Wood Composting

Advice from the Wood Doctor on composting chips, shavings, or sawdust for use in landscaping or gardening. January 30, 2013

From Gene Wengert, President, The Wood Doctor’s Rx, LLC

Fresh wood chips or other fine wood "waste" can look attractive in a garden. In fact, you can buy wood chips that are colored--red, green, blue, brown--or you can get wood from a sawmill by the truck load. Two problems when using fresh wood:
1) danger from natural herbicides in the wood
2) nitrogen starvation in adjacent plants.

Herbicides Some wood species have natural herbicides, insecticides and fungicides. The most notable herbicide is walnut... wood and bark, and even leaves. Fungicide and insecticide species include cypress, cedar, and white oak. Wood from these species, fresh or composted, will act as a fungicide or insecticide for years.

Nitrogen Starvation As wood decomposes, decay activity will immediately tie up all the available nitrogen. Any nearby plants will suffer nitrogen starvation (yellows; slow growth; etc.) So, never use fresh wood around plants that you want to keep alive and healthy.

Why compost? To address this nitrogen problem in garden mulches, compost wood chips first before using for mulch. Prior composting means that the wood will no longer absorb nitrogen (time frame is about six warm months). Compost wood in a separate compost pile that is at least four feet from any desirable plants. Follow normal composting procedures (moist but not soaking wet; aerate often; etc.). We do not want anaerobic decomposition (end result is pH = 3; byproducts are bad), so aerate often. Add nitrogen so that the composting occurs at a good rate. Add heat too?

Adding nitrogen to the compost Add a few pounds of blood meal (which is essentially pure nitrogen, 12-0-0) or add fresh grass clippings weekly; try for 50% brown and 50% green. Chicken manure is fine too. An alternative is 1 pound of ammonium nitrate per 20 pounds of wood. If properly and fully composted, it will no longer look like wood.

Wood chips have a C:N (carbon to nitrogen) ratio of around 500; Newspaper, 700; Leaves (in the fall), 50; Manure, 10; and Grass clippings 20.

As decomposition continues when the compost is used in the garden as a mulch or amendment, the wood will, over coming months, release the nitrogen it has tied up.

Sawdust With more surface area, sawdust will tie up nitrogen more rapidly, and will also decay faster than chips--short benefit to the garden. Further, sawdust does not hold water well. Generally, avoid sawdust.

Sour Mulch A landscaper delivered a load of mulch to a residential client on Saturday. On Sunday, the homeowner spread the mulch around plants. On Monday, tulips, phlox, violets and astilbe had turned white as though bleach had been put on the leaves.

Here is what happened: The wood mulch had been composted in a large pile (over 10 feet high) that was not turned frequently. Therefore, the wood material on the inside had a high temperature and decomposed (fermented) anaerobically, producing low molecular weight fatty acids, including acetic, propionic and butyric acids. The pH of the mulch was lowered (4.6 to about 2.6) creating the so-called "sour mulch." Such mulch has a pungent odor.

Extremely heavy water can leach out the undesired chemicals and likely make the mulch usable again, but turning the pile or using piles no higher than four feet is appropriate when composting wood.

Wood Used For Mulch and Soil Amendments Mulch--Definition: Mulch is a protective covering (as of sawdust, compost, or paper) spread or left on top of the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, prevent crusting, or keep fruit (as strawberries) clean. This layer of composted wood, not fresh wood, should be 2” to 6” thick.

As mentioned, sawdust does not make a good mulch due to the high surface area to volume. Plus sawdust can easily compact to a dense layer and water holding is poor. So, it is best to avoid fines.

Mulching Trees Use no more than 2” of composted wood chips. Add a bit of nitrogen if not fully composted prior to use. Chopped leaves or pine needles are often better for trees. Apply in spring after soil is warm. No "mulch volcanoes." Keep mulch from tree stem to avoid risk of decay of tree bark itself.

Garden Pathway Mulch--No Composting Required A great spot to use the "green" wood chips is in pathways or other areas where you don't plan on planting anything (nitrogen starvation is ok). Fresh wood serves a twofold purpose:

1. It's a great mulching/composting material

2. It serves as an herbicide of sorts, preventing weeds from sprouting
For pathways, the wood layer is usually 4 to 6” thick.

Soil Amendment--Definition Nature, in order to build richer soil, relies heavily on wood mulches--fallen limbs, leaves, cones, seeds and, eventually, the tree stem itself. Adapt this plan to your garden by using a composted wood mulch that is covered and mixed with the soil; three steps are compost, mulch and then amend. Using wood mulch as soil-building material (soil amendment) is a strategy that promises huge, immediate and long-term returns.

A soil amendment changes one or more properties of the soil and thereby makes dirt:
improve physical properties (texture/tilth/structure) (Wood = A+)
increase water holding capacity (Wood = A+)
increase nutrient-holding capacity
improve aeration (Wood = A+)
improve water infiltration (percolation)
improve cation exchange capacity (CEC)
adjust pH (Wood increases the acidity)
add nutrient/mineral values for plant growth (Wood = ok)
suppress soil microbes that could harm plants
increase the beneficial soil microbe community (Wood = A+)

Note: Wood is made mainly of carbon, hydrogen and oxygen. However, there are trace elements in wood, even in wood ash, including calcium, magnesium, phosphorus, potassium, sulfur, aluminum, iron, manganese, zinc, boron, chromium, copper, nickel, cadmium, cobalt, and lead. For best nutrient addition, add organic wastes, or plant a legume cover crop.

For additional info, see Building Soil Organic Matter with Organic Amendments, from the UW-Madison, College of Agriculture at www.cias.wisc.edu/wp-content/uploads/2008/07/soilorgmtr.pdf

Presentation to Fitchburg Gardeners, November 12, 2012
Gene Wengert, President, The Wood Doctor’s Rx, LLC