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C/N Ratio

This leads us into a discussion of perhaps the single best indicator of how rich or poor your compost mix is ... the C/N RATIO.  C stands for Carbon, N stands for Nitrogen; and the Ratio is the comparison of the amounts of carbon to nitrogen any material might contain.  Carbon supplies energy in carbohydrates and nitrogen provides growth in proteins.  All living things need these elements to survive  (including the microorganisms that make compost); and they use roughly 30 parts carbon to each 1 part of nitrogen ... this would make a C/N Ratio of 30:1, simply expressed as 30.  If too little nitrogen is present in the pile, not enough heat will be generated, and decomposition will proceed slowly (taking perhaps many months).  If there is an excess of nitrogen present, the microorganisms release it into the air as ammonia and carbon dioxide.  This is not good, as it results in a loss of valuable nitrogen from the pile and is also likely to cause odor problems  (ammonia has a "soiled diaper" smell; and if there is an excess of nitrogen, you will notice the odor, especially when turning the pile).


Therefore, the ratio of carbon to nitrogen in the pile should generally be in the right proportion to get it hot, and to keep it hot.  The right proportion to keep those microorganisms happy, is a C/N of 25-30.  A general guideline to follow is:

*  materials that are still green and moist are high in Nitrogen  (ex: fresh grass clippings, green weeds, most vegetative kitchen wastes)
*  materials that are dry are generally high in Carbon  (ex: sawdust, paper, dried grass, dried leaves)

On the average, the plant material that goes into a compost pile has a C/N of 50; which as the material decomposes, reduces to a C/N of 15-20  (finished compost); the C/N of stable humus in fertile soils is generally in the range of 10-15.

Below is a list of the C/N Ratio of some common materials used in composting. Use it as a guide as you assemble the materials available to you.

Some Common C/N Ratios
sawdust  150-200   leaves from oak, maple  40-60
peatmoss  50   sun-dried grass clippings  20
straw  50-150   hay from legumes  15
raw garbage  25   fresh grass clippings  15
cow manure  30   fresh garden debris  20

The main thing to remember is that non-juicy, dry materials are high in Carbon, and if you use a lot in your compost you can throw your C/N Ratio off to the high side; and you will need to add Nitrogen in some form  (lots of kitchen wastes, blood meal, etc.) to get that C/N Ratio into the 30 range.  There are charts and formulas to enable one to get fairly scientific about figuring out the C/N Ratio of a compost pile.  These are not included here because I'm not that scientific about the whole thing.  You don't need to be absolutely precise in making your compost pile; just be aware, in a general sort of way, what a C/N Ratio is, and the approximate C/N of the materials you have assembled.  C/N is figured on a dry weight basis, and some of your materials may be moist ... take this into account, and just try to put in about 30 times more carbonaceous material  (estimate wt.) than nitrogenous material  (guess at dry wt.).  Experience will make this easier.

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