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Perfect compost

Steve Solomon's compost pileDespite relying on complete organic fertilizer as his primary source of soil nutrients, Steve Solomon is clearly an expert compost-maker.  For those of us who use a slapdash approach to composting, Solomon's composting description in Gardening When It Counts is a must-read.  He explains that a good compost has more than 1.5% nitrogen, a C:N ratio no higher than 15:1, and lots of micronutrients.

The carbon to nitrogen ratio, often shortened to "C:N" or "C/N" is at the heart of building a perfect compost heap.  The availability of nitrogen in the compost is largely determined by this ratio, which can be thought of as a bit like the pH scale.  A C:N ratio of 12:1 (twelve parts carbon to one part nitrogen) is "neutral" --- this is the C:N ratio of soil humus and is relatively stable.  Most plant matter has a C:N ratio higher than 12:1, which means that decomposing microorganisms require a net input of nitrogen while they're breaking down the first of the carbon --- this is why putting fresh wood chips on your garden does more harm than good, locking up the soil nitrogen for the first few years before the chips break down (and their C:N ratio drops low enough) so that they begin releasing nitrogen.  On the other end of the spectrum, chicken manure has a C:N ratio of about 6:1, which means that the manure provides nitrogen to the soil, and actually breaks down soil humus in the process.  The table below lists the C:N ratios of various compostables.  For immediate use on your garden, you can mix and match until the C:N ratio of your pile is around 15:1.

6:1 or lower
12:1
25:1
50:1
more than 100:1
Meat scraps

Chicken manure

Hair/feathers

Urine (0.8:1)
Vegetables

Garden weeds

Horse manure (no bedding)

Cow manure (no bedding)

Spring grass

Garden soil

Comfrey leaves

Coffee grounds
Summer grass

Legume hulls

Fruit waste

Grass hay (green)
Cornstalks (dry)

Straw (cereal)

Grass hay (poor)

Corrugated cardboard

Tree leaves (deciduous)

Autumn grass
Sawdust (500:1)

Paper (175:1)

Tree bark

Pine needles

Steve Solomon's gardenLuckily for those of us with large masses of high C:N waste, a compost pile's C:N ratio will slowly drop over time as microorganisms break down the carbon-based compounds and make nitrogen more available.  If you mix an incipient compost with a C:N ratio of 30:1 into warm soil, for example, it will break down in about six weeks.  Compost grinders and tumblers will expedite the process, but your compost will overheat in the process and you will lose a lot of your nutrients as gases like methane, ammonia, and carbon dioxide.  Steve Solomon instead recommends composting slowly, starting a pile in the fall, turning it when the apples bloom, and then turning it again a month later if it's not done.

One of my favorite things about Steve Solomon's book is that it's full of numbers and formulas.  Here's his favorite recipe for compost:
  1. Spread garden and kitchen waste in a pile 5 to 7 feet across, five feet long, and eight inches thick.
  2. Add half an inch of good garden soil (to inoculate microorganisms.)
  3. Add a low C:N component --- 5 to 10% of the mass of layer 1 if you're using poultry manure or cow or horse manure with no bedding, or 2 to 5% if you're using seed meal.
  4. Water well.
  5. Repeat steps 1 through 4 until the pile is 4 to 6 feet tall.
  6. Cover the pile with a thin layer of soil.
Steve Solomon notes that his garden and kitchen waste only produces enough compost to cover a third of his beds to one quarter inch deep, so he has to compost a neighbor's cow manure too.  We're currently on a compost quest, so his suggestions of adding in lawn clippings, autumn leaves, and comfrey leaves were much appreciated.

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This post is part of our Gardening When It Counts lunchtime series.  Read all of the entries:





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