In the short term (two years), wood mulch can potentially lower soil fertility, but in the long term their value in building garden soil is beyond question.
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.)