1. What Does Mulch Film Change in the Soil of a Field?
Films covering farm ridges are called mulch (mulching). Their main jobs are preventing soil from drying out, softening changes in ground temperature, stopping mud splashes that cause disease, and making it hard for weeds to grow [7]. Black films block light to suppress weeds, while clear films are considered suitable when you want to raise the ground temperature [7].
There is also an old study that measured these effects. A 1969 report on tobacco cultivation found that plots covered entirely with polyethylene film had higher maximum and average surface and underground temperatures than uncovered plots. The fluctuation in soil moisture was reported to be smallest in the fully covered plots [8].
Because they are convenient, mulch is said to make up about 40% of all agricultural film sales [2]. This means many films remain in the field after harvest. What happens to them next?
2. Why Is Soil Stuck on Regular Mulch Film a Problem When It Is Removed?
In fields where regular poly-mulch is removed by hand after harvest, soil sticks to films that have been stuck to the ground for a long time. A document from the Ministry of Agriculture, Forestry and Fisheries states that dirt like soil becomes a problem with used mulch, and heat recovery (burning to use heat) is often used for post-collection processing [2].
Farmers separate films by type (such as agricultural vinyl or polyethylene), remove foreign objects like mud, pack them, and send them to shops, JA (agricultural cooperatives), or industrial waste processors. Burying them in the field or burning them openly is prohibited. This procedure is based on guidelines from Shiojiri City, Nagano Prefecture, and may not be identical nationwide [3].
Material recycling, where the material is reused, is considered desirable if possible [2]. Companies that wash soil-disinfecting mulch to make recycled raw materials, and companies demonstrating mulch containing 50% recycled material, are introduced in Ministry documents [2].
For all agricultural plastics, the rate of recycling processing rose from a low level in 1993 to 76% in 2014. However, this "recycling processing" includes heat recovery and does not mean the plastic was turned back into mulch [9].
3. How Does Biodegradable Mulch Disappear When It Is Plowed Into the Soil?
There are films that do not need to be removed after harvest and can be plowed into the soil with crop residues. These are biodegradable mulches. During cultivation, they regulate ground temperature and prevent weeds just like regular poly-mulch. After harvest, when plowed into the soil, microbial activity breaks them down finally into water and carbon dioxide. Work for removal and disposal becomes unnecessary [1].
The disappearing process has two stages. In stage one, hydrolysis by moisture creates small cracks in the film, reducing its strength. In stage two, after harvest, when chopped up and mixed into the soil, enzymes produced by microbes advance decomposition into water and carbon dioxide [5].
The first biodegradable mulch in Japan is introduced as being born in 1995 [5]. However, the influence of legal systems cited as reasons for adoption is not asserted here.
4. How Does Decomposition Speed Differ From Field to Field?
The speed of decomposition changes with conditions. In high heat and high humidity, decomposition proceeds rapidly, and the film may lose function during cultivation. In low temperatures and dryness, it may not progress even after half a year [4]. This is why you cannot say "it disappears in X months" with one number.
Because long storage weakens the film through hydrolysis, made-to-order production is considered basic [4].
There are also rules for use. Conventional poly-mulch must not be plowed in. Even products claiming to be biodegradable are treated as industrial waste if they contain non-decomposable components. Therefore, choosing products with the "Green Mark" identification label is recommended. If plowing is insufficient and film remains on the soil surface, local governments may give guidance [4].
What about cost? The price is about 3 to 4 times that of regular poly-mulch, but total costs including removal and disposal are not necessarily higher [1]. In 2020, the shipment volume was 3,822 tons in resin weight. Compared to about 35,000 tons per year for poly-mulch use, this is just over 10% (about 11%) [1]. This is one way of comparing shipment volumes and does not show the current adoption rate.
5. Why Does Biodegradable Mulch Blown Out of a Field by Wind Not Decompose?
This is the most important point to watch. Biodegradable mulch blown out of the field by wind during cultivation may not decompose. It is explained that decomposition does not progress even in water like rivers [1].
Decomposition involves microbes and their enzymes found in farm soil. Outside the field, these functions are likely not fully available. Therefore, after use, it is required to plow them in thoroughly at least twice as soon as possible [1].
From the name "disappearing film," one might think it disappears anywhere. However, it only disappears "inside a field with suitable conditions." This is an important condition for use.
6. What Research Is Being Done on Making Mulch Disappear at a Chosen Time and on Small Fragments Left in Fields?
A group centered on the National Agriculture and Food Research Organization is developing technology to sprinkle enzymes on biodegradable mulch after harvest to speed up decomposition, allowing it to decompose at a chosen time. This is based on an article from 2024; it is under development and cannot be said to be in practical use [6].
Research on small fragments remaining in fields is also progressing. A study in northwest China showed that strong mechanical tillage might make mulch fragments even smaller, potentially worsening pollution [11]. This is overseas research and does not show the actual state of Japanese fields. The amount of mulch-derived fragments remaining in Japanese fields cannot be shown with numbers in this article.
The path of collection has a wall called "soil," and the path of plowing-in has a wall called "conditions." Neither is perfect, and research is ongoing.
7. What Can You Check From the Road When You See Mulch Film in a Field?
If you find a field during a walk, look at it from the road. Do not enter the field. Note in your notebook what color the film is, how the ends buried in soil look, and if wind has peeled it up. You can also see what happens to the film after harvest by visiting in different seasons.
If you want to read materials, the Vegetable Information page of the Agricultural and Food Products Council (ALIC) has shipment volumes and usage conditions for biodegradable mulch [1]. The Ministry of Agriculture, Forestry and Fisheries "Plastic Resource Circulation" page has recycling examples [2]. Both are readable in Japanese.