Better than Miracle Gro: Turn Fish Scraps Into Fertiliser

I want to let you in on a little secret that hardly anyone knows: there’s a way to boost plant growth that costs almost nothing and gives results better than any commercial fertiliser...

How to Make Fish Fertiliser

Our plant-boosting recipe begins with fish, using byproduct that might otherwise be thrown away, so this is a great way to avoid waste. Any fish will do for this: river, lake or sea fish. What we’re going to end up with is a fast-acting fertiliser that feeds soil life too, making plants even stronger. It’s really fab stuff! (But if the thought of fish fertiliser makes you squeamish, try making fertiliser from weeds instead.)

Collect around 9 lb (4 kg) of fish scraps – all the guts, heads and bones left after filleting the fish for eating. If you don’t make enough scraps from your own cooking, try asking your local fishmonger, but here’s a top tip: when you go to collect scraps like this, take along a lidded container. A carrier bag runs the risk of smelly liquids dripping out, and believe me, you really don’t want that on your clothes – or worse still, leaching out onto the seats of your car!

Cutting the scraps up into smaller chunks will mean they break down a lot quicker than larger bits to give a more effective fermentation. If you have a mincer, pop it through that, or whiz everything up in a powerful blender.

Commercially sold fish fertiliser, or fish emulsion, is made using the remains of processed fish or fish meal, but off-the-shelf emulsions are typically made by heating the fish waste up to at least 175ºF (80ºC) to extract the oils and kill off all the bacteria (including the good bacteria). It gives an okay fertiliser, but we can do better than that – and we can make it for pennies!

The stuff we’re making uses more of the fish and nothing is driven off or extracted, so the microbial life remains intact. This means we’ll have a living fertiliser with a wide range of beneficial microorganisms still in it. Our emulsion – or technically a ‘hydrolysate’, because we’re not heating it, just fermenting the fish in water – should retain a much broader spectrum of nutrients too, which our plants will love.

Put your fish scraps in a five-gallon bucket with a lid – this is essential to stop the fishy pong travelling too far! Add a cupful of unsulphured molasses to feed all those lovely fermenting microbes. This will also add some additional micronutrients to the fishy brew – mmm, delicious! If you haven’t got molasses, you could instead use an equivalent amount of soft brown sugar or raw cane sugar, either of which will add fuel to the microbial fire.

Add roughly three parts rainwater by volume to every one part fish, and give it all a really good stir. At this point I like to mix in two optional extras:

First, some liquid from a tub of sauerkraut. Just like sauerkraut or kimchi, our fishy ferment uses lactic acid bacteria to give us a lactoferment, so by adding a little of this liquid we’re kickstarting the whole fermentation process with immediate effect – there’s no delay!

And the second extra is a couple of handfuls of sawdust. Sawdust is a ‘brown’ material, and just as in your compost heap, adding some browns to the mix will help to balance the nitrogen-rich ‘greens ‘– which in this case is our fish waste. It also helps to knock back the stench a little (and I emphasise ‘a little’ – it will still smell, I’m sorry to say!). The sawdust should also help with aeration, creating little pockets of air to keep the mixture healthier and more open. Make sure to use plain sawdust from wood that hasn’t been treated in any way.

Fermenting Fish Fertiliser

And that’s your soon-to-be fish fertilizer all set up! Don’t forget to put the lid on the bucket to avoid the stench upsetting your neighbors, and to stop flies from getting in and laying eggs. Please don’t be put off by all this! It will smell, but it really isn’t that bad – and the lid will keep it in check.

As the mixture ferments you’ll need to ‘burp’ it every few days by jlifting a corner to release the buildup of pressure and ensure fresh air gets back in. You may be able to see the lid bulge slightly, which is a clear sign your fishy brew needs burping.

Leave this fermenting fish outside for anywhere up to a month. The ferment is fastest at a warm room temperature, so summer is an ideal time to make this.

After about a week you will have reached what is, hands down, the smelliest stage of the process. Give the mixture another good stir to get some extra air in there and ensure everything’s well blended. At least the smell means something’s happening… right?!

Another week on and the smell will have mellowed – thank goodness! – into a milder, more vinegary smell, just as we’d expect from any other lactoferment like sauerkraut. Give it another thorough stir about now, and again a week later.

After four weeks you should have a fairly smooth and consistent liquid. It really doesn’t smell that bad any more, and almost all of the original fish has disappeared, decomposed away into the liquid.

Strain it off into an empty bucket. Any solids left behind will have lots of the bacteria you’ll need to start the next batch of fish fertiliser. This is now your primer, so you won’t need to add any sauerkraut next time round.

Bottle up your concentrate and keep it in a cool, dark place. Try to use it up within about two months.

How to Use Homemade Fish Fertiliser

This stuff is surprisingly powerful! It’s got a typical N:P:K (nitrogen, phosphorus and potassium) ratio of around 4:1:1, which means 4% of the liquid is nitrogen, 1% phosphorous and 1% potassium. It’s also pretty high in sulphur and has good levels of calcium and magnesium, along with plenty of other nutrients.

Before using it, you’ll need to dilute it with water. Mix two tablespoons of fermented fish concentrate to every gallon of water. Use it to water your plants once every two weeks while plants are growing, ramping up to once a week at the peak of the growing season. Be sure not to splash any edible parts of the plant.

Not only will this fish fertiliser add nutrients that can very quickly be taken up by plant roots to give an immediate effect, it will also help to feed the beneficial microbes and fungi that live in the soil – and that will all help to strengthen plants to better withstand pest or disease attacks. How fab’s that?!

You can also spray it directly onto the leaves of your plants as a foliar feed. It’s especially good for leafy plants that will appreciate a little extra nitrogen, and great for any plant in the brassica (cabbage) family because its fairly high sulphur levels helps develop the full flavour of these vegetables. Pour your diluted fish fertiliser into a spray bottle, and mist the leaves of your plants. Just don’t use it on the edible parts of plants that you’ll be harvesting in the next few weeks as you won’t want potentially fishy tasting produce!

Just a small word of caution: some fish, especially larger, predatory fish, accumulate mercury in their bodies, which is why pregnant women are advised to limit their consumption of certain fish. And it’s the same with plants – some are very good at accumulating metals like mercury in their leaves as self-defence against grazing – so fish fertiliser is probably best avoided or at least used sparingly around so-called hyper-accumulators like lettuce and amaranth.