Building Garden Soil With Wood Mulch

Learn why wood mulch may lower soil fertility in the short-term, and why its long-term benefits far outweigh this slight disadvantage.

  • building garden soil - wood mulch
    By providing food for fungi, wood mulch builds garden soil by increasing levels of organic matter in your garden beds.
  • mushrooms in wood chips
    One of the best wood decomposing fungi — king stropharia — produces edible mushrooms.
  • wood chipper
    If you have lots of trees to tend on your property, you may want to buy a chipper-shredder and make your own soil-building mulches.

  • building garden soil - wood mulch
  • mushrooms in wood chips
  • wood chipper

Most organic gardeners find that following nature’s patterns serves them well. When it comes to building richer soil, nature’s plan relies heavily on trees — fallen limbs, leaves, cones, seeds and, eventually, the massive trunks. Adapting this plan for building garden soil by using a wood mulch — such as wood chips, sawdust or other woody residues — is a strategy that promises huge, long-term returns.

Field studies dating back to the 1950s — and as recent as this year — suggest that a high-fiber diet of woody materials is exactly what many soils need. Rotted bits of wood persist as organic matter for a long time, enhancing the soil’s ability to retain nutrients and moisture, which results in bigger, better crops.

But wait: Woody materials are high in carbon and cellulose, so they need nitrogen and time in order to decompose. If you ignore these facts by mixing fresh sawdust or wood chips directly into your soil, the materials will bind up much of the soil’s nitrogen and render the spot useless for gardening for a season or two.

The outcome changes, however, if you add nitrogen or time. For example, when researchers planted a new organic apple orchard in northern Maine in 2005, fresh wood chips combined with blood meal (a very high-nitrogen organic material with a typical analysis of 12-0-0) and tilled into the top layer of the soil — plus a surface mulch of wood chips — proved better than three other treatments at promoting rapid tree growth. And, in less than two years, the organic matter content in the chip-amended plots went from near zero to 2 to 3 percent.

Sawdust has much more exposed surface area than wood chips do, so incorporating fresh sawdust into soil is not a good idea chemically (because of nitrogen tie-up) or physically (the mixture won’t hold water worth a flip). But sawdust makes a spectacular mulch for perennial crops. As long as you scatter a bit of organic fertilizer, poultry manure, or other nitrogen source over the surface each time you throw on a fresh layer, sawdust makes unsurpassed mulch for blueberries, strawberries, and raspberries, and it can work well with asparagus, too.

Garden paths paved with sawdust-covered newspapers feel like carpet underfoot. After it has rotted, sawdust contributes mightily to soil’s texture, because the spongy tidbits persist in the soil for a long time. The concern that woody amendments acidify soil is a myth. Only in the early stages of decomposition is there a fast flush of acids, when cellulose fibers begin to degrade. Long-term studies of the effects of wood chips and sawdust in soil actually show a slight rise in soil pH, which is good news for most crops in most gardens. (The lower the pH, the more acidic the soil.)

1/5/2020 1:22:52 PM

During November 2018 I had 2 clearings of an acre created with a large mulching grinder on my 80 acres of mostly 20 year old regenerating aspen In Northern MN. My forestry plan recommended it and I needed a couple more openings in heavy tree cover. Spring of 2019 I broadcast a mix of clover and bluegrass and nitrogen over these areas and lightly dragged a piece of chainlink over it. By September the clover and bluegrass was getting established and spreading over large patches. Spring 2020 I will overseed and fertilize to establish heavier growth. The clover and bluegrass are doing very well and thrive in the wood chips that the land clearing created. Thank you for a informative article.

1/15/2019 12:12:06 AM

Good idea by adding sawdust to the soil, but why wait for it be usefeul, cannot it be added to your compost bin/pile and achieve a better and faster result? Ken

10/3/2018 1:31:18 PM

I have a 80ft pine tree that has to be cut down. I would like to make that area into a perrennial garden. What do I need to do with the saw dust that is left when the stump is ground out. Totally move as much as possible or add something to it



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