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I've got a pile of hot compost going which consists primarily of:

  • Straw (about 75% by volume)
  • Kitchen Scraps
  • Rabbit Manure
  • Fall leaves and grass clippings

At the moment it's giving off a pretty decent ammonia odor when I turn it. In fact the straw (which was out in the rain and beginning to break down before it was even added to the pile), had an ammonia smell even as I was building the pile.

The broad consensus seems to be that ammonia smells indicate too much greens in the pile, i.e., a high nitrogen levels. As ready as I am to accept that this may be the case, I'm a bit confused about why the straw alone would have been smelling of ammonia since it's suppose to have something like a 75:1 ratio (C:N), which is much lower in nitrogen then the recommended 30:1 for Berkeley compost.

To complicate matters further I've just read this article which seems to contradict itself on the topic of browns and straw in particular:

Green material can be grass clippings, old flowers, green prunings, weeds, fresh garbage and fruit and vegetable wastes. Dried material can be dead, fallen leaves, dried grass, straw and somewhat woody materials from prunings.

And then...

Any material which is cut green and is allowed to dry is considered green.

Huh? Straw is definitely cut green and allowed to dry - yet it's explicitly listed as "green" and not "dried" (brown).

I was thinking of amending the pile with some very carbon heavy substance like wood chips but before I do that I'd like to understand more about why this might be happening.

UPDATE: Here's an alternate link to the pdf mentioned above since some users had trouble accessing it: http://se-59312.dev.zuma-design.com/29958.pdf

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  • Did you note this part under point (top of page 2): "Too much moisture will make a soggy mass, and decomposition will be slow and will smell". Given that you said that the straw was already wet and decomposing when you added it to the pile, could this be the source of the ammonia smell rather than too much green material?
    – Jurp
    Jun 25, 2021 at 13:01
  • @Jurp - this compost is very hot and certainly breaking down very quickly. I'm pretty sure that note is in regards to what happens when compost begins to decompose anaerobically which is a different kind of smell. Also, although the straw had previously been wet, it wasn't wet when I started the pile.
    – But those new buttons though..
    Jun 25, 2021 at 13:23
  • But you did say the straw smelt of ammonia when you were building the pile...
    – Bamboo
    Jun 25, 2021 at 13:29
  • Yes - the very center of a couple of the bales was still damp. Certainly not an excess of water though.
    – But those new buttons though..
    Jun 25, 2021 at 13:30
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    the other cause of ammonia smell ls because its anaerobic decomposition, which it would have been... but its likely there was still some nitrogen content present, because the solution is the same - turn and add browns. General info on that here gardeningknowhow.com/garden-how-to/soil-fertilizers/…
    – Bamboo
    Jun 25, 2021 at 14:05

2 Answers 2

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It's likely to do with the condition of the straw you used - straw should be completely dried out, turning from high nitrogen to primarily carbon, and that would be considered brown or carbon material. But your straw was wet and decomposing on its own with nothing else before you put in your compost because it was not stored properly; it may not have been fully dried out at that stage either, so still had a nitrogen content, in which case it was effectively a green material.

The other possibility is the amount of rabbit droppings you added in ratio to the other contents of the heap - rabbit droppings are high in nitrogen. If the smell from the heap is ammonia, then yes, you need to add carbon or browns to it and turn in well, see here https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/ammonia-odors-in-garden.htm

I was unable to access the article you provided a link to, can you add a link that works please - the extracts you provide are contradictory, so I would like to see the whole article. One other question - when you say you added fall leaves, how many were there? A large amount or just a few? Although a few leaves on a compost heap are fine, large amounts should be composted separately to make leaf mould.

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  • The link in original post works for me. It's to a PDF from the University of California, so it's probably location-restricted and won't work for you in the UK without a VPN.
    – Jurp
    Jun 25, 2021 at 12:58
  • ok - its odd though, because absolutely nothing happens when I click on the link, it does not respond at all...University of California is a reputable source, but it looks like I'll never be able to read it! Jurp's comment above on the document itself is likely relevant.
    – Bamboo
    Jun 25, 2021 at 13:21
  • @Bamboo - I added a self hosted link to the pdf in my question above. Regarding the straw, it was very, very dry when I got it. It sat outside for a few weeks, got wet, dried out, and only the very center of the bales had the ammonia smell. I'm thinking you might be right about the manure - perhaps the contributor to the excess N here isn't the straw. The leaves very only a few clumps of partially decomposed stuff from last fall.
    – But those new buttons though..
    Jun 25, 2021 at 13:31
  • Maybe there's a problem with my tech this end, because its ignoring the second link you provided as well.
    – Bamboo
    Jun 25, 2021 at 14:09
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It sounds like you're confusing hay with straw. They're different agricultural products, but non-farmers frequently are unaware of the difference, and use the terms interchangeably. (This can lead to less than satisfactory results when using the wrong one of the two, for example, if you spread hay instead of straw on your freshly seeded lawn.)

Straw is the stems of a cereal grain crop, such as wheat, oats or rice. Those crops are left standing in the field until they dry out and turn brown/tan. At the point when they're harvested, the entire stem is dead already.

enter image description here photo source

Hay is just normal grass such as you would have in your lawn (usually different species of grass, but you could certainly make hay from a lawn). It's cut while green, and allowed to dry in the sun before it's collected and combined into bales or stacks. The grass plants are still alive when they're cut.

enter image description here photo source

Notice the color difference in the photos. Straw is tan or golden-brown. Hay is pale green.

By the definitions you quoted above, straw is a "brown" and hay is a "green" contribution to the compost pile.

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  • I do know the difference and this was definitely straw. But yes I see your point as the straw would not have been cut green. That said, I can't imagine why that would change anything about the C:N ratio. I'm eager to know why if it does.
    – But those new buttons though..
    Jun 25, 2021 at 18:46

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