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March 23, 2018  |  Login
The Perfect Compost Pile: Ratios and Temperature Control
By Ann Whitman and The National Gardening Association

Getting Ratios Right

In composting corners, you often hear about the C/N ratio or carbon to nitrogen ratio. Basically, all organic matter can be divided into carbon-rich (brown stuff) and nitrogen-rich (green stuff) materials. Using the right mixture of brown to green stuff when building a compost pile encourages the pile to heat up and decompose efficiently. Although nearly any combination of organic materials eventually decomposes, for the fastest and most efficient compost pile in town, strike the correct balance (C/N ratio) between the two types — usually 25 to 1. 

Table 2-1 shows which common compost materials are high in carbon and which materials are high in nitrogen. Notice that the softer materials, such as fresh grass clippings, tend to be higher in nitrogen than hard materials, such as sawdust. Mix these together to form a pile with an average C/N ratio of 25:1 to 30:1, and you’ll be well on your way to beautiful compost.

Use the ratios in Table 2-1 as guidelines. Actual ratios vary depending on the sources of the materials and other factors. And speaking of sources — be sure that your compost materials haven’t been contaminated with pesticides or other chemicals.

Table 2-1  Carbon/Nitrogen Ratios of Various Materials
 Material C/N Ratio
 Fresh alfalfa hay 12:1
 Table scraps 15:1
 Grass clippings 19:1
 Old manure 20:1
 Fruit waste 25:1
 Corn stalks 60:1
 Old leaves 80:1
 Straw 80:1
 Paper 170:1
 Sawdust 500:1
 Wood 700:1

Keeping Your Pile Happy

A hot pile is a happy pile: Heat makes the ingredients break down faster and kills weed seeds and diseases. If you follow the method of just throwing everything together, the pile rarely heats up. If you follow the method of building the pile carefully with the recommended C/N ratio, the pile will start to cook within a week. Now you need to keep it cooking. Here’s the procedure:

  1. Keep the pile moist by periodically watering it.

    Dig into the pile about 1 foot to see whether it’s moist. If not, water the pile thoroughly, but not so that it’s soggy. The pile needs air, too, and adding too much water removes air spaces. If you built the pile with moist ingredients, such as kitchen scraps, it won’t need watering at first.
  2. Turn the pile when it cools down.

    Using a garden fork, remove the outside layers and put them aside. Remove the inside layers into another pile and then switch. Place the outside layers in the center of the new pile and the inside layers along the outside of the new pile.
  3. Let it cook again.

    How hot it gets and how long it cooks depends on the ratio of C/N materials in the pile and whether you have the correct moisture levels.
  4. When it’s cool, turn it again.

    You should have finished compost after two to three turnings. The finished product should be cool, crumbly, dark in color, and earthy smelling.

Sometimes, a compost pile smells bad, or contains pieces of undecomposed materials, or never heats up. Chances are that one of the following conditions occurred:



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