How can I tell whether compost is too hot to plant into?

Gardening & Landscaping Asked on October 24, 2021

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.

One Answer

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.

Answered by Bamboo on October 24, 2021

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