The Truth About Composting in Plastic: Pros and Cons

One common query among garden enthusiasts pertains to the safety of using plastic for composting. During my investigation on this topic, I’ve found a wealth of enlightening data. The forthcoming article will elaborate on these discoveries to assist you in finding the solutions you need.

Experts agree that composting in plastic is perfectly safe. It’s been a tried and trusted method for many years with no obvious issues. However, there is the consideration of long-term safety, as very few long-term studies have been done on the subject.

A major concern among gardeners when using plastic for composting is that over time the plastic will release toxins and unwanted substances into the compost.

While it is true that plastics can give off toxic byproducts in some scenarios, it’s unlikely that this will occur if you use a plastic container to compost in.

Tony O'Neill collecting leaf mold from compost bin

Heat is one of the most important factors to consider. When plastic gets hot, it releases toxins at a fast rate. This explains why it’s advised not to use plastics in a microwave, as the heat will draw unwanted materials into the food.

Similarly, when composting, if your container is likely to get hot regularly, this will likely increase the chance of toxins being released into your compost – something that must be avoided.

The potential dangers of composting in plastic

In the following section, I’ll delve into the concerns surrounding using plastic for composting.

Although it’s generally considered safe to use a plastic container for housing compost, there are some areas of concern that you should be aware of and some precautionary measures that should be taken to ensure that toxins and unwanted materials aren’t released into the organic matter.

Composting is a great way to recycle organic waste. There is a long list of materials suitable for composting, including vegetable and fruit peels, fat trimmings, eggshells, and cuttings from the garden.

The benefits of composting are unquestionable, but the materials used for the containers and linings are a hot topic of debate among scientists, gardeners, and experts.

Composting in plastic and the environment.

A recent study carried out at a university in Germany found that fertilizers that are produced by the process of industrial composting often contain minute traces of plastic.

These tiny particles can then make their way into the environment. So why is it a bad thing?

Well, plastic is known to contain toxic substances to both the human body and the environment.

If these substances are present in your compost, they will make their way into the soil, adversely affecting the organisms in your garden.

But much of the plastic found in compost is actually due to people not fully removing the wrappers from their vegetables or other household waste before putting it into the compost bin.

Most plastics are impossible to break down and therefore sit in the compost, excreting toxins into the mixture.

Indeed, it’s justified to be concerned about chemicals from plastic leaching into your compost. In reality, the chemicals are only considered unsafe at high levels.

The amount of plastic-born chemicals found in compost is minimal and is, therefore, unlikely to cause harm to the human body. If there’s a lot of plastic in a compost mix, this is another story.

The chemicals found in plastic are harmful to humans and the environment at high doses.

Can plants absorb plastic chemicals?

A major concern among gardeners is whether the unwanted toxins and substances found in plastics can be absorbed by plants, causing irreparable damage.

If chemicals make their way into the compost mix and the soil, they can only get into the plants if their roots absorb it. This can be the case sometimes, but even when the roots absorb the chemicals, only a minuscule amount is transported into the top parts of a plant, if any at all.

sweet potato slips in tray

Can soil absorb plastic chemicals?

A common misconception is that chemicals found in plastic are non-biodegradable. Indeed, there is debate surrounding their effect on soil, but chemists define them as organic and capable of being decomposed by microbes.

The varying factor is how long it takes for the chemicals to be decomposed and their effect on the soil’s health.

The half-life of a chemical indicates how long it takes to remove it. If one of the chemicals found in plastic had a half-life of 5 days, this would suggest that half of it would be removed after five days in the soil.

Soil quality plays a big part in determining how long the chemicals from plastic composting take to decompose. If the compost is washed frequently and has good aeration, the water will run out of the bottom of the plastic compost container, and in the process, much of the chemicals will be washed away.

It’s worth being aware that watering your compost too much will also cause fertilizers to wash away, so if you are using them, bear this in mind.

Finally, the process of absorption of the chemicals by organic matter is different from that of generic garden soil. If your compost is full of organic materials, fewer chemicals will be present, and therefore less will be absorbed by any plant roots.

Essentially, the more organic matter you use, the less chance of chemicals leaching from the plastic and causing any issues.

Phthalates, plastic & compost

One of the biggest concerns surrounding using plastic for compost purposes is the presence of plasticizers, technically known as phthalates. These are commonly found in soft plastic products like plastic bags, bins & containers, or garden hosepipes.

Studies have shown that phthalates are present in fruit, vegetables, and most other food groups. They were also found in almost all cosmetic products, like cleansers and moisturizers.

So what’s so bad about these plasticizers, and what do they mean for composting in plastic? Here’s a brief overview of phthalates:

  • They are a group of chemicals used to increase plastics’ flexibility and durability.
  • Often referred to as plasticizers
  • Sometimes used to dissolve other materials
  • Used in numerous products such as adhesives, cosmetics, containers, raincoats or vinyl flooring
  • Transmitted into the human body through foods that have come into contact with plastic containers
  • Can enter the body through breathing in particles or through eating & drinking

BPA & compost

Most plastic containers have BPA (or Bisphenol A), which is highly toxic to plants. This area concerns gardeners, but the risk is minimal when using plastic for composting. 

Firstly, BPA is not present in all plastics, so many compost containers will not even contain it in the first place. Also, it is unlikely that the chemical would leach out into the compost at room temperature – it would have to be significantly hotter for this to be possible.

Plastic is difficult to break down, hence the problem of washing up on many beaches and taking centuries to degrade fully. Therefore, the chance of it beginning to degrade when used for composting is minimal.

Plastic Types

Some types of plastic are recommended for composting, and some are advised to avoid using. Differentiating between the various types of plastic can be difficult for the average gardener like me. Still, thankfully, there is a universally recognized system to indicate the safety levels of certain plastics for compost usage.

You’ve probably seen the small triangular symbols which encase a number on plastic products. If you haven’t, look at the next plastic product you use and see if you can identify the number.

plastic waste

These symbols signify each plastic type’s characteristics, safety, and suitable uses. They are numbered from 1-7, with each number meaning a different thing:

  1. PETE
  2. HDPE
  3. V
  4. LDPE
  5. PP
  6. PS
  7. Other

Let’s look at what each type of plastic is, its common uses, and whether it is safe for composting. 

PET – Plastic type 1

The plastic marked with number one consists of PET, which is short for Polyethylene Terephthalate. It’s most commonly used for packaging foods and drinks, like plastic bottles and condiment jars. It is known to absorb the scent of the food or drink that is stored in it.

Type 1 plastic is one of the most frequently recycled varieties. It is usually used for single-use because it breaks down when subjected to heat and sunlight for substantial periods.

Plastic-type 1 is not recommended for composting. It should be avoided for all gardening purposes since the garden is a place that is usually exposed to sunlight and heat for long periods of the day.

If you use it as a container, the chances of type 1 plastic leaching into the compost are still fairly slim, but why take that chance when plenty of better plastic alternatives fit for composting?

HDPE –Plastic Type 2

If there is a number 2 within the triangular symbol on a plastic product, this means it consists of high-density polyethylene. HDPE is a common plastic used for cleaning product bottles or milk containers.

Type 2 plastic is considered one of the safest and best varieties. This is because it is resistant to the sun’s UV rays and can tolerate high temperatures without the risk of chemicals leaching into your compost.

Overall, Type 2 plastic is a reliable choice for your composting needs and is very unlikely to cause any harm to your garden or the environment if used responsibly.

V – Plastic Type 3

Plastic marked with a number 3 consists of PVC, or Polyvinyl Chloride, as it is formally known. This is a fairly common type of plastic used for liquid detergent bottles, plastic piping, and irrigation systems.

As I mentioned earlier in this article, phthalates are an area of concern regarding composting. PVC type 3 plastics contain these abundant chemicals because they make them more malleable and robust.

Including phthalates is brilliant for construction purposes, making the plastic much more durable. However, it is not good for composting. It should be avoided whenever possible.

Although some may argue that phthalates are unlikely to cause damage to compost unless present in large quantities, why take the risk when there are other better-suited varieties of plastic for use as a compost container?

LDPE – Plastic Type 4

If a plastic container is marked with the number 4, it is made from low-density polyethylene. Plastic bags, bin liners, and packed lunch containers are common purposes for this variety of plastic.

You might have realized by now that the plastics used for food storage are also likely suitable for composting. This is the case with type 4 plastics. Like HDPE, LDPE plastics are safe and can handle various temperatures and conditions.

Similar to plastic-type 2, type 4 is also a fine choice for composting.

PP – Plastic Type 5

Plastic marked with a number 5 consists of a material known as polyproline. Its common uses include bottle caps, food containers, and plastic straws. Anything that requires injection molding is usually made from type 5 plastic.

This variety of plastic isn’t as tolerant to heat as LDPE or HDPE (plastics 2 & 4), but it is still considered OK when used for composting. However, it’s best to err on the side of caution because some concerns about leaching have been brought to light due to recent research in Canada.

PS – Plastic Type 6

Perhaps the most common type of plastic is type 6, Polystyrene. This variety makes everything from plastic cups, plastic cutlery, trays, and containers. It’s multi-purpose and used all over the world.

Due to the popularity of Polystyrene, it has been studied by many scientists to determine whether it is safe to be used so frequently by humans. Generally, the conclusion is that it is safe to use for gardening and composting.

Although it’s perhaps not as well suited to composting as Type 2 or 4 plastic, type 6 is a suitable choice that won’t cause any harm to your garden.

OTHER – Plastic Type 7

Finally, if the plastic is marked with a number 7, it falls outside the categories of the previous six types. This usually indicates that it is made of Polycarbonate or Polylactide. Polycarbonate is known to be one of the most harmful types of plastic ever created.

It leaches BPA at an alarming rate, making it unsuitable for composting, gardening, or most other uses. Therefore, I recommend steering clear of Plastic Type 7 so that you don’t cause harm to your compost or your garden in general.

So now you know all the info about composting in plastics. I will continue to use them as I feel the process is safe. I created a How-To Video explaining the entire process for many people who struggle to compost, whether in plastic or any other form. You can view it below.

FAQ’s

What materials should not be composted?

When used in compost, meat, fish, or poultry can cause problems with rodents, pests, and smells. Dairy products are similar, so they should be avoided too. Fats and oils also attract pests, and disease-ridden plants can cause issues in a compost bin.

Can you compost toilet paper?

Both toilet and kitchen rolls can be composted. These paper rolls are considered a good source of carbon, which is an important part of the composting process. It’s best to shred the toilet or kitchen paper before putting them in the compost, as this will speed up the process.

Do pesticides break down in compost?

The majority of pesticides are broken down during the composting process. This results from the heat and microbial action that occurs in the compost. The pesticides are diluted down until there are very low levels present.

Conclusion.

In conclusion, composting in plastic is, for the most part, safe and recommendable. The chances of harmful toxins and chemicals making their way into the compost are slim, barring extreme weather conditions or using harmful varieties of plastic.

Using the numbered guide I explained in the previous paragraphs, you can instantly recognize whether a certain plastic container is suitable for composting.

There’s no reason you can’t create thriving, organic compost that is free of harmful chemicals, provided you are aware of the type of plastic that makes up the containers you intend to use.

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