Do Wood Chips Deplete Soil of Nitrogen? Here Are the Facts!

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After researching the best mulches to use in my no-till garden, I have found a great deal of conflicting information about using wood chips. I certainly don’t want to use them if they are going to rob my soil of nitrogen, so I did a lot of research to find out.

Will wood chips remove nitrogen from the soil? Yes, wood chips do affect nitrogen levels in the soil. Wood chips deplete nitrogen from a shallow zone of soil close to the surface. However, below the first few inches of the earth, nitrogen levels actually increase after the application of wood chips. This difference occurs because microorganisms use nitrogen to decompose organic matter (in this case, wood chips), causing a depletion.

The decision to use wood chips in the garden depends greatly on the application. There are many benefits to using wood chips. In some instances, they are the perfect mulch. However, in other situations, they are detrimental to the soil and may hurt the growth of plants.

The Best Uses for Wood Chips

Landscaped areas, specifically around trees and bushes, benefit significantly from a wood chip mulch. In general, trees, shrubs, and bushes have deeper roots and aren’t affected by the lack of nitrogen in the top layer of soil. They actually benefit because, as the wood chips decompose, nutrients are added back into the soil where they can be easily absorbed by the trees.

A study done of 15 mulch options, including grass clippings, leaves, yard waste, and bark, demonstrated that:

“Wood chips were one of the best performers in terms of moisture retention, temperature moderation, weed control, and sustainability.” (Source: Washington State University)

The Benefits of Using Wood Chips

Less Weeds!

Nitrogen-depleted soil reduces the number of weed seeds that germinate. Weed seeds can’t access the nutrients needed to sprout, which means fewer overall weeds. A layer of wood chips also prevents many weeds from reaching the sunlight they need to grow. Consequently, even those weeds that do manage to sprout aren’t able to grow to maturity.

Aesthetics

Wood chips look nice. They provide a uniform look and don’t get compacted by rain or foot traffic. Mulches such as sawdust and leaves will compact over time and look downtrodden and messy.

Wood chips lessen the impact of rainwater as it hits the ground, reducing mud splashing and other unsightly effects. A layer of wood chip mulch is an easy filler, covering empty spaces in an attractive way.

Moisture Retention

A layer of wood chips prevents water from evaporating quickly. Wood chips also absorb water, holding onto it longer than any other mulch choice. Water is less able to run off and instead is retained for later use. The water is then slowly released to the plants, all the while keeping the soil cool.

The use of wood chips reduces the overall demand for watering. For drought-sensitive plants, like Rhododendrons, this is invaluable.

Other mulches, like leaves, sawdust, and cardboard, become impenetrable mats, and water will struggle to penetrate them.

Prevent Erosion

Plants surrounded by an appropriate layer of wood chip mulch establish themselves strongly in the soil. In areas where flooding or excessive rainfall is an issue, strong roots are critical. Erosion-proof plants aren’t just important aesthetically; they also create better quality soil and build up the landscape.

Moderate Temperature

An even application of wood chips will keep the soil cool and moist in hot temperatures. And, it will warm the ground during cold temperatures. This automatic temperature regulation keeps plants happy and healthy, and it also reduces the caretaking work of the gardener.

Economic

Locally produced wood chips are often free. Arborists produce wood chips as a by-product of their work. In many cases, they then must pay to dispose of them.

Gardeners who will take this expense away from them are greatly appreciated! They may have to be picked up and transported by the consumer, but that is a small price to pay for free wood chips.

Environmental

Using locally produced wood chips is sustainable and supports the local economy. It also cuts down on transportation costs, which are associated with the wood chips being shipped to a landfill. Additionally, it reduces fuel and transportation costs necessary for big chain stores to truck in the mulch.

Wood chips that aren’t repurposed are added to landfills. This is not great for the environment or the economy. Overfull landfills are increasingly becoming a problem that affects all of society.

Additionally, wood chips don’t need to be replenished as often. They break down much slower than other mulch options, which means they last longer.

Hinder Pests and Pathogens

Many types of wood chips are natural insect repellents. Cedar chips contain thujone, which is a natural deterrent to termites, cockroaches, moths, Argentine ants, carpet beetles and house ants.

Pests, like termites, aren’t attracted to wood chips because they aren’t nutrient-dense compared to other options.

Supply nutrients

The slow decomposition process of wood chips is beneficial to the soil. They provide nutrients over a long period of time. These nutrients include nitrogen, tannins, lignin, and suberin.

Do you not know? Have you not heard? The Lord is the everlasting God, the Creator of the ends of the earth. He will not grow tired or weary, and his understanding no one can fathom.
Isaiah 40:28

When Should I Not Use Wood Chips?

Many people would like to use wood chips in the vegetable garden. This is fine, as long as there is a buffer layer between the wood chips and the soil. The buffer layer can be compost or another type of mulch, like leaves or straw. It is best not to put the wood chips directly on top of the soil.

The majority of garden vegetables have shallow roots. This puts their roots directly into the nitrogen-depleted zone that wood chips cause when applied on top of the soil. The lack of nitrogen will negatively affect the growth of the vegetables.

Annual flowers have the same shallow root problem and will struggle to grow. Many vegetable gardeners use wood chips in their garden successfully, using compost as an in-between layer.

Note that this compost layer may just be aged wood chips. One way to successfully use wood chips in the garden is to not use them fresh and age them first for a year or so in a compost bin before applying to the top of the garden.

First-year perennial bushes and plants will also struggle in soil that is nitrogen-depleted. They are just establishing themselves, and their roots are long enough yet to get beyond the top layer of soil. After the first year, however, they will be fine with a wood chip mulch.

Other plants with shallow roots, like yarrow, bellflowers, and peonies, need careful mulching. Mulching them too early in the spring, before they have sprouted, may prevent them from growing at all. Wait until they appear, and then mulch carefully around them.

How to Fix Nitrogen Depleted Soil

Gardeners who are concerned about the nitrogen content of the soil have options. For first-year perennials, shrubs, and trees, a “mulch sandwich” can be built.

A layer of nutrient-rich compost is placed on top of the soil before the wood chips are laid down.

This gives the young or sensitive roots a significant barrier from the wood chips and also provides extra nutrients to assist in their growth.

Another option is to apply a nitrogen-rich fertilizer to the soil before adding the wood chips. Look for organic fertilizers with a high nitrogen supply.

Precautions to Take Before Using Wood Chips

A few precautions should be taken before using wood chips. Never use wood chips from an unknown source.

Wood chips may be sourced from infected trees, and that disease could pass on to other trees. Not all diseased trees will affect any trees, though. It all depends on the type of disease and whether it is a concern for that type of tree. In general, though, this is not common. A healthy tree rarely is infected from wood chip mulch.

Those who are worried about wood chips carrying disease can let the wood chips age by sitting outside for a year or two. The downside to this is that some nutrients are lost in the aging process.

Some commercially produced mulches contain shredded discarded wood-based products. These wood chips are then colored to look like natural wood chips.

How To Apply Wood Chips as Mulch

Apply the wood chips to the bare soil in the spring before weeds have a chance to establish themselves. If this isn’t an option, rid the area of weeds as best as possible before laying the wood chips. The wood chips can also be applied in fall after the weeds have died and before cold weather sets in.
In instances where gardeners are creating a “mulch sandwich”, apply the compost or leaf mulch layer several inches deep before adding the wood chips on top.
Lay the wood chips 4-6” deep around landscaped areas. If the wood chips are shallower, weeds will come through. If they are higher, they may suffocate the soil and plants. Areas that are being rehabilitated need a mulch layer 8-12” deep.
Do not lay mulch directly against tree trunks or shrubs. Always leave an area around the base so the trunk can breathe. Wood chip application should resemble a donut-shape. Piling the mulch close to the trunk encourages fungal diseases and pests. Fungi and opportunistic pests like wet, dark environments, and mulch piled up against a tree trunk is the perfect habitat. Leave 6-8” open around the trunk.
Apply the wood chips in a circular method, working outwards. Use fewer wood chips closer to the base. Start with 1” of mulch, then increase it to the desired height as you move outwardly.
Reapply wood chip mulch each year to maintain the desired height.
Do not “fluff” the mulch before adding more each year. Add it on top without disturbing the under layer. Disturbing the under layer defeats a critical purpose of the mulch, the decaying, and adding of nutrients to the soil. (Do Not Disturb!)

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