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

Sun Scorch and Trench Composting

Sun scorch. It's okay to eat with a little trimming.
Source: gardeningthehudsonvalley.com

So far August's weather leaves a lot to be desired - way too much rain and  very hot streaks followed by cooler and wet nights.  Although this has been good for my compost piles, it has badly affected my tomatoes and a number of other veggie favorites although the winter squashes seem to like it. 

The excess moisture results in cracked tomatoes that are then exposed to bacterial infections.   Although  tomato vines grow okay in temperatures that reach into the low 90°s,  their fruit ripens best with temperatures in the 70°s! If you've been wondering why your fruit has been slow to ripen, that's the reason.  Want a little more info? Check this note from Cornell's Vegetable Program website.

If you have over pruned your tomato vines or removed too many leaf stems to let more sun light shine on the fruit to hasten ripening, SURPRISE, you may be creating the conditions for "sun scorch."  High temperatures actually inhibit the production of the pigments that provide the glorious tomato red gardeners look for... Oh, well.

Trench Composting

A full trench ready to be covered.
Source: flowering.com

If you have a small garden AND possibly have some space in it now from your pulling up your blighted tomato vines or sun scorched fruit AND you still do not have a compost pile, now is the time to reserve a little of your precious  gardening space for compost.  Dig a trench about 12" deep (or just a small pit in a raised bed) leaving the excavated soil to one side and start dumping your vegetative table scraps into it. If you don't like the looks, simply cover the scraps with the soil that you left  along the side.  The  scraps will get a good start on decomposing.  In the spring, even if your trench is not full, plant a favorite vegetable parallel to the trench to benefit from the added nutrients.  

Another approach to composting directly in your garden is called the "sheet" method.  Gardeners simply scatter their table scraps around the garden and then gently till them into the soil throughout the season. I do this with coffee grounds,  but I prefer trenching (or using a pit)...  

FWIW:  Note that this photo shows corn cobs and corn husks as well as other whole fruits and vegetables. Leaving items whole will slow down their decomposition.  Corn cobs and husks will take 2+ years to rot. It's much better to to cut these scraps or stale vegetables into pieces to expedite composting.

And remember:

"Kings and cabbages go back to compost, 

but good deeds stay green forever." - Rick DiMarinis, American author

What the world needs now

What the World Needs Now? Compost, Sweet Compost

Compost is decomposed organic matter from plant or animal sources.  It’s the stuff of magic, both loosening the soil and increasing drainage while simultaneously increasing water retention.  It feeds micro-organisms – those billions of fungi, bacteria, and unknown critters found in each teaspoon of soil – as well as provides nutrients to plants.  Most soils are severely deficient in organic matter.  While Dionne Warwick reminded us the world needs love, sweet love, more organic matter wouldn’t hurt, either.

How can you find good compost?  Unfortunately, there are no mandatory standards in place to help us.  Bagged materials may be labeled as cow manure or yard waste (dead leaves, lawn clippings, etc.), but indicating the parent material on the label is not required.  Often, packaged composts contain human biosolids – a nice way to say sludge – from a waste treatment plant.  While most of these products are probably okay for landscape use (excluding food crops), with so little information, their use is a crap shoot.

I like local sources of compost.  Take a trip to the pile and have a gander and sniff before requesting five yards be dumped in your driveway.  Compost should be largely free of debris, including trash, glass, rocks, twigs, wood chips and plastic.  It should have a dark brown color with a pleasant, earthy aroma.  If it’s too stinky, it either isn’t finished (and may harm your plants) or something has gone terribly wrong.  For some reason, many folks believe that compost is acidic and requires lime, while in reality most finished compost is near neutral and requires no adjustment.  And the poop on parent material is that if it is still identifiable, the compost isn’t properly aged.  Horse muffins or cow pies are fine, they just need more time to mellow. 

The best compost, like the best spaghetti sauce, is made at home, and the two share some similarities.  To the compost pile you can add the things at hand:  dead leaves, grass clippings, weeds (those without seeds), various trimmings, vegetable scraps, egg shells, coffee grounds.  Keep out meat, bones, grease and pet doo due to concerns with attracting pests and promoting diseases.  The pile should be roughly 3 feet tall, high and wide, although you can go up to five feet.  Put it somewhere secluded and give it time, as most home compost piles are cold and take years to decompose.  Eventually, you’ll find “black gold” at the bottom, ready-to-use.  Don’t ask for my gravy recipe – I compost much better than cook.   

David Chinery is the Senior Resource Educator for Horticulture & Turf Management for Rensselaer County Cooperative Extension.