James,
You are correct about the risks of only using a wood mulch in gardening. In his book Gardening When It Counts, Steve Solomon discusses the normal carbon/nitrogen ratio in soil — 12:1 — and compares that to various fertilizers. Woody products such as tree bark can have C/N ratios in excess of 100:1, causing "nitrogen robbing": the nitrogen already in the soil is retained by soil microbes until the wood decomposes and the excess carbon is burnt off, leaving even less nitrogen for the plants in the meantime. As you mention, it can take years for wood to decompose, leaving the garden starved of nutrients in the meantime.
Interestingly, the author of the "Back to Eden" does not use wood mulch as his only fertilizer — he also uses chicken manure. Though he emphasizes the wood mulch as the key to his success, I suspect that the chicken manure plays a much bigger role in fertilizing his garden, because its C/N ratio is around 6:1 — it provides excess nitrogen, which helps counterbalance the high carbon levels in the wood. – Nate in Pennsylvania
Dear James,
I have enjoyed reading you blog nearly everyday for over three years. I have learned so much.
Wood chip use as referenced in the Back to Eden Film are to be placed on top of the soil [JWR Adds: and removed in the Spring.]. Do not incorporate them into the soil as that is when they bind nitrogen. Until the wood chips break down, the chips are to be pushed aside and the seeds planted directly into the soil below. As the plants mature the wood chips can be tucked in around the plants to hold moisture and prevent weed growth. As the wood chips break down, they start providing nutrients to the soil below and when fully decomposed planting can occur directly into the chip mulch.
I started with tilled soil two years ago and fought the weeds and grass. Last fall I put down a layer of maple leaves, composted chicken manure and then 4-5 inches of fresh wood chips (mostly fir). I am thrilled not to be fighting the grass and weeds. My plants grew well with no indication of nitrogen depletion in the soil. I also highly recommend watching the Back to Eden film. – M.R.N. in the foothills of the Central Cascades of Washington