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If you have a composting service at work or school, you might have noticed an odd pattern. They can take as many apple cores and paper towels as you care to give them — but they will not accept meat or dairy.

And if you compost at home, you might avoid adding these things to your pile. But the problem isn't that it's hard to break these things down. The problem is that it's easy — too easy.

Composting is a way to convert things like food scraps and yard trimmings into fertilizer, instead of sending those things to landfills. So it might seem annoying that you can't toss all your leftovers in. And it turns out you can—if you're careful.

Some of the problems with composting meat are practical. It can attract pests, and you probably don't want to have to fight off a bunch of rats and raccoons in your own backyard. Plus, some folks worry about disease-causing bacteria being able to grow in home compost piles.

But the solution there is just to make sure the pile reaches a high enough temperature to kill those pathogens off. Temperature is a super important factor in composting anyhow — one any home composter should be keeping an eye on. But the weirdest reason it's tough to compost meat and dairy is how delicious soil microbes find animal protein.

And even though composting is all about getting friendly soil organisms to break stuff down for us, we don't want them to get too excited. Good composting relies on the ratio of two elements: carbon and nitrogen. Carbon makes up the chemical foundation for practically all of life's favorite molecules, including proteins.

But proteins also contain a lot of nitrogen. This means that animal products, which are more densely packed with proteins than veggies, contain more nitrogen. Experts have determined that the best ratio of carbon to nitrogen in compost is somewhere between twenty and thirty to one.

Veggie scraps are generally in that sweet spot at twenty-five to one, but something like a chicken carcass is more like five to one. And when bacteria see all that nutritious nitrogen in a compost pile, they go a little bonkers. They start to grow really fast.

That uses up oxygen, and when the bacteria use up all the oxygen in the pile, that favors the growth of other bacteria that don't need oxygen to live. Basically, the pile switches to anaerobic, or oxygen-free, decomposition. And the chemical products of this process are very smelly.

Like hydrogen sulfide, which smells like rotten eggs. So if you just casually toss your meat and cheese into your compost pile, you'll probably end up with a slimy, putrefied mess instead of lovely fertilizer. And putrefied compost can actually contain chemicals that are toxic to plants — though you might be able to salvage it if you dry it out and try again.

To avoid taking those extra steps, expert composters might add extra wood chips or paper products to the pile, since they have a lot of lignin — a tough component of plant cell walls which doesn't contain any nitrogen at all. Bacteria can still eat lignin-packed materials — so they just slow down those overly- enthusiastic microbes. And if you really want to compost meat at home, you could plan ahead.

One way is to use a method known as bokashi, which relies on anaerobic processes on purpose. Basically, you add cultures of friendly anaerobic bacteria instead of bad, smelly ones. Once things like meat scraps have been treated with the bokashi method, they can be added to a compost pile with less risk of the whole thing going stinky.

So just because your municipal compost service won't take meat, that doesn't mean it can't be composted — you just have to know how to rein in your bacterial buddies. Thanks for asking, and thanks to our patrons for helping us bring you these answers. You guys are the best.

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