I have always been an organic gardener, so the fact that my hydroponic kits are currently fuelled by chemical fertilisers irks me somewhat. The main problems with chemical fertilisers (from my perspective) are the pollution they cause once they’re out in the world, and the energy required to make them. While the former is not an issue in my indoor garden (I give any wastewater to houseplants or plants in the garden), the latter most certainly is. So I would like to move to a more sustainable fertiliser solution in the future.

So I was intrigued when Hexafly got in touch with me about their Organic Insect Fertiliser Frass. Hexafly is the only company in the UK licensed to farm Black Soldier flies (Hermetia illucens). The idea is to produce fly larvae for food – a sustainable source of protein for pets or fish farms. Hexafly also produces chitin, which is a biopolymer which has industrial uses but is also used in agriculture and can encourage plants to produce defensive chemicals. And all those flies poop, so the company sells a fertiliser made from the fly waste – frass. 

Hexafrass fertiliser

“Hexafly Insect Fertiliser is a natural by-product of black soldier fly larvae, which is collected during larval rearing and requires no further processing. The frass is a natural and sustainable soil enhancer for plant nutrition, and also mediates plant immune responses, leading to a vastly reduced susceptibility to diseases and pests.”

The frass arrives in a 12 kg bag, which begs the question – how many flies are needed to produce that much poop on a commercial scale? Scary.

I have set up a comparison trial in the hydroponicum. 

Nitrogen 3.07 % 
Phosphorus 0.64 % 
Potassium 0.39 % 

The website says the application rate for hydroponics is 1 cup of frass per 10 gallons of water. By my reckoning, that’s 6.5 ml per litre, or 32 ml (roughly 2 tbsp) in my 5-litre bottle. They recommend straining the solution before putting it into the hydroponic system, to avoid blocking pipes. But my passive system doesn’t have any pipes to worry about.

The comparison is to the IKEA VÄXER fertiliser I have been using since I set up the hydroponicum.

Total Nitrogen (N): 5 % 
Water-soluble phosphorus pentoxide (P2O5): 3 % 
Water-soluble potassium oxide (K2O): 8 % 

The NPK levels in the Ikea solution are higher, particularly the phosphorus and potassium. The solutions also have differing levels of micronutrients.

Top left tray - Hexafrass fertiliser
Top left tray: Hexafrass fertiliser solution

For the trial, I have set up identical trays in the hydroponicum. Each tray holds eight plants, so I decided on two plants each of four different species. I chose varieties I have grown before that grew well, and which we enjoyed eating – green oak leaf lettuce, mizuna, rocket and pak choi. I sowed the seeds into my hydroponic seedling tray on Saturday 16th November.

I planted up the new trays on Sunday, with two plants of each variety. To make the experiment fair, I thinned each plug to its strongest seedling, snipping off the others to avoid any root disturbance. (In prior hydroponicum runs I have left them all to grow, which can get a bit chaotic, so it will be interesting to see how these plants grow without any rivals!)

Top right tray - Ikea Vaxer fertiliser
Top right tray: IKEA VÄXER solution

The top left tray is filled with Hexafrass solution, and the top right tray has the IKEA VÄXER solution. The seedlings are settling into their new home, and I will add more fertiliser solution to the trays as the water level drops (‘on demand’ feeding!).

I will be watching how fast the plants grow and how well. I will record the harvest weights from each tray, and whether there are any problems. We will also be taste testing to see whether we can discern any differences in flavour between the two crops. I’ll keep you posted!