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We have a black tumble composter that is about one-quarter full of organic plant food and garden clippings. Now that the weather is heating up the bin has become quite smelly and is attracting flies.

The contents are wet, without adding water, and drip throughout the day. The inside is about 97 degrees (f) for 7 hours then slowly cools down to about 63 degrees (f) for about 3 hours until the sun warms it up. I have added some soil from the yard hoping to get some microbes to grow.

I'm stuck on how to troubleshoot the compost to get the balance right. Any thoughts as to what might be done to improve the break down of the material?

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If the compost becomes smelly, it is probably a sign of too much greens (or lack of browns). Try to add more browns to get the ratio right. You should have a C:N ratio of 30:1, ideally. This is however not the ratio greens:browns, don't be confused. If you look on the web you'll find they advice a greens:browns ratio of 2:1 or 4:1. It depends a bit on the greens or browns you use.

So add more browns when the smell starts, browns like dried autumn leaves, hay, or branches. Check here more about ideal composting conditions.

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You said it contains "organic plant food and garden clippings" but it's unclear to me if those are carbon sources or nitrogen sources. You'll need a balance of those 2 things for the compost to work best.

It should be as wet as a wrung out sponge. Pick up a handful and sqeeze it in your hand. If liquid drips out, it's too wet. To address the wetness, there's a few things you can do:

  1. If the items inside are matted down from the wetness (e.g. grass or newspaper) you will need to break them up to get them to dry out and stop the anaerobic decomposition.
  2. If the bin is constantly wet, find a way to open existing vents or add vents to it.
  3. Add some dry materials (dry woodchips, dry leaves, sawdust if you can really mix it in and not let it be in a big pile).

About the smell there are a few reasons depending on the kind of smell:

  • If it smells like ammonia, that's too much nitrogen and you need more carbon.
  • The wetness + flies makes me think that the things you added could be too wet and in too big of chunks. Did you chop them up at all before adding them? If not, take them out and chop them up and try again.
  • You should mix it more than normal for a while (every day or 2) to help dry it out and get the materials and microbes working better together.

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