What's in Your Garden Soil:  The Carbon : Nitrogen Ratio 

Master Gardeners are always talking about using  cover crops or compost as two very important means of providing nitrogen (N) to a garden, but there is another element that influences how much N and in what form N may be present in soil. That other element is carbon (C), and we will now  review why that relationship is important as well as why  backyard gardeners should be atuned a little into the carbon/nitrogen (C:N) ratio.

Plants obtain carbon from the atmosphere by converting carbon dioxide via photosynthesis into sugars that are used to fuel their cellular processes.  Not all of this captured carbon, obviously,  is used as fuel. Plants also incorporate carbon into their cellular structures in  the forms of cellulose and lignins. A tree, remember, is one big edifice of mostly cellulose.

Where does nitrogen (N) come into play?

Nitrogen Cycle. Source: scied.ucar.edu

N is needed by living cells  to make the proteins  that are essential for growth. Plants obtain their N from soluble compounds such as nitrate  (NO3-), nitrite (NO2-), ammonia  (NH3), and ammonium (NH4+) that are available in the soil. These compounds are the result of a number of different processes involving the conversion of either atmosperic N or N from  biologic sources by the actions of various mircroorganisms. A few plants, like legumes, also can convert atmosperic N into a usable form.  Few plants, by the way, can handle ammonia directly; most plants need it to be  converted by bacteria into either nitrate(NO3-) or nitrite (NO2-

How does carbon (C) come into play?

Lots of carbon   actually can be  found in  soi and comes mainly from the   remains of decomposing plant  cellulose. This carbon  soil is not used by living plants, but it is the bread and butter for bacteria.  These bacteria, however,  also use available N sources in the soil as they  breakdown cellulose and multiply.  In the short term high amounts of   C in soil can spur rapid bacterial growth subsequently depleting  the available N for  plants.  In the long term because bacteria incorporate relatively a lot of N into their proteins, when they die and their proteins decompose, soluble N will become available again to plants albeit maybe later in the growing season when plants may be in a different growth stage and need less N.

So how much carbon (C) is too much?

As you can see, the amount of carbon versus nitrogen in soil can become a seesaw-like activity.  The ratio of C to N can vary considerably  during a season. Because soil microorganisms usually become active before plants, they can get a head start on plants for available N. If the C/N ratio is high,  e.g. greater than 33:1, bacteria may easily out compete your raddishes. This is why gardeners and farmers use cover crops and  fertilizers in the spring - to ensure that there is enough N in the soil for young plants to thrive.  

• FWIW: Gardeners  know that  using sawdust as a mulch in a garden (its C/N ratio is around 400:1)  is a "No, No" because all of the available N might be taken up by feasting bacteria.  Gardeners using straw as a mulch (its C/N ratio is about 80:1) run a similar but reduced risk, although these gardeners are probably  also adding  compost  to their gardens to maintain an overall healthy C/N ratio. 

What is a good C/N Ratio?

Depends... there is  lattitude,  and the C/N ratio naturally fluctuates due to moisture levels, temperatures, soil characteristics and plant growing cycles during the course of a growing season.   Typically a ratio between 12 - 15:1 is considered good for vegetables. In that ballpark range bacteria populations seem to be relatively stable. There is enough available N   for bacteria to break down cellulose and for  plants to grow. On the other hand, a low C/N ratio  (e.g. 9 - 10) may not be necessarily bad. Decomposition will be slowed, but will continue at a steady rate assuming you have a soil with sufficient humus content.  

What about the C/N Ratio in my compost pile?

New green organic material like clover, coffee grounds,  grass clippings & other common yard/garden waste added to a compost pile  will moderately increase the C/N ratio. The ratios of these items are in the range between 20 - 30:1.  Weeds, vegetable scraps and hay   bring higher C/N ratios  in the range of 20 - 35:1 to a compost pile. By turning over your compost for aeration and keeping it moist, you can promote the nitrification process. You can also add manures (C/N 15:1 typically) to give it quick boost of N.  When decomposition is completed, your pile will eventually wind up with ratio of around 12:1.

There are methods to determine the C/N ratio of large compost piles. These require  that you know the quantities of the materials,  e.g. tonnage,  in the pile and their basic chemical compositions.     For the backyard gardener,  however,  the        fundamental    question simply is  "Are your plants looking healthy and green?"  If yes, no problem.  If no,  you have some investigating to do because it may not be just a whacky C/N ratio...

And remember...

"If you wish to grow anything, you must understand it and to understand it in a very real way." - Russell Page