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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.

Tom W
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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.

Bamboo
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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 '20 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 '20 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 '20 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 '20 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 '20 at 19:20
  • "Raised bed" in the sense of thickly mulching (3-5") directly onto repurposed lawn, yes. – Tom W Jul 17 '20 at 19:24
  • By the way, am I correct in understanding that "hot" compost that "burns" plants because it is too fresh is so because of microbial rather than chemical causes? Obviously the metabolism of microbes makes it indirectly a chemical process either way but...I'm wondering whether I've muddled the intent of the question. This answer seems to focus very much on the microbial point in this question, but that's an assumption of mine. – Tom W Jul 17 '20 at 19:36
  • If you fill a "raised bed" with nothing but this, the bed will be "empty" again within a year or two as the material decomposes. It is certainly a valuable *addition* to the soil in a raised bed, but the bed should be more like 10 - 20% of this stuff mixed with 80 - 90% soil, not 100% "compost". – alephzero Jul 17 '20 at 19:37
  • The use of "hot" isn't entirely scientific here. The reason the temperature is high is because the microbes are working to decompose the material. If you plant things directly into it while the microbes are working at their current (fast) rate, the plants will get "chemical burns" from the decomposed products, in the same way as they would get chemical burns if you applied an overdose of "chemical" fertilizer. – alephzero Jul 17 '20 at 19:39
  • @alephzero that's fine by me; I'm not making the beds "raised" for the sake of them being higher, just recognising that my adding matter will necessarily make them higher. I'm comfortable with continually refreshing these beds with new material in practical quantities. – Tom W Jul 17 '20 at 19:40
  • Well, in the UK, a "raised bed" usually means something that is physically higher than the surrounding soil, and you can't make that from a pile of rotting manure and nothing else. – alephzero Jul 17 '20 at 19:42