Breaking down the art of backyard composting

With warmer weather on the way, many people will be turning to backyard composting to dispose of household scraps and create fertilizer for their gardens.

Paul Arnold, a chemical engineer with the Ivan Curry School of Engineering at Acadia University in Wolfville, N.S., said there are a few simple ways to ensure the best composting results and keep pesky rodents and other pests away, including burying fresh organic waste deep in your compost pile.

He said meat, dairy, fish — any oily or fatty products — should be avoided. Instead, Arnold suggests filling your compost pile with fruits, vegetables, eggshells, tea bags, coffee grounds, plus leaves and other yard waste.

"That's why it's so important to have a stockpile of leaf and yard waste available. You can really overcompensate on that side, while you can often put in too much food waste into a pile," Arnold told CBC Radio's Maritime Noon on Thursday.

If rodents keep coming around, Arnold said it might be best to switch gears.

"You just have to bite the bullet and divert your organics to your green bin at curbside under certain situations," he said.

The remainder of Arnold's interview with Maritime Noon host Bob Murphy has been edited for length and clarity.

I want to begin with compost piles that have been around for a while. Maybe you have been at this for a while or you haven't paid too much attention to your compost pile. How do you know when one is ready to use and put on the garden? 

Composting [is] really a decomposition process and it's a continuous process over a spectrum of time where you go from something like a banana peel or an apple core down to eventually dirt, because eventually this compost will mineralize into its original constituents.

Measuring compost maturity ... It's not really easy or consistent. 

One of the best and most reliable methods is in terms of the oxygen that is actually consumed in the pile.

For the average backyard composter, your nose is still going to be a pretty reliable indicator when it has that earthy smell. That's an indication that the maturity process has proceeded to a point where you can actually add it to your lawns or your garden. 

You don't want to be turning it too often, do you? Is there also some concerns there?

I'd say if you do it once a week, that would be considered an active management plan. The best analogy I can use is one of a buffet where you put all kinds of fresh food in there and the microorganisms there, anaerobic organisms — meaning that they consume oxygen just like we do when we eat our food — will use up that oxygen over time.

When you have fresh material, you've got an open buffet. As the food starts to become depleted, the oxygen levels will go down because there's just going to be less microbes in there to consume the food.

That's why it's important to continue turning it and maintain aerobic conditions in the pile so that the organisms can continue to attack the complex, fresh food waste in there and turn it into finished product. 

What about carbon and nitrogen? How much time do you spend thinking about those? 

Predominant sources of carbon would be your leaves and your drier material that you would get from the outdoors, and the nitrogen-rich material would be the fresh food you would be adding.

The easiest way to measure material in terms of additions to a compost pile is in terms of volume to the average homeowner. When you think about it in terms of volume, it's usually one volume of food waste from the kitchen to two to three volumes of leaf and yard waste that you would add. 

And that gets that the proper mix?

It would give you your proper mix and it also gives you your proper mix of water as well. Because, like aeration, water control is very important to make sure that you continue to maintain your pile in an unsaturated state where these little pockets of air aren't filled up with water.

We often will underestimate the amount of moisture that we have in a pile, but usually there's enough moisture supplied from our fresh food waste to keep that in balance. And we want to keep it in what we call an unsaturated state.

The best test for that, for the backyard composter, is what they call the squeeze test, where you just take a handful of the material and give it a good, hard squeeze. And if you get a couple of drops coming out, that's probably wet enough. And you don't want to have to add any more water.