Balancing C/N

Here’s a simple arithmetic problem that illustrates how to balance carbon to nitrogen.

QUESTION: I have 100 pounds of straw with a C/N of 66:1, how much chicken manure (C/N of 8:1) do I have to add to bring the total to an average C/N of 25:1.

ANSWER: There is 1 pound of nitrogen already in each 66 pounds of straw, so there are already about 1.5 pounds of N in 100 pounds of straw. 100 pounds of straw-compost at 25:1 would have about 4 pounds of nitrogen, so I need to add about 2.5 more pounds of N. Eight pounds of chicken manure contain 1 pound of N; 16 pounds have 2.  So, if I add 32 pounds of chicken manure to 100 pounds of straw, I will have 132 pounds of material containing about 5.5 pounds of N, a C/N of 132:5.5 or about 24:1.

It is far more sensible to learn from experience.  Gauge the proportions of materials going into a heap by the result.  If the pile gets really hot and stays that way for a few weeks before gradually cooling down then the C/N was more or less right.  If, after several turnings and reheatings, the material has not thoroughly decomposed, then the initial C/N was probably too high.  The words “thoroughly decomposed” mean here that there are no recognizable traces of the original materials in the heap and the compost is dark brown to black, crumbly, sweet smelling and most importantly, when worked into soil it provokes a marked growth response, similar to fertilizer.

If the pile did not initially heat very much or the heating stage was very brief, then the pile probably lacked nitrogen.  The solution for a nitrogen-deficient pile is to turn it, simultaneously blending in more nutrient-rich materials and probably a bit of water too.  After a few piles have been made novice composters will begin to get the same feel for their materials as bakers have for their flour, shortening, and yeast.

It is also possible to err on the opposite end of the scale and make a pile with too much nitrogen.  This heap will heat very rapidly, become as hot as the microbial population can tolerate, lose moisture very quickly, and probably smell of ammonia, indicating that valuable fixed nitrogen is escaping into the atmosphere.  When proteins decompose their nitrogen content is normally released as ammonia gas.  Most people have smelled small piles of spring grass clippings doing this very thing.  Ammonia is always created when proteins decompose in any heap at any C/N.  But a properly made compost pile does not permit this valuable nitrogen source to escape.

There are other bacteria commonly found in soil that uptake ammonia gas and change it to the nitrates that plants and soil life forms need to make other proteins.  These nitrification microorganisms are extremely efficient at reasonable temperatures but cannot survive the extreme high temperatures that a really hot pile can achieve.  They also live only in soil.  That is why it is very important to ensure that about 10 percent of a compost pile is soil and to coat the outside of a pile with a frosting of rich earth that is kept damp.  One other aspect of soil helps prevent ammonia loss.  Clay is capable of attracting and temporarily holding on to ammonia until it is nitrified by microorganisms.  Most soils contain significant amounts of clay.