Abstract
Industrial insect rearing is expected to increase as a feedstock to meet growing global food demand. This will lead to greater production of insect excreta known as frass, a nutrient-dense organic material that has shown promise as a natural fertilizer source with potential environmental benefits. In this study, black soldier fly (Hermetia illucens) frass (BSFF) was compared with a synthetic fertilizer (SF) during production of containerized ornamentals grown under greenhouse conditions. Fertilizers were incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) planted with coleus (Plectranthus scutellarioides) plugs. Growth index, shoot dry weight, and leaf quality were assessed for a period of 6 weeks. In addition, coleus fertilized at 0.3 kg⋅m–3 N and a control had leachate collected and analyzed weekly for volume, pH, electrical conductivity, and nutrient losses. Black soldier fly frass was found to produce marketable coleus plants at 0.3 kg⋅m–3 N and reduce cumulative N leaching by 87% compared with coleus fertilized with SF at the same rate. Therefore, BSFF can be a suitable fertilizer source for coleus production without compromising growth and leaf quality while potentially decreasing nutrient leaching losses.
Industrial insect rearing as a protein source for animal, aquaculture, and human consumption is expected to expand in the coming years (Paul et al. 2016; Terfa 2021; Wang and Shelomi 2017). Insect species including but not limited to grasshoppers [Schistocerca sp. (Paul et al. 2016)], black soldier fly [Hermetia illucens (Pastor et al. 2015)], and crickets [Acheta domesticus (Kipkoech et al. 2017)] are being evaluated as food products across a range of industries and as potential components of an efficient biodegradation process for solid wastes (Klammsteiner et al. 2020; Lopes et al. 2022; Pastor et al. 2015). Annual food wastes account for close to one-third of global food production, or 1.6 billion tonnes (Food and Agriculture Organization of the United Nations 2011), thereby making food waste a readily available and economically viable substrate to support expansion of the insect-rearing industry.
Black soldier fly (BSF), an insect native to North America, has emerged as a candidate species capable of decomposing solid waste while possessing positive feedstock nutritive characteristics. Black soldier fly is composed of 30% to 40% protein, decomposes food waste quickly, and is not a pest to animals, plants, or people, nor is it a known vector for disease transmission (Liu et al. 2019). However, ramping up BSF production for feedstock is subject to the challenge affecting other animal production operations: how to dispose responsibly or repurpose increasing volumes of waste—or, in this case, excreta—referred to henceforth as frass.
The decomposition of solid waste by insects results in a nutrient-dense material (Anyega et al. 2021; Beesigamukama et al. 2020; Menino et al. 2021; Sarpong et al. 2019) with characteristics of a natural fertilizer depending on feedstock composition (Fielding et al. 2013). However, research regarding BSF frass (BSFF) use as a fertilizer in the United States is limited compared with studies conducted in other countries. For example, in areas of sub-Saharan Africa, BSFF has been shown to increase soil health through organic matter deposition with nutrients mineralized to support maize production (Beesigamukama et al. 2020). In experiments conducted in India, BSFF proved suitable as a biofertilizer for pak choi (Brassica rapa) production, with the added benefit of increasing soil microbial activity compared with conventional fertilizer (Agustiyani et al. 2021). In Italy, Setti et al. (2019) reported that BSF processing residue could be used successfully as a slow-release fertilizer and a peat substitute for baby leaf lettuce (Lactuca sp.), basil (Ocimum basilicum), and tomato (Solanum lycopersicum) when used up to 20% of container volume. It has also been suggested that BSFF may have environmental benefits of reducing nutrient movement with slower nutrient solubility as well as enhancing plant disease resistance through greater chitin concentrations (Chavez and Uchanski 2021).
Experimentation using frass has shown great promise as an alternative nutrient source to synthetic fertilizers (SFs) used in industrialized agriculture and horticulture operations. However, expanding adoption of BSFF as a viable alternative to SF will require validation for ornamental container-based production systems as well as characterization of potential environmental benefits. Therefore, the objectives of this research were to evaluate BSFF compared with an SF and compare nutrient leaching during container-grown production of a short-cycle ornamental.
Materials and methods
Experimental design
A study was conducted on container production of coleus (Plectranthus scutellarioides) under greenhouse conditions following the methods outlined by Sanders et al. (2019) used for examining fertility and irrigation practices on coleus. Experiments were initiated Apr 2021 and Jun 2021 for 42 d at the Ornamental and Turf Research Facility of the Louisiana State University Agricultural Center Botanic Gardens located in Baton Rouge, LA, USA (lat. 30°24'25.3"N, long. 91°06'09.5"W). Thirty-two coleus liner plants were selected for uniform height and quality before transplanting into 1-gal containers. All containers were filled with a bark-based substrate composed of 3:1:1 coarse bark:peatmoss:vermiculite with a size distribution of 16% >6.3 mm, 32% between 6.3 and 2.0 mm, 26% between 2 and 0.7 mm, and 26% <0.7 mm. The substrate had a pH 6.08 ± 0.02 with a bulk density of 0.18 ± 0.007 g⋅cm–3, a container capacity of 55% ± 0.046%, and air porosity of 28% ± 0.048% for a total porosity of 83% ± 0.046%.
Substrate was amended with a micronutrient mix (Micromax Micronutrients; ILC Specialty Fertilizers-Americas, Summerville, SC, USA) at 0.3 kg⋅m–3 and dolomitic lime (MK Minerals, Inc., Wathena, KS, USA) at 4.8 kg⋅m–3. Treatments included a natural fertilizer, BSFF, obtained from Fluker Farms (Port Allen, LA, USA) containing nitrogen (N), phosphorus (P), and potassium (K) (5N:1P:2K) or an SF composed of ammonium sulfate [21N–0P–0K (Hi-Yield; Voluntary Purchasing Group, Inc., Bonham, TX, USA)], triple super phosphate [0N–20P–0K (Hi-Yield; Voluntary Purchasing Group, Inc.)], and muriate of potash [0N–0P–50K (Hi-Yield; Voluntary Purchasing Group, Inc.)] to equal the N:P:K analysis of BSFF. The BSFF used in this study is the result of BSF decomposition of kitchen food wastes obtained from Louisiana State University Food Services in Baton Rouge, LA, USA. Fertilizers were incorporated into the substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 N before planting. Plants were irrigated at 0.8 inch/d with municipal water treated with sulfuric acid to achieve a pH range of 6.5 to 7.0. Containers were arranged across a bench in a completely randomized design, with four replications per fertilizer treatment combination.
Plant growth measurements
(Irmak et al. 2004). Leaf quality measurements were assessed based on visual appearance on a scale of 1 to 9, with 1 representing poor leaf size and color, and 9 representing ideal leaf size and color. Coleus shoot dry weight was collected at 6 WAPs and dried at 40 °C for 72 h, and dry weight was determined gravimetrically.
Leachate collection and analyses
Leachate was collected using 3-gal plastic reservoirs fitted with lids that allowed the containers to drain directly in the reservoirs for SF- and BSFF-fertilized coleus at 0.3 kg⋅m–3 N and an unfertilized control (0 kg⋅m–3 N SF-coleus). Leachate volume was measured every week for 6 weeks with 250-mL subsamples collected and analyzed for pH and electrical conductivity (EC) (model 9813-6; Hanna Instruments, Woonsocket, RI, USA), inorganic extractable nitrate (NO3–) and ammonium (NH4+), and dissolved total P (DTP). Analysis of NO3– and NH4+, was conducted at the Louisiana State University Agricultural Center W.A. Callegari Environmental Center Water Quality Laboratory (Baton Rouge, LA, USA) with quantitation using A flow analyzer (Quickchem 8500 FIA; Lachat Instruments, Milwaukee, WI, USA) that resulted in standard recoveries of >97% for NH4+ and >95% for NO3–. The amount of N leached is reported as a combined total of inorganic N (NO3– + NH4+) henceforth. Samples were also analyzed at the Louisiana State University Soil Testing and Plant Analysis Laboratory (Baton Rouge, LA, USA) for quantitation of DTP using inductively coupled plasma optical emission spectroscopy (SPECTRO ARCOS model FH E12; SPECTRO Analytical Instruments GmbH, Kleve, Germany).
Statistical analysis
Containers of coleus subjected to fertilizer treatments were arranged as a complete randomized design with four replications. Fixed effects included fertilizer source (SF, BSFF, and control) at four levels of application from 0 to 0.3 kg⋅m–3 N using 0.1-kg⋅m–3 N intervals. Experimental run was considered a random effect within the statistical model. Coleus growth indices, leaf quality, leachate volume, pH, EC, N, and DTP losses were analyzed over time using the statistical analysis software R-3.6.2 (R Foundation for Statistical Computing), with the mixed-effects model package lme4 (Bates et al. 2015), with standard errors applied to means when results were graphed. Means for shoot dry weight and cumulative N and DTP losses were separated according to Tukey’s honest significant difference procedure (P = 0.05).
Results and discussion
Coleus growth
Application of fertilizer to accelerate plant growth is a widespread practice in the ornamental container production industry particularly for short-cycle plants that require readily available nutrients. In our study, coleus fertilized with SF or BSFF exhibited greater growth and shoot dry weight by the end of the 6-week production period as fertility rates increased from 0 to 0.3 kg⋅m–3 N (Table 1, Figs. 1 and 2). The only detectable differences in coleus GI between fertilizer sources occurred at 5 and 6 WAPs for 0.1 kg⋅m–3 N and 6 WAPs for 0.3 kg⋅m–3 N, with SF-coleus having a greater GI. Although differences in coleus GI between fertilizers applied at the same rates were not evident over much of the production period, pairwise comparisons calculated for coleus GI at each rate along with shoot dry weight 6 WAPs show SF-coleus accelerated coleus growth consistently compared with BSFF-coleus, which seemed to lag one 0.1-kg⋅m–3 N rate unit (Figs. 2 and 3). In further support of this observation, coleus leaf quality ratings of ≥8 at 6 WAPs were attained for SF-coleus at >0.1 kg⋅m–3 N, whereas a minimal rate of 0.2 kg⋅m–3 N BSFF was needed to achieve similar ratings (Fig. 3). Black soldier fly frass applied at ≤0.1 kg⋅m–3 N and SF at 0 kg⋅m–3 N resulted in consistently low or declining leaf quality over the 6-week production period.
Pairwise comparison of fertility rates on coleus growth during a 6-week production period using black soldier fly frass or a synthetic incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) under greenhouse conditions.
Average growth of coleus fertilized using black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. The bars represent standard errors for growth index data illustrated over time. Growth index = {Plant height + [(Plant width1 + Plant width2)/2]}/2; 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Average growth of coleus fertilized using black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. The bars represent standard errors for growth index data illustrated over time. Growth index = {Plant height + [(Plant width1 + Plant width2)/2]}/2; 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Average growth of coleus fertilized using black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. The bars represent standard errors for growth index data illustrated over time. Growth index = {Plant height + [(Plant width1 + Plant width2)/2]}/2; 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Shoot dry weight of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen at 6 weeks after planting under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 kg⋅m–3 = 1.6856 lb/yard3, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Shoot dry weight of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen at 6 weeks after planting under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 kg⋅m–3 = 1.6856 lb/yard3, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Shoot dry weight of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen at 6 weeks after planting under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 kg⋅m–3 = 1.6856 lb/yard3, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Leaf quality of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. Leaf quality measurements were assessed based on visual appearance on a scale of 1 to 9, with 1 representing poor leaf size and color, and 9 representing ideal leaf size and color. The bars represent standard errors for data illustrated over time. 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Leaf quality of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. Leaf quality measurements were assessed based on visual appearance on a scale of 1 to 9, with 1 representing poor leaf size and color, and 9 representing ideal leaf size and color. The bars represent standard errors for data illustrated over time. 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Leaf quality of coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0, 0.1, 0.2, or 0.3 kg⋅m–3 nitrogen (N) during a 6-week production period under greenhouse conditions. Leaf quality measurements were assessed based on visual appearance on a scale of 1 to 9, with 1 representing poor leaf size and color, and 9 representing ideal leaf size and color. The bars represent standard errors for data illustrated over time. 1 kg⋅m–3 = 1.6856 lb/yard3.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Slow nutrient release from natural fertilizers is a legitimate grower concern for those that are looking to accelerate plant growth to expedite production. Nutrients in natural fertilizers are often components of complex organic matter that require microbial decomposition to release nutrients for plant uptake (Lazicki et al. 2020). The nutrient composition of natural fertilizer, environmental conditions, and method of application affect the timing and amount of nutrients released (Bergstrand 2022; Sradnick and Feller 2020), and thus a fertilizer’s availability for ornamental crops produced under abbreviated schedules. In our study, BSFF incorporated into the substrate at 0.3 kg⋅m–3 N resulted consistently in marketable coleus both in terms of plant size and leaf quality, indicating the synchronicity of nutrient release supported plant growth within the allotted production period. Therefore, BSFF is a suitable natural fertilizer for coleus production. Additional research is needed to evaluate BSFF to determine proper application rates and timing for species with production cycles greater than 6 weeks.
Nutrient leaching from container-grown coleus
The greenhouse and nursery industries recognize the importance of using fertilizers that release nutrients slowly as a best management practice to limit nutrient movement (Bilderback et al. 2013). This has led to the use of controlled-released fertilizers or low rates of fertilizer applied frequently through irrigation. Fertilizers applied in agricultural operations have been identified as nutrient sources that contribute to impaired surface waters (US Environmental Protection Agency 2019). Therefore, it is appropriate to characterize any environmental benefit or drawback that may occur when using BSFF applied at 0.3 kg⋅m–3 N during short-cycle, container-plant production.
Leachate volumes among the fertilizer treatments evaluated (BSFF and SF at 0.3 kg⋅m–3 and an unfertilized control) did not differ at weekly samplings across the production period even though weekly leachate volumes among fertility treatments decreased from ranges of 2.7 to 3.0 L at 1 WAP to 1.6 to 1.8 L at 6 WAPs [data not shown (fertilizer treatment: P = 0.65, 2 df)]. Plant growth leads to enhanced rooting, which slows container drainage (Tran et al. 2020) but also increases water absorption to support greater leaf transpiration rates. The similarity in leachate volumes among treatments provided experimental conditions that allowed for a more direct comparison of nutrient movement among fertility treatments by limiting confounding effects associated with highly variable leachate volumes. Nutrient movement from containers has been shown to vary depending on leachate volume (Tyler et al. 1996; Warsaw et al. 2009).
At 1 WAP, leachate pH ranged from 6.4 to 7.3 among fertility treatments with leachate pH increasing slightly at 6 WAPs to 7.6 to 7.8, whereas changes in leachate EC differed among fertility treatments particularly during the initial WAP (Fig. 4). Coleus fertilized using SF resulted in a high initial EC that declined 3 WAPs, but remained elevated compared with the control, which exhibited static losses within a range of 0.22 to 0.25 mS⋅cm–1 for the duration of the production period. Leachate EC for BSFF-coleus had a slight decline from 0.38 mS⋅cm–1 at 1 WAP to 0.29 mS⋅cm–1 at 6 WAP, but was elevated consistently compared with the control. High initial leachate EC from SF-coleus indicates high nutrient losses, with EC serving as an efficient but rudimentary indirect measure of container fertility that can be used during ornamental-container production (Cox 2019).
Leachate pH and electrical conductivity from coleus fertilized with black soldier fly frass (BSFF) or a soluble fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 nitrogen (0.51 lb/yard3) and an unfertilized control during a 6-week production period under greenhouse conditions. The bars represent standard errors for data illustrated over time. 1 mS⋅cm–1 = 1 mmho/cm.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Leachate pH and electrical conductivity from coleus fertilized with black soldier fly frass (BSFF) or a soluble fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 nitrogen (0.51 lb/yard3) and an unfertilized control during a 6-week production period under greenhouse conditions. The bars represent standard errors for data illustrated over time. 1 mS⋅cm–1 = 1 mmho/cm.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Leachate pH and electrical conductivity from coleus fertilized with black soldier fly frass (BSFF) or a soluble fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 nitrogen (0.51 lb/yard3) and an unfertilized control during a 6-week production period under greenhouse conditions. The bars represent standard errors for data illustrated over time. 1 mS⋅cm–1 = 1 mmho/cm.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
SF-coleus resulted in cumulative losses of 178.7 mg N and 65.8 mg DTP for the 6-week production period compared with 22.6 and 15.1 mg N and 49.7 and 20.9 mg DTP using BSFF and the control, respectively (Figs. 5 and 6). More specifically, N and DTP losses from SF-coleus followed a similar pattern exhibited by SF-coleus EC, with high initial N and DTP losses occurring within the first 2 WAPs that accounted for 94% and 81% of the cumulative N and DTP lost, respectively. Black soldier fly frass–fertilized coleus and the control exhibited less steep declines in N losses, which resulted in reduced cumulative N losses of 87% and 92% compared with SF-coleus. However, BSFF-coleus and the control resulted in statistically similar cumulative DTP losses as SF-coleus, even though BSFF-coleus and the control reduced cumulative DTP losses 24% and 68%, respectively.
Nitrogen and dissolved total phosphorus leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) nitrogen and an unfertilized control during a 6-week production period. The bars represent standard errors for data illustrated over time. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Nitrogen and dissolved total phosphorus leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) nitrogen and an unfertilized control during a 6-week production period. The bars represent standard errors for data illustrated over time. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Nitrogen and dissolved total phosphorus leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) nitrogen and an unfertilized control during a 6-week production period. The bars represent standard errors for data illustrated over time. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Cumulative nitrogen (N) and phosphorus (P) leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) N and an unfertilized control during a 6-week production period under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Cumulative nitrogen (N) and phosphorus (P) leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) N and an unfertilized control during a 6-week production period under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
Cumulative nitrogen (N) and phosphorus (P) leaching losses from coleus fertilized with black soldier fly frass (BSFF) or a synthetic fertilizer (SF) incorporated into a bark-based substrate at 0.3 kg⋅m–3 (0.51 lb/yard3) N and an unfertilized control during a 6-week production period under greenhouse conditions. Means not followed by the same letter are significantly different according to Tukey’s honestly significant difference procedure at P = 0.05. 1 mg = 3.5274 × 10–5 oz.
Citation: HortTechnology 33, 3; 10.21273/HORTTECH05093-22
High initial nutrient leaching losses, especially for N, from SF followed by nutrient losses tapering over time has been well documented in the scientific literature for plant container-grown studies across several species and production schedules (Catanazaro et al. 1998; Du et al. 2011; Owen et al. 2008; Sanders et al. 2019). Many SFs are composed of salts that disassociate readily as irrigation application volumes increase. This leads to nutrients being more susceptible to movement in the porous substrate that has low water and nutrient retention properties, which are used frequently throughout the ornamental container production industry (Owen et al. 2008). For example, Sanders et al. (2019) reported increasing irrigation volumes led to greater nutrient movement in container-grown SF-coleus, especially during the initial weeks of production, but nutrient leaching from controlled-release–fertilized coleus was unaffected.
Alternative fertilizer sources to SF such as controlled-release fertilizers, which rely on polymer coating technologies (Du et al. 2011), or natural fertilizers, which rely on mineralization (Bergstrand 2022), have all been reported to govern nutrient availability and thus losses from container-grown plants. This is particularly important during the initial period after planting, when nutrient leaching is common and plant nutrient uptake is limited physically by developing root systems (Goss et al. 1993). In our study, the slow mineralization of BSFF that hindered coleus shoot growth, biomass accumulation, and leaf quality when applied at rates <0.2 kg⋅m–3 N also limited initial leaching losses the first 2 WAPs for BSFF applied at the highest rate of 0.3 kg⋅m–3 N, but without deleterious effects on plant growth.
It is also interesting to note N and P losses were similar for BSFF-coleus, most likely because of similar mineralization rates, with greater inorganic N uptake occurring to account for differences in application rates. Factors that contributed to greater SF N losses compared with DTP include NO3– being highly susceptible to leaching (Fernandez-Escobar et al. 2004) as a result of its low ion exchange affinity as well as differences in application rates between the two nutrients. Phosphorus was applied at a ratio of 5N:1P for each fertilizer. Varying composition ratios of N:P are common with natural fertilizers derived from animal excrement or other by-products (Bergstrand 2022). A recent container-grown ornamental study has shown P application rates can be greatly reduced during production for several species (Shreckhise et al. 2019). Continual overapplication of natural fertilizers with high P concentrations have led to scenarios of P soil saturation and increased offsite P movement from agricultural lands (DeLaune et al. 2004). Phosphorus losses into waterbodies can be particularly problematic compared with N because concentrations as low as 0.06 mg⋅L–1 P have resulted in impaired water vs. 0.57 mg⋅L–1 N in the mid-southern United States [Texas-Louisiana Coastal and Mississippi Alluvial Plains Ecoregion X (US Environmental Protection Agency 2019)]. The ratio of greater N:P in BSFF suggests losses may have been curbed further if the experiments had compared BSFF to a complete SF with an equal ratio of N:P:K. However, for the purpose of our study, the ratio of nutrients for N:P:K were equivalent based on the BSFF analysis to allow a direct comparison of plant growth and nutrient leaching losses between fertilizer sources. The application of BSFF at 0.3 kg⋅m–3 N not only reduced N losses, but also provided sufficient P to support proper coleus growth when compared with an SF.
Conclusion
Increasing industrial insect rearing as a feedstock to meet growing global food demands will lead to greater frass production as a by-product. Frass, a nutrient-dense material, has shown promise as a natural fertilizer source with potential environmental benefits. In the production of container-grown coleus, BSFF produced marketable coleus when applied at 0.3 kg⋅m–3 N and demonstrated reduced N leaching of 87% compared with SF-coleus during a 6-week production period under greenhouse conditions. Therefore, BSFF is a suitable nutrient source for the production of coleus without compromising growth and leaf quality.
Units
References cited
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