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I have created a monster of a compost heap on my property (3 meters, by 2 meters, by almost 2 meters tall) next to an old granite rock wall. Anyway, I mostly put all my grass clippings (primary offended of volume of material) as well as leaves and other material. I just started doing this because I didn't think it would be good to have the local rubbish company haul this off.

As of late, the pile has started getting a bit bigger than I want it to, and it exudes a sickly sweet smell. What can I do to speed up the decomposition of all this vegetation? I don't mind the smell that much, since it's pretty far from the house. However, will any process I initiate cause any other odours to come from it that may be more noticeable than the current smell?

I'm not after dirt, just trying to lessen my impact a bit (and stop that smell).

Will some of those organic disintegration agents work (like a stump dissolver)?

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When you finally 'make dirt', even if you don't have a garden, you can still spread the compost over your lawn. –  Peter Turner Jun 22 '11 at 14:51
@Larian: Re: disintegration agent, I don't know what those are, but the main things your pile needs are (1) carbon and (2) air. My answer below discusses both. An additional way to introduce carbon would be to add a layer of wood chips or sawdust on top of clippings every time you mow. The bacteria, worms, etc that do all the work are already there, you just need to feed them a bit and let them breathe. –  bstpierre Aug 22 '11 at 12:18
@Larian: Re: the edit about the smell. Have you tried any of the suggestions? Does the pile still stink? (Or were you just making an edit?) –  bstpierre Sep 21 '11 at 2:56
@bstpierre yes I have been doing the adding of charcoal and dirt. I have tried to break it up as well (and that's when the smell became worse). Although that may just be a matter of time. (And yeah, was going for my first edit too.) –  Larian LeQuella Sep 21 '11 at 10:34
Charcoal probably isn't a great idea -- too literal an interpretation of "adding carbon". If you're adding briquettes, they can take a long time to break down and don't actually feed the carbon that the pile needs. (For people who actually want the compost, briquettes have other nasty stuff you don't want to feed your plants.) You want material with a lot of surface area -- sawdust, wood chips, straw or fall leaves (that aren't matted down). Wood ash would have a lot of carbon, but is undesirable because (especially in the quantity you would need) it will raise the pH of the pile too much. –  bstpierre Sep 21 '11 at 11:56
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7 Answers

up vote 25 down vote accepted

If your pile has a sickly sweet smell (not ammonia) it is probably not from a huge overdose of nitrogen. You say that you've added leaves and other material -- so there is some carbon in your pile. My guess is that the composting process has gone anaerobic. In other words, you've got the wrong batch of bacteria working on breaking down the pile. Anaerobic breakdown will be very slow and give off this kind of smell.

You can get the right batch of bacteria (aerobic) working on breaking down the pile by allowing air to penetrate the pile. Right now your pile is so big that the center is not getting any air and these bacteria can't go to work.

Some bits of advice:

  • Keep your piles to about 1.0-1.5m cubed; this is an ideal size for a compost pile. It is easier to work with, but large enough to get the aerobic bacteria working. Alternatively, you can make a windrow of about that size but as long as you want. When you're composting this way, the pile with shrink a lot as it breaks down -- you don't have to worry about a mile long row of compost.
  • To introduce air and make your piles more manageable, you can: Take your existing pile and break it into about a half dozen piles, thoroughly mixing it as you go. Or try poking a long pole (as thick as you can find and are able to insert) into the pile to introduce air into the center. Your pile is big enough that the latter solution may not work well.
  • If you break your existing pile into separate piles, and you have some carbon-containing materials that you can add, it might be a good idea. Carbon is hard to come by this time of year (no leaves, etc but you may be able to get hold of some spoiled hay that you can mix in) so if you don't have anything just adding air might work.
  • As you're adding material, if you can make a layer of hay or straw every 10-20cm or so, you'll allow air to naturally move into the pile. If you don't mind keeping a bale or two of straw handy, I would just add a layer of 2-3 flakes of straw, and then dump the clippings on top of that. This will prevent it from getting too wet and matted down, and at the same time help to balance your carbon to nitrogen ratio.
  • If you can't or won't keep hay/straw on hand, you can add carbon to the compost pile by keeping a pile (or barrel, or bags) of wood chips or sawdust near the compost pile. Add the chips/sawdust as described above for hay -- about a 0.5-1.0" layer of wood to a 4-6" layer of grass (experiment to find out what works best with your materials).
    • Avoid charcoal briquettes because they take a long time to break down (you need more surface area) and they can have additives that will make your compost toxic to plants.
    • Avoid wood ash because it will raise the pH of the pile too much and put the wrong microbes in control.
  • You can make a big pile of autumn leaves this year right next to your grass compost pile. If you can keep it dry they will be easier to use next summer, but even wet, matted leaves will do the job. This is another good carbon source you can use as described above to balance the ratio.
  • If you have topsoil or mature compost that have not been sterilized you can add to the pile as you build it, this will inoculate the pile with all sorts of bacteria and other organisms that will help it break down faster. A thin sprinkling (0.25") of soil on top of every 6-12" layer should do the trick.
  • You probably should not add any "compost accelerator" products. These contain bacterial and fungal ingredients as described above for soil/mature compost. They also usually contain nitrogen, which is the last thing you need to add to your pile -- you've got a nitrogen overdose.

The pile should heat up and break down faster when you introduce air and balance the carbon-to-nitrogen ratio.

If you have no interest in managing your pile:

  • If you don't mind the current smell, you can let it sit the way it is indefinitely. It will rot over time.
  • Check out your town's recycling guide under "Compost Materials". It includes the number of a local landscaping company who will presumably take your compostables.

If we were just a little closer (I'm about an hour away from you) I'd offer to come and take it off your hands. 10+ cu meters of raw compost sounds like a dream but I'd be trucking back and forth all day :)

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Slow decoposition with the materials listed, does sound anerobic to be ("anoxic" is the term I use - but same difference). It is a problem we often have but for us it is primarily because our compost tends to get too wet. Either way, getting air into the pile is essential. –  winwaed Jun 20 '11 at 14:25
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I've been composting for the past few years. In my research, there are a few two main causes (besides the excess nitrogen pointed out by shane): too much moisture and adding things that are not compostable.

You should not add things like dairy products and meat scraps, they might a part of your problem. The solution for moisture is the same for to much nitrogen. Adding dried leaves, wood chips, newspaper can help soak up some of that excess moisture. Also, turn the compost regularly. I turn mine about once a month.

With that said, you can considerably cut back on the material you are adding by using a mulching lawnmower. Some people say that using a mulching mower is one of the best things you can do for your lawn.

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+1 for mulching lawnmower. Grass clippings (as long as the grass isn't too wet or long when cutting) should remain on the lawn ie Free fertilizer. –  Mike Perry Aug 22 '11 at 18:07
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You need to aerate it. It will probably smell ghastly at first, but that will pass. Anaerobic bacteria dying stinkily upon exposure to air is one of the biggest causes of stench in a compost bin or pile. So if you can get in there with a pitch fork and turn it, it will probably smell pretty shocking, but in the long run the smell will dissipate and the more often you turn it the faster it will break down.

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As @bstpierre mentioned, the pile you are working with is huge and suggested making it into smaller piles. You also said you don't want to use the results of your composting personally.

My suggestion is to take the material that is already well composted and place it bags and sell it on the sidewalk next to your house for a reasonable price. I have seen this done quite often where I live.

If I can get a whole bag of good quality compost for about $5 it is much better than spending $15 for the same quantity of poorer quality stuff from a store.

This way you are reducing the size of your pile, someone is getting good use out of your work and you can continue with your goal of lessening your impact.

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It's almost time for the leaf-peepers. Perfect timing - mix in the fall leaves with clippings, roughly 50-50, until you run out of leaves. If the clippings are already somewhat composted, use less leaves.

Once you get the pile balanced, keep all your extra fall leaves (mower-shredded, if convenient) in one pile. Then the next year, whenever you add clippings, stack them in another pile, spreading them an inch or two thick, and cover with the same amount of packed leaves - more if loose. Repeat, layering this up to around 3 feet, then start another pile.

If you don't have enough leaves, you may have neighbors who'd be happy to be rid of theirs.

After one of these piles has been brewing for several months, it will be time to turn it, to get more oxygen into it. Being lazy, I don't really do that. When building a pile of leaves and clippings, I also layer in partly-made compost from an adjacent pile that's been going for a while. That way I don't have to turn the other pile, and the new pile gets a lot of good composting critters.

The bottom of the adjacent pile may be ready to use long before the top. You'll want to pull this out to make room. If you've nothing else to do with it, September is a good time to top-dress the lawn with compost - up to a half-inch. It will look like heck for a couple of weeks, then be fine.

On the question of whether it's has much nitrogen, or has gone anaerobic - probably both are true. Clippings that have gone anaerobic will often turn dark green, almost black, and get slimy.

As noted earlier, splitting up the pile will make things easier, and work better.

Re smell: yes. When you make a pile with fresh clippings, and mix in the right amount of carbon (leaves), and have the right amount of moisture, the pile will get hot, turning to compost rapidly, and killing any weed seeds in the process. It may put off a smell like a cow barn, and if you stick a fork into the pile, you'll see steam come out with a strong ammonia smell. I don't know whether this will be more pervasive than the smell now. To me, it's a lot more pleasant smell, but it's a matter of er... taste.

A gardening fork is a must-have tool for compost - much easier to use than a shovel.

If you have a vegetable garden, and don't apply herbicides or pesticides to your lawn, consider using some clippings for mulch, instead of composting them. They work very well, but will bring in a few weed seeds. Don't use more than an inch or so of fresh clippings - they'll burn your plants, just like a pile of clippings in the yard will turn the grass underneath yellow. Because of the weed-seed problem, I wouldn't use clippings in a flower bed, simply because I'm not willing to spend the extra time weeding there. Your mileage may differ.

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The other answers are very correct, but they left out one important aspect, which is to add some dirt to your heap between adding more organic matter. You've got a lot of surface area to cover, but that grass'll break down to nothing if you keep up on it.

Also, you probably should break it into more manageable piles by putting partitions in between your huge piles of dead grass. It would be good to not put new stuff right on top of last years stuff so you can actually get at it if it ever finally breaks down. 3 partitions is probably ideal. One you add to, one you use and one that is is currently decomposing.

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Bad smelling compost is usually a sign of too much nitrogen material in your compost. If you have too much nitrogen in your compost it will start to generate ammonia. You need a balance of 1 part nitrogen to 30 parts carbon based material to have a good compost pile. Your grass clippings are a source of nitrogen and are probably the main problem. It's not a big deal if you can balance it with enough carbon based material. Dried leaves, wood chips, straw, and shredded newspaper are a good source of carbon.

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