Fish Fertilizer – Is it Worth Buying?

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Robert Pavlis

Fish fertilizer is very popular. It is reported to be a good source of nutrients and a good source of proteins, amino acids and oils – for your plants. Can plants use fish proteins and oils? Is fish fertilizer a good source of nutrients?

Before I go any further let me say that there is nothing wrong with using fish fertilizer. It will help make your plants grow. I have two problems with fish fertilizer; it is extremely expensive compared to other sources of fertilizer and many of the claims for it have no basis.

making fish fertilizer
Making fish fertilizer

Fish Emulsion vs Fish Hydrolysate

What is the difference between fish emulsion and fish hydrolysate? The difference from a plants point of view is minor, but if you are trying to sell product–there are big differences.

Fish emulsion and fish hydrolysate start with the left over bits from the fish industry–the parts no one else wants. These are then treated with various chemicals and enzymes to break down larger organic molecules into nutrients and other small organic molecules. Further treatment can take one of two paths; it is either heated or cold processed. Fish emulsion is the end product if the heating process is used. Fish hydrolysate is the result of using cold processing.

Growing Great Tomaotes, by Robert Pavlis

There is great debate between the benefits of emulsion vs hydrolysate–which is better? The reality is that plants can’t use most of the large or even small organic molecules from either process. Normally microbes in the soil degrade these to nutrients plants can use. So the argument that heat in the emulsion process is detrimental, makes no sense. it is true that heat will denature proteins, but they need to be denatured for the plants to use them.

I think the arguments for or against either process is just marketing hype. I have seen no scientific evidence to support the superiority of either process.

Fish Fertilizer Benefits

Fish fertilizer is an organic product–for the most part. So it does have the benefits other organic soil additives have. It feeds plants, microbes and improves soil structure.

But proponents of fish fertilizer make claims that do not apply to other organic fertilizers. Most seem to be centered around the fact that the liquid fertilizer contains proteins and oils. We all know fish oils are very important for our health and so they must be good for plants, right? Wrong!

Plants can’t make use of large molecules such as oils and proteins; see Organic Fertilizer – What is it’s Real Value? for more details. When these molecules are added to soil, microbes digest them and turn them into small molecules like nitrate, and phosphate. It is only then that plants can make use of these molecules.

Since the large molecules need to be degraded before plants can use them, there is little difference – to the plant – between proteins and oils from fish, cows (manure), or even plants. I have found no support for the claim that fish fertilizer is better than any other organic fertilizer.

Microbe Science for Gardeners Book, by Robert Pavlis

The main thing plants need from fertilizer is a source of nitrogen. Garden soils usually have enough P and K and the other minor nutrients. Nitrogen is the thing that is missing in soils. Given this fact, fish fertilizer is no better or worse than other types of fertilizer.

Fish fertilizer has about 2% nitrogen, which is the same as most organic fertilizers; compost, manure, and coffee grounds.

Is Fish Fertilizer Organic?

This probably seems like a dumb question–fish are organic so why would fish extracts not be organic? Here’s why. In the process of turning fish scraps into fertilizer companies add a number of chemicals, including phosphoric acid, and odor inhibitors. Apparently, as long as these ingredients form less than 1% of the finished product, the product can still be called organic. Who knew–organic fertilizer only needs to be 99% organic!

Cost of Nitrogen

I checked several fish fertilizers and a common analysis is 2-4-2 and if you buy in large containers you can get 9 lb (3.8 Kg) for $25. Small quantities are even more expensive. This fertilizer has 2% nitrogen, and so the cost for the nitrogen is $33 for 100 g of nitrogen. Wow! Even fresh caught Atlantic salmon doesn’t cost that much!!

What is the cost of 100 g of nitrogen if you buy a commercial fertilizer? Scotts sells a 30-0-9 at $17 for 6.2 Kg, or $0.91 for 100 g.

Fish fertilizer is 35 times more expensive than commercial fertilizer and plants can’t tell the difference between the two sources of nitrogen.

I can hear your objection – BUT … fish fertilizer is organic. That is true, and organic fertilizers do more than just provide nutrients. They also help build better soil by feeding microbes. Fish fertilizer is about 14% protein which is the same as manure. A 30 lb bag of manure will cost you $4 compared to $75 for the same amount of fish protein. Manure bought in bulk is even cheaper.

I really can’t think of any good reason to buy fish fertilizer if other sources of fertilizer are available.

References:

1) Photo Source: Cheryl’s Garden Goodies

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

243 thoughts on “Fish Fertilizer – Is it Worth Buying?”

  1. I use homemade fish fertilizer for my garden. It only has 3 ingredients. They are fish, blackstrap molasses and water. Yeah it has a strong odor. Especially when the fish are breaking down. I just put a tight lid on a 5 gallon bucket. Usually turn the bucket once a week to stir everything up. I used nothing but it, molasses and epsom salt in my garden last year and it did great. Just dilute it a few ounces to a bucket of water. Used it once to twice a week and it did great

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  2. I guess we never have to worry about a food shortage with all the knowledge in your comment session. EVERYBODY seems to know what is RIGHT and what is WRONG…only everybody seems to disagree with everyone else. This all makes the idea of gardening a bit intimidating having to decide which “expert” to listen to. LOL I’m just a grandma who does a small garden for my and my family’s use and I just do what I want with regards to fertilization, etc. It seems to work for me!

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  3. I have the extreme misfortune of living immediately adjacent to and downwind of a farm that sprays fish fertilizer on their hay fields. As in, all day, all night, for many weeks in the heat of the summer, microdroplets of dead fish permeate every breath we take. It is inescapable even if we could seal ourselves up in our own home — but we have our own farm to look after, which means we spend many hours inhaling this stuff. It stinks! The air even tastes fishy. It’s thoroughly disgusting. Does anyone know of any research regarding the long-term health implications of exposure to fish fertilizer?

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    • Probably not as bad as here in the south where every farm has chicken manure spread on it and the stench is everywhere for weeks.

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  4. “ Each soil has their own capacity to support microbe life. At any given time the soil is saturated with all the microbe life it can support.”

    At any given time? Do you have any evidence for this? Are you suggesting that chemical vs organic will not make any difference? Please provide some evidence.

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  5. Fish fertilizer increases the amount of decomposers in the soil. I am not aware of any chemical fertilizer capable of doing this. FF is a great fertilizer for sandy and loamy soils. I have to say measuring this fantastic fertilizer simply by its N content is pretty shallow. Too much N is never a healthy thing anyways.
    FF increases the fertility of the top soil especially when combined with pumice.

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    • “Fish fertilizer increases the amount of decomposers in the soil.” – Provide some evidence that this is true. It is not true because soil is already full of these decomposers and can’t support more.

      https://www.gardenmyths.com/buying-soil-probiotics/

      nitrogen is the nutrient that is most likely deficient in any soil so it is a good measure of the value of any general fertilizer.

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      • “Provide some evidence that this is true. It is not true because soil is already full of these decomposers and can’t support more.”

        Are you suggesting that all soils are equally full of microbiological activity so they can not support more? Not all soils are equal my friend. The biological activity in the amazons vs your back yard are not the same. I have seen you recommending compost, if they are all equal and can not support more then why would you recommend the use of compost if I may ask?

        You wanted evidence, here you go:

        https://pubmed.ncbi.nlm.nih.gov/21043120/

        – Lookup https://www.researchgate.net/. Hundreds of research available about soil compaction, bio activity in soils and organic fertilizers (specifically research fish fertilizer)

        https://dergipark.org.tr/en/download/article-file/1409647: Read the conclusion part

        https://sustainablefarming.com.au/product/fish-emulsion/#:~:text=Fish%20Emulsion%20is%20a%20nutrient,Reduce%20root%20disease

        I am a commercial certified organic blueberry grower, I have thousands of plants and they all grow in peat based containers. I used to go with chemical fertilizers up until two years a go. Here is my own personal experience:

        1) Worms (all sorts), Bacteria, Fungi and Lichen (which is a symbiotic relationship between fungus and algae) are all common decomposers. Specifically symbiotic relationship is important in blueberry production. Blueberry plants dont have root hair and they rely on this relationship. Chemical fertilizers make this relationship unnecessary for the plants (something they have evolved in thousands of years). I am not sure if this is a good thing.

        Anyways, when I was chemically fertilizing I never saw a single earth worm in my containers, I was fooling myself and thinking that the soil PH was too acidic for earthworms. Though guess what after I switched to fish fertilizer the earth worms were everywhere, not just in the containers but under the weed mat as well.

        2) Fish fertilizer is only active when the soil is warm, this is excellent to avoid bacterial diseases. As soon as the soil cools down the activty stops. The N content in chemical fertilizers on the other hand is readily available for the plants. When I was fertilizing chemical my plants were going dormant too late and getting frost injury in winter. Everybody knows this is an invitation for canker and blight like diseases. Never had this issue with fish fertilizer.

        Again too much N is never a good thing, the amount of N in the chemical fertilizers is unnecessarily high. Plants can only take up a fracture of it. The rest goes to underwater systems and triggers pollution. I am all in favor of indirect fertilization, more robust and disease resistant plants I get. Chemical fertilizers are direct fertilizers with enormously high amount of NPK available for a short period of time in the soil. Overkill in my opinion, the nature does not work this way.

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        • “Are you suggesting that all soils are equally full of microbiological activity so they can not support more?” – no, never said anything like that.

          Each soil has their own capacity to support microbe life. At any given time the soil is saturated with all the microbe life it can support.

          Not sure what you are trying to prove with your references. The first one proves that when you add nutrients to soil you see an increase in microbe activity. As the amount of nutrients goes up, you get higher and higher activity. – Correct we know that and nobody disputes that.

          Your evidence proves micros use nutrients.

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  6. One thing is not right here: “plants use the same chemicals being organic or not”. That is not true. For example in N, in organic managment plants absorb it in the form of aminocids, wich are more efficient for them, for the contrary in the conventional practices, plants absorb N in the nitrate form, wich accumulates in the sap and is not 100% readly usable. Source: Dr. Christine Jones Phd in soil microbiology

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  7. I was reading about all the red tide and bad algae in waters creating dead zones and killing sea life and harming humans. It made me want to start making fish fertilizer from fish that would otherwise be thrown in the water to dispose of. I agree the fertilizer is expensive and I know it is diluted when used quite a bit. Still, I was thinking of selling it at a lower cost if it is still profitable, like $10 a gallon vs $15 to $27 a gallon for other ones. I don’t live near the ocean anymore so I would have to start with farmed fish but my goal would be to help the waters, not make huge profits so if I succeed, maybe it can get down to $5 a gallon someday. When diluted, that would stretch a long ways.

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  8. What is your scientific proof that fish fertilizer will make the microbes starve to death as it is lacking macro nutritions? Please also advise reference.

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    • “What is your scientific proof that fish fertilizer will make the microbes starve to death as it is lacking macro nutritions?” – I never said that

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  9. Hi, Robert. Thanks for all information I find in your website and the comments here. What fertilizer would you suggest I use for tomatoes that will also feed the microbes in the soil for healthy soil and plant?

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  10. Thanks for your rationale, science over anecdotes, debunking myths, ‘diplomatic-crucial-conversations’ writing style, and educating me about bio-chemistry plant / soil affects & economy of price per unit of fertilizer. I’m a new gardener in Vegas (zone 9a) & am a new fan Robert.

    Reply

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