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Mulch – How Does It Affect Soil?

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.

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
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

36 Responses to 'Mulch – How Does It Affect Soil?'

  1. 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.

  2. 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!

  3. Thanks again for a great post!

  4. Lar says:

    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

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

    • Thomas Brophy says:

      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.

  5. phanmo says:

    Can pine bark be considered as functionally identical to wood chips?

    • 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.

      • phanmo says:

        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.

        • Thomas Brophy says:

          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.

  6. Pamela Murphy says:

    Hello, I’ve been trying to figure out the amount of potential sequestered carbon that my leaf compost pile is capable of when it is finished compost. Using the poundage of finished dry compost a cubic yard is 540 pounds. If the pile is 70 % maple leaves and 18 lbs of coffee grounds, how much carbon would be finally available to add to the soil after it’s finished composting? I was using 47:1 for carbon to nitrogen. Look forward to your response. Thanks!

    • What is the content of carbon in the finished compost? That will tell you how much carbon you will add to the soil.

      The problem is that what ever amount you add to soil, it will change. Finished compost is not finished yet. It will continue to decompose for at least 5 years, and if you accept that humus does not exist, it will continue to change forever. It will continually lose some carbon as CO2, which is then no longer sequestered.

      Compost is not a good way to sequester carbon long term. But if you add it every year, you do build up the organic layer and the soil holds more and more carbon.

  7. rena says:

    You just made my day. Spent a lot of money and sweat equity mulching existing beds and creating new ones. If you have an email list please sign me up.

  8. Richard Egan says:

    Excellent article. Thank you.

  9. ConstantWeeder says:

    What size wood chips are they talking about? I must rely on buying wood chips “by the bag” and want to get the best size for best results.
    On another note, for ease of reading, please spell out your abbreviations the first time they are used! Thank you!

  10. Roger Brook says:

    Very impressive results for the wood chips!
    I have always argued that a mulch that does not intercept water but lets it pass through with minimum absorption into itself gives the best result. My own favourite mulch is gravel!
    Excellent table – worth publishing twice!
    I hope you approved of my recent mention of your good self in my latest post1

    • How well do wood chips let water go through? This is an interesting question for which I have not been able to find an answer. I almost started some trials this summer – but never got around to it. My guess is that a 4 inch layer would probably prevent water from getting to plants unless you have a heavy rain.

      Thank you very much for the plug on your site.

  11. Jude says:

    Very interesting results, and gratifying for those of us who look for gardening shortcuts to save work.

    I can get quite a large quantity of leaves in autumn. A few years ago, I ran out of space for making leafmold so I dumped a thick (about 4 inches) of leaves on a bed that had rather claggy soil. I’d planted mostly shrubs, that mostly sulked and had just about survived for two years. By midsummer, the leaves had disappeared and the shrubs started to look happy. I expected some soil improvement but I was surprised at how much in such a short time. One more year’s leaves and the soil is in excellent condition. My shrubs, grasses and herbaceous flowers are halthy and thriving.

    I don’t have a ready source of woodchips so I don’t use them, but in a previous garden I mulched with tree/shrub shreddings, mostly on paths. The results were slower than leaves but they created a lovely woodland floor effect that my trees thrived on.

    Now I’m off to gather some more leaves and spread autumn joy on my beds.

    Thank you for this post.

  12. Lloyd says:

    Interesting – I have encountered nursery-landscaping-urban horticulturists whose advice is supported by the findings noted in the article. The recommendation to me for progressing the growth of my sub-tropical rainforest plant garden (not a rainforest, but a garden using rainforest plants), in a suburban block with a shallow top soil profile over schist-shale, was to mulch with a product akin to your “woodchips”. The product is called “forest mulch” here and comprises the milled prunings from local trees (natives and exotics) and shrubs – a mix of woodchip (carbon) and greenery (nitrogen), but in no particular ratio. When you order it you get whatever they have been grinding recently.
    I was recommended to use 100 millimetres annually – initially on top of a thick newspaper layer and then just directly onto the surface in subsequent years. The addition of fertiliser to balance any nitrogen drawdown was also recommended – the preferred product is chicken manure processed into prills. The formula seems to have worked.
    So thankyou for providing the confidence to continue!

  13. Craig says:

    It’s no surprise that for this trial the treatment that performed the best long term was the one that appears to have best maintained soil moisture and temperature. All of the soils appear to have had good levels of soil organic matter at over ~5%, and other than a nitrogen deficiency created by disturbing and compacting the soil ecosystem, I doubt any were very nutrient deficient. Moisture on the other hand is just so important for soluble nutrients and for the microbes that cycle those nutrients, without that moisture they go dormant (typically 3-4 weeks depending on climate) or die. Microbes and microfauna mine and exude nutrients and also create glues and macroaggregates that improve soil tilth and soil bulk density such that roots can then penetrate. Without microbes creating and recycling themselves and these glues, and producing organic matter, the nutrients leach away. To see how just how transient nitrate can be, and why microbes are important in replenishing their levels, look up The Scientists Garden.

    It would have been interesting to compare an inorganic mulch with one of similar moisture and temperature properties.

    The higher potassium and lower phosphorous in comparison to the compost would have also promoted arbuscular mycorrhizal fungi colonisation that can increase plant growth, and they also produce glues like glomalin and related proteins, and a hyphal network that further improves soil bulk density.

    The standout for me is how comparable ACT was with compost at reducing soil bulk density, and also the correlation between SOM and nitrogen. ACT in combination with ramial wood chips would be interesting for a shorter term trial as it’s important to note that the type and amount of wood chips (or other forms of mulch) matters when choosing a mulch to meet your needs. A recent study indicates an ideal Carbon:Nitrogen of less than 50:1 if you want to sequester the carbon from the mulch and that there likely is diminishing sequestration over 100:1 and limited at 300:1. If you dump stemwood chips with a C:N of 600:1 then for a while you’re going to see plant nitrogen and phosphorus along with other nutrients temporarily reduce in the O layer as microbes take it up, that is until the equivalent or more is released from digesting the mulch. Too much or the wrong type of mulch can also smother, create waterlogging, or even spread disease.

    Also, for what it’s worth, I’ve read a gene sequencing soil microbial study on a number of inorganic, organic, and biodynamic farms that showed that while inorganic fertilizer farming does still contain many microbes, the abundance and diversity is far less than in organic or biodynamic systems, the latter of which showed the most abundance and diversity. Whether most of them are asleep and waiting for a rainy day or litter to fall, I can’t say. However the differences could also largely be down to farming practices, till vs no till, and whether fields are left fallow, and cover crops are grown or not, etc.

    Growing just has so many variables and mulch or ground cover is one of the most important!

    • Would be interested in a reference about the various C:N ratios for mulch.

      Microbe diversity is definitely lower on agriculture soils. No till increases the diversity as does added organic matter. Mono-cultures probably also play a role.

    • Thomas Brophy says:

      Wonderfully informative (and humbling) post, as was Robert’s. Thanks so much to you both.

    • Thomas Brophy says:

      Love your scientific orientation and explication. Thanks so much.

  14. Useful. My lawn and surrounding soil would appear to match that of the study. I’m all for the increase in tree biomass yielded by the wood chip mulch.

  15. tolga erok says:

    perhaps you could share your thoughts on the “garden of Eden” theory where he only uses compost and loads of loads of wood chips

    • I have not spent an in depth time looking at Garden of Eden. I should review it at some point.

      • Thomas Brophy says:

        Always a great informative or “thinkable” post. Many ( if not all) biodegradable materials are great for mulch, and upon disinteregrating, are beneficial for the tilth of the soil, if not necessarily for the fertility, of the soil. I know this from first-hand experience, having grown vegetables and flowers for more than 40 years. There is, however, always more to learn, as Robert so frequently demonstrates.

  16. Karin says:

    Again, excellent information…thank you!

  17. Elizabeth says:

    Wow, what a rigorous and well conceived study. In so grateful to the original researchers (not to mention whomever granted the research funds) and to you for making it accessible.

  18. Mike Ricci says:

    I think that the key word in this discussion is “mulch!” We had yards of wood chips one year from roadside clearing operations. After about 3 years, I hauled it on to the field, dumped it, and then harrowed it in. I could definitely see yellow areas in my cover crop (tillage radish) over the areas where the chips were dumped. Was it long term? No, the yellowing was gone when the next crop was planted. I felt that the positives outweighed the negatives-increased OM and tilth (and probably N). And like I said, “mulch” is the key word here. I harrowed my chips in and didn’t mulch with them. Interesting to see that the microbes weren’t effected by the fertilizer-you are right, all I hear about is how the microbes are killed off!

  19. johnoot says:

    Another great post. I’ve read the study, and wonder what the numerical differences would have been for an established garden as compared to the study plot (typical new suburban development lawn with tree on top of compacted base).

  20. Lar says:

    I would have guessed that fertilizer would have made the most difference. I may adjust some of my gardening practices based on this. One wonders what the results would have been if these were used in combination. Very interested to see the Compost Tea results. Thanks for the post.

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