Mulch – How Does It Affect Soil?

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Robert Pavlis

Mulching the garden is a very common recommendation. Mulch will reduce the number of weeds and it will hold moisture in the soil but how does mulch affect the quality of soil?

There are lots of claims that mulch improves soil but have you ever seen numbers to validate them? I haven’t either. How exactly does mulch improve soil? It should add organic matter, but how much? It should increase the number of microbes, but is this really true? Unfortunately, almost nobody studies landscapes and gardens because no one will fund the work.

Luckily I was able to find one very good research paper that looked at this exact problem.

Effect of fertilizer and mulch on soil, by Garden Myths (based on reference 1)
Effect of fertilizer and mulch on soil, by Garden Myths (based on reference 1)

Overview of the Research Project

The research paper is called “Wood Chips and Compost Improve Soil Quality and Increase Growth of Acer rubrum and Betula nigra in Compacted Urban Soil”, by Bryant C. Scharenbroch and Gary W. Watson.

Testing took place in an urban-like setting designed to mimic a new development. Top soil was removed, the soil was compacted with standard construction type equipment and 3 cm of top soil was replaced. Trees as well as grass were then planted to mimic a normal backyard. The trees were treated in a variety of ways; only water, compost tea, commercial bacterial concoction, wood chips, compost or fertilizer.

Compost Science for Gardeners by Robert Pavlis

The purpose of the study was to look at the effect of each treatment on the soil and on tree growth.

Each tree received the same amount of water, either as part of the treatment, eg compost tea, or as a separate watering. Analysis of soil samples were done by independent labs. Half of the 60 trees were removed after 4 years, and the remaining ones after 6 years. Average results are reported for 5 years.

During the test period, the liquid additives were applied on a regular basis, and compost and wood chips thicknesses were renewed yearly.

Discussion of Compost Tea

This is discussed in Compost Tea – Does it Work?

Effect on Tree Growth

To measure tree growth, the total mass of the tree, including roots, was weighed.

After five years the total tree mass under wood chips was 170% greater than the control trees which received just water. The mass of trees receiving compost were 82% higher, and the ones receiving fertilizer were 69% higher, than controls.

Both compost and fertilizer provided additional nutrients, and helped the trees grow. But neither worked as well as wood chips.

Effect of fertilizer and mulch on tree growth, by Garden Myths (based on reference 1)
Effect of fertilizer and mulch on tree growth, by Garden Myths ( reference 1)

Effect of Mulch on Soil

The following soil parameters were measured; density, moisture, organic matter, respiration, pH, nitrogen, phosphorus and potassium.

Density is a measure of the degree of compaction. A lower density indicates that the soil is less compacted and of better quality.

Respiration is a measure of the amount of CO2 produced. A higher level indicates that the microbe population is higher and more active in decomposing organic matter – the soil is healthier.

Effect of fertilizer and mulch on soil, by Garden Myths (based on reference 1)
Effect of fertilizer and mulch on soil, by Garden Myths (based on reference 1)

Fertilizer did improve the density of soil, probably because the extra nutrients fed microbes in the soil. Their activity as well as that of the tree roots made the soil more porous.

Contrary to what many organic gardeners preach, fertilizer is clearly NOT killing the soil microbes. This study clearly shows an increase in respiration due to microbe activity compared to using just water.

Both types of mulch improved density, moisture and organic matter; significantly improving the soil. The levels of phosphorus and potassium released from compost were quite high and would probably lead to runoff and pollution of ground water.

A common belief is that wood chips rob the soil of nitrogen, but this work clearly shows that over time they actually increase nitrogen levels, even above that of fertilizer. This is just one of many studies that have proven wood chips do not rob soil of nitrogen.

The numeric values can be seen in figure 2, reference 1.

Conclusion

This study confirms the fact that wood chip mulch is the best mulch for the garden. Over time it loosens compacted soil, adds organic matter, keeps moisture levels up and slowly adds nutrients to the soil.

Compost woks too, but it can add too many nutrients to soil. This problem is being seen by more and more organic gardeners who are experiencing very high nutrient levels, even to the point of becoming toxic. You can have too much organic matter.

References

  1. Wood Chips and Compost Improve Soil Quality and Increase Growth of Acer rubrum and Betula nigra in Compacted Urban Soil; joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=3337&Type=2

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

39 thoughts on “Mulch – How Does It Affect Soil?”

  1. I’ve heard that mulch provides hiding places for slugs and other pests, as a con for using mulch along with the nitrogen depletion potential, but I heard that as long as you just leave it on top of the soil layer and don’t mix it in, there isn’t the issue of nitrogen depletion at all for the root systems under the soil. When planting, they advise to push the mulch layer aside before making a hole in the ground for the transplants. But what about the harboring of pests, is that an issue?
    Also, is there a need to add soil amendments or supplements then after mulching? Does that require moving the mulch off the area to be amended and then planted before moving the mulch back over again?
    I’m a beginner gardener and don’t know where to begin. Even though I’ve tested my soil and added the appropriate fertilizers and amendments like rock dust, compost, manure and mychorrizae at planting, I installed automatic irrigation with timer. I’ve buried kitchen compostables under the soil to decompose and enrich the soil before planting, I still have issues with my vegetables not thriving, they stay small, or the fruit is small, they take longer to grow to maturity, eventually become plagued with downy or powdery mildew, aphids, slugs, caterpillars, leaf hoppers.
    I suspect my soil is so compact that there is root rot (I’m in the Tacoma, Washington, area, zone 8b. Maybe I’m not adding enough stuff? There are so many products out there, some very expensive and I don’t know if what they claim is true, or if it’s just marketing. For instance, there is this soil optimizer that is primarily composed of, as they describe, “concentrated, fully decomposed humus and other essential soil additives” to optimize the soil. Supposedly 3 lbs of it will cover 1,000 ft and be the equivalent of “working in hundreds of pounds of compost humus” into your garden. Could that be true? It’s $28 for 3 lbs of this stuff in a catalog that I was viewing.
    Does anyone have experience with such a product?

    Reply
    • Things will live in mulch, but remember 9 out of 10 bugs are good guys, so you are also providing a home for them.

      I just add any additional organics right on top of the mulch.

      Reply
  2. The results of this work correspond exactly with my personal experience in my large vegetable garden in northern France. I have carted in somewhere between 40 and 45 cubic metres of semicomposted chipped logging debris from a huge pile outside our village (free for the taking). Dumped on the surface, it has rapidly increased the ability of those beds to retain moisture and survive dry spells.
    One bed in particular was layered with 10cm of dead leaves then 10 of this wood waste. I planted directly into that, and the plants there (squash, beets, tomatillos, chard) were far bigger than in surrounding beds. Without any watering during drought. I pulled up plants and the roots were holding together big masses of rotting material, not down into the standard soil below.
    I highly recommend everyone experiment with this. Layer at least one bed in your garden and see how it does.

    Reply

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