Compost – Is It An Organic Fertilizer?

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Robert Pavlis

Compost is good for the garden, in part, because it adds nutrients for the plants. That sounds like a fertilizer. But almost everything you read says that compost is NOT a fertilizer. Something doesn’t make sense—let’s have a closer look at this myth. Is compost an organic fertilizer?

Compost Bin for making organic fertilizer
Compost bin for making organic fertilizer

What is Fertilizer?

Before we look at compost we need to understand the term fertilizer. Here are some definitions from the internet dictionaries:

  • A chemical or natural substance added to soil or land to increase its fertility.
  • Any substance used to fertilize the soil.
  • Any of a large number of natural and synthetic materials, including manure, nitrogen, phosphorus, and potassium compounds, spread on or worked into soil to increase its capacity to support plant growth.

It certainly sounds as if compost fits into this category, but none of the definitions I found actually list compost.

It turns out that the above definitions are definitions in common use. When we ask about the legal definition of fertilizer we get a very different answer. Legally, at least in some parts of North America, fertilizer is “a soil amendment that guarantees the minimum percentages of nutrients (at least the minimum percentage of nitrogen, phosphate, and potash)” (ref 1) . An organic fertilizer is a fertilizer from organic sources.

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The key words are “minimum percentages”.

Since compost is made from plant material, and the inputs are very variable, manufacturers have a hard time controlling the amount on nutrients in the final product. Because they can’t or won’t guarantee “minimum percentages” from batch to batch, they can’t legally label the compost as fertilizer.

Interestingly, in Manitoba, Canada, the definition of fertilizer does not mention minimum percentages. So in Manitoba, even under the legal definition, compost is fertilizer.

Fertilizer Numbers in Compost

A lot of information sources refer to compost as a ‘poor source of nutrients’. They say that compost has ‘very low levels of nutrients’. Is this true?

Home made compost contains nitrogen, phosphorus and potassium—the NPK fertilizer numbers—in levels around 3-0.5-1.5. The key nutrient that might be deficient in soil is nitrogen and home made compost has about 3% nitrogen, with a range of 1 to 6%. Commercial sources for compost report levels of 1-1-1. I suspect that home made compost has higher levels of nitrogen because it is less finished.

That is a low level of nitrogen for fertilizer, but consider this. Fish fertilizer is considered to be a great fertilizer, and it has average fertilizer numbers of 5-1-1 with a range of nitrogen between 2 to 5%. It is not all that different from home made compost.

Compared to commercial fertilizer that might have 20% nitrogen, compost does have lower levels of nutrients. But 3% nitrogen is a fairly good fertilizer when compared to other natural organic products.   For example it is higher than coffee grounds which are considered to be a “good source of nitrogen”.

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It is also important to know how much fertilizer is being added to a garden. Usually you add very small amounts of commercial fertilizer, compared to compost. If commercial fertilizer is 20% and compost is 2%, nitrogen, and you add 10 times as much compost as commercial fertilizer – then both applications are providing the same amount of nitrogen to the soil.

Compost also contains a reasonable amount of micro-nutrients. This makes sense. Since plants need micro-nutrients to grow, and compost is made from plant material, the compost must also contain the micro-nutrients that were absorbed by the plant.

Will compost provide all the nutrients your plant needs? That is a question for another post.

Does Compost Feed Plants?

I think this is a dumb question, but I ask it because many web sites say something like this:”In the simplest terms, fertilizers feed plants. Compost feeds the soil”–clearly implying that compost does NOT feed plants.

That is a dumb statement. First of all soil does not need to be fed–it is not living. Things live in soil, but soil itself is not living. Things, also live in air, but we don’t go around calling air a living thing that needs to be fed! Compost makes nutrients available to living organisms. As we will see in future posts it also improves soil structure so it improves the environment for living things.

There are certainly differences between compost and fertilizers, but the differences have nothing to do with feeding soil. Both provide nutrients for living things, including plants.

Is Compost an Organic Fertilizer?

From a legal perspective, compost is NOT a fertilizer especially if you make it in your back yard. From the point of view of a gardener, compost is a fertilizer. It certainly adds nutrients to the soil, which can then be used by plants.

Most sources of gardening information say that it is not a fertilizer and although legally speaking this is correct, this is one instance were a small white myth is OK. Gardeners would understand their gardens better if they think of compost as a fertilizer.

What Type Fertilizer is Compost?

First of all it is organic. More importantly, compost is also a slow release fertilizer as explained in my post; The Real Value of Organic Fertilizer . Unlike commercial fertilizer, compost adds nutrients to the soil very slowly over several years. In my last post I discussed the fact that so called ‘finished compost’ is still decomposing. This continued decomposition provides a steady slow release of nutrients to the soil.

A new term that is being floated around is ‘biofertilizer’ and this term would seem to be a good fit for compost. Unfortunately, the term biofertilizer has been butchered by many in the organic movement and the term no longer has an accepted definition. Until this mess gets sorted out it is probably best not to use this term to describe anything.

In conclusion, compost is a good organic fertilizer–unless you are a lawyer who lives outside of Manitoba!


1) Legal definition of Fertilizer and Organic Fertilizer:

2) Chemical numbers in home made compost

3) Definitions for Biofertilizer:

4) Photo Source: grabadonut

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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!

26 thoughts on “Compost – Is It An Organic Fertilizer?”

  1. Very good article, and I think compost provides way more benefit than the NPK numbers would suggest.

    Here’s one of the main reasons I say that…

    Those comparisons you made between compost and other organic sources at 1-2 versus 20 for a chemical nitrogen fertilizer don’t tell the full story. If you read the article above, you’ll see that plants are only able to uptake about 5-7 percent of the total nitrogen applied through chemical fertilizers.

    That’s the real reason the numbers are so high. It’s not that it is a better source of fertilizer, it’s just not bioavailable, so they have to inflate the numbers to make it work.

    Compost and organic fertilizers like fish emulsion definitely work better in my opinion, even with the lower NPK numbers.

    • Compost does provide more benefits than the nutrients – they help aggregate soil and they provide the nutrients in a slow release form.

      In your link the first sentence is “Feeding plants is an important aspect to organic gardening. ” – which is an incorrect concept. We do not feed plants. Our soil for improving soil is to provide the nutrients that are missing. Most soil has the nutrients plants need, so there is very little requirement for this.

      We also want to increase the organic component in most soil, so that aggregation can take place.

      This statement is also wrong; “Nothing could be further from the truth! The numbers are higher to make up for the fact that only 7-8% of the nutrients in those products are used by your plants. Mineral uptake is limited when the nutrients are not delivered in a bio-available form.” The nutrients in synthetic fertilizer are exactly the same as those released by organic material – 100% identical. If synthetic fertilizer is applied correctly all of the nutrients can be used by plants since they are already in “bio-available form”.

      Contract that to organic material, where most of the nutrients are tied up in large molecules and are NOT available in bio-available forms.

  2. I’d argue that soil is everything you have in your hand when you pick up a handful of soil. To remove life from soil would take a filtering process of some sort. Anyway great article!

  3. great work Robert . I’m an extension officer and i really look forward to using your information during my training with coffee and turmeric growers in Belize.

  4. Robert,

    Thank you for this great information. I appreciate your accurate information and all the effort you do presenting it to people to finally hear truth about gardening. Most sites leave you more ignorant that before you read them.

  5. Interesting stuff Robert but my head hurts trying to make sense of all your readers comments – many are rather profound

  6. You’re asking two questions here… (1) Is compost a fertilizer, and (2) Is it an “Organic” fertilizer? As far as #2, it depends who’s making the compost. I’m not sure there are any commercial “true organic” fertilizers out there, including commercially produced compost. Shells are used as a source of calcium, yet clam and mollusk shells are also used commercially to absorb heavy metals in water treatment operations. In 100 grams of Kelp there is 12mg of Arsenic, 2.1µg of Mercury, 30µg of Lead, 21mg of Aluminum, and 3700mg of sodium. I would have left “organic” out of the title. Question #2 is the scary one, while question #1 is just semantics. I don’t always agree with you, but I love your blog.

    • I don’t understand your comment that there are no commercial organic fertilizers – there clearly are, unless you are using a different definition of organic than the rest of us. for example, fish fertilizers using only organic ingredients, and they list the fertilizer numbers, qualifying them as a fertilizer.

      The fact that Kelp contains lead does not make it any less organic. The term hear is using the common gardeners definition, not the chemists definition. for a discussion of what organic is see

      Question 1 is not semantics. There is a legal definition of fertilizer. You either use the word correctly or you don’t.

    • What is a vitamin? Definition: any of a group of organic compounds that are essential for normal growth and nutrition and are required in small quantities in the diet because they cannot be synthesized by the body. The fertilizer nutrients are not organic, so by this definition they can’t be considered to be vitamins. The term ‘organic’ here is the chemical meaning of the word, not the popular use as in ‘organic food’. More on this at What Does Organic Mean

      What is food? One definition is: any nutritious substance that people or animals eat or drink, or that plants absorb, in order to maintain life and growth. Fertilizer nutrients seem to fit this definition quite well.

      Are the nutrients food for plants? Plants need the nutrients in order to grow. They also need a carbon source which is CO2 and an energy source which is the sun. Plants use these basic building blocks and produce the other compounds that they need. Photosynthesis is part of this complex process. Animals use a different, but similar process. We ingest complex molecules (protein, fat etc) and break them down into smaller molecules–building blocks. We then have a complex system of building the chemicals we need to grow. For animals food is water and the complex molecules that we start with. For plants it is CO2 and fertilizer nutrients.

  7. I totally agree that compost provides nutrients that will be used by plants and very important improves the soil structure (which is another difference from the chemical fertilizers).
    I would note that certain people (especially the ones who feed the soil 😉 might also understand that when using compost as an organic fertilizer it equals using organic compost – we become entangled in ‘legal’ terminology.. 🙂

  8. After 40 years as a professional and at home gardener I came to the conclusion that compost feeds the soil, enabling plants to absorb fertiliser more easily. To feed the plant you need some form of artificial or organic fertiliser that makes itself available to the plant. Two different things.

    • I don’t understand the comment “Two different things”?

      If you are feeding the soil–how exactly are you feeding it? And how does it eat? The proper definition of soil is the inanimate, mineral component–it has no life.

      • Oh dear, I am obviously on the wrong blog. Feed the soil: maintain the organic content of the soil so that it is able to transmit its nutrients to the plant. The soil should contain many organic elements that need feeding. Not the same thing as adding nutrients that will be directly absorbed by the plant.

      • The definition you gave (mineral content) is not the definition of soil, but perhaps of dirt in this context. Soil by definition, and in distinction from dirt, includes at least mineral component (sand/silt/clay), organic matter, and the organisms. Per the father of soil science, Vasily Dokuchaev (1846–1903). Defining soil as mineral component, is like defining the ocean as water. Soils and the oceans are ecosystems, and as ecosystems cannot be defined apart from their living constituents. Correct me if I am wrong.

    • I thought that plants produced their own food by photosynthesis. You can’t feed plants. However, you can provide essential elements that they need. As you say Robert these elements are in plants that you compost. Therefore they are in compost. When plant matter decomposes these elements such as nitrogen, phosphorus and potassium are released into the soil solution and can be accessed by plant roots.

      All this “feeding” thing is nonsense.

      • The nutrients plants absorb is their food. They then use these chemicals to make other chemicals that they need for life. Part of this latter process is photosynthesis.

        • Confusion on confusion which always happens when scientific terms and every day terms are mixed up. Even in science, the term organic means different things. In biology it means anything that is or has been alive and is related to the term organism. In chemistry it means chemicals that contain carbon. What it means in everyday language is completely obscure.
          Bottom line – compost is extremely good for plants. Put as much on your garden as is reasonable.
          The term food is difficult one for a biochemist. If you look in a text book like ‘Biochemistry’ by Stryer the word food is not mentioned at all. However the whole book is about how chemicals are used to produce energy (catabolism) or make the building block molecules (anabolism) to allow an organism to live. Usually, if I have to, I define food as a substance that can be used in both catabolism and anabolism which does not help when looking at some substances like water and salts. In this case I hedge my bets by calling everything nutrients. (Remember there are lots of salts – not just sodium chloride.)
          A biology professor I worked with used to say, “Trees don’t have great holes beneath them where they have eaten the soil!”
          Fertiliser is just concentrated plant nutrient – such as nitrogen in the form of the salt ammonium nitrate. Fertiliser is analogous to refined sugar and compost is analogous to a fruit like an orange which contains sugar.
          Animals are heterotrophs which get their nutrients from eating other organisms or parts or remains of organisms. Fungi are heterotrophs because they can get their nutrients from dead organic material. (I use organic here in both the chemical and biological meaning of the word because they are fantastic in cleaning up pernicious polluting manmade chemicals, (which can add further confusion to the word food!). Fungi are not plants, which is why they are put into a new group of their own.
          Plants are autotrophic, which means they produce chemicals for anabolism and catabolism using captured sunlight energy, carbon dioxide and water.
          Plants, and by implication humans, are just wisps of air.
          This is the process called photosynthesis which not only produces carbohydrates for the plants structure but also some energy that is needed to absorb chemical nutrients in the soil by active transport. (However most energy in the form of ATP is produced in the mitochondria but that’s another story.) It’s not that easy to get things into and out of plant cells – or animal cells for that matter. These absorbed chemicals are then used – after photosynthesis in cascade reactions usually starting with glycolysis and the citric acid cycle – to produce chemicals that make up the structure of the plant.
          So is there any plant ‘food’ in compost? As far as everyday language goes, definitely yes. It is much diluted but plants need such tiny amounts of nutrients like nitrogen, phosphorus and potassium that compost provides adequate amounts – unless you are part of a monocropping industry. Compost has a high cation exchange capacity meaning it captures nutrients and releases them slowly so that they can be used by the plant. It reduces bulk density, increases air filled and water filled porosity, improves the soil structure by forming aggregates, provides habitat for soil organisms that decompose organic matter and provide further nutrients for plants. Some of the compost will eventually end up as humus that resists decomposition, which means that the soil is a carbon sink storing carbon so that it does not produce the greenhouse gas carbon dioxide.
          There is nothing bad you can say about compost; put it on your soil.

          • Very good information. I agree that the term ‘nutrients’ is a better term than ‘food’ when dealing with plants.

            I do want to clarify two items which I will be dealing with in upcoming compost posts:

            1) You can have too much compost.
            2) Compost may not provide enough nutrients for your plants.

            A discussion about what is Organic can be found at What Does Organic Mean.

      • Any dead organic material that is returned to the soil, whether compost or natural litter from dead plant debris or dead animal remains is food for the soil life. Bacteria, fungi, earth worms, micro arthropods, any species that gets nutrients from organic plant matter. Soil is a combination of organic plant matter in various stages of decay, minerals and soil life, adding compost is feeding the soil. Soil without soil life it’s just dirt. Proper nutrient cycling is dependent on soil life, so plants in an organic ecosystem are dependent on soil life for a great deal of the nutrients in the right form that they need. Saprophytes feed on dead organic matter, predators feed on other organisms and so on.
        The soil food web is complex and pretty amazing.

        • I agree with almost everything you said. Especially “The soil food web is complex and pretty amazing”. However,..

          None of the common dictionaries that I looked at include ‘living organisms’ in the definition of soil. The definition is generally something like: The top layer of the earth’s surface in which plants can grow, consisting of rock and mineral particles mixed with decayed organic matter and having the capability of retaining water.

          Even your own reference says “The soil food web is the community of organisms living all or part of their lives in the soil”. If the organisms live ‘in’ the soil, then clearly they are not part of the soil.

          • Robert, common dictionary semantics aside (who writes those definitions anyway, they are definitely not soil scientists), soil is not just sand, silt, clay and decayed organic matter. How did the decayed organic matter get there? Microorganism are the main decomposers of organic matter, organic matter wouldn’t decay without them. Nutrients present in the mineral fraction of the soil would be unavailable to plants without the microorganisms that mine the mineral particles for those same nutrients they need themselves. As you know, nature doesn’t use Miracle Grow in an inert medium, plants and the soil life have evolved together to form symbiotic relationships that supply food and protection from pathogens and predators.

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