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Composting Basics: What (and What Not) to Compost

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Deciding what to add to your compost begins with understanding what different kinds of organic materials bring to the microorganisms that will be breaking them down. All living things—or things that have once been alive—contain carbon and nitrogen, and this carbon and nitrogen is what microbes consume to cause decomposition. Carbon is used give the microbes energy to do their job, and nitrogen is essential for them to grow and reproduce. Making sure that your microorganisms have all the food that they need to keep active and reproducing is essential to fast composting.

Different types of organic waste have different levels of carbon and nitrogen. Each thing that you can add to your compost can be either high in carbon or high in nitrogen, and with each addition of these items to your compost, the ratio of carbon to nitrogen can change. Knowing how much carbon and nitrogen you’re adding to your compost is important—the recommended ratio of carbon to nitrogen is about 30:1. This is the ideal ratio for decomposition speed and the reduction of foul odors. Items high in carbon are referred to as “browns,” as they are mostly comprised of dry or dead materials. Organic waste that is high in nitrogen are called “greens,” and include living or moist plant materials. The table below shows common examples of compostable materials falling into the brown and green categories:

High in Nitrogen High in Carbon
Vegetable and fruit scrapsTwigs
Used coffee groundsDead, dry leaves
Used tea leaves and bagsStraw
Cut flowersNewspaper and paper bags
Live plants and trimmingsEgg shells
Horse and cow manurePine needles
HayNut shells
Fresh lawnmower clippingsDried lawnmower clippings

This is an extremely brief list when you consider all of the organic materials that exist in the world, and with some research, you will discover many more materials that are compostable, including some that may surprise you (did you know that you can compost hair, nail clippings, and certain brands of dryer sheets?).

Unfortunately, just because a material is organic doesn’t mean it should be composted. In your research to become more familiar with what you can compost, you are bound to run into some discussion about the things you can’t. Certain organic materials can not only slow the decomposition of your compost, but they may also be harmful to you and your plants. These are a few items you should consider before introducing them to your compost:

Waste MaterialWhy to Avoid
Meat and boneMeat and bone are very slow to decompose as they do so anaerobically. While it is possible to compost meat, it takes a lot of attention and time, and the stench it will produce can cause a rodent and maggot problem. It’s recommended by most that the home composter steer clear of meat.
DairyLike meat, attempting to compost dairy products like yogurt, sour cream, or milk will cause an extremely foul odor very quickly, attracting creatures you definitely don’t want in your compost.
FecesDroppings can totally be composted. Manure from herbivores like horses, rabbits, cows, and sheep is an excellent source of nitrogen for your compost and nutrients for your plants. Droppings from carnivorous animals, however, may contain bacteria or disease that can make you very sick. While dog and cat waste can be composted (as can human waste via composting toilets), it is unsafe to use in an edible garden.
SawdustSawdust is compostable, and if you are well acquainted with the source of the sawdust, you can use it to bump up the carbon in your compost. The problem with sawdust and other little shavings and bits of wood is that a lot of it tends to come from projects around the home wherein treated wood may have been used. Pressure-treated wood contains toxic chemicals, and should not be put into compost. If you are not sure about whether or not your leftover wood is hazardous, do not use it.
Oily or cooked foodOils are also slow to break down, as are any foods that were cooked in them or produced them when cooked. Oils and oily food will putrefy and stink, like meat and dairy, inviting pests to come and poke around in your compost.

Again, these are only a few of the items that are up for debate bycomposters, so be aware of what you are putting into your composter to achieve the highest quality and safest product for you and your garden.

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BruceMcAdam