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I have just had delivered a tonne of bulk compost. My intention was to mulch flowerbeds and also lay out a new no-dig vegetable bed so I ordered what I expected to be mature compost.

The compost is what I'd consider extremely hot; I can feel radiant heat from it upon turning over a trowel full. Filling up a clay flowerpot (with which to carry it; not to plant into) with it instantaneously heats up the pot to the point that I can easily feel it from the outside. However it has a very fine texture which is what I would identify as well-rotted. I can identify white fungal activity spread throughout. It is also bone dry. I'm sure that if I spread it, it would reduce in (thermal) heat significantly, but what I am concerned about is the chemical "hotness" which is often talked about as compost being too fresh to plant into.

Is there any way I can objectively gauge whether this (thermally) hot compost is also too (chemically) hot to plant into - other than trying it with a few "canaries" and seeing what happens?

edit 1: I have pinned some strawberry runners into small pots with this compost, and sown climbing beans, beetroot, and lettuce into a new plot following the no-dig method. I've also mulched over an existing bed also created like this that is growing tomatoes, courgettes and strawberries. The runners look fine so far and there's no way the seeds could have germinated yet; so I'll come back with further updates when I would expect to see something. I've also bagged some of the compost and this has gone cold, so the heat I assume was residual from the commercial composter it was delivered from.

  • is it warm to the touch? – black thumb Jul 18 at 2:04
  • When adding large amounts of soil amendments, including compost, I would recommend testing it on some plants first anyway (even if it's not hot). – Brōtsyorfuzthrāx Jul 18 at 3:15
  • Thank you, please continue the excellent updates – M H Jul 21 at 13:27
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In the UK, 'compost' is an incomplete general term; it can mean potting composts such as John Innes, multi purpose, ericaceous, seed and cutting compost, all which are sterile and suitable for use in pots. However 'compost' also refers to soil conditioning composts such as your own garden compost, or composted manure or other composted materials which are not necessarily sterile and are not therefore recommended for use in pots. Given you bought a bulk supply, I imagine what you purchased was soil conditioning compost; its intended use is on open ground rather than in containers, where the fact it is hot won't be an issue once it is spread out over garden soil.

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  • It has enough of a manurey smell that I suspect a large part of it is that - even though the description says "made from natural Garden matter". I would rather not link to the exact product at this point as I wouldn't want this post or my subsequent comments to be construed as a complaint. – Tom W Jul 17 at 19:15
  • If the word 'potting' or 'container', or the term 'can be used in containers' is not present, it's not suitable for use in pots. – Bamboo Jul 17 at 19:16
  • I will also add that the product description specifically says it is ideal for filling raised beds. I didn't have a primary intention of potting with it, except for using up the scraps on experiments I'm not precious about. – Tom W Jul 17 at 19:19
  • And I see now where I have introduced the confusion. I was filling up a flowerpot to carry it in, not to plant into. – Tom W Jul 17 at 19:19
  • Raised beds are often sitting on open soil, which is fine. Not so good with raised beds not in contact with the soil though. Ah, I see... so is your raised bed in contact with open ground? – Bamboo Jul 17 at 19:20

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