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There are types of compostable plastic that you can compost yourself at home (as opposed to compostable plastic that can only be composted in industrial composting facilities). I was wondering if there would be any benefit in doing this, other than getting rid of the plastic?

If I understood correctly, during composting microorganisms consume the carbon in the plastic and produce mainly CO2 and water. I don't think there is any NPK in the plastic, so I suspect there is no benefit for plants. Is this true? Or do the microorganisms produce something else that is useful?

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Depends on the plastic to some extent - those made from cellulose may leave some NPK residue, but will also leave other, possibly more noxious residues. The term 'compostable' is a loose one with regard to plastics - the definition of 'home compostable' means the product breaks down into '...carbon dioxide, water, inorganic compounds and biomass...', which is all very well, but that covers just about everything in the universe, so it certainly doesn't rule out toxins being left behind, and when composted at home, methane will be produced. The term 'inorganic compounds' alone starts my alarm bells ringing, it's all terribly non specific.

If you were thinking of getting a load of 'compostable' plastic and having a dedicated heap with the expectation of using it on the garden, I'd strongly recommend you leave it just as a thought and not carry it out. Also bear in mind that 'biodegradable' does not mean the same as 'compostable' - some plastics are biodegradable, but that doesn't mean they'd be welcome additions to your compost heap. By and large, and as you suspected, biodegradable or compostable plastic is simply about disposing of the plastic rather than any useful by products from the process.

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  • Most biodegradable plastics just break up into smaller and smaller pieces as the weaker bonds oxidize and break up or undergo ultra-violet delinking. No food value as they truly are not assimilable by anything. They just don't stay around for 10 years as complete product.
    – Fiasco Labs
    Commented Mar 13, 2014 at 1:06
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    @FiascoLabs I'm not asking about biodegradable plastic, I'm asking about home-compostable plastics (a subset of biodegradable plastics).
    – THelper
    Commented Mar 13, 2014 at 8:21
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    @Bamboo The reason why I am asking this is because where I live all compostable plastic is incinerated. I was wondering if composting them instead would be more beneficial/environment friendly or not. I do share your concern for toxic residues, but on the other hand there are national standards (in the US, Europa, Australia and Japan) that specify when you a plastic meets the criteria for 'compostable'. Those standards say that the residue may not negatively influence the compost and set a limit to the amount of heavy metals and toxins that are allowed.
    – THelper
    Commented Mar 13, 2014 at 8:22
  • @Bamboo BTW, do you have any references for the NPK residue in cellulose compostable plastic?
    – THelper
    Commented Mar 13, 2014 at 8:23
  • @THelper, nope, absolutely not, but since cellulose is of vegetative origin, its vaguely possible, unlike other plastics constructed entirely of petrochemicals.
    – Bamboo
    Commented Mar 13, 2014 at 11:53

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