Is Fish Fertilizer Better Than Compost?

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Robert Pavlis

Fish fertilizer is a common organic fertilizer that is recommended for both houseplants and the garden. It has a strong following and its proponents feel that it is better than any other type of fertilizer, but how does it really stack up against the most common organic fertilizer; compost?

Fish fertilizer – are the accolades true, or a bit fishy?

Compost vs Fish Fertilizer, by Garden Fundamentals
Compost vs Fish Fertilizer, by Garden Fundamentals

Benefits of Organic Fertilizer

There are four key reasons to use organic fertilizer.

  • Sustainable and eco-friendly
  • Provide nutrients in a slow release form
  • Feed the Microbes
  • Soil Improvement

How does compost and fish fertilizer compare in supplying these benefits?

Sustainable and Eco-friendly

Compost rates fairly high in this category since most of it is made from waste organic matter. There is some concern about the release of CO2 during the composting process, and I have dealt with that concern in Does Composting Contribute to Climate Change?

Use of fish guts for making fertilizer is also good use of a waste product. A few companies are collecting invasive species, like the carp, and making fertilizer from them – that is also eco-friendly.

Unfortunately, some fish fertilizer is made from live fish that is specifically caught for the purpose under the guise of using a useless species. The reality is that these so-called ‘useless fish’ are an important food source for larger fish, and we are starting to see declines of this food source. The practice of catching live fish and using them to make fertilizer is anything but eco-friendly.

Food Science for Gardeners, by Robert Pavlis

Provide Nutrients

One of the main reasons to use a fertilizer is to provide nutrients for plants and these can be provided quickly or on a slow release basis. I commonly see statements about the slow release of nutrients from organic fertilizer, but this is not always true. The source of the organic matter is critical.

Compost is a slow release source of nutrients and on average it takes about 5 years for all of the nutrients to be released to the soil.

Fish fertilizer is commonly made from fish emulsion which decomposes in a few weeks, not years.

If you are feeding plants that need a quick feed, such as a vegetable patch or houseplants, fish  fertilizer would be a better short term option than compost, but synthetic fertilizer provides an even quicker feed.

For landscapes, fruit trees and berry bushes, there is little benefit from a fast feed and compost is a better option.

Most organic fertilizer has some free nutrient ions immediately available, and for both compost and fish fertilizer this is about 3-10% of the total nutrients.

Cost of Nutrients

The nutrient that usually limits plant growth is nitrogen and the cost of nitrogen varies greatly between various types of fertilizer.

Microbe Science for Gardeners Book, by Robert Pavlis

Synthetic fertilizer is by far the cheapest, both in terms of actual cost, and in terms of handling costs, since it has less relative bulk than other types of fertilizer.

Compost is fairly inexpensive, but has a higher handling cost because it’s bulky.

Fish fertilizer is the most expensive source of nitrogen. Its four times higher than blood meal nitrogen, which most consider to be very expensive. A major part of fish fertilizer is water, making it heavy to handle.

Fish fertilizer nitrogen is 20 times more costly than compost nitrogen.

Soil Improvement

The real value of organic fertilizer is its ability to improve soil. It increases microbe biology, decreases compaction, increases aggregation and holds more moisture in the soil. The right type of organic fertilizer also holds onto nutrients and slows down their travel through soil – the chelating effect. All of these contribute to better plant growth.

Microbe growth and activity is increased due to both the presence of nutrients and energy sources like sugars and other carbohydrates. This activity then leads to improved compaction and an increase in aggregation, making it easier for roots and air to get into the soil. The added organic matter holds more moisture and nutrients, making them available to both plants and microbes.

The amount of nutrients added to soil by both compost and fish fertilizer are about the same. Since all fertilizer adds nutrients, the quality of any organic fertilizer is determined mostly by how much organic matter it adds to soil.

On a dry weight basis, both compost and fish fertilizer have about the same amount of organic matter, 70-80%. The picture is quite different on a wet basis, where fish fertilizer has much less organic matter.

Some fish fertilizer is a clear liquid in which case it adds no solid organic matter. It is essentially the same as a liquid synthetic fertilizer.

Other brands consist of a slurry; a 50/50 mixture of water and solid organic material.

One pint of Neptune fish fertilizer (NPK of 2-4-1) contains 510 gm. Assuming it is a slurry, it contains 255 gm water and 255 gm organic matter. This concentrate makes 18 gallons of fertilizer. Interestingly, the instructions do not tell you how much to use to fertilize the garden, which makes it next to impossible to use it correctly.

Let’s say that you add 1 gal to every sq meter (1.2 sq yard) of soil. You therefore add 14 gm of organic matter per sq meter, the weight of 14 paperclips.

Compost varies quite a bit but average values give us a density of 540 Kg/cubic m with a 28% water content. Assume it is spread to a thickness of one inch, or 2.5 cm. Each sq meter would receive 540/40 = 13.5 Kg of compost. A third of this is water, so the dry weight added is 9.7 Kg, or 9,700 gm.

Compost adds 700 times as much solid organic material as fish fertilizer.

Contrary to what manufacturers tell you, this calculation makes it very clear why fish fertilizer does very little to improve soil.

Fish Fertilizer is NOT Fish

Fish have been a traditional way to fertilize plants and I see many comments about burring fish in soil.

When we talk about fish fertilizer it is important to realize that this can’t be compared to using whole fish or even fish guts.

The process to make fish fertilizer starts by extracting fish oil. This is followed by some type of digestion process and the removal of as much solid material as possible, which is used to make fish meal. The remaining liquid is boiled down to concentrate the small amount of goodness left after processing. This is then sold as fish fertilizer.

Most of the fish is removed before they make the fertilizer. It is similar to making a good stew and then removing all the vegetables and meat before serving it.

Three Classes of Fertilizer

It is time to start thinking of 3 classes of fertilizer; synthetic, organic solid and organic liquid.

Synthetic fertilizer adds nutrients for a fast feed, but does little to improve soil.

Organic solid fertilizer is the traditional compost and manure. It is bulky, has low nutrient levels and contains a lot of organic matter which feeds the soil over a long period of time. The real value of this material is its contribution to improving soil quality and microbe biology.

The third class of fertilizer, organic liquid, is becoming more popular and includes things like fish fertilizer, seaweed extract, kelp ferment, and compost tea.

These products are organic, but they contain almost no organic matter. You simply can’t dissolve a lot of organic matter in water. The problem with these products is that, producers make claims which are similar to the valid benefits of solid organic fertilizer, and people believe them. Without the bulky organic matter, these products do almost nothing for soil improvement.

Most of these products also decompose quickly, meaning they don’t provide the long term feed benefit of solid organic fertilizer. Neptune’s Harvest recommends you add their fish fertilizer to your garden every two weeks. On the other hand, you add compost in the spring and it feeds the garden for a year or more.

There is nothing wrong with fish fertilizer. They do provide a low level of nutrients which are the exact same as found in synthetic fertilizer, but at a much higher cost. Their other benefit is the sustainability and eco-friendly qualities and that may be enough to justify their use, provided the products are not made from native fish caught in the ocean.

Is fish fertilizer more sustainable or eco-friendly than synthetic fertilizer? Or is this just something people believe because one is labeled organic?

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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!

15 thoughts on “Is Fish Fertilizer Better Than Compost?”

  1. Fish Meal is valuable : made from fish bones it’s a natural source of Phosphorus. Phosphorus as a mineral you can mine is in a rather short supply on earth (especially if you need to purify it from its high sulfur content. So it makes sense to try to use Phosphorus found in bones (fish or animal) ….

  2. I would like to point out one major flaw in your article.
    “The reality is that these so-called ‘useless fish’ are an important food source for larger fish, and we are starting to see declines of this food source. The practice of catching live fish and using them to make fertilizer is anything but eco-friendly.”
    This statement is very untrue concerning the Mississippi river and the Asian Carp. (Plus Many other examples of invasive species)
    These Asian carp are killing off the natural species of fish by completly filtering out all the food from the waters.
    One whole asian carp per tomato plant hole and lookout because they will be 10′ by fall time.

    • That is why I said “A few companies are collecting invasive species, like the carp, and making fertilizer from them – that is also eco-friendly.”

      and “Their other benefit is the sustainability and eco-friendly qualities and that may be enough to justify their use, provided the products are not made from native fish caught in the ocean.”

  3. Thanks for this. Robert. I had thought to contact you about a week ago to see if you’d do a post on whether Sea Soil is better than standard compost. It’s related to this, so it might be a good follow-up piece.

  4. It would seem liquid organic does nothing more than provide an eco-friendly quick release of nutrients (origin factors considered). Are we certain nothing remains in this liquid on a micro-scale that wouldn’t feed nitrogen fixing bacteria, protozoa, etc? I’m sure in this context, a bulk amount of solid organic might be considered inert.

    • I am sure there is something on a “micro-scale”, but nothing that is not also in compost in much larger quantities. There are no magic chemicals in fish.

  5. I have one thing to say about fish meal it suck it great for your plants you can’t get into your field for the fire ants that’s like ringing the dinner bell

  6. I agree with all you state above Robert, but one thing to remember is that the cheapest form of Nitrogen for use as a quick boost fertilizer is human URINE & its FREE & readily available several times a day, gardeners should think of it as a free supplement to their regular composting activities etc. However if urine is stored it needs to be kept in an air tight container as the nitrogen content quickly depletes if it comes into contact with the air. So gardeners go out into the garden & discretely fertilize your garden directly or in the privacy of your own home into a closed container & use as soon as is possible, & don’t forget to use the compost from your compost bin.

    • I was especially glad to see your comment. Having been interested in urine as a way of converting an unavoidable waste product to useful material, I began saving urine. Wound up over last winter with about 50 gallons stored in five gallon buckets, covered but probably not air tight. Wound up spilling it all out because I could not be confident it wouldnt damage my garden soil/microbes. Threw it out in an area of weeds. So far have been no observable effect beneficial or negative.

      • Hi Thomas,
        In the garden I use approximately 1 litre of urine in a 9 litre watering can, recommended ratio is one to ten of water, but I go a bit higher on the urine.

        Sometimes I apply urine directly to the soil in which case I quickly water it in with a hose pipe.

        I also apply it to the compost bin from time to time if the compost looks a bit dry.

        My favourite use for urine is soaking dry brown autumn fall leaves in a dustbin/garbage can. The dry brown leaves are rich in carbon whereas the urine is rich in nitrogen, thus hastening the composting process of the leaves. Generally I soak the leaves for a couple of weeks then add them to the compost bin to do their thing & rot down.

        The nitrogen in urine is quickly lost once in contact with the atmosphere. Therefor I try to use ASAP & on regular basis, have never noticed any negative effects, & my garden is thriving.

  7. I tend to use a fish emulsion/seaweed blend on my potted plants but in the garden I just alternate different composts — one year I’ll put down mushroom compost, the next leaf compost, then one year I’ll bite the bullet and spring for earthworm castings. Last year, I was recuperating from a broken elbow so couldn’t lift/drag 50 lb. bags around so just threw on some greensand. What’s your take on Greensand?

    • I wish Robert had commented on greensand, because I don’t know anything about it. But I will say this: I envy your access to mushroom compost, which in some places is made from composted horse manure, and can be made available locally. Also, the variable materials you rotatioally use seem wonderful to me. I made a large garden out of just composted leaves (all kinds) , which worked out beautifully. Would love to have lots of earthworm castings, but I’m too cheap. So I’d rather provide the kind of materials earthworms like to reproduce in. When I was younger/stronger and acquired horse manure with red wiggler worms, they didn’t last. But other types took their place because the environment was right for them.


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