Frass: The Better Option - 9 July 2024
Before agrochemicals, fields were fertilised with manure (poop) and we had lower crop yields. Modern agrochemicals eliminate pests, increase yields, deplete soil life, and cause significant issues downstream due to run off. Frass on the other hand gives us the best of both. It increases crop yield on par with agrochemicals, increases microbial life, and doesn’t suffer with run off issues.
Bonus fun facts.
Mealworms can eat a variety of pesticide free crop by-products. When given a slight nutrient boost, they do very well with lignocellulosic crop wastes (He, et al. 2021) such as rice stalks. They can even break down some soft plastics (Yang, et al. 2018), but I wouldn’t recommend that frass as a fertiliser. So, maybe start a mealworm farm - they don't bite.
Fermenting?
Some places recommend fermenting the frass before application. This could be done to fresh frass by simply placing it in an appropriate bucket, mixing it with water, and waiting a few days – ratios and additional supplements vary. This process has a few benefits. It will kill off any existing eggs or larvae. The frass, now suspended in water, is no longer an atmospheric allergen, and is easier to apply. Also, the acidity from the fermenting process can help break down the chitin in the frass, which helps with improve plant vitality. Here's a video from a mate of mine:
Microbially active?
Frass is biologically active, and it contains prebiotics which feed existing soil microbes. The bacterial profile of the frass can vary wildly depending on their food source (He, et al. 2021). Nitrogen-fixing bacteria that naturally exist in the soil (and mealworms) reproduce rapidly when frass ratios get over 2% (Watson, PreiBing & Wichen, 2021). This results in an accumulation of nitrates, which become toxic to plants in high concentration. It also increases fungal growth which can harm the plant (Fuertes-Mendizabal, et al. 2023). However, treatment with a bacillus-based bio-stimulant (or simply fermented for a few days) changes the biological profile enough to mitigate both issues (Fuertes-Mendizabal, et al. 2023).
Should it be sterilised?
There is the issue of introducing mealworm eggs into your farm or garden. If the frass is fresh, sterilization via cooking or freezing will kill any eggs. However, any eggs should have hatched and died after a 6-month rest period. Best practice is to wait. When considering bacteria, there is very little difference between the results for sterilised and non-sterilised frass (Blakstad, et al. 2023). Except when the plants are subjected to stress (Poveda, et al. 2019). Plants fertilized with microbially active frass outperform plants treated with sterilized frass on all metrics. This effect was especially prominent with plants subjects to both salinity and flooding, with dry weight being doubled for the plants fertilized with microbially active frass.
How do I apply it?
For field application a pot experiment was done with Italian ryegrass (Houben, Daoulas, & Dulaurent 2021). An application rate of 10 tonne/ha worked best. This application rate gave similar results to a mineral fertiliser with the same NPK values. Here's the chemical analysis:
Carbon (C) - 39.3 %
Nitrogen (N) - 5.0 %
Phosphorus (K) - 4.6 %
Potassium (P) - 2.0 %
The best results for potting mix were 1-2% frass by volume, as going over 2% frass causes a dramatic increase in soil life which can negatively affect the plants (Watson, PreiBing & Wichen, 2021) . However, potting mixes with a bio-stimulant were successful with as high as 5% frass by volume (Fuertes-Mendizabal, et al. 2023).
WARNING: Mealworm frass is a fine-grained allergen; facemasks should be worn. Best practice for broad acre application is to mix with water and apply it as a tea. Alternately, pellets may be available.
Composting?
Frass of all types can be composted, however there is a significant loss of nitrogen in the form of nitrogen-dioxide through the composting process (He, et al. 2021). Although illegal in the EU, I would personally recommend adding the frass directly. Alternatively, frass can be used to boost fertility in the worm bin (Dulaurant, et al. 2020).
Additional benefits.
Chitinolytic bacteria break down the chitin in frass by producing chitinase, which prompts an immune response in plants resulting in more vibrant plants and (possibly) increased volatile compounds (Carrion, et al. 2019). This does not happen immediately, as the breakdown is dependent on the soil’s pH and biome. As Houben et al (2020) points out P is one of the biggest issues with mineral fertilizer leaching. Frass in its unaltered form is a slow-release fertilizer, particularly when it comes to P release (Houben, et al. 2020). Frass can also be used to increase fungal growth and amend soils contaminated with heavy metals (Watson, et al. 2021).
What is frass, how do I get it, is it affordable?
Frass is insect poop. It comes in many colours, smells, and flavours – not that I’ve tried. It is procured from insect farms. The frass is a by-product, and these farms sell their frass relatively cheaply – sometimes giving it away for free. The three most popular insects being farmed are crickets, mealworms, and black soldier flies. They’ve all been deemed suitable as fertilisers (Beesigamukama, Subramanian, & Tanga, 2022). However, I don’t have 50kg of cricket or black soldier fly frass sitting in my backyard, so for this article we will be focusing on mealworm frass.