Gardening

Will mulching reduce soil nitrogen?

Posted on April 3, 2025

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Last Updated on April 3, 2025 by Aaron von Frank

Does mulch cause nitrogen deficiency?

If you mulch your garden soil, will it remove or lock up nitrogen in your soil, thus causing your plants to become nitrogen-deficient? The short answer: no, so long as you mulch correctly. In fact, proper mulching can serve as a slow-release fertilizer that actually boosts bioavailable nitrogen in your garden soil.

Below, we’re going to take a deeper dive into this topic so you understand why mulching doesn’t deplete soil nitrogen. We’re also going to make sure you know how to mulch properly so you don’t cause soil nitrogen deficiency or harm your plants.

Quick note about the mulch nitrogen myth

Recently, my mother sent me a post from a gardening influencer recommending that people don’t mulch their garden beds because it robs the soil of nitrogen, thus harming your plants. We’ve seen this myth circulated so widely and so often that we felt the need to provide a rebuttal.

For the record, we’ve been using no-till (aka no-dig) organic gardening methods using mulch for 15+ years, so we have a lot of hands-on experience regarding this topic. However, we’ll also include information from published research that further corroborates our positions.

Not all mulch is the same

First, it’s very important to note that there are different types of mulch. Common examples: chopped leaves, wood chips, straw, etc..

Each type of mulch has different features, as well as pros and cons. While we can’t possibly detail every type of mulch, our personal favorite for both ornamental and edible gardening is arborist wood chips.

A handful of one week old arborist wood chips. If you’re like us, the smell of fresh wood chips makes you oh-so happy and putting them out in your garden makes you even happier.

What are arborist wood chips?

Arborist wood chips are freshly chopped wood chips from local tree service companies. Arborist wood chips have not been aged; rather, they’re so fresh they still contain green chopped leaves in them (assuming the tree/s still had leaves).

Why do we like arborist wood chips?

They’re very economical or potentially even free since local tree companies will often drop them at your location without charging you if they have a nearby job. Otherwise, the company has to pay to dump them at a local landfill.
They contain a good amount of carbon (wood and bark) and nitrogen (green leaves or recently dessicated leaves).
Since they decompose very slowly, they last a long time – potentially up to a year, depending on the depth you apply them (more on that below).
They drastically improve soil health.

On the point of soil health, one recent study summarized the benefits of using mulch as follows:

“…reducing evaporation, improving temperatures, adjusting the microbial biomass, maintaining the soil organic carbon balance, increasing nutrient cycling, promoting soil enzyme activity, enhancing soil aggregate stability and suppressing weed infestation.”

A new strawberry bed made using free, local forest waste products: wood logs that fell during a hurricane outline the bed edges and wood chip mulch is used to feed and protect the soil while blocking weeds.

How does carbon-rich mulch INCREASE soil nitrogen?

Pretty much any mulch you use is going to be a lot higher in carbon than nitrogen. (Exceptions: mulches like freshly cut grass clippings or cover crops.) So how can a carbon-rich material increase nitrogen in the soil?

When placed on the surface of the soil, mulch begins forming a microbe-rich ecosystem teeming with bacteria, fungi, arthropods, worms, etc. The slow decomposition of the mulch is less due to mechanical processes (like weathering) than biological processes. In other words, the decomposition of mulch is primarily due to all the micro and macro organisms consuming the mulch and/or consuming each other. It’s a jungle in there!

What happens after an organism eats? It produces a waste product. That waste product is high in nitrogen and other nutrients. In the case of the microbes living in and around your mulch, the nutrients from those waste product slowly leach deeper into the soil during rainfall events and are also transported deeper into the soil via fungal networks, worms, and other larger soil organisms. These nutrients are then absorbed by plant roots.

To be clear, nitrogen levels ARE low in the actual mulch layer. And nitrogen levels are also relatively low in the “mulch-soil interface,” which is the transition zone between the mulch and your soil. However, in the soil below, nitrogen levels will actually increase as mulch decomposes.

The transition zone between the mulch layer on top of the soil and the deeper soil below is referred to as the mulch-soil interface. Side note: it wasn’t many years ago that the soil in this bed was hard-packed red clay. Now it’s full of life and fertility.

A review in the Journal of Environmental Horticulture summarized:

“Experimental research reveals that neither nitrogen immobilization nor growth suppression occurs as a result of using woody materials for mulch (51, 100). To the contrary, many studies have demonstrated that woody mulch materials actually increase nutrient levels in soils and/or associated plant foliage (5, 99, 114, 125). A zone of nitrogen deficiency exists at the mulch/soil interface (Chalker-Scott), possibly inhibiting weed seed germination while having no influence upon established plant roots below the soil surface.”

You’ve actually seen this exact process at work in healthy forest ecosystems where thick layers of leaves blanket the forest floor each fall before slowly breaking down and feeding the soil food web, until the process repeats again the next fall.

How do you apply wood chip mulch?

This is where things get a little nuanced — and possibly where the myth about mulch causing soil nitrogen deficiency comes from: some people don’t apply mulch correctly, then they blame mulching for the poor results.

The three most common mulching mistakes new gardeners make are:

Tilling or digging mulch into the soil instead of top-dressing the soil with mulch. Doing this will definitely deplete soil nitrogen!
Transplanting annual seedlings into mulch rather than into the soil.
Putting thick layers of mulch against the trunks or stems of plants.

How you apply arborist wood chips is going to vary a bit depending on the type of plants you’re mulching and what you’re trying to accomplish, as detailed below:

1. PERENNIAL edible plant mulching

When applying wood chips around plants such as fruit & nut trees, shrubs, vines, cane berries, etc, a few rules apply:

In fall or early spring, apply a 2-3″ layer of compost or worm castings around the plants right BEFORE putting down the mulch, especially if you’re creating new beds or dealing with poor soil fertility. Yes, you can apply this compost layer over the top of last year’s mostly decomposed wood chips, or simply rake back a preexisting thicker mulch layer that’s not fully decomposed yet. However, you can skip the compost application in well-established beds with high soil fertility.
Put 4-6″ of mulch down around the plants OVER the compost layer in order to provide adequate seed blocking and soil health benefits.
IMPORTANT! Don’t ever pile mulch or compost against the trunks or stems of your plants. This will damage or kill them over time. Instead, taper the compost or mulch down from the maximum depth to 0″ as you approach the trunks or stems.

Notice how the mulch around this blueberry plant tapers off towards the base of the plant so that the crown and canes are not buried. Never pile mulch against the stems or trunks of your perennials!

How far out from the plant you mulch will vary… Are you mulching a stand-alone plant or mulching numerous plants within a larger bed? As a general rule with shrubs and trees, you want to mulch at least to the edge of the overhead canopy, but the further out you mulch, the more you encourage the roots to grow outwards.

2. ANNUAL edible garden plant mulching

When direct sowing seeds – When direct sowing seeds in your annual garden beds, you do not want to cover the seeds with mulch. When preparing seed beds, we’ll rake back the mulch and put a 2-3″ thick layer of compost or worm castings down, then sow our seeds into the compost. From there, we’ll lightly mulch (2-3″ thick) the areas adjacent to the rows of seeds. This means the seeds don’t have competition from weed seeds, don’t have to struggle to push through mulch, and have plenty of microbial friends and nutrients to aid optimal growth.
When transplanting seedlings – Immediately after transplanting 4-8 week old seedlings, you can put down a thicker 4″ layer of mulch in the garden bed. Taper the mulch off as it gets close to the stems of the plants, unless those plants happen to be capable of forming adventitious roots (tomatoes, ground cherries, etc.). Otherwise, the stems will rot, killing the plants.

3. Garden paths and/or “mulch & wait” soil building

When mulching our garden paths, we’ll put down very thick layers of mulch, up to 8″ deep. If we’re making new paths over very weedy areas or areas of grass, we’ll also put down a layer of brown cardboard under the mulch to serve as an additional weed blocker (aka sheet mulching).

We’ve also used what we call a “mulch & wait” method for establishing new garden beds slowly. We’ll put down a very thick layer of mulch (1-2′ thick), then just wait a year for the mulch to break down into beautiful, rich soil. Then we plant.

Are there any downsides to mulching?

Life is full of tradeoffs. While we’ve found the benefits of mulching far outweigh any downsides, there are a few problems with mulching you should be aware of and plan to mitigate:

1. Slugs, snails, pill bugs

Mulch creates a food-rich ecosystem for myriad creatures, large and small. Some of these creatures might also want to dine on the same plants you intend to eat.

Case in point: slugs, snails, and pill bugs (aka roly poly) tend to proliferate in mulch. However, we also raise ducks and have learned how to use them to keep the populations of these pests in check (smaller predators in our ecosystem also help). There are plenty of other organic control methods to be found on the internet if you aren’t fortunate enough to be owned by ducks.

2. Hydrophobic soil in hot, dry conditions

The fungal species that inoculate our mulch will sometimes cause the mulch to form into a dense connected mat which is hard for rain to penetrate or permeate — especially when temperatures are near 100 degrees with no rain for multiple weeks. Thus, during a prolonged hot dry period, the soil beneath the mulch can become hydrophobic, aka so dry that it actually repels water.

Supplemental irrigation during the summer is a good way to prevent this from happening. Use your fingers to periodically test to make sure the soil below your mulch is staying slightly damp.

3. Increased vole activity

When we started gardening on our land, we had hard-packed red clay soil that was like trying to grow food on a brick. Now, thanks largely to mulch, we have beautiful, black, soft, nutrient-rich soil that’s over a foot deep in many spots.

If you’re a burrowing animal that likes to eat roots (hello voles), this is a good place to call home. We’ve come up with all sorts of ways to stop voles from eating our food.

4. Smothering native bees

Many people don’t realize that most native bees lay their eggs in underground nests. While many adult bees die before winter, some overwinter under ground as well.

Unfortunately, a thick layer of mulch can block bees’ entry and exit from the ground. So be sure to leave plenty of un-mulched areas in your yard or garden to provide habitat for the bees. Once you start mulching an area, the bees are less likely to use it for nesting if there is better habitat nearby.

Hopefully it goes without saying, but “good habitat” for bees also means plenty of native flowering plants and no pesticides.

A bee crossing sign inside a freshly mulched bed at our son’s school. Since many native bee species nest underground, mulched areas are not good native bee habitat, but the un-mulched areas behind the sign are.

Wood chip mulch FAQs

Now you know why and how mulch increases soil nitrogen plus how to mulch properly. Here are answers to a few other questions you might have:

How does mulch prevent weeds?

Mulching prevents weeds primarily by inhibiting the germination of weed seeds present in the soil seed bank below the mulch. Weed seeds that do manage to germinate below the mulch struggle to reach the surface and photosynthesize.

Established weeds present prior to mulching can be managed by some combination of: a) pulling and removing the weeds prior to mulching, b) sheet mulching with a layer of brown cardboard prior to application of mulch, c) mowing or weed whacking the weeds as low as possible to the ground, and/or d) applying at least a 4″ layer of mulch.

Does mulching change the Ph of soil?

No, studies have shown that different types of mulch will not change the Ph of the soil below the mulch-soil interface where plant roots grow.

Will allelopathic chemicals in mulch kill your plants?

Some trees such as hickory, junipers, and cedars contain allelopathic chemicals that can inhibit seed germination or possibly even kill young plants trying to grow nearby. However, when these trees are used as wood chip mulch in accordance with the mulching guidelines above in this article, they will not harm your plants.

We have used wood chips from hickory, juniper, and cedar and can speak from experience on this issue. Our best guess as to why is that these chemical compounds are degraded and not released deep into the soil or at high enough concentrations to harm the plants growing there.

However, if you’re concerned about this issue and/or you’re growing shallow-rooted annual plants, you can opt to use aged wood chip mulch or other mulches such as straw instead.

Can mulch made from a diseased tree cause your plants to become diseased?

It is very unlikely that a plant pathogen infecting a tree that is chopped into wood chips will be able to infect the roots of another healthy plant/tree where the mulch is applied.

Will mulch attract termites?

Mulch is not an attractive food source for termites. They much prefer larger pieces of wood such as logs or fallen trees. In 15+ years of using mulch in our gardens, we’ve never seen termites in our wood chips.

We hope this article gives you a better understanding of the benefits, downsides, and best practices for using mulch in your garden — plus helps you understand why mulch actually increases your soil nitrogen despite internet myths claiming the contrary!

Happy gardening!

KIGI,