When filling a new raised bed, gardeners often look for ways to reduce the cost and volume of expensive topsoil. Using organic material, or “filler,” in the base is a popular technique for saving money and utilizing yard waste. Since wood chips are frequently available and inexpensive, the core question is whether it is advisable to use them in the bottom layer of a raised bed.
Understanding the Risk of Nitrogen Drawdown
The primary concern with placing wood chips beneath the planting medium is nitrogen immobilization, commonly known as nitrogen drawdown. Wood chips are a high-carbon material, often having a carbon-to-nitrogen (C:N) ratio as high as 400:1. To break down this large amount of carbon, soil microorganisms require a source of nitrogen.
Because the nitrogen content in the wood chips is low, microorganisms temporarily draw nitrogen from the surrounding soil to fuel decomposition. This action effectively “locks up” the nitrogen, making it unavailable for plant roots. This can lead to deficiencies, stunted growth, and yellowing leaves. However, this nitrogen deficit is temporary, as the nitrogen is eventually released back into the soil once the microorganisms die.
The risk to plants is mitigated when the wood chips are isolated from the main root zone. If the layer of soil above the wood chips is deep enough, most plant roots will not reach the zone where microbes are utilizing the nitrogen. Nitrogen drawdown only occurs where the soil and the wood chips physically meet.
Physical Effects on Drainage and Soil Volume
Beyond chemical concerns, the physical structure of a coarse wood chip layer offers several benefits. A thick layer of wood chips at the base significantly improves drainage by creating large air pockets and preventing water pooling. This is particularly helpful in deeper beds, as it prevents waterlogging and saturated soil conditions in the lower sections.
A major practical benefit is the reduction in the total volume of planting mix required for the bed. Using wood chips as filler saves a substantial amount of money, as purchased topsoil and compost are often the most expensive components. While the organic material will slowly decompose and compress over time, the coarse structure of the chips settles more evenly than loose soil. This decomposition process also slowly adds organic matter, enhancing soil structure over the long term.
Guidelines for Layering and Material Selection
To safely incorporate wood chips, create a physical separation between the high-carbon material and the plant root zone. A layer of finished compost, partially decomposed leaf mold, or landscape fabric can act as a buffer. This isolation layer ensures that nitrogen-consuming microbial activity remains below the area where roots are actively feeding.
For best results, use a generous layer of finished compost, ideally 4 to 6 inches thick, placed directly on top of the wood chips before adding the final planting mix. If using fresh wood chips, which have the highest C:N ratio, let them age for several months or a full season. This allows the initial, most intense decomposition phase to pass.
When selecting wood chips, avoid materials that have been pressure-treated, as they contain chemicals that can leach into the soil. Also avoid wood from black walnut trees, which contain juglone, a natural chemical that suppresses the growth of certain plants, especially nightshades. Favor aged or partially composted arborist chips, which often include a mix of leaves and smaller green material. This mix provides some nitrogen to assist the decomposition process.