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I've a small compost heap (1.2m x 1.2m x 0.8m), which is mostly formed of grass clippings. Thanks to this answer to another composting question, I now understand the C:N ratio.

Unfortunately, there isn't a good source of carbon in my mostly-lawn garden. What is the best thing I can do to increase the carbon content in my compost? The cheaper the better!

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Coffee grinds are a good source of nitrogen for the compost heap. –  leancz Jun 12 '11 at 20:05
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Grass clippings actually have a ton of nitrogen. You need to add carbon to your pile. –  Daniel Bingham Jun 12 '11 at 22:04
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Yup, totally wrote N:C instead of C:N, and now I have a lot of editing to do... –  Tragedian Jun 13 '11 at 12:59
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3 Answers

up vote 12 down vote accepted

If you're using mostly grass clippings you are nitrogen heavy and need to add more carbon. Lawn clippings are an excellent source of nitrogen.

Cheap/free sources of carbon:

  • autumn leaves -- mixing these with grass clippings makes great compost
  • newspapers (sticking with black & white newsprint is safest)
  • corrugated cardboard (uncoated/unprinted is safest) -- shredding will compost better
  • straw (if you can get it cheap/free locally)
  • wood chips ("arborist chips", which are available free from some municipalities)
  • sawdust (cheap/free as a waste product from a nearby sawmill)

Just don't add too much wood -- it will eat quite a bit of nitrogen as it breaks down. IMO, leaves are the best: they're natural, abundant, renewable, bulky, and free.

Additionally, you could grow your own carbon. If you can afford to convert an area of the lawn into a garden bed, you could grow:

  • oats, which are easy to grow and produce seed (for next year's crop) and straw (which is high carbon). (Other straw-producing crops like wheat or barley would also be possible; I have had good experience with oats.)
  • sunflowers, which are nice to look at and have woody stalks that contain a lot of carbon (chop them up before adding to the pile so they break down in less then a decade!)
  • timothy or other "hay" crop (perennial); cut after it's 2' high or so during dry weather before seed heads form and allow to dry for a couple of days. If you add the undried cuttings to your compost pile, you'll just be adding a bunch of nitrogen.
  • bamboo, be sure to select a variety you can keep under control
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Wood chips if you have them. Vegetable kitchen waste will also help to balance C:N. (meat is usually advised against because it can attract rodents)

[edit] Wood chips can be quite small - eg. from prunings from shrubs. Also if you really are short of prunings and veg kitchen waste, you could always buy a bag of deciduous wood chip mulch from a garden center. The stuff I'm thinking of comes in big relatively lightweight bags. So it should go a long way, especially as you mix it into the grass clippings.

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No wood at all. I'm basically all lawn and bushes. –  Tragedian Jun 12 '11 at 18:53
    
What about the clippings from the bushes when you prune them? And you've got the veg waste (see above) and if you're really short, you could buy some deciduous wood chipping "mulch" from your local garden center. –  winwaed Jun 12 '11 at 19:01
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I've never read this anywhere, but my experience is that if you leave grass clippings spread out to dry in the sun for a couple of days, they aren't nearly as "hot" (nitrogenous) if you then add them to the pile. I'd guess this is true for anything else, too.

(I'm coming at this from the opposite direction - I never have enough nitrogen.)

[six months later] In hindsight... I should have begged the question. If the pile doesn't show signs of too much nitrogen, don't fix what ain't broke. A pile with an good mix will get hot inside, maybe even steam, and smell like a dairy barn - a smell similar to urine. You may see a lot of it inside turn white. This is all great for the pile. If hot enough, it will even kill weed seeds in the pile. If it's situated where the smell is a problem, though, you may want to add carbon, as you thought, just to reduce the smell.

If a pile made mostly of grass has too much nitrogen, it will do one or more of:

  • smell bad, especially inside, like spinach or lettuce that's been in the refrigerator too long
  • turn black inside
  • turn gooey/slimy inside
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Yes, hay is nothing more than dried grass... if you look up the relative C:N ratios, hay is less nitrogen rich than fresh grass. Hay still could be considered "high nitrogen", but the ratio is almost neutral in terms of composting 25:1, where the goal is about 30:1. –  bstpierre Sep 12 '11 at 17:52
    
@Programming Hero - see updated answer above. –  Ed Staub Dec 14 '11 at 16:25
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