By Steve Reiners, Professor and Chair, Horticulture Section, School of Integrative Plant Science, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic.
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April 9, 2021

You’ve read this advice before:  “Spread one inch of compost across your garden.”  But what is compost? How do you make it? And can you make it in your yard?  Doesn’t it attract rodents and other undesirables?  Let’s answer some of these questions.

Compost is simply the rotted remains of plants and animals (or animal wastes).  This organic matter is exactly the type of material we want to add to our soils.  It adds nutrients just like fertilizer. But it does much more.  Organic matter helps sandy soils hold more moisture, so they don’t dry out as much.  It can improve drainage in heavier clay soils.  And it feeds beneficial soil microbes. 

Compost also helps fight climate change. Being about 40% carbon, applying compost increases the carbon in the soil which helps decrease the carbon in the atmosphere. Plus, by composting in your backyard rather than sending scraps to the landfill, you reduce the amount of methane released to the atmosphere.  Methane is produced when organics decompose in landfills, and is about 30 times more potent as a heat trapping gas than carbon dioxide.

Ideally, a well-made compost pile should be a balance of materials that are high in nitrogen (the greens) and those that are high in carbon (the browns).  We classify compostables  by the ratio of carbon to nitrogen or the C:N ratio.  For example, sawdust is high in carbon and may have a C:N ratio of 500 to 1, 500 units of carbon for every one unit of nitrogen. 

At the other extreme, poultry manure is high in nitrogen, with a ratio of just 10:1. Coffee grounds are in the middle, about 20:1.  See Table 1 for a list of C:N ratios of common compost ingredients.

C:N ratios of high nitrogen materials:

• Poultry manure: 10:1
• Hair/fur: 10:1
• Vegetable waste: 11:1
• Alfalfa: 12:1
• Sheep Manure: 17:1
• Vegetable trimmings/scraps: 17:1
• Grass Clippings: 20:1
• Coffee Grounds: 20:1
• Manure (Cow): 20:1
• Fresh weeds: 20:1
• Horse Manure: 25:1

C:N ratios of high carbon materials:

• Nut Shells: 35:1
• Manure (Horse) with bedding: 60:1
• Pine needles: 70:1
• Autumn leaves: 80:1
• Corn Cobs: 100:1
• Straw: 110:1
• Paper towel: 110:1
• Office paper: 129:1
• Shredded Newspaper: 170:1
• Cardboard (shredded): 350:1
• Sawdust (fresh): 500:1

The ideal C:N ratio for compost is about 30:1.  Higher values have too much carbon and not enough nitrogen for hungry microbes.  The process stays cool and breakdown is slow.  Compost with lower values have too much nitrogen and can become slimy and smelly.

Examples of green materials include freshly cut grass, garden waste, vegetable and fruit scraps, coffee grounds, tea bags, weeds, fresh manure from horses, rabbits and guinea pigs, hair and fur.  Browns include fall leaves, straw, wood chips, shredded paper, paper bags, and sawdust. 

You don’t need to make precise calculations when you mix your compost ingredients together. But I find it helpful to have some browns handy so that when I have greens that need composting I can layer in some browns to get a good blend.  Also throw some soil or old compost in to help introduce microbes.  No need to add worms, they will find it on their own.

For a backyard compost pile, don’t put in things that will attract rodents and other animals.  Meat, bones, fish, dairy products and bread should be saved for a municipal compost facility.  Check to see what’s available in your town.  Waste from cats and dogs should also stay out of the compost as it may contain human pathogens.  Don’t use ash from a coal-burning stove as it may contain heavy metals.  Ash from wood burning fires is okay but use sparingly as it can raise the compost pH much too high.

Some people complain that making compost is a lot of work.  That depends on how quickly you want the compost to go from its raw form until it’s ready to use.  For example, you can make a big pile of leaves in the fall and not do a thing.  It may take two years before it turns into leaf mold compost. 

Or you can take those leaves, chop them with a mower and mix it with some high nitrogen materials like fresh horse manure.  Then every 2 weeks, “turn” the compost.  That means taking a pitchfork and turning it over – the top becomes the bottom and the bottom the top.

Turning the compost puts a lot of air into the pile and the microbes that make the compost breathe air just like you and me.  A well aerated pile with the right C:N ratio can even get hot, over 150F.  Temperatures like that can help kill weed seeds and disease organisms.  In a home compost pile these high temperatures rarely occur.  But when they do, you can get compost in a couple of months.