Giving farming advice while sitting at a desk can be a perilous endeavor. I know, because I do it all the time. To keep from screwing things up I rely on a lot of other people to share their knowledge and information with me, so I can pass it on. That includes farmers, researchers, Extension personnel, and those special folks that do some of each.
The following information comes from one of those people, Brian Caldwell. He’s been an organic farmer for many years, worked in Extension, then as organic farming educator, and now he’s a technician on an organic farming study at Cornell University. We share an interest in long-term soil health, along with many of you.
The problem. Annual cultivation of vegetable crops tends to beat up the soil. It typically involves intensive tillage, mechanical cultivation, exposure to the sun and rain, and plenty of traffic from people and equipment. These things can destroy soil organic matter and soil structure while increasing soil compaction. That in turn reduces yield over the long run because it suppresses root growth and soil biological activity.
The solutions. Farmers know they have to address this problem, and they do it in one or more ways, by adding compost, manure and mulches, reducing the intensity of tillage; growing cover crops; and restricting most traffic to grass aisles or drive lanes.
Add one to the list? There may be another practice that can help maintain soil quality despite intensive production: adding wood chips. From 1951 to 1965 an experiment was carried out on a Soil Conservation Service research farm in Marcellus, NY. The project is written up in a 1971 Cornell bulletin called, “Soil Management for Vegetable Production on Honeoye Soil with Special Reference to the Use of Hardwood Chips” by G. R. Free. Over 15 years this study used a 5-year vegetable rotation of sweet corn, beans, tomatoes, cabbage, and peas. It compared 14 different treatments, including several in which 10 tons per acre fresh weight (or 7 tons dry weight) of wood chips were added each year.
Other treatments looked at using overwintered ryegrass or bromegrass cover crops, and more extensive rotations in which legume sod hay crops were substituted for the beans and tomatoes. The hay crops were harvested and removed, not plowed under. The crops were fertilized with chemical fertilizers and probably sprayed for pests and weeds using conventional materials of the time. Crops were also mechanically cultivated for weed
control.
The results. Yields of most crops were improved with the addition of wood chips and were best when the chips were top dressed on the soil surface after the crops were planted, instead of being plowed under. Over the years, soil organic matter (SOM) and nitrogen increased in the chip-amended plots, while they dropped slightly in the chip-free plots without cover crops. Including yearly grass cover crops allowed soil organic matter and soil nitrogen to stay at about an even level over the 15 years.
Important findings. Stated simply, adding 10 tons/acre of wood chips each year did more to maintain soil quality than adding grass cover crops or resting the soil with harvested alfalfa sod hay crops. However, there has been little follow-up on these findings. Perhaps the expense of the wood chips discouraged their use by farmers, and therefore researchers, in the years after this study. Nonetheless, it seems that such positive benefits should have stimulated more research and wider awareness.
More recent research on wood chips has been done in Quebec, though with a different emphasis. While the Marcellus project used chips from stems up to 6” in diameter, the Quebec research focused on hardwood chips less than 3” in diameter, called “ramial”. The percentage of nutrient-rich bark and buds is much higher if the branch diameter is held to less than 3”. In practical terms, the volume of chips produced is also greatly reduced, but the wood over 3” in diameter can be used for firewood.
The Quebec researchers feel that smaller diameter wood also contains less developed lignin that in turn is converted by fungi into long-lived humus in the soil. They also obtained positive results on both crop yields and soil quality using ramial applications. A drawback of ramial is that it is hard to get.
How to try it. Combining the results from these two studies, the ideal way to use wood chips for vegetable production appears be to topdress 5 to 10 tons of remial chips per year, worked into the soil surface during cultivation. If an area is to be bare over winter, chips could be spread after harvest to protect the soil before being tilled in the spring. Hardwoods chips are preferred over those from coniferous species. Wood chips that heat up and partially decompose can produce volatile organic compounds that inhibit seed germination and plant growth. These should be avoided; chips should either be used fresh or after they cool down.
Avoiding N tie-up. The carbon/nitrogen ratio of chips is rather high, about 200:1 or so. If chips are worked into the soil, they will tie up some nitrogen as they decompose. However, the effect is far less than it would be for the same amount of sawdust, since the surface area for sawdust to interact with the soil is hundreds of times higher than for chips. In fact, nitrogen tie-up was not seen in the Marcellus study, which surprised the researchers. Since the study was done on a rich soil relatively high in soil organic matter, enough nitrogen may have been released to overcome any tieup that occurred.
Try it, but carefully. Take a very small area of a relatively vigorous row-crop (not onions or carrots, for example) and apply wood chips as a surface mulch, putting them on in between rows shortly after planting or transplanting. The application will be pretty thin – 10 tons of chips per acre amounts to only about half a pound per square foot. Try one area without added N and one with a small amount of fertilizer added to see if there is any difference. You should be able to cultivate for weeds OK right through the chips in crops that allow for use of shovels, sweeps or other aggressive tools.
Another relatively safe way to use chips is to spread them on legume crops such as alfalfa or vetch before plowing them under. Since these crops release an abundance of N, any tie-up by the wood chips can even be beneficial, as some N will be held and released slowly over time. Again, start with a small area, mark it clearly, and observe subsequent crop growth for signs of N deficiency.
After conducting on-farm trials on a small scale, you can get a sense of whether extra nitrogen needs to be added with wood chips on your soil for good crop growth. Then you can gradually try applying chips to larger areas in pursuit of long-term soil benefits.
July 2007
Vern Grubinger, Vegetable and Berry Specialist
University of Vermont Extension