In order for a compost pile to decompose efficiently, you 
need to create the right ratio of carbon (C) to nitrogen 
(N). Piles with too much nitrogen tend to smell sour, 
because the excess nitrogen converts into an ammonia gas. 
Carbon-rich piles break down slowly because there's not 
enough nitrogen for the microbe population to expand. An 
ideal compost pile should start with a 30:1 C/N ratio. 
Fresh grass clippings alone have about a 20:1 C/N ratio. 
Building your pile with one part grass clippings or other 
green matter to two parts dead leaves or other brown matter 
will give you the right mix. 

it is proportion of amount of carbon presents in soil or 
organic matter or given material in soil it it10:1or 
12:1&in organic matteris 100:1