Organic Fertilizer – What Is Its Real Value?

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Robert Pavlis

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.

Compost Science for Gardeners by Robert Pavlis

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

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!

34 thoughts on “Organic Fertilizer – What Is Its Real Value?”

  1. Hi Robert,
    In regard to the soil, how harmful are antibiotics that are fed to animals?Are they residual in manure? Will they have an affect on the biome?
    Thank you in advance for your answer.

    • Antibiotics as a chemical are of no real concern. The problem with them is that bacteria can mutate to become immune to them. Then we have the potential of bacterial infection we can’t treat.

  2. Wow, scientific method in gardening. You demonstrate advance in knowledge is as much about losing bad ideas as gaining correct ones. This is not always appreciated as history of science does not get due attention.

    I just wonder, sometimes things may be more complex than a simple study might suggest. e.g. multiple variables, and some traditions may have value that is not easily supported.

    I need a motivation to continue composting as for me this involves several cubic metres a year and a lot of manual labour…

  3. Dear Sir,

    In your blog, you have clearly explained, that, as long as the concentration of the mineral fertiliser is adequate, it provides the same basic substances to the roots of the plant as the organic ones do thanks to soil microbes.
    The thing that I cannot fully understand is that every spring, there is a talk in the media about green leafy plants as spinach or lettuce that due to mineral fertilisation contain hazardous levels of nitrates. I am curious to read your opinion on that.

    with kind regards,

    • High nitrates in vegetables can be the result of over fertilizing crops. This happens with both synthetic and organic fertilizers. Admittedly, it is easier to over fertilizer with synthetic because a given amount of nitrogen weighs less and it is released quicker, but the problem is not due to the fact that the nitrogen is from a synthetic source.

  4. Hi! Robert,
    How come I didn’t see mention of Worm Castings. This I believe is the ultimate fertilizer for any garden. From the novice to Professional. Won’t burn plants, Natural insecticide, and a Great Starter medium.

  5. The fish Emulsion, molasses, teas have organic compounds in them. Things like gibberellic acids which are plant, bacteria, fungi hormones. Look up plant hormones and see what their role is for bacteria and fungi.
    There are many microbial produced volatile organic compounds which help balance a microbiomone.

    • Yes plants contain gibberellic acid. But what happens to it when it is added to soil? Does it get used by microbes before it reaches the roots?

      If it does get to the roots, can plant roots absorb it? They don’t absorb most chemicals found inside the plant.

      If they do absorb it, how much do they need to absorb before there is any difference in plant growth?

      Now relate that back to how much you have to dump on the soil to have an effect on the plant.

      Until you can answer all these questions you don’t know if gibberellic acids in organic fertilizer has any effect.

  6. Hi, I am an urban backyard composter. The main reason I compost is to keep organic materials out of the landfill. I have started composting dog and cat poop as well in my insulated tumbler. This compromise means a sterile end product as well as a risk of pathogens remaining. But isn’t composting animal waste better than throwing it in the landfill?


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