Despite its name, Bokashi composting is a fermentation process for handling kitchen scraps right in the home. It is fairly easy to do and produces few odors. The majority of the material ends up as fermented kitchen scraps and some is drained out the bottom as a brown liquid, commonly called Bokashi tea or Bokashi leachate. Advocates of this process claim that the Bokashi tea is a good fertilizer, full of all the nutrients your plants need.
In my previous discussion about Bokashi, I speculated that the tea contained few nutrients. At the time I found no chemical analysis of Bokashi tea, not even from proponents of the process. One of my readers has now found such a study and I will have a close look at the data in this post.
What is Bokashi Composting?
The process is called Bokashi composting, but in reality it is not a composting process. As discussed in my previous post, this is a fermentation process. It pickles the food scraps and the end product is called Bokashi ferment which is then added to a compost pile, or incorporated into soil to start the real composting process.
The excess water and juice from the food scraps migrates to the bottom of the Bokashi pail, and are drained away to prevent the fermenting material from getting too wet. This liquid is called Bokashi tea, or Bokashi leachate.
Chemical Analysis of Bokashi Tea
In this study, kitchen scraps were obtained from several places including a home, and I am going to use that data since it is more representative of the material a gardener would process. The collected material was then fermented in a lab location so they could control the environment. The table above shows nutrient levels in pure Bokashi tea (concentrate) and the diluted form (middle column) which is 2% of the concentrate. The 2% value is considered high by the researchers, whereas 1% is considered low.
To compare this to normal fertilizer values, I have added the values for the MSU fertilizer which is used for growing a wide range of plants including orchids.
Note that the above table shows the macronutrients in mg/l and the micronutrients in ug/l. The values for the MSU fertilizer have been converted to element mass numbers to match the ones reported in the research (ie P values instead of P2O5).
Nitrogen levels in Bokashi tea are very low. Phosphorus and potassium are on the high side.
The level of several micronutrients are also very low.
Levels of Sodium and Chloride
In addition to the plant nutrients reported above, the Bokashi tea also had high levels of sodium (1200 mg/l) and chloride (4300 mg/l). Once diluted to the 2% level used in the above table, the leachate will have a sodium value of 24, with 50 being toxic to plants. The chloride is at 85 mg/l and a value of 70 is considered toxic.
Much of the sodium chloride would be from salt added during cooking, or salt added as a preservative in commercial food. People using Bokashi leachate on plants should be careful which food they process.
Growing Plants With Bokashi Tea
The study also tried growing pak choi in a number of different medias using both Bokashi tea and standard fertilizer.
Plant growth using Bokashi tea was much slower than control plants, in both sand and peat substrates. It is assumed that this is largely due to the very low nitrogen levels.
“The low concentration of inorganic nitrogen in the leachates can be assumed to be due to denitrification processes taking place during the fermentation, as the food waste was kept anaerobically in air-tight containers, thereby promoting denitrifying bacteria. ”
Should You Use Bokashi Tea as a Fertilizer?
Bokashi tea could be used as a fertilizer if additional nitrogen is added. Without the added nitrogen, it is a poor fertilizer at best and at worst, the high sodium chloride could be toxic to plants.
I would not recommend it for use on potted plants. Add it to the garden where additional nitrogen might be available, and where excess sodium is easily washed away.
It should be diluted to at least 1% to ensure the sodium chloride levels are not toxic.