Bokashi vs Composting

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Robert Pavlis

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.

Compost Science for Gardeners by Robert Pavlis

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.

Building Natural Ponds book, by Robert Pavlis

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.

This description for CO2 is common but may not be right. A student study found that the CO2 produced by both composting and bokashi were similar. It is also known that the first stages of fermentation produce CO2.

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 weighs 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
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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!

56 thoughts on “Bokashi vs Composting”

  1. Thank you for the education. I do appreciate the awareness as I am new to gardening. I have a compost pail that developed the white fungi. I ended up adding biochar to absorb the nutrients and smell. This was around February 2021. I just built some raised garden beds (August 2021) and mixed in with worm casting, fish fertilizer, other organic fertilizers, soil amendment, and compost. While the smell was awful, I’m looking at what the future may bring.

    PS the reason for the wait is that we were living in an apartment. Now that we have a house, we are free to grow in our backyard

    Reply
  2. I believe it was mentioned on this thread that worms don’t like the fermented waste. I have been using the Bokashi method for a year where I leave the bins to create white mould and then I add it to my outside composting bins with a layer of waste and a layer of dampened waste tissues from the bathroom and kitchen bins. I have never had so many composting worms in my bins in the 50 years I have been gardening. My composting are literally heaving with them and they can be scooped out by the handfuls!

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      • The Swedish experiment compares CO2 emissions from soil treated with 3 “bokashi” varieties – with live inoculum, inoculum killed by heating and without inoculum – and a fourth treatment with compost. The student seems to be equating bokashi with the wheat bran substrate, whereas to me bokashi is all about live bacteria. Maybe I missed something in translation.

        This produces a bizarre result: all 3 bokashi varieties produce a very similar response (see figure 3 on page 28), and two of them – live inoculum and without inoculum – are virtually identical. Eh? If the experiment and its measurements were done correctly this means that the response is due to an independent variable not identified by the report.

        I hazard a guess that this variable is the act of mixing treatment with soil at the start of the experiment, very thoroughly. Basically it is like the response to tillage. This is supported by a rise in emissions from the compost treatment. This rise is smaller, which will make compost fans happy until they discover that the compost had already emitted much CO2 prior to the experiment. Overall, emissions from compost were 4% higher.

        4% disappoints me, but who knows what a professional experimenter would find. On reflection it shouldn’t surprise, because when a soil ecosystem gets hold of bokashi ferment it must respire frenetically to achieve the observed speed of absorption.

        The experiment doesn’t take into account the numerous advantages of bokashi over composting, some of them being what the (now larger) ecosystem does: e.g. transport of remaining carbon to greater depths; protection of carbon within soil aggregates enclosing excreta and dead biomass. These increase the residence time of carbon in soil and thus keep more out of the air.

        Reply
        • Seems like a very interesting study! Considering the emissions impact of various composting techniques is really interesting to me and I wish the study were available in English so I could make heads or tails of it. Question here: is most of the C loss in bokashi taking place during the fermentation stage, or after it’s added to the soil? Presumably the latter, because bokashi product is not particularly degraded when it is “finished” after its two week fermentation period. The typical recommendation with Bokashi is either to add it to your compost pile, or dig a hole and bury it with at least 6 inches of soil over top. Since burying woody, carbonaceous material is a serious policy recommendation in our crazy sequestration quest, presumably putting stuff in a hole is actually a pretty good sequestration technique, but from the articles I’ve read, they’re also recommending burial take place at places like retired surface mining sites, where the “holes” are relatively deep. But perhaps this is really a recommendation due to scale? I guess what I’m getting at is, at a depth of, say, a foot underground, how much C escapes? To what extent would the C savings be mitigated by C loss due to the disturbance of digging? Presumably it’d still be a net C storage.

          Since C loss in traditional composting takes course over the whole degradation process, and because it’s generally going to take place at a slow rate (most households just don’t produce that much food and yard waste to have a really active pile year round), wouldn’t the immediate burial of the relatively non-degraded bokashi be better for subsoil sequestration? On this point, in the original article Robert says his composting process is just as easy as throwing things in a bucket, because he puts everything in a big passively-composted pile. From a labor standpoint, I’m all about this and do much the same thing, but passive composting like this tends to be less aerobic, and the methane release that result can really be a net negative in terms of emissions. From my understanding even though Bokashi is anaerobic, the low pH suppresses methanogenic organisms.

          What an awesomely complex topic!

          Reply
  3. For me, the main point is to rid myself of kitchen scraps that become a foul, malodorous, seething bag of maggots in a sub-tropical bin. Bokashi gets rid of all of that and then I drop it in the composter-tumbler for about a fortnight with some brown leaves before the worms get it. It turns from mush to slush in days – Wins all round. Bones are fine, too if you don’t mind them in your garden soil for a while.

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    • I did a small Bokashi bucket in the early winter. Especially being in a northern region (Ontario Canada), composting is not possible in the winter. I do have a worm bin but had to go with a small foot print as my house is quite small and I do not have a garage. The worm bin cannot handle all of my veggie scraps and no dairy, etc. I did a test of the Bokashi method and included some veggie scraps so as not to overwhelm it with fats, dairy and meats. So, my reasons for the benefits of Bokashi is that it can be done inside in winter and it can handle some things that do not do as well in a worm bin or will attract nastiness (smell and vermin) in an outside compost bay and tumbler. We have some currently unused parts in our raised beds so I will bury it and try planting in about a week. We will see. Should it not work out so well, we do have the option of sending our organics off to the city for composting but I’d like to keep it in-house as much as possible. And, the city doesn’t do a very good job of making it accessible when it is ready for the garden. But, if I had the option of composting year round, my reliance on Bokashi would likely be very limited.

      Thank you for examining this topic. I have learned lots, as always.

      Reply
  4. At the very beginning you start with Bokashi bran….no definition…then further I see it is from Japan and EM addition is mentioned…ok I am living in Japan and see the EM…on products. Eggs, produce…and looked it up it is Effective Microorganism …but what is really in it..? That is what I would like to know.

    Reply

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