Carbon-nitrogen ratios are an important part of understanding soil, as explained by Crow Miller in this piece earlier published in Acres USA magazine:
There are two chemical elements in organic matter that are extremely important, especially in their relation or proportion to each other: they are carbon and nitrogen. This relationship is called the carbon-nitrogen ratio. To understand what this relationship is, suppose a certain batch of organic matter is made up of 40 percent carbon and 2 percent nitrogen. Dividing 40 by 2, one gets 20. The carbon-nitrogen ratio of this material is then 20 to 1, which means 20 times as much carbon as nitrogen. Suppose another specimen has 35 percent carbon and 5 percent nitrogen. The carbon-nitrogen ratio of this material then would be 7 to 1. Anyone who handles organic matter, who mulches, or who composts, regardless of which method is used, should have some idea about the significance of the carbon-nitrogen ratio.