There are some really scary DIY fish fertilizer recipes out there — this is not one of them.

You will not need to stir every day, for weeks on end, a putrefying mess of fish guts while fighting off flies and maggots. You can make your fish fertilizer the easy, anaerobic way. Mix it, seal it, and wait.... when it's done (if it's done right) your fertilizer should smell no better or worse than edible fish sauce. In fact, you could probably use it as seasoning...

This instructable also includes directions for making sauerkraut from scratch because one ingredient for the fish fertilizer is the live lacto bacilli cultures you will find in sauerkraut -- but if you want to skip this step and the 3-4 weeks needed, you can buy some ready-made (as long as it is unpasteurized... you want the live cultures).

About Fish fertilizer

Have you noticed the three numbers on bags of fertilizer? They stand for nitrogen (N), phosphate (P) and potash (potassium - K). Learn more. Fish fertilizer has a high proportion of nitrogen, which encourages shoot and leaf growth, but it is low in potassium, which is why I added banana peals to my recipe.

There are two types of fish fertilizer: emulsion and hydrolysate. Both types use enzymes to digest and liquify the fish, breaking it down into chemical elements the plants -- or the communities of microbes and fungi in the soil which help the plant -- can absorb. But the fermentation process is slightly different.

Commercial fish emulsions are heated and the oils are skimmed off to sell as separate ingredients, making the remaining fertilizer poorer. DIY fish emulsion is produced through an aerobic fermentation, which is why you have to leave your fish gut mixture exposed to the air and stir it regularly -- just like regular compost. And just like compost it generates heat and requires the addition of "brown" organic matter (most recipes I've seen call for sawdust which can be hard to come by for some people...) In addition to being inconvenient, it's more work and unbearably stinky (full disclosure: I haven't tried it; just reading the recipes and imagining the process made me gag).

Fish hydrolysate, on the other hand, is a cold process which leaves all the beneficial amino acid chains intact, so it is more nutritious for the soil -- but more importantly, for a DYI situation, it is an anaerobic fermentation so you can seal it off completely and you won't smell a thing during the whole process. It uses lacto bacillus to break down sugar, which produces lactic acid, which lowers the Ph, which stabilizes the mixture once the other enzymes have done their job and have digested the fish.

Commercial fish fertilizer is expensive! High quality fertilizer can cost to $70/gallon, so by making your own you are not only reducing waste, you are saving lots of money. Yes it takes time, but it's completely worth it. Fish fertilizer is much better than commercial bright blue powder or pellets because it doesn't just provide the three primary chemicals plants require, it nourishes your soil, helping your entire garden ecosystem… but also, quite frankly, fermenting fish is a blast.

Please note, regarding my links:

If the link directs you to Amazon, you should know that I am an Amazon affiliate and I may get a small commission on anything you purchase (but the price you pay is the same). If the link is to another vendor or website, it is purely FYI. The only website I am directly affiliated with is makepopupcards.com, a marketplace I created for paper designs and pop-up cards which obviously has nothing to do with rotting fish. But feel free to check it out!

Supplies

Ingredients:

Lacto bacilli, aka sauerkraut juice

Fish guts, heads, bones scales, etc

Molasses. Any sugar could work, but molasses has a lot more micronutrients and minerals your plants and the microbes and fungi who love them will appreciate.

Optional: banana peels. I haven't seen this in any other recipes, but fish fertilizer is high in nitrogen so I figured a little extra potassium could balance it out.

Equipment for sauerkraut:

large crock or jars with lids. You want your container to be airtight BUT it must allow gas to escape. This means using a water seal of one kind or another. The crock photographed here is a ceramic container I made myself with a moat for the water seal. You can buy a similar one in ceramics or glass, or there are plenty of good alternate (cheaper) options. You can buy these water airlock mason jar lids, or these with rubber airlocks, or you can use any large jar and seal the top with cheesecloth and a rubber band, to keep out insects. Or drill a small hole in the lid, screw it on over a layer of cheesecloth as shown in the 4th photo. With this (not 100% air-tight) option you'll have to pay closer attention and make sure your water level stays high enough to cover all your cabbage, or mold will grow, and you definitely need...
weights These can be proper glass weights, a plate weighted down with something clean and non-porous, like a bag of marbles (keep them in their plastic mesh bag), or seal some ice cubes in a plastic bag with a food saver vacuum sealer machine -- but do not use the vacuum function, the air bubble left inside will ensure the bag stays at the top once the ice melts.

Equipment for fish fertilizer

If your sauerkraut fermentation crock is big enough, you can use it for the fish fertilizer as well.

Ideally you'll use a wide-rimed carboy like this 3 gallon one (it's the one I used) or you can get smaller ones, depending on how much fish you get. A traditional narrow rimmed carboy will work too, but if you have the choice get the wider rim, it will make your life easier. If you have the time and ability, you can find a bucket with an airtight lid drill a hole to fit a piece of tubing inside (must be air tight!) which will then sit in another water-filled jar.

Regular Blender (or inexplicably expensive heavy duty blender). You can also use a meat grinder.

Step 1: Make Sauerkraut

You can skip this step and just buy sauerkraut (as long as it has live cultures) -- but what's the fun of that?

I've always hated sauerkraut... too mushy and I didn't like the taste either -- until I made my own! You can control the mushiness by limiting the time you let it sit before refrigeration, and the flavor is just SO much better -- even if you keep herbs and seasonings to a minimum!

All you need is cabbage and salt -- plus herbs or even other vegetables if you want to get fancy and creative. The traditional German seasoning is juniper berries.

Sauerkraut instructions:

Boil some water and let it cool down to room temperature -- or let it sit in a bowl for 24 hours so the chlorine will evaporate. You want to get rid of the chlorine, which is designed to kill the microorganisms you need to transform boring raw cabbage into delectable sauerkraut bathing in lactic acid. No need for a starter or any kind, all the microbes you need are already on the cabbage; with the salt and lack of oxygen you are just creating an environment which favors the good guys who pee lactic acid (and fart carbon dioxide) and kills the bad guys who grow mold.

Chop up a cabbage. Small chunks. How small is a matter of your own preference. Set aside a couple full leaves for later.

Use a potato masher or a wooden spoon or whatever you have on hand to pound the cabbage and get it to release its juice, Salt it (about 2 tablespoons for a regular sized cabbage should do), mix, and stuff it into your container. Add other spices if you want, like juniper berries, dill, etc. You can also add other vegetables like shredded carrots, but I say keep it simple for your first try. When you press down the natural juice should almost cover all the shredded cabbage. Cover with a couple full size cabbage leaves, which will help prevent small chunks from floating to the surface and potentially becoming moldy. Add your weights, press down, and top it off with some of your de-chlorinated water so that the sauerkraut and weights are submerged.

Let it sit for about two weeks at least, but make sure (if you are using a moat or a water lock) that the water doesn't evaporate and break the seal. It's fine to put regular chlorinated water in your moat. If your jar is just covered with a cheesecloth, be sure to top it off with de-chlorinated water (tap water which has been boiled and cooled to room temperature or left out for 24 hours) so that the cabbage stays 100% submerged.

After 2 weeks you can start tasting it. The time it takes depends on ambient temperature, the amount of salt you use (more salt = more time but less mushy), and then your personal preference for how ripe you like it to be! After three weeks I took out half the cabbage from the crock and put it in the fridge, leaving the rest to ripen some more. Even after 6 weeks the sauerkraut was crunchy and delicious.

Step 2: Get Your Fish Waste

Depending on where you live, you can go to fishing docks or ports and ask anyone you see cleaning fish to give you the guts. Or go to a fish store, or the fish section of your supermarket (assuming there are fishmongers there, not just a cooler) and ask them for their trash. You might have to ask in advance -- they don't typically save this stuff, but if you ask nicely, and possibly pay a small fee, most fishmongers will do this small favor for you. Maybe buy something nice for dinner from them when you ask; I got two whole branzinos, a delicious trout sized fish, and I highly recommend this NY Times recipe with arugula sauce -- but I digress.

I asked the fishmonger to make fillets and give me ALL the waste, including, scales, heads, bones and especially the guts (which contain excellent digestive bacteria). I was worried the waste from my branzinos (branzini?) wouldn't be enough, so I also asked for the bones and heads of two (massive!) salmon. Turns out the smaller fish would have been fine for a gallon of fertilizer... With the extra fish I just made a three gallon batch instead.

You can also buy and use a whole fish, but then you won't be doing anything to reduce waste and you'll miss out on a tasty dinner.

Step 3: Prepare Fish Mix

First I thought I would be using a one gallon glass carboy (which I had on hand), so I needed to cut up and blend the fish chunks to get them into the bottle (blend fish in batches with just enough water to get the texture of vomit, top off water when adding to the carboy). Then I realized half way through that my carboy would be too small for the amount of fish I had, so I ran out and bought this wide mouth 3 gallon carboy (at this point I had a mess of fish on my hands, so time was of the essence and I couldn't come up with a clever cost-saving DIY rig).

If you are using a wide-rimmed container you don't need to chop up and blend the fish, though the smaller the pieces the faster the process, so you might as well go a ahead and blend it. As long as your fish is fresh it's not particularly gross or smelly. Still a wide rim is helpful for the hard to chop pieces like a giant salmon head (it would have broken my blender if I'd tried putting it in whole — the head you see in the blender photo is the small branzino, not the giant salmon).

Ingredient proportions

All proportions by weight. I find it's easiest to use the metric system since 1 liter of water = 1 kg. You don't need to be super precise with the measurements -- especially the sauerkraut juice, you can just add what you have. A little extra can speed things up at the beginning, but it will all be the same once it's finished.

1 part fish to 3 part DECHLORINATED water (to dechlorinate water, boil it and then let it cool to room temperature -- or just leave a big pot of water out, uncovered for 24 hours. The chlorine will evaporate).

3 parts fish to 1 part sugar (molasses or brown sugar is best, but anything will work)

1-2 tablespoons sauerkraut juice per liter of water

I had 2.5kg of fish, so I needed 7.5 liters of dechlorinated water, and a little under 1 kilo of sugar. I didn't haven enough molasses so I finished my jar, added some old rock hard brown sugar and topped it off with white sugar. I added two banana peels to the mix because I had them on hand, and they added both carbohydrates and potassium, another desirable element for fertilizer.

When all was said and done, with these quantities I ended up with about 9 liters of fish fertilizer concentrate... not bad!

Step 4: Seal and Wait!

This was the first time I've tried fermenting fish, so I was a bit nervous... I didn't want the fish head bobbing up on the surface, exposed to air and possibly developing nasty molds, so I made a bag weight with ice cubes in sealed plastic (see supplies step) and shoved it in the top. I don't think this step was necessary, and I don't recommend it because it made the bottling more difficult and messy, but it didn't hurt... Certainly not needed if you've managed to grind all your fish parts.

Now the fun part!

Scroll through the photos to watch it change over time... Video shows day 9, at the peak of the gas production. It should be self evident, watching that, why I chose the tube in a jar method rather than the small waterlock, which couldn't have handled that amount of gas... but if you're just making a gallon the regular water lock should work. Last photo shows day 26, when I first dared to sniff it -- it was ready for bottling, but it took me till day 31 to collect the bottles and find the time to finish.

Step 5: Bottle It

Once all the bubbling has stopped and you've waited a few extra weeks just to be sure (or not, if you are more gutsy than I am), open up your container and take a sniff; if it just has a faint fishy and acidic smell it's done. You can even dip your finger in and taste it! It should be sour, and if you like it, you can reserve some as flavoring or add a spoonful to your pet's water as an allegedly healthy supplement (full disclosure; I was too much of a wimp to taste it myself -- but I put a teaspoon in my cat's water dish. Turns out he's also a wimp).

Strain it (you can toss the remaining solids in your regular compost), and bottle it. If you are planning on using some as a spray, strain it a second time though a layer of cheesecloth so it won't clog up the nozzle).

Bottle the liquid and store in a cool dark place -- but you do not need to refrigerate it. The fertilizer is shelf stable because it is acidic -- this is fine for the plants because before you use it you will be diluting it.

WARNING: although the smell is not noxious, because of the fat your hands will still smell fishy after handling the concentrate. Once diluted however your garden will not smell bad, and so long as you don’t spill the concentrate on yourself neither will you.

Step 6: Use It in the Garden

This is a potent concentrate -- so you need to dilute it before you use it. You can use it to soak the roots or you can spray the leaves directly.

Shake the bottle. To soak the roots, dilute the solution 1:10 which comes out to about 1 1/3 cup of concentrate per gallon of fresh water.

If using as a foliar spray, dilute it more, about 1:50 which comes out to a little less the 1/3 cup for a gallon, or 4 teaspoons per liter. You can add a small squirt of dish soap in your spray to help it get the foliage evenly wet. Supplement with a little neem oil if you are having trouble with pests, or some baking soda to remedy powdery mildew blackspot or rust (1 tsp per liter for both supplements).

Once diluted, your fertilizer mix will not keep.

Also keep in mind that this fertilizer (unlike pellets) becomes available to the plant immediately, not slowly over time, so keep on applying it when the plant is growing actively, about once a week or every couple weeks, preferably in the early morning.

Stop fertilizing in the fall, because you do not want to encourage the growth of tender new foliage before the first frost when the plants will need to prepare to go into dormancy for the winter.