1. Steaming Leaf Piles in Winter: The 360-Year-Old Musashino Leaf Mulch Farming Method
In the Musashino region of Saitama Prefecture (Kawagoe, Tokorozawa, Fujimino, and Miyoshi), the "Musashino Leaf Mulch Farming Method" has continued for over 360 years since the Edo period. In July 2023, it was designated as a Globally Important Agricultural Heritage System by the United Nations Food and Agriculture Organization (FAO). This method involves sweeping fallen leaves from flat forests (called "yama") to make compost, which is then added to fields to improve the soil [2].
Explanations of the sweet potato production area in Kawagoe note that leaf compost generates heat. They also mention using the heat from fermented leaf piles, where leaves are trampled down, as seedbeds [1]. The steam seen on cold mornings is not confirmed in this specific source but is described as a general observation. Regarding the size of the piles, they are said to be piled higher than a person’s height. In the past, it was said that one "tan" of woodland was needed for each "tan" of field (a "tan" is a unit of area, about 1,000 square meters or 0.25 acres), which shows how many leaves were used [3].
Earlier notes on leaves explained that piles become slightly warm because of insulation and heat from fermentation. This article follows up on that. What needs to be present for a pile to become hot enough to produce steam?
2. How Microbes Decomposing Leaves Produce Heat
The source of the heat is microbes that decompose the leaves. Bacteria handle most of the decomposition and heat generation. The heat is a byproduct released during decomposition [5]. In leaf compost, aerobic microbes such as actinomycetes and filamentous fungi decompose the leaves and multiply, releasing heat. This heat maintains the temperature, making fermentation more active and speeding up the compost-making process [6].
According to the National Institute for Environmental Studies, the reaction heat when easily decomposable organic matter breaks down raises the internal temperature of the compost pile, sometimes reaching 50 to 80°C (122 to 176°F). During this time, the number of bacteria increases rapidly from about 10,000 to 100,000 per gram to about 1 billion to 10 billion per gram. This is an increase of 10,000 to 1,000,000 times [4].
This reaction proceeds slowly near room temperature, so it is sometimes called "slow combustion" [4]. However, it is not a fire burning. It is a reaction where microbes use enzymes to decompose organic matter. Note that 50 to 80°C is a value that can be reached in managed composting, not the temperature of a leaf pile in a park.
We now know heat is produced. However, the way the pile warms up seems to change depending on its size.
3. How Pile Size and Air Gaps Keep the Heat Inside
The size of the pile matters for storing heat. The smaller the pile, the larger the ratio of surface area to volume, so more heat is lost through conduction and radiation [7]. This is the same idea as why a large bathtub cools slowly while water in a small cup cools quickly. However, unlike water, leaf piles continue to generate heat internally.
Another factor is the air in the gaps between the leaves. Still air does not conduct heat well. This is why sweaters are warm. Air is trapped between thin threads, making it difficult for heat to escape. Glass wool, used as insulation in house walls, works on the same principle: it traps air between thin fibers to prevent air movement and insulate [8]. It is thought that the same logic applies to the gaps in leaf piles. This is an estimate based on building material principles.
If you make the pile larger, heat accumulates more easily. According to the Fire and Disaster Management Agency's Fire Research Center, when materials like wood chips or compost are piled in large quantities, heat dissipation does not progress, and the internal temperature gradually rises. As the temperature rises, reactions progress further, and another reaction called oxidation by air can occur, leading to spontaneous combustion [9]. However, it is said that even if a farmer’s compost pile becomes warm, it rarely catches fire.
Heat is produced and hard to escape. Yet, in compost making, the piles are turned over from time to time. Why do they do this?
4. Why Too Much Heat Stops Decomposition and Piles Must Be Turned
Higher heat is not always better. According to Cornell University, if the temperature exceeds about 65°C (149°F), many microbes die, and the speed of decomposition slows down. Therefore, people managing compost add air or mix it to prevent the temperature from exceeding this limit. On the other hand, it is said that many pathogens harmful to humans and plants die when the temperature reaches 55°C (131°F) or higher [5]. The 50 to 80°C mentioned earlier is a value that "can be reached" inside the pile, while 65°C is a rough upper limit for management. These are different measures.
In leaf compost, if oxygen becomes scarce inside the pile, microbes have difficulty breathing. To prevent this, farmers perform "kirikaeshi" (turning over). This is the work of swapping the top and bottom of the piled leaves, which helps keep oxygen from running short inside the pile. This is done about every 20 days, adjusting moisture levels. The compost is considered complete when the leaves become crumbly about 200 days after starting [6]. This is a procedure for home gardening. The period varies greatly depending on the method; for example, the piles of farmers in Kawagoe are said to take about 2 years [3].
Research by the National Agriculture and Food Research Organization shows an example of the effect of mixing. When moisture was adjusted to about 60% and the pile was turned over once a week, raising the fermentation temperature to 70°C (158°F), E. coli in the compost became undetectable after 2 weeks. In improper composting like simple stacking, some survived [10]. However, this result is from 2006 compost involving livestock manure, not numbers for leaf-only compost.
Farmers have adjusted the balance by hand: producing heat, not letting too much escape, and not letting too much be produced. There is a way to use this heat for something other than compost.
5. Using Fermentation Heat to Grow Seedlings in Traditional Hotbeds Made from Trampled Leaves
A traditional hotbed made by trampling leaves is a method where rice bran and water are added to leaves while trampling them down. The heat from microbial fermentation is used to grow seedlings. In one farmer’s method, rice bran is always added because it helps the microbes work. The internal temperature can reach about 40°C (104°F) in as little as 2 days, and it drops to the outside air temperature in about 1 month [11].
In another explanation dealing with raising summer vegetable seedlings, temperature changes are divided into three periods. The first about 8 days is the rising period where temperature continues to rise. Days 9 to 20 are the stable period. Days 21 to 30 are the declining period. It is made by laying leaves and stepping on them to compress them to about half their height, repeating this 6 to 8 times [12]. The number of days is an example from home gardening and is not a fixed value. In the farmer's method above, the inside can reach about 40°C in as little as 2 days, while the summer-vegetable seedling guide says the temperature begins to rise after 3 to 4 days if things go smoothly.
Heat does not continue to be produced forever. In the farmer's method described above, the temperature drops to the outside air temperature in about 1 month. The pile is warm only while it is producing heat.
6. Safe Ways to Investigate Leaf Piles Yourself
Do not touch or dig into leaf piles. They may be hot inside or contain insects. Avoid climbing on them. You can observe and investigate from a distance.
With a family member, look for places in your neighborhood where fallen leaves are collected and piled up. From a distance, look at the size of the pile and the amount of leaves. Check if steam is rising on a cold morning. Piles without steam can also be clues for comparing size and stacking methods. Consider whether they fit the three conditions in this article: size, air, and moisture.
Read the page on Saitama Prefecture’s "Musashino Leaf Mulch Farming Method" [2] and the National Institute for Environmental Studies’ "Trivia on Recycling and Waste" [4]. Compare how composting is explained as a reaction that produces heat.