Why Frass Fertiliser is the Future of Soil Health

Healthy soil is not built by one ingredient. It depends on nutrient supply, organic matter, physical structure, water management and biological activity working together. Mealworm frass is interesting because it can contribute to several of those functions while also turning a residual stream from insect farming into a useful garden input.

That makes frass a strong fit with the idea of circular growing—but it is more credible to explain what it can contribute than to present it as a cure-all for soil.

What is mealworm frass?

Mealworm frass is the fine material produced as Tenebrio molitor larvae eat and grow. It contains insect excreta, small fragments of shed insect material and residual feed particles. In a garden, its most straightforward role is as a nutrient-containing organic fertiliser and soil amendment.

RichRoots frass: the numbers we actually have

Our current independent analysis found:

Those figures matter because frass composition changes with insect diet, production system and source. A generic NPK number from another producer is not necessarily a good description of the material in a RichRoots pouch.

Why frass fits circular agriculture

In insect farming, frass is produced alongside the insects themselves. Using that material as a fertiliser creates an additional pathway for nutrients and carbon to return to productive soil rather than treating the material simply as a waste stream.

Research on insect frass increasingly discusses it in this circular-economy context. The important point is not that every insect system automatically has a lower environmental footprint than every alternative; a proper environmental comparison requires life-cycle data. The circular advantage is more direct: one production stream can generate both an insect product and a useful soil input.

What can frass contribute to soil?

1. Plant nutrients

Mealworm frass contains nitrogen, phosphorus and potassium as well as other mineral nutrients. In a greenhouse barley study, Houben and colleagues found that mealworm frass supported biomass and nutrient uptake at levels comparable with a mineral NPK treatment under the conditions tested.

2. Carbon-rich organic matter

RichRoots' current analysis contains 38.5% carbon. Organic carbon is part of the material soil organisms process and is one component of soil organic matter management. Frass should still be viewed as one input within a broader soil-building program rather than a replacement for compost, mulch, crop residues or good soil management.

3. Soil biological activity

Houben et al. reported increased soil microbial activity after mealworm-frass addition. This suggests that frass can influence the living component of soil as well as provide nutrients.

That does not mean the value of frass depends entirely on live microbes inside the product. A 2023 study by Blakstad and colleagues found that the frass microbiota did not drive the fertilising capacity observed in their experiment.

4. Chitin and plant responses

Mealworm-derived material contains chitin, which is associated with insect exoskeletons. Research has investigated whether frass can influence plant defence signalling. Blakstad et al. found systemic resistance against Botrytis cinerea in their controlled experiment, but did not detect the local root callose response they tested.

That is scientifically interesting, but it is not a reason to promise that frass will prevent pests or diseases in every garden.

Why source-specific testing matters

Amorim and colleagues found meaningful compositional differences between mealworm-frass sources and emphasised that rearing substrate and supplements can affect the final material. This is one reason we prefer to show the actual RichRoots nutrient analysis rather than repeat broad claims such as “all frass is 2-3-2” or “all frass is pathogen-free.”

Frass is useful—not magical

A healthy-soil strategy still depends on the basics: appropriate nutrition, enough organic matter, suitable pH for the plants being grown, water management, soil structure and good horticultural practice. Mealworm frass can sit comfortably within that system, but no single fertiliser replaces all of it.

The amount that makes sense also depends on the plant, pot or bed size, growth stage, season and existing soil fertility. That is why our practical guidance starts conservatively rather than treating “more” as “better.”

Where to start

If you are new to frass, start with our How to Use Mealworm Frass Fertiliser on Plants guide. For the research in more detail, read The Science Behind Mealworm Frass Fertiliser, or browse our FAQ.

For repeated feeding across veggie beds, larger pots or several plants, see the 360g Large Garden Pouch.

Shop the 360g Large Garden Pouch →

References

1. Houben, D. et al. (2020). Potential use of mealworm frass as a fertilizer: Impact on crop growth and soil properties. Scientific Reports. doi:10.1038/s41598-020-61765-x
2. Blakstad, A. et al. (2023). Frass from yellow mealworm (Tenebrio molitor) as plant fertilizer and defense priming agent. Biocatalysis and Agricultural Biotechnology. doi:10.1016/j.bcab.2023.102862
3. Amorim, H. C. S. et al. (2024). Insect frass composition and potential use as an organic fertilizer in circular economies. Journal of Economic Entomology. doi:10.1093/jee/toad234