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?

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.

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