Anaerobic Decomposition


Fill one raised bed with animal manure and bedding mined from a foot-deep deposit in an old barn, hire two guys to walk on it for about a month to press out all the air, and what do you get when you dig it up?

A smell only a dog could love–equal parts old-fashioned floor-cleaner and soiled diaper sitting in a hot car.

I’m really glad my neighbors on either side have their houses shut up and their air conditioning going because my backyard is officially reeky.  I cleared the paint chips from the back part of the raised bed I built last fall and then started digging up the soil that had been seriously compacted during the house-painting process.

That “soil” was mostly the aforementioned mined brown gold, which didn’t smell bad when I got it–sure a little bit of ammonia odor, but it was mostly absorbed by the bedding contained in it, and the top part dried out pretty quickly and became completely odorless.  But once all the air got pressed out of it, the whole thing under that top crust turned into an anaerobic ammonia pancake.

I dug it out in three sections down to the landscape fabric I’d laid beneath. It was so tromped down, the texture of the fabric was imprinted on the underside of the big chunks I carved out with my shovel. Once I dug each section, I emptied a bag of hedge trimmings I offered to take off my neighbor’s hands (and keep out of the dump).

I’m hoping the carbon in those stems will serve to sequester some of the excess nitrogen contained in the manure/bedding mix. After the hedge trimmings, which also added loft (air–also good to avoid anaerobic stinkiness), I shoveled out the partially-decomposed contents of my compost barrel over the twigs, then pitched the manure chunks back on top, which broke them up at least partially on impact.

Out in the gardens, I often have more carbonaceous stemmy materials (from fall and spring clean-up) than nitrogenous green stuff–though the weeds I’m pulling are certainly helping bring that into balance.  Here in town, I’ve generally got plenty of nitrogen from all the kitchen scraps, but less abundant carbon unless it’s fall and I’m collecting leaves.

In Gardening When It Counts, Steve Solomon points out that the ideal C/N ratio (that of humus) is 12:1, and claims that spreading or composting with a lower ratio will burn off nutrients and kill microorganisms.

This is same thing that happens when you add nitrates as chemical fertilizer– it causes fast but weak growth (more susceptible to insect predation and disease) plus a lower overall nutrient value in the crop yield.

A higher C/N ratio takes longer to decompose, and the nitrogen is tied up as soil microorganisms need it to break down the carbon in order to reach that 12:1 balance. That’s why using wood chips (very high carbon) on paths works well to suppress weeds, but you should never use wood chips to mulch your vegetables because they simply won’t grow well, or sometimes at all.

Considering the low C/N ratio of the manure/urine-soaked bedding combo, I’m trying to add a little more carbon to my mix with the twigs in order to keep the nutrients in the bed from burning off as well as protecting my little buddies in the soil.

I’m tempted to mix in a little half-rotten straw, but I might not, as I’ve been tearing up lightweight brown cardboard cereal and pasta boxes and adding those to the contents of the compost barrel that’s now spread in the bed.

One good sign I noticed as I was carving out wedges of compacted manure and bedding was a great population of earthworms big and small–including some real lunkers. I haven’t seen earthworms that big in a long time.

They were happily tunneling their way through all that good stuff–now they’ll have even more tasty treats to make into good humus.

At any rate, that part of the raised bed, which wasn’t planted because it really wasn’t even close to full, will have to sit and get worked on by all the worms and microorganisms until next spring. The only thing I can foresee doing (if I can get my hands on any) is to throw a layer of topsoil over all the composting goodies.

It’ll be like icing on the cake!

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2 responses to this post.

  1. Have you ever tried some home made charcoal for a garden bed?
    I dug some into my plot last fall and have gotten very good results. It’s also kept the stink down in my anaerobic compost pile while soaking up some of the liquid that would normally just drain out the bottom.
    We had a few friends over last weekend for a little firepit s’more party and ended up with a lot of charcoal left over from the burning – I guess it wasn’t getting enough oxygen to completely burn.
    After I noticed that, I took the pile of trimmed branches I was planning to take to the dump and burned them down to charcoal too. Should have enough to spread a good amount on the garden this fall.

  2. Posted by flyingtomato on July 7, 2009 at 6:14 pm

    S.D.–

    I am a little wary of charcoal (and wood ash) in the beds themselves, as they are very basic (as opposed to acidic), and our soils in eastern SoDak are already pretty basic (unless, perhaps, you have a lot of oaks or evergreens).

    But charcoal would seem like a good addition to an anaerobic compost pile. Wood ash+(rain)water=lye–caustic stuff.

    But homemade charcoal might not be as problematic, though I am noticing the squash I planted in a place where there was a wooden greenhouse that burned (and I can still see charcoal in some places) is not doing quite so well as the squash planted elsewhere. It’s possible that’s due to other factors.

    I have also seen a number of garden writers recommend sprinkling wood ash around the base of plants that are susceptible to root maggots (esp. brassicas); however, I have not thusfar had a problem with them.

    I usually use the wood ash from my fireplace as a way to keep down weeds in informal pathways, and sometimes add it to the compost pile in small amounts.

    In the end, what works in your garden is what works!

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