Bokashi Compost for Cut Flower Health
We farm on a hilltop in a former hayfield. It’s a beautiful setting but the light loess and heavy clay soil was depleted well before we started growing four years ago, so we have, from the beginning, focused on improving the health of our soil within a no-till system. For the past two summers we have integrated bokashi compost into new beds and jumped at the chance to evaluate the impact of this source of organic material on both soil and plant health.
After seeing the transformation of our heavy clay soil over just a single summer we are beyond enthusiastic about bokashi and hope our experience will benefit other farmers. In this article we will present bokashi, explain how it’s produced and used on our farm, and discuss what soil tests and our observations tell us about the changes to our farm’s soil.
Nearly all compost produced in the United States is aerobic and produced by combining a nitrogen source such as food waste with a carbon source such as chipped wood. Microbes feeding on this moistened mix cycle carbon, and in the process bring the working compost pile up to temperatures of 140 to 160F. In the course of successive cycles of watering and turning to evenly mix this pile and incorporate oxygen for the microbes, the raw input materials are gradually transformed into the undifferentiated, crumbly, dark brown or black material we all know and love.
Although aerobic compost is top-dressed as a soil amendment on nearly every small farm in the country, anaerobic compost—bokashi—has received much less attention from farmers. Rooted in Asian farming practices, the term “bokashi” refers to the process of fermenting organic matter in airtight containers by adding helper microbes. At our farm we use EM1 as a source for these microbes, a collection of acid-producing lactobacillus, yeast, and other species developed by Dr. Terua Higa.
The microbes need a carrier to be most effective, and as Midwesterners we are fortunate to have ample affordable organic wheat bran available from a local mill. We expand the EM1 with water and molasses, and mix with the bran in food-grade buckets which are sealed to allow the EM1 to colonize the bran without competition from airborne microbes. With its small particle size, this bran provides excellent jumping-off points for the EM1 into our bokashi substrate.
The process of making and using bokashi is simple: fill the vessel with material while sprinkling bokashi bran between layers, then close the container tightly, and drain the liquid that’s produced every few days. Once thoroughly pickled and no longer producing leachate, it gives off the strong, pleasantly yeasty scent of spent brewing grains, and is often covered with a thin layer of fuzzy white fungal mycelium. The entire process should take between 2 and 3 weeks. Note that the material will be fairly acidic due to the lactobacillus bacteria driving the initial stages of the process, as will the leachate.
While aerobic compost piles need to be a minimum of one cubic yard to have a chance of heating up quickly and evenly, bokashi can be produced in any container. For our household scraps we use a 4-gallon tub, but for the farm we used a number of 96-gallon roller trash bins with spigots fitted into the base, which we can move around easily. Each roller can hold around 350 pounds of material, so every six is a ton of inputs.
Uses for Bokashi
At first, how to use bokashi is a puzzle. Unlike aerobic compost, bokashi still very much resembles its constituent components—fermented carrots are still obviously carrots—and an approach other than spreading along the beds with buckets is in order. We do the following:
● Bury in the root zone 6-10” below soil surface.
● Feed to composting worms for castings.
● Aerobically compost.
● Dilute leachate and use as soil drench.
Burying: The first, burying, is the focus of this article. The fermented material is pre-digested when buried, making it a very accessible food for indigenous soil microbes which then, along with the microbes originating with the EM1, finish the job in the soil. Trenching and placing 6” or so beneath the soil surface means that once digested, the organic material forms a band in the root zone.
Vermicompost: Although we were uncertain whether composting worms—red wigglers at our farm—would eat acidic bokashi, we found that after a short period of adjustment they were happy with this food.
Compost: We find that bokashi used as an input for aerobic compost results in piles that heat up more quickly and break down more evenly, so we also use the bokashi process as a precursor to our aerobic composting.
Leachate: The by-product of fermentation is also a valuable input that we dilute (for acidity) and use a root drench. We typically discard the first 3-4 days’ liquid as we think it’s immature.
Bokashi in a No-Till Setting
Our goal is to avoid disturbing the soil once our permanent beds are formed, which effectively rules out the subsurface burial for established sections of the field. Luckily, as a new and growing farm we have been adding new blocks each year and thought it would be a great time to add a significant amount of bokashi and evaluate the impact.
Early this spring (2021) we made somewhere between 12 and 14 thousand pounds of bokashi from nitrogen-rich food waste which required around 500 pounds of bokashi bran, about two food-grade 50-gallon barrels, and half a sling bag of bran. While we chose to use post-consumer food scraps due to their high levels of nitrogen and (hopefully) trace elements, it’s also possible to bokashi surplus agricultural materials such as soybean or rice hulls. Typically this is done by tightly covering the material with a non-breathable tarp to allow bokashi at scale.
When this was nearly done fermenting, we dug a series of 3-4’ wide trenches roughly two feet deep using a rented excavator, into which we placed a layer of bokashi 12” thick, topping with cardboard as a carbon boost. Upon covering with the removed soil we sowed a cover crop of cereal rye to suppress weeds and keep the soil from eroding.
After 8 weeks to allow the bokashi to fully break down and before the rye set seed, we mowed it and used our BCS rotary plow to form beds. These are the beds we are comparing to adjacent areas that did not receive bokashi.
Qualitatively, the change in the soil texture was the most striking outcome of this trial. While we have been successfully improving the soil in other non-bokashi beds by rotating in fast-growing cover crops and managing fertility, the first season (or two) in new beds is usually a struggle—the heavy clay beneath a thin layer of loess is a challenge for tender transplants and makes a slippery sticky mess when it rains.
Once we had the beds full of growing plants and the soil had settled back from the plowing, we tracked the texture of the soil over the season. The difference between bokashi beds and “normal” beds appeared midsummer, and by August it was incredibly vast. The beds with bokashi incorporated had a superb tilth and crumbled easily while retaining its structure. Nonetheless, all our soil is on the clay to clay-loam line, so the changes weren’t the result of simply incorporating more silt or sand.
Soil tests provided quantitative data. Soil samples were pulled for 0-6” deep and testing was done by Midwest Labs using the Soil Health Complete test suite, which we like because it includes measuring the amount of CO2 released from the soil sample in a 24-hour period, a good proxy for biological activity in the soil. The lab results also include a “Soil Health Calculation” based on this CO2 burst, together with organic carbon, organic nitrogen, and the C/N ratio which represents the overall health of the system on a 0-25 scale.
For this article we are comparing two sets of beds, and a table of the soil test values follows:
The first difference is in organic matter. Unsurprisingly, there is more organic matter in the bokashi beds, and a correspondingly higher cation-exchange capacity (CEC). Also unsurprisingly, the nitrogen parts per million and phosphorus were also higher as we added nitrogen-rich food waste.
No clear impact on either pH or percent base saturation of potassium was seen, and surprisingly the trace elements we sampled were just as low in the bokashi beds as the control set. This was a real disappointment because we have been struggling to increase levels of these essential micronutrients and had hoped bokashi would help.
Turning to the two test values that speak directly to soil health—the one-day CO2 burst, and the soil health calculation—we see clearly the impact as both values are significantly higher in the test beds.
As a flexible, carbon-free method of preparing organic waste materials for incorporation into farm soil we think bokashi should be a part of many more farm systems.