a day and managed following recommended practices (Omondi, 2014). The seedlings were then transplanted into the experimental plots 4 weeks after germination.

A randomized complete block design (RCBD) with three replications was used to establish the experiment. The RCBD was chosen to cater for soil heterogeneity. Plots of 2 m × 3 m with a spacing of 1 m between blocks and 0.5 m between plots were used. The solid organic fertilizers were applied 3 days before planting, using band placement method to ensure the timely onset of mineralization and synchrony of nutrient release for plant uptake. The liquid BSFFF and chitin-fortified liquid BSFFF were applied twice weekly, starting from the 3^rd week after transplanting up to the harvesting time. This was achieved by diluting 200 ml of each liquid fertilizer into 20 liters of water. The split-application was meant to reduce nutrient leaching and improve fertilizer use efficiency. The liquid fertilizers were applied by drenching the soil around the plant base using a knapsack sprayer. The fertilizers were applied at rates equivalent to 250 kg N/ha (Vågen et al., 2007). To meet this nitrogen requirement, the amounts applied were6775.1 kg/ha, 5567.9 kg/ha, and 8333.3 kg/ha for solid BSFFF, solid BSFFF fortified with 3% BSF chitin, and Safi, respectively. Broccoli was planted at a spacing of 60 cm between rows by 45 cm between plants.

During the experiment, weeding was done twice a month using a hand hoe, while irrigation was performed three times per week. The study was conducted in the same plots for two consecutive cropping seasons. The first season ran from June to October 2023 while the second season began in December 2023 and ended in April 2024. For clarity, throughout this manuscript, the seasons are referred to as season 2023A and season 2023B for the first and second seasons, respectively.

Broccoli growth was assessed by collecting data on plant height, chlorophyll content, stem diameter, leaf length, leaf width, number of secondary curbs/heads, and the number of leaves every week, from the 8^th week after transplanting up to the 17^th week when harvesting was conducted. Ten randomly selected plants were tagged per plot and used to measure the different growth parameters. The plant height, leaf length, and width were measured using a tape measure. Plant height was measured using a tape measure that was stretched from the ground level up to the tip of the shoot. Leaf chlorophyll was measured using SPAD-502 (Konica Minolta, Tokyo, Japan), which was placed on top of six fully grown leaves. Digital vernier callipers (Mitutoyo, Kanagawa, Japan) were used to measure the stem diameter at 10 cm from the ground.

The total biomass and yield were determined at the harvesting stage. At harvesting, the plants from each plot were uprooted and soil washed off from the roots using tap water. The edible flower heads were harvested by cutting using a knife and non-edible parts were discarded. The yield was determined by determining the weight of heads per treatment using a weighing balance. Part of the harvested broccoli heads was chopped, air-dried to reduce moisture for 5 days, and then oven-dried (SDO-225-CLAD-F-200-HYD, Wagtech Projects Limited, Thatcham, United Kingdom) at 60°C for 72 hours to drive off all the moisture and determine dry matter yield on a kg per hectare basis. The dried samples were then crushed into powder using an analytical mill to measure proximate (crude ash, carbohydrates, crude fiber, crude fat, and crude proteins using standard methods (Van Soest et al., 1991). The minerals were determined using atomic absorption spectrometry (AAS) (iCE 3300 AA system, Thermo Scientific, Beijing, China) at respective wavelengths (Okalebo et al., 2002).

The concentrations of N, P, and K were determined in section 2.5 (Okalebo et al., 2002), and yield data were used to calculate nutrient uptake in broccoli heads (Equation 1). Agronomic efficiency (AEN), which is a measure of yield produced per unit N supplied from each treatment, was also calculated using broccoli yield from each treatment (Equation 2).

Where,

F represents plots that received fertilizer treatments

C represents the control treatment (unfertilized soil)

The profitability of broccoli production using the different forms of BSFFF and commercial organic fertilizer was assessed by determining net income, benefit-to-cost ratio, return on investment, and gross margins (Equations 3-6) (Chia et al., 2019). The variable costs considered during the study included the cost of seeds, fertilizers, labor, and pesticides. The pesticide was used to manage aphids during the experiment. We used Degree Max 200EC (Osho Chemical Industries limited, Nairobi, Kenya). The application rates were 2.5ml per 20 liters of water, and it was applied once a week for 5 weeks. The prices of solid fertilizers were sourced from the websites of Mzuri Organics (https://mzuriorganics.co.ke) and Safi Organics (https://safiorganics.co.ke) while the price of green sprouting broccoli seeds was obtained from Simlaw seeds (https://www.simlaw.co.ke). The labor costs during land preparation, fertilizer application, planting, and weeding were determined using the hourly rate of US$1.25 (The Labor Institutions Act (No. 12 of 2007), 2022). Broccoli fresh yield was considered a source of revenue.

The Shapiro-Wilk test was used to check the normality of data. Data on broccoli growth (number of leaves, leaf area, plant height, number of curds, chlorophyll content, and stem diameter) parameters were analyzed using the linear mixed effect function from the model with “lmer” package “lme4” in R Studio (R Core Team, 2022) with fertilizer treatment and growth period as the fixed effect, replication as a random effect). The following data were analyzed using Analysis of Variance (ANOVA) because they met the assumptions of normality: broccoli growth parameters, including plant height, number of leaves, leaf area, stem diameter, and chlorophyll concentration for both seasons. Also, the number of edible flower heads, fresh and dry yield, proximate composition of broccoli (crude protein, ash, fat, fiber, and carbohydrates), mineral content (potassium, magnesium, calcium, sodium, manganese, copper, boron, molybdenum, zinc, sulfur, and cobalt), nutrient uptake (potassium and phosphorus for both seasons), and agronomic nitrogen use efficiency for season 2023B, were analyzed using (ANOVA). Conversely, data that did not meet normality assumptions [fresh biomass (2023A), dry biomass (2023A and 2023B), nitrogen uptake and agronomic nitrogen use efficiency (2023A), phosphorus and iron concentrations in broccoli, economic returns (2023B), benefit-to-cost ratio and return on investment (2023A), and dry matter content] were analyzed using a Generalized Linear Model (GLM). In case of significant differences, the means were separated using Tukey’s Honest Significant Difference (HSD) test. All statistical analyses were conducted separately for each season using R software version 3.6.1 for Windows (R Core Team, 2022).

The net income, gross margin, benefit-to-cost ratio (BCR), and return on investment (ROI) of broccoli production are presented in Table 4 . Broccoli grown in plots amended with chitin-fortified solid and chitin-fortified liquid BSFFF generated higher net income compared to the vegetables grown in unfertilized plots. The net income yielded by broccoli grown using liquid BSFFF, chitin-fortified solid BSFFF and chitin-fortified liquid BSFFF was 18.9-fold, 25.7-fold, 19.5-fold higher than the values achieved with Safi fertilizer during season 2023A. Furthermore, in the same season, chitin-fortified solid BSFFF (30.5-fold) and chitin-fortified liquid BSFFF (2.9%) yielded significantly higher net income than non-fortified formulations (solid BSFFF and liquid BSFFF). Economically, the chitin-fortified solid BSFFF was the most effective treatment during 2023A which coincided with the long rains, whereas, the liquid formulation was more profitable during 2023B, characterized by short rains.

The gross margin, BCR, and ROI exhibited similar trends, with BSFFF treatments outperforming Safi. Additionally, chitin-fortification further enhanced the economic returns of broccoli production compared to non-fortified BSFFF products. Notably, the application of Safi or solid BSFFF was less profitable than the production of broccoli with no fertilizer (control treatment).

The improved broccoli growth and its association with different black soldier fly frass fertilizers as compared to commercial organic fertilizer and unfertilized soil has been previously reported using vegetables and other crops (Chavez et al., 2024; Chepkorir et al., 2024; Rejeki et al., 2023; Abiya et al., 2022; Dzepe et al., 2022; Anyega et al., 2021; Ouda and Mahadeen, 2008), and this can be ascribed to the high nutrient levels in black soldier fly frass fertilizer and chitin-rich exuviae that is readily available to plants (Anedo et al., 2025; Beesigamukama et al., 2020a). Furthermore, the BSF frass fertilizer does not have phytotoxicity challenges (Beesigamukama et al., 2021b), and has a low C/N ratio, a high mineralization rate (Adin Yéton et al., 2019), and better synchrony of nutrients for plant growth (Beesigamukama et al., 2020b).

Past studies have also demonstrated the benefits of frass fertilizer in addressing soil acidity and salinity, suppressing pests and pathogens, and enhancing soil microbiota, thereby improving soil health for better plant growth (Kisaakye et al., 2024; Anedo et al., 2025; Beesigamukama et al., 2021b; Poveda, 2021; Poveda et al., 2019; Kagata and Ohgushi, 2012). The chitin fertilizer contained in the BSF frass also enhances soil fertility and microbiota. In leafy plants, dry weight is frequently a good sign of above-net primary productivity (Smart et al., 2017), partly due to increased photosynthetic activity (Mendoza-Tafolla et al., 2019) indicated by high chlorophyll content, and root development (Boudabbous et al., 2023) which are consistent with the findings of this study. Past studies (Kisaakye et al., 2024) have reported improved root development in vegetables grown with chitin-fortified black soldier fly frass fertilizer. The enhanced plant growth associated with chitin-fortified fertilizers and BSFFF could be attributed to the synthesis of amino acids that play an active role in plant growth and tolerance to environmental stresses (Trovato et al., 2021).

The higher broccoli growth and yield achieved in the second season can be attributed to residual benefits of the BSFFF amendments and higher rainfall, which enhanced the higher nutrient release and uptake as reported in the study. This also caused the leaves in season 2023B to mature faster than those in season 2023A, thus the difference in peaks of the graphs. The high nitrogen, phosphorus and potassium uptake of broccoli grown in soil amended with BSF frass fertilizer, compared to Safi has been previously reported (Chepkorir et al., 2024; Anyega et al., 2021; Beesigamukama et al., 2020a) and can be attributed to better root formulation linked to phosphorus availability within the rhizosphere (Beesigamukama et al., 2021b).

Soils amended with BSF frass fertilizer improves mineral nitrogen in the root zone, resulting in higher nitrogen uptake and crop yields (Beesigamukama et al., 2021b). Therefore, the higher agronomic nitrogen efficiency achieved using frass fertilizer products when compared to Safi, accrues from the multiple benefits of these novel fertilizer products on soil health and aligns with previous studies (Anyega et al., 2021; Beesigamukama et al., 2020a). Our findings show that farmers would need to apply fewer quantities to realize higher increases in crop yields (Beesigamukama et al., 2022b), hence reducing the burden of high fertilizer purchase costs. It should be noted that high prices are one of the hindrances to fertilizer use in Africa and most developing countries. Therefore, adopting high-quality frass fertilizer could provide a sustainable solution to this challenge and transform agri-food systems.

The differences in broccoli growth and yield observed due to the different BSFFF products highlight the crucial role of fertilizer source and quality in influencing soil health, crop productivity, and overall food security. The higher broccoli yield achieved using BSF frass fertilizers may be attributed to their the additional benefits, such as pest and disease suppression, which is not typically provided by conventional fertilizers like Safi (Anedo et al., 2025; Tanga and Kababu, 2023; Wantulla et al., 2023; Barragán-Fonseca et al., 2022). It should be noted that due to climate change and soil degradation, crop production faces several abiotic challenges and adoption of multipurpose inputs such as insect frass fertilizers provide a holistic solution. These inputs not only supply essential nutrients but also contribute to pests and disease control, improve soil moisture retention, and enhance biodiversity (Tanga and Kababu, 2023). The regenerative insect frass fertilizers also contribute to safe food production due to reduced pesticide and agrochemical residues in food crops.

The differences in the performance of solid and liquid frass fertilizers observed can be attributed to the distinct characteristics of each product. The liquid frass fertilizers contain readily available soluble nutrients for plant uptake compared to the solid which requires time, water, and microbes for the mineralization process. The attribute of fast nutrient release associated with liquid frass fertilizers enables them to rapidly address nutrient deficiencies, even during critical crop growth stages, leading to better growth and yield. Therefore, the superior performance of liquid BSFFF observed during the dry season 2023A could be largely due to its dual-purpose role of supplying both moisture and nutrients, making it valuable during periods of moisture stress. This characteristic positions liquid BSFFF as a promising input for climate-resilient agriculture, helping cropping systems adapt to the impacts of climate change (Abiya et al., 2022; Barragán-Fonseca et al., 2022).

On the other hand, the higher agronomic efficacy of chitin-fortified BSF frass fertilizers compared to non-fortified BSF frass fertilizers can be attributed to the benefits of chitin in enhancing crop growth, nutrient availability, and plant defense through suppression of pests and diseases (Anedo et al., 2025; Kisaakye et al., 2025; Quilliam et al., 2020). Past studies have reported reduced aphid infestation of pests in vegetables grown in soil amended with chitin-fortified fertilizer (Kisaakye et al., 2024). Additionally, chitin-fortified fertilizers also promotes the growth of chitin-degrading microbes, which enhances microbial activity and diversity, which in turn improves soil fertility and soil microbiota (Barragán-Fonseca et al., 2022). Future research should explore the effect of the different forms of black soldier fly frass fertilizers on the management of broccoli pests and diseases to generate accurate recommendations.

The increased concentrations of crude protein, fat, ash, and carbohydrate observed in this study following the application of the different forms of BSFFF shows that, alongside vegetable yield, BSFFF is efficient in boosting the nutritional quality of vegetable crops. These results are align with previous studies that reported improved nutritional profiles of different food crops grown in soils amended with insect frass fertilizers (Anyega et al., 2021; Yoder and Davis, 2020; Meena et al., 2018; Mukta et al., 2016; Chakwizira et al., 2015). The observed improved nutritional quality of broccoli could be associated with nutrient uptake, as discussed in section 3.1. In particular, the enhanced levels of minerals such as potassium, phosphorus, calcium, and manganese corroborate earlier reports on use of BSF frass fertilizer (Menino et al., 2021) and highlight the role of BSF frass fertilizer in eliminating hidden hunger and malnutrition, contributing to sustainable development goal to zero hunger. Moreover, recent studies have revealed a high willingness to buy vegetable foods produced using insect frass fertilizer and animal feed on insect-based feeds (Traore et al., 2024), paving the way for full-scale development of insect-based value chains. However, to ensure consumer acceptance and marketability, organoleptic tests are necessary to evaluate taste, texture, and overall appeal, thereby supporting the refinement of insect-based fertilizers and the formulation of recommendations for producing nutritious and palatable food.

The higher economic returns observed with the application of different forms of BSFFF formulations, compared with Safi could be largely attributed to their high quality and affordability of BSFFF, as previously reported (Chepkorir et al., 2024; Tanga et al., 2021). In particular, the higher net income, gross margin, benefit-to-cost, and return on investment achieved with chitin-fortified frass fertilizers relative to other fertilizer products assessed could be largely attributed to dual benefits of controlling pests and diseases while supplying nutrients (Anedo et al., 2025; Kisaakye et al., 2024). Furthermore, the enhanced profitability of liquid BSFFF formulations during the dry season suggests additional advantages, such as contributing moisture during periods of water stress. This not only mitigates irrigation costs but also improves crop performance and revenues. The liquid formulations are also more portable than their solid counterparts, reducing transportation costs and making them more accessible to smallholder farmers (Tanga and Kababu, 2023). These findings show that by adopting diversified BSF frass fertilizers, farmers can reduce fertilizer purchase costs, increase income, and contribute to improved soil health, and ultimately improved food security (Beesigamukama et al., 2022b).