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.
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 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.
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
- 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
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.
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!
There would not be any draw down of nitrogen, but adding some is probably good for the garden.
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.
Wonderfully informative (and humbling) post, as was Robert’s. Thanks so much to you both.
Love your scientific orientation and explication. Thanks so much.
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.
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.
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.
Again, excellent information…thank you!
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.
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!
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).
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.