The secret to keeping bad odors and rats out of your compost pile


When done well, food composting is an excellent way to dispose of waste while creating garden-friendly soil additives. But, when done in a slipshod manner, all that organic waste becomes a convenient and smelly buffet for rats, mice and other vermin. Here’s how to keep bad odors and rats out of your compost pile.

The secret to keeping bad odors and rats out of your compost pile
Mark King, director of the Maine Compost School, creates DIY compost bins out of lengths of lobster trap “fencing” that he rolls into large cylinders and fastens with bent wire coat hangers. They are strong enough to hold the composting materials and the open squares allows for proper aeration. When done properly, these bins are effective, odorless and do not attract pest like rats or other vermin. | Mark King

“For the home composter, if you don’t pay attention to your compost, fats, meat and dairy [waste] in your pile can be a problem,” Mark King, specialist with the Maine Department of Environmental Protection, said. “That’s why people are often told to not compost those items, but as long as you pay attention to the compost, it does not have to create problems.”

King helped found the Maine Compost School in 1993 and serves as its current director. He tells aspiring composters that moisture management is the key to effective and clean composting.

“It’s all about the moisture,” King said. “When you compost food waste it’s around 80 percent water [and] when folks get too busy and add too much food [waste] the compost pile can get too much moisture.”

When moisture levels rise past 70 percent, King said, it displaces oxygen and creates conditions for anaerobic metabolism — the creation of energy in the absence of oxygen. The byproduct of this anaerobic process are organic acids that smell like garbage or rotting food.

“That is what attracts rats and other vermin,” King said.

It’s also what annoys any neighbors downwind of the compost pile.

“The easiest way to control those odors is not to create those conditions which cause them in the first place,” King said. “You need to break up the compost materials to let the oxygen flow.”

King said he is a fan of what he terms “Ronco composting” a term he coined playing off the “time saving” kitchen appliances sold by Ronco.

“Their motto for a countertop appliance was ‘set it and forget it,’” King said. “I talk about ‘set it and forget it’ composting.”

Compost is happiest and at its most productive when it is mixed at a ratio of one part “green” — or organic — waste to two parts carbon producing “brown” waste, such as leaves or manure.

“With that ratio you are going to get 55 percent moisture,” King said. “That is perfect for composting.”

When done properly, left to its own devices brown and green waste will break down into usable compost in about eight months, he said.

Students of King’s compost school often visit Kinney Compost in Knox, where Wes and Gwen Kinney have been turning fish waste into Maine Organic Farmers and Gardeners Association-approved compost for 20 years.

And, if anything was going to create some nasty odors and attract vermin, it would be rotting fish.

“You have to be right on it,” Gwen Kinney said. “You can’t have [the fish] just sit and accumulate.”

Every morning waste products like fish skin or entire salmon that are not suitable for human consumption arrive at the Kinney’s facility.

“Every afternoon my husband goes out and stirs it in so it gets well combined with sawdust, cow manure or other [brown] compost,” Gwen Kinney said “By doing that, you don’t get any bad smells and it makes excellent compost.”

King’s go-to brown waste is horse manure, which he said is just about the best thing anyone can use.

“It has a lot of ‘fluffy’ carbons that create a good airflow,” he said. “It also has high nitrogen content and is high in bacteria that are favorable for composting.”

When starting a new compost pile, King said he begins with a six-inch layer of horse manure in the bottom of the compost bin, leaving six inches between the manure and the sides of the bin.

Next, he adds a layer of food waste on top of that manure and then another six-inch layer of horse manure that covers the food and extends down into that space between the bottom layer and the sides of the bin.

“That seals off the food layer and prevents any odor,” he said. “If the food does start to smell during the composting process, those smells don’t get through the layer of manure [because] it acts like a natural biofilter.”

King then keeps layering his compost in a sort of food waste-horse manure parfait all season.

“There is no smell at all from the manure,” he said. “The cool thing about horse manure is it will compost on its own and the food is just along for the ride.”

Composting in Maine, King said, just makes good sense.

“Compost is not a fertilizer, but it enhances and rebuilds poor soil,” he said. “We have weak and poor soil in Maine [and] by composting you help the soil and recycle waste.”

Gwen Kinney said they have never had an issue with rats or rodents, thought the occasional skunk has been spotted checking out their compost.

“My son takes care of those,” she said with a laugh. “You don’t want to know how.”

If mice or rats are sniffing around looking for a free meal at the composter, King said the problem is probably too much moisture, and can be solved easily and fairly quickly.

“The good news, it’s an easy fix,” he said. “Add in more brown compost material, give it a good stir and you should solve the problem.”

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