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
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.
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.
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!
Thanks again for a great post!
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.
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.
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.
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.
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.
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.
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.
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Excellent article. Thank you.
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!
Larger is better. An inch in diameter is great.
Which abbreviations did you not understand?
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.