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Bokashi vs Composting

Bokashi is a fermentation method used for processing kitchen scraps in the home with very little mess and no foul odors. Some claim that bokashi is a method of composting, but that is incorrect, as discussed in Bokashi Composting Myths.

What exactly is bokashi? We call it a fermentation process but what does that mean? How does fermentation compare to composting on a molecular level? Does one method have advantages over the other?

bokashi vs composting, by GardenMyths.com

What is Bokashi?

In a future post I will review this in more detail and show that there are many different ways to do bokashi, but for this discussion I’ll consider it to be the process that is most common in North America and Europe, for home owners.

Food scraps are added to a special bucket and then covered with some bokashi bran. As more food scraps become available they are also added, along with more bran. Once the bucket is full, it is closed and allowed to ferment for 2-3 weeks. During the process liquid is produced which escapes to the bottom of the pail and needs to be removed through the spigot. This “bokashi tea” can be used to fertilize your house plants or the garden.

At the end of the process you need to do something with the fermented material (the ferment). Some people add it to potting soil and use it for potted plants. Others spread it in the garden, add it to the compost pile or even use it to feed worm bins.

Bokashi vs Composting

I’d like to compare the bokashi fermentation process to composting because there are some very interesting things we can learn about both processes.

Decomposition vs Preservation

Composting is a process where you encourage microbes to break up organic matter into small molecules including nutrients. This is a very aerobic process.

Fermentation is a preservation process. Think pickled food. During fermentation, oxygen is eliminated so that the right microbes can produce organic acids, which will lower the pH and preserve the food. In the case of sauerkraut the organic acids are made by microbes, and in the case of some pickles, vinegar is added to speed up the process.

Because the environment in which bokashi and composting take place is very different, they each use different organisms.

The microbes used in fermentation consume very small amounts of organic matter to produce a range of organic acids, including lactic acid, butyric acid and acetic acid (vinegar). They also produce antibiotics that help preserve the organic matter and kill pathogens. Under these conditions, normal decomposition does not take place and eventually the environment also kills the microbes doing the fermenting. The resulting material is quite stable as long as oxygen is kept out.

Production of CO2

The microbes in compost use the energy in the organic matter and through a process of respiration, convert complex organic molecules into CO2 and water (H2O), both of which are lost to the air as gases. The amount of carbon (C), Oxygen (O), and Hydrogen (H) lost depends on many things including the C:N ratio of the starting material, temperature and duration (ref 1). After several months cold compost is hardly changed, but hot compost can lose half of it’s carbon. The released CO2 is not good for the environment.

During fermentation, very little of the organic matter is decomposed producing very little CO2 and water. The liquid that drains to the bottom of the pail is mostly water from the food scraps.

Loss of Nitrogen

In composting, the activity of microbes is significant, and requires nitrogen. As they use nitrogen and convert it from one form to another, some is lost to the air as ammonia or N2 gas. The amount lost is usually in the range of 25% to 75%, but it can be as high as 90% (ref 1).

The limited microbe activity in bokashi means that very little nitrogen is used and none is lost to the air. In fact most of it remains tied up in large molecules.

Loss of Energy

All of this activity in compost produces a lot of heat – that is why compost piles get warm. This heat energy has to come from somewhere and it comes from the stored up energy in the organic material added to the compost pile. This may not sound like a big deal, but this energy is valuable for plant growth in the garden. If it is lost during composting, the resulting compost is of less value to plants.

At this point you might be thinking to yourself that cold composting is better. It loses less nitrogen, carbon and energy, but cold composing does not kill pathogens.

Fermentation does not use up the energy trapped in large molecules and is therefore a cold process. It does kill pathogens by creating a very acidic, anaerobic environment.

Ideal C:N Ratios

Composting is most efficient when the C:N ratio is around 30:1 which is the right mix of carbon to nitrogen for the microbes. Water is created during the process and therefore the water content of the starting material can be lower.

Fermenting uses different microbes that work best with a lower C:N ratio of 10:1 and a higher water content. Kitchen scraps are perfect for this.

The traditional material for composting has too much carbon and not enough water for bokashi.

Available Microbes

The microbes that carry out composting are found everywhere and they do not need to be added to start the process. Those that carry out fermentation are much loss common since they prefer anaerobic environments and therefore they usually need to be added to get the process started.

The End Result

The material left after composting consists mostly of large stable organic molecules that are not easily decomposed. This has traditionally been called humus, but we now know humus does not exist. In a home compost pile a lot of the nutrients are washed away by rain, but if composting is done in a closed system where the leachate is collected, all of the mineral nutrients will still be there. The only things lost are C, O, H, and N.

The so-called “finished compost” is far from finished. It will continue to decompose for another 5 years.

The result of fermentation, the “ferment”, is chemically unchanged from the starting material. A few organic acids have been created from sugars, but the quantity of these are minor compared the original organic matter. None of nutrients have been lost, and most of the C, O, H and N are still present in their original molecular forms.

Bokashi vs Composting

The two processes are clearly very different, but what does this mean to the gardener?

Nutrient Value

Nitrogen is one of the most important nutrients for plants since it is the most likely one to be deficient in soil. From a nutrient point of view, ferment is better than compost since it contains more of the original nitrogen.

However, it is important to understand that the concentration of nutrients is greater in compost. This might seem like a contradiction to the above statement, but it’s not. Ferment weights the same before and after the process. Compost on the other hand is much reduced in weight mostly due to a loss of carbon and oxygen. At the same time the nutrients, except for nitrogen, have remained the same.

What this means is that you need a larger amount of ferment, to provide the same nutrients as compost.

Since compost has already undergone significant decomposition, more nutrients are immediately available. Ferment has not yet started the decomposition process and most of its nutrients are still tied up in large molecules.

pH Value

The pH of compost is around 7 or slightly above and ferment is around 4. This acidic nature of ferment will not likely affect soil pH unless it is sandy soil or very large amounts are added. The low pH is largely do to simple organic acids which are readily consumed by microbes, besides soil has a great buffering capacity.

Unwanted Chemicals and Pathogens

It is assumed that the pH conditions in ferment kill off most pathogens, but there has been very little science to investigate this. Until the research work is done, it can not be assumed that ferment is pathogen free. It is however known that hot composting kills pathogens. The problem is that most home composting does not get hot enough to kill them. From a home owners perspective both systems could be transferring pathogens to the garden.

Since fermentation preserves instead of decomposing, it is much more likely to contain pesticides that were present on the input material. Provided that the input material is mostly kitchen scraps, this should not be a worry for home gardeners.

Effect On Soil Life

Soil life requires organic matter as a source of energy and nutrient. Since ferment contains higher levels of energy and nitrogen it should be better for soil life. This extra benefit is short lived and since some people are convinced ferment decomposes quickly in soil, the benefit might be of limited value.

The bottom line is that this has not been studied and this is just a guess on my part.

CO2 Pollution

At first glance it seems as if composting contributes significantly to the CO2 levels in air, while fermenting does not. But what happens once the ferment hits the soil? It is possible that it slowly decomposes and that much of the carbon is absorbed by microbes during the process. This would be akin to cold composting which releases very little CO2 to the air. But if the ferment decomposes quickly, it may release significant amounts of CO2 in the process. Studies don’t exist to point in one direction or the other.

Beneficial Microbes

Whenever microbes are added, people claim that they have special powers. They improve soils, make plants grow bigger and reduce diseases. Similar claims are made for compost tea and bokashi and there are studies that show such benefits. But there are also studies that show either no benefits or actual harm from the microbes. On the whole, there is poor scientific evidence of any real benefits except maybe in some very special situations.

A number of studies have compared bokashi made with EM mixtures to bokashi made from non-EM mixtures and in some cases the EM microbes produced better plant growth. I have not seen a good summary of this work.

From a home gardener perspective they should not expect to see any magical benefits from microbes in the garden, but using EM microbes to control the fermenting process may be beneficial.

Bokashi Is Fast

A common claim is that the ferment from bokashi decomposes quickly once it is added to soil. This is based on the fact that it “disappears” quickly. Decomposition is a chemical process whereby large molecules are converted into small molecules. There is no way to verify this process by “looking at it”. Fermentation produces mushy material that may simply fall apart when mixed with soil, to a point where we no longer see bits and pieces. That does not mean the material is decomposed.

I have yet to find a study that looks at what happens to ferment in soil.

The fermenting process is quick – takes 2-3 weeks. There is no evidence that decomposition is also quick.

What Does It All Mean?

Provided bokashi ferment decomposes slowly in soil, it would be more environmentally sound than composting because it retains nitrogen and does not produce as much CO2. But we don’t know this to be true.

Bokashi works well for kitchen scraps, but it does not work as well for the high carbon material traditionally found in fall gardens. Composting is better at getting rid of the organic material generated by most gardens. Has anyone tried bokashi on fall leaves?

Traditional bokashi seems like a good option for managing and recycling kitchen waste, but does not seem like a good option for garden waste.

References:

  1. Treating Food Preparation ‘Waste’ by Bokashi Fermentation vs. Composting For Crop Land Application; https://www.bhu.org.nz/future-farming-centre/ffc/information/soil-management/treating-food-preparation-waste-by-bokashi-fermentation-vs-composting-for-crop-land-application-a-feasibility-and-scoping-review-2012-ffc-merfield.pdf
Robert Pavlis
Editor of GardenMyths.com
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.

18 Responses to 'Bokashi vs Composting'

  1. John Colwill says:

    Another interesting read, thank you Robert. My perspective on the issue is a little different but I should declare it is from the other side of the world in Western Australia.
    I have long had an issue with making compost. Even though the process is well understood, there still seems to be an air of mystique about it that allows non scientific gardeners to give the process and its end product magical powers. This also feeds the demand for a massive range of (sometimes really weird) books on the subject. It has got to the stage where you are considered a failed gardener if you can’t or don’t make compost!
    Stand back a moment though and look at the process. It involves gathering material from all around the garden and taking it to a central point where it may need further chopping up before it goes into a (often very expensive) compost bin. My estimate is that 90% or more of this compost is not made properly, as you say it does not get hot enough. Then there is the problem of what to do with the compost. In our hot dry climate spreading it on the soil is wasteful and increases moisture loss*. Digging it in is fine but not easy in an established garden.
    The alternative is simple. Spread the raw material out as a mulch which will protect the soil, help with moisture conservation and allow nearly all the nutrients to be utilised. To get the message across I refer to mulching as horizontal composting. Less work, greater returns and a lot cheaper.
    * See ‘Mulch ado about something’ at plantsman.com.au

    • I agree 100%. I have over an acre (1/2 hectare) of actual flower beds. I produce half a bin of material each year for composting. Almost everything stays right in the garden, in what I call the cut and drop method. The only thing I remove is really thick woody grasses that I can’t be bothered to cut into small pieces. In spring any perennial laying on the ground stays there, soon to be hidden by new growth.

      Here is a hot link to your article Mulch ado about Something.

  2. Lee Reich says:

    Excellent article, with thre caveats.

    Pathogens can be killed in cold composting as well as in hot composting. It just takes more time. Basically, some combination of time and temperature kills pathogens; colder temperature, more time. (See “The Humanure Book” for more details on this.)

    As far as CO2 release, pretty much any organic material decomposes to release its carbon as CO2. Bokashi and cold composting might do it more slowly, but they still do it, eventually.

    And finally, as far as N loss from a compost pile: That can be minimized by adding sufficient carbon material (such as straw or wood chips), and keeping a pile covered. I grow vegetables very intensively, with succession cropping, close planting, interplanting, and season extension, and the only nitrogen the N-hungry vegetable plants get is from the 1″ of compost that I spread on top of the planting beds once a year.

    • Good to hear from you Lee. I got a copy of “The Humanure Book” – it is quite good. According to the data in the book, most pathogens are dead in 6 months, even in cold soil. Some worms may persist for several years.

      It is true all organic material will eventually be converted to CO2. But if the material is used as a food source the smaller organic molecules can be used to build larger ones, preventing the immediate production of CO2. By building up the organic level and number of living organisms in soil over time you extend the time before CO2 is produced. It would be interesting to see real data on these processes.

  3. Hi Robert, another thought provoking blog. If Bokashi fermentation is put onto the compost heap or onto top soil to decompose, I cannot see the advantage. All we are doing is adding an unnecessary process. Why not just compost straight away? You say that the fermentation still has all the nutrients locked up in it – well yes because it has not even started to decompose. The only way that crops will be able to access the nutrients is by the mineralisation of organic matter through decomposition. I really cannot see what advantage Bokashi fermentation brings to the process of mineralisation. Whether the organic matter has gone through the process of Bokashi or just composting, when it decomposes the same elements will be lost or retained. You seem to end up at the same place whatever. Unless I have missed something whereby when there is fermentation somehow nutrients are retained then there is no advantage.

    • Your summary is right on the mark. However, if bokashi results in a product that decomposes without releasing CO2 and nitrogen, then it would have a valid advantage.

      The reality is that for most home composting, much of the nitrogen is lost either to the air or to the soil under the pile. Even other nutrients are lost because the compost piles are not closed system. Bokashi would keep more of it in the soil around plants.

      At this point the only real advantage we can be sure of is one of convenience. Some people have good reasons why they can’t compost.

      • Brian Tremback says:

        Decomposition implies the breakdown of organic matter, which is largely cellulose, a carbohydrate. If cellulose didn’t decompose, organic matter would build up like it does in a peat bog. Because carbohydrate is used as an energy source by the bacteria and fungi that are feeding on it, CO2 is produced. Less CO2 is produced in oxygen-starved bokashi, and consequently less decomposition is occurring. But the point in making bokashi is to ultimately add it to soil where a low-temperature “composting” will finally happen. That little CO2 is released in the bokashi process itself isn’t particularly relevant because it’s only a temporary situation.

        I think Tony’s question about the utility of bokashi is a valid concern. Why go to the trouble if the end result – incorporating kitchen scraps into garden soil – is the same? The only advantage I would see is if the bokashi process makes the kitchen scraps unpalatable to pests like rats.

        Regarding nitrogen, proper C:N ratio will reduce production and loss of ammonia in an aerobic compost bin. And leaching of nitrates can be avoided by controlling exposure to rain.

        It seems that to productively use bokashi means digging it into the soil. Which means having an area of temporarily unproductive soil. The other alternative doesn’t appeal to me: sloppy, kitchen scrap kimchi as mulch. The compost bin lets me confine the unappealing stages of decomposition to a limited area and doesn’t require land as a burial ground for bokashi.

        • You are correct in your description in the first paragraph. I agree – there may not be any benefit to Bokashi, if composting is an option. Not everyone has a yard, and not everyone can compost.

          If the decomposition of ferment in the garden is a slower process, there are benefits in carbon and nitrogen preservation as I suggested in the article. These are not insignificant benefits – but we don’t know if decomposition is slow.

          Even with proper C:N control – and who actually has that, there is some nitrogen loss.

          I agree, I don’t see the advantage of bokashi – yet. In my next article on the subject I will present a way of adding it to soil quickly which might make it more appealing to you.

  4. Nadav says:

    Interesting post as allways. I didn’t think CO2 is such an issue concerning composting. In Israel, it is by far concidered better to compost every kitchen waste instead of landfield it because of methane gas during anaerobic process.

    • Agreed. There are much bigger CO2 problems than compost, but most people are not aware that this is going on. One of the most “natural” and “organic” things a gardener can do is actually adding to the CO2 problem – in at least a small way.

      • Ray Roberts says:

        I don’t see where CO2 produced by decay is a problem at all since it’s a critical part of the carbon cycle. Plant’s need the CO2 they take in through their leaves. The CO2 levels are much higher closer to the soil where CO2 is produced from the aerobic respiration of soil life, which gives plants a more ample supply they wouldn’t have higher in the atmosphere.

  5. Andy says:

    Surely when the ferment is put onto into soil it is exposed to oxygen and the microbes die because their habitat has been destroyed and then soil microbes start decomposing the matter as normal?

    • I am not sure if they die, but some certainly would. For example EM contains yeast – how sensitive is that to air?

      Soil microbes would certainly take over and start the decomposition process. Is this process “as normal”? I can’t find anyone who has looked at the process.

      • Ray Roberts says:

        Yeast are single celled members of the fungus kingdom, and like many bacteria species, they can be facultative anaerobes which means they can change their metabolism from aerobic to anaerobic or back again to adapt to environmental conditions. That can be a very useful survival tactic in the microbial world. EM1 cultures contain Saccharomyces cerevisiae, a common yeast species that humans have be using for centuries for making wine, brewing and baking

  6. John says:

    Thanks for another great article. Given the available information, I especially appreciate you highlighting what is known AND what is unknown. Good clarity!

    Given then what is known and unknown about the impact/ benefit of Bokashi when it is added to a garden, what are your thoughts, from a backyard garden perspective, of Bokashi vs simply burying small amounts of kitchen scraps. Is there any potential “value add” from the Bokashi process in this perspective?

    • I don’t think so. Small amounts of kitchen scraps will decompose with a cold process. Bokashi may speed things up – but is that a good thing? When we add organic matter to the garden, one of the big benefits is that it is a slow feed over many years. It is hard to see why a faster process adds value. if you need a fast feed, just add some fertilizer.

      Personally, I don’t compost or bokashi – I just drop everything on the soil. All of my garden waste is cut and dropped in place. My kitchen scraps are usually dropped behind a bush or tall perennials. I see no point in digging it in.

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