First published by Amanda Foxon-Hill on Mid Lachlan Landcare

Back in July we gathered up some of the carp caught during the Canowindra Carp Muster & took it back to Emu Park farm, Canowindra to turn it into fertiliser. We did this by (sort of) following a tried-and-tested biological fermenting method promoted by Gerry Gillespie of Returning Organics to Soil. You can read about the first part of that process by following this link to our earlier blog post.

Around 6 weeks after putting the fish in the barrel they seemed sufficiently digested and so we sent some of the juice off for analysis.

I opted for a test protocol commonly used for liquid fertilisers which made sense given what we’d (hopefully) created. But as simple as that sounds, selecting a test and test facility isn’t always easy so I did consult with a few people in our Landcare network first, to check what information would be most useful for them.

Like any (once) living thing, Carp can also accumulate toxins as they are going about their business. However, we made a collective decision not to spend a significant sum & time testing for those at this stage. Carp in the Belubula river has already been tested for PFAS quite recently and by groups better set up to deal with the data. In terms of heavy metals, fish in the area are also regularly tested for these with results informing fishing guidelines. Lastly, with only a small bucket load of fish making up a small scale, potentially one-off experiment, we figured the risks associated with any potential contamination would be relatively small and likely in the order of that one may find with commercially bought mulches & recycled soil adjuvants.

So, if the purpose of our experiment was to investigate the ease of which feral species can be turned into liquid fertiliser, what was the result?

Four weeks into our experiment, Eliza from The Land newspaper came to grab some pics for the paper and that gave us a chance to go fishing once again!

The official instructions call for you to chop up your protein source but I was really keen to avoid any extra steps that might make this a complete pain for farmers. So, we popped our carp in whole!

Keep in mind, we’d been ‘cooking’ up this biological soup in the depth of winter, the fish appeared to be breaking up very well, mostly from the inside out which made sense to us.

The following week Robbie got into the barrel with a mixer to see if he could chop up what was left and apparently there wasn’t that much so it was clear the microbes were doing some of the hard work for us. That said, it totally makes sense that the process would go much faster if the surface-area-to-volume-ratio was a little larger.

We left the barrel for a few more weeks before getting a sample for test. We probably could have called it earlier and sent off a sample at the 7 week mark (around 8th September) but life got in the way and we ended up sending it off on ten days later.

Incitec Pivot’s Nutrient Advantage brahcn ran the testing for us and we chose the C3 test, the protocol for which can be found here, along with the prices.

The Results:

Our Yield of ‘soup’ would have ended up being around 100 litres.

While I am a chemist, I’m not an agronomist so I’ll leave the detailed analysis to those of you who are suitably equipt to do so. However, what I did work out is the N,P & K are within appropriate ratio’s and that by diluting what we ended up with 10x, we’d get a product that, in that regard, was the same strength as the commercially available Charlie Carp. So, we turned a bucket of dead fish, around 9 litres of milk, a few cups of molasses and a small amount of rice water costing approx $20 into about $50 worth of All-Purpose Charlie Carp. On that basis, the effort seems a little mis-spent…

What’s next for our Carp Soup?

We’re way too late to spray this on the current seasons crop but the aim now is to filter it then decant it into 20 litre containers and store it until the next crop goes in when we’ll give it a try.

Is this worth doing?

Ultimately that’s a question with an applied answer – it depends. Here’s some of the thought’s we’ve had around the subject.

Dead Stock/ Ferals

Farmers will always end up with dead protein sources to manage. The easiest way to handle that is to just leave things where they lay. A little harder is to move it to a strategic site where it can help reduce water run-off or perform some other physical function as it breaks down. Both of these leave the dead thing available for predation by living things and in some cases, this opportunity can bring with it more problems -feral pigs, foxes etc. If that doesn’t happen, you will return the carcus and it’s nutrients to the soil but they’ll be concentrated in an animal-shaped heap at the end of the process. This may work out just fine but could create a patch (or patches) of soil that are super-fertilised Or even hotspots for toxins if the protein source was contaminated.

Verdict: 

Turning protein into liquid, sprayable fertiliser spreads any benefits across a wider area & dilutes any risks.

Time & Motion Study

Our project was small scale & experimental & therefore innefficient in terms of time and output. However, it wouldn’t take much to integrate this into an efficient farm-waste protocol where the stock ‘juice’ was made up in bulk ahead of time and barrels ready to go. The set-up just needs a couple of drums with lids you can fully open, a place to chop stuff up, access to water & a stick mixer of some sort to get into the mixture occasionally. Filtering can be done using gravity, some fine mesh and another bucket & spraying could be carried out using something like a hose pipe rather than a boom spray (although that could be possible if the mixture can be filtered finely enough). The ‘cooking’ doesn’t actually require any input – the mixture sorts its self out – so it’s not too time consuming that way.

I wouldn’t see this as a way to replace fertiliser costs, more of a way to a) maximise value from a problem (protein waste) that already exists on-farm and b) prevent or reduce follow-on problems that arise from dead stock/ ferals.

Sense & Sensibilities

It feels good to make a (reasonable) quality product out of waste. It feels like you’ve cleaned something up and make the world a better place WHILE creating something of value to yourself.

Rotting things stink but the biological soup way of digesting dead things reduces the smell considerably. This is of great benefit where there’s a lot of dead things all of a sudden – Pig Culls for example – or where you are on a smaller block / home garden and don’t want to stink out your space for the 5-7 weeks or so while the process goes on. Before doing this my biggest concern was that I’d have a barrel load of reaking fish but no, the barrel smelled quite nice most of the time – a licquorice beer type of smell. That alone made me value this process.

Verdict: 

Doing this is doing good.

We hope you’ve enjoyed hearing about our experiments in Carp digesting and would definitely encourage you to have a go!

We’d also like to thank Watershed Landcare and in particlar Cheryl Nielsen for inspiring us to give this a go. To Weddin Landcare who introduced us to Dr Ernie Harbott from Compost Culture & created an opportunity for us to talk about our experiment at a recent workshop we co-hosted with them.