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.

Microbe Science for Gardeners Book, 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
  3. Dear Robert, having discovered your site today i have read many of your fine articles with great pleasure. However, I was not able to find an article on biochar, recently given attention due to its discovery in terra preta, dark soils found in pre-Columbian deposits in South America. I would enjoy seeing the application of your keen research and writing skills to this topic of interest to gardeners.

    Reply
    • You are right – but it is on the drawing board along with several hundred other myths. Short answer – no real demonstrated value for gardens so far.

      Reply
  4. I can tell you with certainty that pine bark chunks in a thick layer (6 to 12 inches or even more) have a very positive effect on avocados. The avocado is a surface feeder and its roots do not like too much heat or too much water. The bark holds just enough water and the avocado grows tiny roots right into the bark. They love it!

    Reply
  5. A few years ago I was attempting to grow vegetables in my Central Florida native soil garden. The soil is said to be 70% sand and 30% silt. Each season I added compost at time of planting but the soil appeared incapable of retaining and value gained and required the continual addition of compost. Then I switched to mainly growing in Raised Beds (RBs) filled with 100% compost. In the RBs my ability to grow vegetables improved dramatically. I add fertilizer (normally 10-10-10) pretty much as I did in native soil.
    On reading this article I am thinking that maybe there is a chance the gardening in Central Florida native soil could be improved to at least match what I am doing in RBs. Specifically I question whether the use of wood chip mulch would potentially improve my very sandy soil to a point where it could be a place to grow vegetables successfully.
    Any thoughts on this would be very welcome.
    Thanks

    Reply
    • Wood chips should only be used as mulch, and in that capacity they decompose slowly. Florida might be quite different with the hot weather.

      Reply
    • Many of us gardeners would like to have your soil issues instead of tenacious clay and/or stony soil. Just keep adding organic materials– whatever will decompose, preferably “organic,” so you are not adding a pesticide/herbicide burden to overcome. Ostensibly raised beds make this easier, except that the damn things require a lot of material to fill– too often wheelbarrowed in by gardener. Whew! Don’t give up. You might find local suppliers eager to get rid of organic materials– like a local horse stable, or other animal enclosure– chickens, rabbits, goats, etc. Hay, grass etc can also be great.

      Reply
    • They are similar but not exactly the same. Bark tends to repel water whereas wood chips absorb it. Some people feel this is a significant difference – I am not so convinced. Bark will keep water in the soil longer, keep weeds down, and will decompose.

      The best option is probably the one that you can get locally.

      Reply
      • I’m seeing buckwheat husks more and more often in potted nursery plants, and I’ve seen it on sale on bags as well. It may be a regional thing, as I live in a big buckwheat producing area.

        Reply
        • Buckwheat husks should be, I would think, the perfect mulch if one could get enough cheaply. A long time ago I was able to get cocoa husks, a waste product, and they were wonderful. And you’d get this occasional wonderful whiff of chocolate.

          Reply

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