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Introduction
Global agriculture sits at a phosphorus crossroads. Phosphorus (P), mined chiefly from finite phosphate rock, is indispensable for plant growth—yet much of what we apply as synthetic fertilizer ends up locked in soils, lost to waterways, or inefficiently used. As farmers scramble for solutions amid environmental concerns and rising costs, insect farming and its by-product—frass—have emerged as a compelling circular economy approach to recycle nutrients and boost soil fertility. A recently published MDPI study, “Evaluation of Insect Farming Residue (Frass) as a Phosphate Fertilizer Within the Context of the Circular Economy” (Nieto-Cantero et al., Agronomy, August 2025), provides fresh data showing that frass from black soldier fly (BSF) and mealworms might outperform traditional mineral phosphate fertilizers. Here’s what they found, why it matters, and what it means for BSF farmers.
What the Study Found
The team conducted a controlled 48-day pot experiment in Seville, Spain, using lettuce (Lactuca sativa) grown in a phosphorus-deficient, calcareous soil. Treatments included frass from BSF (Hermetia illucens) reared on olive mill pomace, mealworm (Tenebrio molitor) frass (from wheat bran-fed larvae), vermicompost, standard superphosphate mineral fertilizer, and combinations thereof.
Nutrient Fractions & Bioavailability
- In both BSF and mealworm frass, more than 30% of total phosphorus was water-soluble inorganic P (Pi), and over 50% was organic P. By comparison, vermicompost had only ~2.3% water-soluble P, with most phosphorus tied up in less available fractions.
- The pH and electrical conductivity (EC) of frasses varied: BSF frass was alkaline (pH ~8.97), mealworm frass acidic (~5.99), affecting how P interacts with soil chemistry. Vermicompost was closer to neutral pH (~7.64).
Plant Growth & Phosphorus Uptake
- When using pure mealworm frass, plant shoot dry biomass was 90% higher than plants treated only with mineral phosphate fertilizer. Root biomass also significantly increased under frass treatments, especially BSF frass.
- Phosphorus in shoots under frass treatments exceeded those under mineral fertilizer: mealworm frass led to ~6.48 mg P per pot, BSF ~5.66 mg, while mineral fertilizer produced less. Root P accumulations were heavily boosted.
Replacement Value and Soil Health Indicators
- Mineral fertilizer replacement values (MFRVs) for frass were remarkable: approximately 150% for BSF frass and 180% for mealworm frass, indicating that frass could not merely substitute mineral P fertilizers, but surpass them in effectiveness under the test conditions. Vermicompost lagged behind with an MFRV of ~81.9%.
- Soil Olsen-P (a measure of plant-available phosphorus) increased more under frass treatments than with mineral superphosphate. Mealworm frass raised Olsen-P to ~18.22 mg P/kg, significantly above mineral control (~13.02 mg/kg).
- Alkaline phosphatase activity (a key enzyme for converting organic P to inorganic, plant-available forms) was highest under BSF frass treatments, over 100% greater than non-fertilized control. Mealworm frass also increased enzyme activity, although less dramatically.
Why It Matters
Reducing Reliance on Finite Mineral Phosphorus
Mineral P fertilizers are derived from finite phosphate rock reserves concentrated in few countries, creating supply risks and geopolitical concerns. Moreover, in calcareous or high-pH soils—which are widespread—applied mineral phosphorus often becomes quickly fixed in forms unavailable to plants, resulting in use efficiencies of only ~15–35%. Insect frass addresses both challenges: it supplies phosphorus recycled from organic sources, and because frass contains organic matter, it helps prevent fixation, maintaining more P in soluble or easily mineralizable forms.
Enhancing Circularity in Agriculture
Frass represents a key loop of circular agriculture. Insects consume organic waste (olive pomace, wheat bran, etc.), converting it into protein and frass. The frass, rich in both inorganic and organic phosphorus plus beneficial soil biology, returns nutrients to the soil. This reduces waste, lowers environmental burdens, and ideally cuts inputs of synthetic fertilizers.
Improving Soil Long-Term Health
Frass delivers multiple soil health benefits beyond just nutrient supply: organic matter input, increased enzyme activity, and enhanced microbial processes that drive P cycling. Such effects help sustain fertility across seasons, buffer stress, and improve organic matter content—potentially sequestering carbon, improving structure, water retention, and resilience. Mineral fertilizers usually don’t provide these benefits.
How This Fits Into Larger Trends in Insect Farming
Other studies in 2025 echo these results. For example, research shows frass improves plant resilience to abiotic stressors like drought or salinity and biotic threats, serving as a biostimulant and contributing to soil remediation. Another study of frass across multiple insect species found that frass properties—including nutrient content and microbial communities—varied significantly depending on species, and that heat treatments (for safety) reduce pathogens without eliminating beneficial soil microbiome effects.
These studies suggest frass is not a one-size-fits-all product: its effectiveness depends on insect species, larval diet, post-processing, and soil context. Still, the momentum is real: research labs, industry players, and policymakers are increasingly attending to frass as part of fertilizer regulation, climate policy, and sustainable agriculture frameworks.
Implications for BSF Farmers
If you’re farming BSF (or considering getting into it), here are key points to consider.
Feedstock & Diet
What you feed your BSF larvae dramatically influences the composition of the frass: phosphorus forms, due to both substrate chemistry and the insects’ digestion, and whether there may be unwanted compounds (e.g. salts, heavy metals). Feeding on safe, nutrient-rich byproducts yields frass likely to perform better and safer as fertilizer.
Processing & Safety
Regulations (for example in the EU under the Fertilizing Products Regulation) often require frass to be pasteurized or otherwise hygienized to ensure safety. Nieto-Cantero et al. used pasteurization at 70 °C for 60 minutes. Such protocols appear not to destroy the organic P or impair enzyme activity in a way that negates benefit (at least in short-term pot trials).
Measuring Efficacy Locally
Because soil type, pH, and existing phosphorus status strongly influence outcomes, it’s crucial to test frass in your own soil with your crops. Measure plant responses—biomass, P content—as well as soil health indicators like Olsen P, enzyme activity, and longer-term residual phosphorus. Small pilot trials can guide fertilizer rates and application schedules.
Potential for Value Addition
BSF farmers producing frass have a potential secondary revenue or cost saving stream: selling or using frass rather than treating it as waste. When frass yields replacement values above 100%, regions with high phosphate fertilizer costs may find frass a cost-effective alternative once logistics and processing are optimized.
Challenges, Unknowns & Next-Step Research
While results are promising, several caveats remain:
- Variability: Frass performance depends heavily on insect species, diet, environmental rearing conditions, and post-processing. What works in a Spanish pot trial may differ elsewhere.
- Scale & Field Trials: Pot experiments—even rigorous ones—don’t always translate directly to field scale. Long-term, field-based trials (multiple seasons, crop types, climate zones) are needed to validate sustainable yield gains and residual effects of organic P. Nieto-Cantero et al. call specifically for such work.
- Regulatory Hurdles & Market Adoption: Safety standards (pathogen control, heavy metal content), consistency of product, and acceptance from farmers accustomed to mineral fertilizers can pose barriers.
- Economic & Logistical Considerations: Cost of processing (pasteurization, drying, storage), transport to agricultural lands, and ensuring reliable supply must be considered.
Sources:
- Nieto-Cantero et al., “Evaluation of Insect Farming Residue (Frass) as a Phosphate Fertilizer Within the Context of the Circular Economy”, Agronomy, MDPI, August 2025: mdpi.com
- Research on frass as biostimulant and soil remediation, Applied Sciences, MDPI, 2025: mdpi.com
- Study on frass properties across insect species, PubMed: pubmed.ncbi.nlm.nih.gov
