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Is It Safe To Compost In Plastic?

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Tony O'Neill collecting leaf mold from compost bin

A commonly asked question amongst gardeners is whether or not it is safe to compost in plastic. When I started to look into this topic, I found a substantial amount of helpful information, which I will outline in the following article so that you can get the answers you need.

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

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    Composting 3
    Composting 3

    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 are simply using a plastic container to compost in.

    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 the unwanted materials into the food.

    Similarly, when composting, if your container is likely to get hot on a regular basis, this is likely to 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 purposes.

    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 that are 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 on the subject.

    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 substances that are toxic to both the human body and the environment in general.

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

    But much of the plastic that is 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, though, the chemicals are only considered unsafe at high levels.

    The amount of plastic-born chemicals found in compost is very 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 though.

    At high doses, the chemicals found in plastic are harmful to both humans and the environment.

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    Can plastic chemicals be absorbed by plants?

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

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

    sweet potato slips in tray

    Can plastic chemicals be absorbed by soil?

    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 what effect they have on the health of the soil.

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

    Soil quality plays a big part in determining how long it takes for 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 there is 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 the majority of soft plastic products like plastic bags, plastic 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 that are used to increase flexibility and durability of plastics
    • 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) in them, and this has been shown to be toxic to plants at high levels. This is an area of concern for gardeners, but the risk is quite minimal when it comes to using plastic to compost. 

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

    Plastic Types

    There are some types of plastic which are recommended for composting, and some which it is advised that you avoid using. Differentiating between the various types of plastic can be difficult for the average gardener like me, but thankfully, there is a universally recognized system set up to indicate the safety levels or certain plastics for compost usage.

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

    plastic waste

    These symbols signify the characteristics, safety, and suitable uses of each plastic-type. 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 take a look at what each of the types of plastic is, its common uses, and whether it is safe to use for composting. 

    PET – Plastic type 1

    The plastic that is marked with a number one consists of PET, which is short for Polyethylene Terephthalate. It’s most commonly used for the packaging of 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 and is usually used for single-use purposes because it breaks down when subjected to heat and sunlight for substantial periods.

    Plastic-type 1 is not recommended for composting. In fact, 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.

    The chances of type 1 plastic leaching into the compost if you use it as a container are still fairly slim, but why take that chance when there are 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 very common type of plastic, used for cleaning product bottles or milk containers.

    Type 2 plastic is considered to be one of the safest and best varieties. The reason for this is that it is resistant to the sun’s UV rays and can tolerate high levels of heat 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 that is marked with a number 3 consists of PVC, or Polyvinyl Chloride as it is formally known. This is a fairly common type of plastic and is used for liquid detergent bottles, plastic piping, and irrigation systems.

    As I mentioned earlier in this article, phthalates are an area of concern when it comes to composting. PVC type 3 plastics contain these chemicals in abundance because they make the plastic more malleable and robust.

    The inclusion of phthalates is brilliant for construction purposes, as it makes 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 they are 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 a number 4, this means it is made from low-density polyethylene. Plastic bags, bin liners, and packed lunch containers are some of the common purposes for this variety of plastic.

    You might have realized by now that the plastics that are used for food storage purposes are also likely to be suitable for composting. This is the case with type 4 plastics. Similar to HDPE, LDPE plastics are very safe and can handle a wide range of temperatures and conditions.

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

    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. Basically, 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 to be OK when used for composting. It’s best to err on the side of caution, however, because there are some concerns about leaching which have been brought to light as a result of recent research in Canada.

    PS – Plastic Type 6

    Perhaps the most common type of plastic is type 6, which is made of Polystyrene. This variety is used to make 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 in your garden.

    OTHER – Plastic Type 7

    Finally, if the plastic is marked with a number 7 this means it falls outside the categories of the previous 6 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. I’d therefore 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 about all the info about composting in plastics. Personally, I will continue to use them as I feel the process is safe. For a lot of people who struggle to compost whether in plastic or any other form, I created a How-To Video which will really explain the entire process. You can view it below.

    How To Make Compost - Composting Process - Compost Methods

    Related Questions

    What materials should not be composted?

    Meat, fish, or poultry can cause problems with rodents, pests, and smells when it is used in compost. 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 to be a good source of carbon, which is an important part of the composting process. It’s best to shred up 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 is a result of 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 very slim, barring extreme weather conditions or the use of harmful varieties of plastic.

    By using the numbered guide that I have just explained in the previous paragraphs, you can instantly recognize whether a certain plastic container is suitable to be used for composting.

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

    If you found value in this article, please consider subscribing to my blog to be notified of future articles. I would appreciate your help in sharing it with other like-minded gardeners.

    Happy gardening 🙂

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