I grew up being warned about using fresh wood products as mulch or soil amendments because, in theory, as the wood broke down it would tie up valuable nitrogen, stealing it away from the plants. More recent conclusions based on old and new field research- and practical experience- suggests otherwise. I have found that wood chips make an excellent soil conditioner and weed suppressant (almost more critical this year).
Soil health should be the primary concern in horticultural and agricultural fields. In the vegetable garden fertility and soil health are paramount if you want healthy plants and a bountiful harvest. The vegetable garden is also where we see the most abuse being done to the soil. Annual tillage, regular cultivation, exposure, human and equipment traffic all take a toll on soil health. Little to no microbial life is left in soil that has been beaten, churned, compacted and eroded.
Conventional wisdom about regular cultivation of the soil is being replaced by a “tread lightly” mentality. Reduce the amount of soil disturbance, add composted organic matter, plant cover crops, hand turn soil instead of mechanical tilling, and mulch traffic areas. All of these practices make sense but how do they relate to the use of wood chips in the garden?
Using woodchips on or in soil that already has a healthy amount of organic matter present, like composted manure; or soil that has a legume cover crop planted provides the extra nitrogen to allow the natural decomposition of the chips without depleting the soil of necessary nitrogen to feed your plants.
Reviews of two studies on the soil building power of wood chips support the efficacy of wood chips. One study, conducted between the years of 1951-1965, showed that crop yield was improved in the beds where wood chips were used as surface mulch after the crops were planted. This 15-year study saw improved soil health each year as the wood chips decayed. There was an increase in organic matter and nitrogen availability each year. More recently research being done in Quebec supports the use of wood chips in the vegetable garden with additional focus placed on the diameter of the chip. Apparently, size does matter in the case of wood chips.
Sawdust is definitely out, too much surface area which ties up too much nitrogen as it breaks done. So, is the bulkier the chip the better chip? Well, there is a happy medium. Woods chips with a stem diameter of less than 3 inches seem to be more nutrient rich than those up to 6 inches in diameter. These measurements are pre-chipper. The Quebec contingent believes that the nutrients in the bark and buds are greater in the 3-inch diameter stem. In simple terms the smaller stems convert to humus in the soil, which is desirable.
Most wood chipped up in a chipper is a combination of big and small so it is difficult to know exactly what you are getting but the point is that we should not be skeptical about using wood chips. Just be sure to provide some additional nitrogen for it to use as it continues to break down. Put down some composted horse manure or plant a legume cover crop like alfalfa before dumping your chips on top (after the cover crop is mature and turned under, of course).
For the home vegetable garden wood chips can help improve the soil, cut down on weeds, reduce erosion and splashing and create a nice, clean walkway between and around your planted rows. These are all the things we are supposed to do anyway when tending to a healthy garden.