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Organic Fertilizer – What Is Its Real Value?

In my last post What is Organic Fertilizer I explained why the nutrients in organic fertilizer and synthetic fertilizer are the same. Plants can’t tell the difference between the two because there is no difference. However, organic fertilizer is better for the garden. In this post we will look at why this is true.

manure - organic fertilizer

manure – organic fertilizer

What is Organic Fertilizer?

An organic fertilizer is one that is composed of organic material. Manure, compost, and blood meal are good examples. Each of these contains complex organic molecules like protein, carbohydrates, fat, DNA etc. It is the existence of these large complex organic molecules that is so important.

Slow Release Fertilizer

When organic fertilizer is added to your garden it acts just like a synthetic fertilizer. The available nutrients leach out fairly quickly into the soil. Plants can start using them immediately. The amount of available nutrients in organic fertilizer is low, making organic fertilizer a poor choice if you need to get a lot of nutrients to your plants quickly. Synthetic fertilizer is much better for a quick feed if you need the higher levels of nutrients.

In a healthy garden a quick feed is never needed, but it might have some benefits in a vegetable garden where you want things to grow quickly. Personally, I have not worried about a quick feed even in the vegetable patch.

After a few weeks of leeching nutrients, the ‘available nutrients’ are gone and that is when the real magic starts to happen. The microbes in the soil (bacteria, fungi, algae) start to eat the organic matter. During the digestion process large organic molecules are converted into smaller nutrient molecules.

This is a slow process and will continue for many years. For example in the case manure and compost, nitrogen is released for at least 4 years. This steady release of nitrogen is a result of microbes eating and digesting large molecules like protein and DNA that contain nitrogen atoms, and converting them into nitrates which plants can use.

Think of organic fertilizers as slow release fertilizers. The organic component is slowly converted to nutrients by the soil microbes and other life living in soil. Synthetic fertilizers are unable to do this – their nutrients are used up fairly quickly and provide no long term food for microbes.

Soil Structure

Have you ever gone into mature woods and dug around the soil? If not you really should do this. The soil there is black and friable (crumbly). It is easy to dig because it has great soil structure. This soil structure is built up over a long time and it is maintained by the microbes in the soil that digest the organic matter, mostly leaves, that cover the ground each fall.

When you add organic fertilizer to your garden you mimic the process taking place in the woods. Microbes eat, poop and die. The dead microbes and microbe poop, I call that ‘microbe juice’, are critical for building soil structure. Think of microbe juice as being a slimy, sticky material like honey. This ‘goo’ sticks together the clay, sand and silt particles in soil to form larger particles called aggregates. Aggregates are the reason soil is friable. Aggregates allow lots of air and water into the soil, and give roots a perfect place to grow.

Microbes are responsible for building soil structure, ie making your soil better. The organic matter added to the garden is the food source for the microbes.

The real benefit of organic fertilizer is that, over time, it builds soil structure.

Are All Organic Fertilizers Equally Good?

The answer is no. The real value in organic fertilizer is not the level of available nutrients. The real value is the long term feeding of microbes, and the slow release of nutrients. A good organic fertilizer is one that contains a lot of large organic molecules that are not yet digested.

Manure is perfect. It has both available nutrients for a quick feed and lots of undigested organic material for the long term care of the microbes.

Compost is also good, but some of the digestion of organics is already done. It was done in the compost pile, and not in the soil that needs to be improved. Compost is not as good for your garden as manure.

Hay, straw and weeds are also good organic fertilizers for the long term. They have almost no available nutrients for a quick feed, but they contain lots of organics for the long term.

Fertilizers that are mostly water based such as fish emulsion, and manure tea may provide a few nutrients immediately, but they provide almost nothing for long the term. They are not a good source for slow release nutrients and they don’t build soil structure because they don’t contain any significant organic matter to feed the microbes. For this reason I would not even consider them ‘organic’, although most people do. Think of them as weak synthetic fertilizers. These extractions of organic material will not improve soil structure or provide nutrients for the long term.


1) Photo Source: slimmer_jimmer

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Robert Pavlis
Editor of
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

17 Responses to 'Organic Fertilizer – What Is Its Real Value?'

  1. A Hudson says:

    Hi Robert

    Ive been using your blog as reference for my organic gardening activities for quite a while now and find it both very informative and invaluable, thanks for all your hard work.

    I was hoping you could clarify an issue for me around two of your posts. In this post, of the benefits of organic fertilizer, you say manure is perfect, better then compost…however in another of your posts ‘too much compost’ you suggest plant based compost is better then manure based compost due to the latters higher phosphorous to nitrogen ratio and the mobility issues for P, or lack of it, and thus high P levels built up over time.

    Are you just comparing fertilizers, manure vs compost, in this post, saying manure is better for both a quick feed and organic matter for soil structure over time….but isnt composting, plant and manure based, used for the same reasons? Could you please enlighten me?


    • To be honest, the best option is to use whatever you can get that needs to be trucked the shortest distance. The trucking and processing hurts the environment.

      In most situations manure is better than compost because it is less decomposed. The process of making compost loses nitrogen to the soil around the compost pile and into the air. If the same material composts right in the garden the loss runs into the garden.

      Once you reach a level of high organic levels in the soil it is quite common to also see a high P level. The reason for this is that P travels very slowly in soil, compared to most other nutrients. So over time it accumulates. Once soil is at this stage, you want to try an minimize the amount of P being added. A better organic source is one with more nitrogen and less P. In fact on such soil, it is better to add nitrogen as a fertilizer instead of an organic product since you can use something like urea and add no P.

      Hope that helps.

  2. troysantos says:

    I used to live in a community in Thailand that makes its own organic fertilizer. One of the main components is eucalyptus trees. Some people reported amazing results with the fertilizer while others reported horrible results. I suppose most users had more moderate results. Given that eucalyptus and other vegetation have an inhibiting affect on the growth of other vegetation, it seems sensible to me to not include these things in fertilizer unless we know the inhibiting substance breaks down completely before it is sold.

    • That makes sense. Here is what I would do. Make tea from it and water seedlings with it. Seedlings are normally more sensitive to inhibitors than mature plants. if it does not harm seedlings, it is not likely to harm other plants.

      Also keep in mind that this could be species specific. We don’t grow eucalyptus but we do have walnut trees which produce juglone. This will kill tomato seedlings but will not harm some other types of seedlings. For some reason tomatoes are very sensitive to it.

      • ashraf says:

        Dear brother, we have molasses. Unfortunately, I do not know how to use and what other additives of fertilizers can be used with molasses. Please wait for your reply and advice. Thank you.

  3. troysantos says:

    So, tell me please … do you believe / know that soil fertility / health has deteriorated over the past several decades (or choose your own time frame)? If so, what do you attribute this to? Tillage? I’ll look in your archives for a post about tillage. I am reading this old post by following a link from your most recent post, “Does fertilizer kill soil bacteria?”

    • I am sure fertility is down in agricultural and cleared land because the level or organic matter is down. This is mostly due to tilling, but also repeated mono-cultures.

  4. dan smith says:

    Hi, just told my wife I put twigs/sticks, leaves etc. In veg. Garden last fall & that was why we have enough surplus to sell. I just sprinkled dirt over it in the rows & watered. Amazing how fast the worms etc. Make it disappear. The worms are my plows & I use all hand tools no ‘rototiller’. Thanks.

  5. Donna says:

    You say manure is among best soil amendment, but you cannot put this straight on garden or mix with soil without heating and composting first, right? Can I use oak leaves as a top dressing for nitrogen? I want to grow Okinawa spinach and that takes nitrogen, so how to get that once the plants are in?

    • You can add manure right to the garden if it has aged. It does not need to be fully composted. I mix horse manure that has sat for a year right into potting soil.

      Oak leaves will be very high in carbon and low in nitrogen. You can use them to top dress but they won’t add much nitrogen. Anything organic adds nitrogen slowly to soil, over many years. That is one reason people use fertilizers on vegetable crops. they need a fast feed. Plants can’t tell the difference between nitrogen from fertilizer and nitrogen from organics.

      • jack burton says:

        From what I have read (and experienced as a rabbit grower) you can add rabbit manure directly to the soil without composting. It does not burn the plants. If you look on the net there are many people who claim it is the best manure to use. I won’t make that claim except to say that it does work for my veggie garden. You might be able to find a supply by asking around since rabbits eat and poop pretty constantly in life.

  6. Bimal Jayasundera says:

    Hi This is Bimal Jayasundera from Sri Lanka, I have tried to make a fertilizer based on fish and it parts. in By country well it is covered by the sea. wish waste from the large fish market even can be transported to by gardern (Tea 250 acres) almost free. Im not an agriculturist, Other than microbial activity, Is there any way of making smaller organic molecule? or what would be the best organic source to make fertilizer? in Sri Lanka, there are almost 10,000 people are dying every year due to the Kidney problems due to residuals made out of agro chemicals / and chemical fertilizers. I have tried 75% Fish / 15% Sugarcane molasses / 5% Neem leaves / 5% Glidisiria leaves. there is no heating process. i only crush to make as slurry and ferment 21 days and filter and use. the ultimate slurry dry and use as compost. it gave me good results but now im confiused. Seek your advise. Bimal

  7. Patrick says:

    This website has really blown my whole concept of gardening completely wide open. I wanted to thank you for taking the time to clear up these ideas about gardening. The logic finally adds up. Sifting through the B.S. of gardening has been quite a challenge.
    Thanks again!

    • Thank you very much. It has only taken me 30+ mumble, mumble years to start figuring some of this stuff out. Add it is still a learning process.

  8. Robert Pavlis says:

    Agreed, microbes are important. Feeding them is also important. The problem with the whole idea of feeding microbes is that people think they need special food. See my latest post on Molasses Magic All they really need is some organic material like compost, manure or wood chips.