(January. 2004)

It’s no wonder most gardeners seldom succeed at backyard composting and, if they do, the process seems to take nothing short of an eternity. How come? Gardeners do everything right up to a point, and that point is creating the very foundation upon which composting happens. Ninety-nine percent of gardeners never knew or were aware of this “foundation,” so let’s take a moment to share the essential information with you. Once you know why composting succeeds, and why it doesn’t, you will hit the target every time you want to make humus from autumn leaves, grass clippings and organic residue from the kitchen.

Composting only works when the proper Carbon-Nitrogen ratio is in place.

You could easily make a fetish of this, but, really, it’s not as complex as it seems. The more you know about it, the easier it gets.

First, we begin with an acronym for this carbon-nitrogen ratio. From now on, you want to call it the C/N ratio. Forget everything except for the C/N ratio. This ratio is going to decide if any organic matter is going to decompose in the first place. This ratio will also decide how fast organic matter will decay to humus. It doesn’t make any difference whether you’re managing a massive compost pile in the backyard, if you’re composting in one of the fancy “designer bins,” or if you’re simply using black plastic trash can liners. No matter how you compost, this C/N ratio governs the process.

The C/N ratio represents the percentage of carbon to nitrogen in the organic matter you are trying to compost, namely change leaves, grass clippings and food residue into rich, organic humus.
Every time you compost, the C/N ratio changes because the organic matter always changes. Some organic matter holds unbelievable amounts of carbon, others have little or no carbon.

Why are carbon and nitrogen essential for composting? Carbon provides the energy for bacteria (microorganisms) to multiply in the soil, nitrogen supplies the food for bacteria to create proteins and cells in the composting process.

In almost every situation, there needs to be some adjusting of the C/N ratio. In most soils, the C/N ratio of organic matter varies from 8:1 to 15:1. When considering the C/N ratio in bacteria, it varies from 4:1 to 9:1.

Why is this C/N ratio so critical? Spectacular composting happens when the C/N ratio is around 20:1. This means there is 20 times the carbon in the organic matter being composted than there is nitrogen. If the percentage of carbon is greater than 20, the composting process will take an extended period of time. If the carbon ratio is less than 20, composting may never happen. If you’re thinking of modifying the C/N ratio, you can by simply adding more nitrogen to bring the C/N ratio closer to 20:1.

Let’s examine some organic products to define their C/N ratios.

Fresh cow manure. In its “fresh” state, it has a C/N ratio of 10:1. This means there is 10 times the carbon in fresh cow manure than there is nitrogen. However, that’s not all. Fresh cow manure is never used due to its high concentration of nitrogen. Manure is left atop the soil to oxidize and leach nitrogen over a period of months (usually the winter). As nitrogen leaches and evaporates from the manure, the C/N ratio gradually increases from 10:1 to a perfect 20:1. For this reason, we know that rotted cow manure has a 20:1 C/N ratio (the nitrogen has decreased while the percentage of carbon has remained the same).

Autumn leaves. If we compost fallen leaves, even if we shred them, the C/N ratio is extremely high due to an abundance of carbon in the foliage. In the early phase of composting, aerobic bacteria work hard at trying to stabilize this ratio. Bacteria attack the carbon and vaporize it as CO(2), carbon dioxide. Meanwhile, the nitrogen is retained by bacteria. If you were to add any organic matter to the compost pile, anything that contains good amounts of nitrogen (like fresh cow manure or grass clippings), the added nitrogen will accelerate the decay of tree leaves. Also, there is an extra dividend here because the final compost (humus) will have a higher nitrogen content because you added cow manure or grass clippings in the composting process. This explains why we always add inorganic 5-10-5 fertilizer when we are composting autumn leaves.

Evergreen needles. If you’ve tried to decompose needles, you know it’s pretty much been a hopeless cause, but why? Needles contain huge amounts of carbon, so you’re looking at C/N ratios like 80:1 and 100:1. This explains why pine and spruce needles take all of five summers to decay to humus, but you can work miracles by adding copious amounts of 5-10-5 inorganic fertilizer to bring the C/N ratio closer to 25:1 and 20:1. When you do, needles will decay almost as fast as lightning.

If you have questions about composting and the C/N ratio, let us try to anticipate them.

* When happens if the C/N ratio is low, like below 20:1? If you come up with a C/N ratio of 15:1 or 10:1, it means there isn’t enough carbon in the organic matter in the compost pile. The result is something you didn’t anticipate. Bacteria can’t access nitrogen in the pile because there just isn’t enough carbon to make bacteria multiply. The liability here is that any nitrogen in the pile evaporates as ammonia, and any eventual humus won’t have any nitrogen to pass on to plants.

* What happens if the C/N ratio is high, like 80:1? Translated, it means there is an excessive amount of carbon in the pile. So, nitrogen is used to continue the composting process, and almost no residue of nitrogen is available in the humus at the end of the decay cycle.

If you compost over late spring and summer, make use of kitchen waste in the pile. Some good reasons for this:

* Banana skins contain no nitrogen, but have 42% potassium which is added to the end humus;
* Cantaloupe rinds are a good source of potassium, almost 12%;
* Coffee grounds average out about 2% nitrogen whether they are wet or dry;
* Cucumber skins average about 27% potassium, good for the compost heap;
* Orange skins contain 27% potassium, all this winding up in the humus;
* Finally, lemon skins contain 31% potassium, even better than oranges.