Building Garden Soil With Wood Mulch

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.

| October/November 2010

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.)

Lee Swerdlov
3/24/2015 4:27:02 PM

Tree mulch is the secret to life!! Leaves , branches, trunks it's all good. Pile it on thick 3-6 inches is great. Don't use wood chips, use shredded wood mulch like from a tree trimmer. Wood chips take too long to break down as the bugs don't like it as much. The authors concern with adding extra nitrogen is incorrect in my opinion. If you add it to the top without tilling there is no need for extra nitrogen. This is by far the better way of doing it. Tilling is both unnecessary and counterproductive. Counterproductive because if you examine any compost pile you will see many many organisms that are actively eating the mulch. In my area I see many millipedes that love to eat the wood particles, and they leave tons of castings behind ( free fertilizer!!), and then these organisms soon die and the nutrients used by their bodies are released and readily used by your plants. The mulch will break down in 3 months, in warm weather, and you have the most wonderful soil. Just remember to always, always keep the soil covered with mulch, and moist, or all the little critters will leave and your wonderful soil will turn to sand in very short order. The organic matter will get used up leaving only inorganic sand. Here's the secret: the mulch is the food. Mulch feeds the bugs, the bugs feed the plants. Water the soil, not the plants, for the same reason. Keep the soil moist and covered and earthworms will soon move in- that's when you will know for sure that you have done your job as the grower. Soon you will not have any need for organic fertilizer . Never, never use chemical ferts, as they will immediately kill all your good bugs turning your soil to sand again. Finally, nix the sawdust, not sure why but bugs don't like it, and it needs to be composted before adding to the soil. You must add nitrogen to it as well, or it will just sit there. Summing it up, tree mulch: keeps the weeds down,conserves water, provides fertilizer, and creates beautiful soil.

6/17/2014 5:22:03 PM

have wood chips...if not growing veggies, do I have to add anything before putting chips down ? also, have peat moss and don not know what to do with it !

5/6/2014 2:07:11 PM

Last year i bought new organic dirt and it seemed more like mulch than soil - the plants did very poorly. I amptied out my container this year and bought new dirt and it looks the same - more like mulch than soil. Am I wasting my time - I can't see inside the bag before I purchase them.

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