Organic Matter

Despite some claims, plants and microbes really do care about the source of their nutrients.

They often do better with organic forms of the elements rather than synthetic chemical forms.

CompostCompost is valuable for far more than its organic matter, as we’ll see later on.

While I definitely recommend using products such as lime in certain circumstances, the thing most of us need to do before all of that is start a regimen of deliberately increasing the organic matter content of our soil.

We can do this with leaves, compost and other organic materials, even cover crops.

In the last century, we’ve burned up more than 90% of the organic matter in many of our soils through tilling, applying chemicals, and clearing plant residue without allowing organic matter to decompose.

Humus is our ultimate goal, but we also want a supply of fresh organic residue as food and shelter for microbes and animals. As this residue is broken down, carbon dioxide becomes available to plants.

Conventional agriculture has largely ignored organic matter, and organic gardeners have relied on it perhaps too much at the expense of other management practices. Still, increasing organic matter and humus should be one of our most important goals.

Humus, made up of biologically active complex carbon chains, is critically important in the soil.

While the term is often used interchangeably with organic matter, it’s really organic matter that has been broken down by multiple organisms to the point where it won’t get broken down much more.

Humus holds nutrients in the soil, including the fertilizers we apply.

According to some, it particularly holds calcium, which otherwise likes to sink down in the soil profile below most of the other minerals.

It definitely holds water.

Walk into your garden in the middle of a rainless summer night and your shoes may very well get soaking wet, IF you have enough humus in your soil. It has many other important functions, such as tying up toxins, contributing to better soil structure, moderating soil temperatures and stimulating microbes.

When we build up our organic matter content in the soil with mulch and compost, it decreases the amount of fertilizer we need to apply because these organic matter sources also contain nutrients. Also, the more carbon you have in your soil, the less fertilizer you need to apply because the fertilizers will stay in the soil longer and will become chelated.

Chelated means they are bound with organic compounds, and consequently are more available for microbes and plants. Let’s look at our main sources of organic matter…

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Thick, dense mulching in the garden with the right material provides an array of benefits.

You can certainly say goodbye to most of your weeds when you apply a thick mulch.

Applying straw mulchLight-colored mulches like straw are extra good at reflecting heat and keeping the soil cool in summer.

Not only does it smother them out both physically and sometimes biochemically by tying up nutrients on the soil surface, it makes the ones that do find their way through so much easier to pull, especially if you’ve been clever enough to hit the garden and the mulch with some water.

Note that it may be necessary to kill some tap-rooted or perennial weeds before placing the mulch on top of them.

When we mulch, we create homes for insects and other animals and provide them (and microbes) with food. They take this food and turn it into available minerals and humus, incorporating it into the soil to create amazing soil structure.

Then all of the other benefits of good organic matter begin to accrue — increased CEC, water-holding capacity and fertility, and decreased compaction.

The mulch itself serves to decrease the compaction caused by us walking on the soil and from heavy rains, and can help to prevent erosion. It moderates the soil temperature, benefiting everyone living there.

Some studies have concluded that mulches and cover crops make soil cold in the winter and increase frost damage, but these studies are using dead soil, and this does not happen on real, organic soil that is actually kept warmer during the winter due to the insulating capacity of the mulch and the activity of microbes and animals.

So mulch actually improves the biodiversity of your entire soil ecosystem by giving all manner of critters a place to live, food to eat and water to drink.

Why not just use compost? A little bit of thought tells us why.

It does a lot of things right but fails to stop the weeds. The same goes for manure. Manure can be a very useful soil amendment, but it needs to be well composted and should really be mixed in a proper compost pile.

On its own, it isn’t a valuable, balanced nutrient source, since the animal kept most of the trace minerals for itself and gave up excess nitrogen and salts.

Indeed, many studies have shown that soils continually amended with the same manure will eventually produce unhealthy plants.

Manure injectorA manure injector filling up. On an industrial scale, using manure on crops is a convenient way to dispose of vast amounts of unwanted animal waste.

We can use compost and manure, but they don’t make the best mulch.

Mulch is often applied two to three inches thick, and should be kept away from tree trunks, which don’t want to be covered in anything.

If you want to promote fungi, such as in a perennial or shrub garden or around trees, put the mulch on the surface. If you want to promote bacteria, such as in an annual or vegetable garden, incorporate it lightly into the top few inches of soil to give the bacteria better access.

For bacteria, also consider chopping or grinding up the mulch into smaller pieces, and make sure it’s plenty wet, as bacteria need moisture more than fungi.

Let’s start with mulches that satisfy very few of our healthy soil conditions and get rid of them right away.

Landscaping Fabric

Landscaping fabric is considered part of our mulch layer because it’s often placed on the soil under various mulch types in order to help control weeds.

The cheap stuff doesn’t work very well, but thicker fabric can work for a while before weeds start to find their way through the cracks or just start growing on top.

Unfortunately, that thick landscaping fabric can also stop water from getting down to the soil, especially on a slope where the water just slides down the fabric to the bottom of the hill. It doesn’t take long for the landscape to show signs of suffering in this case.

But the biggest problem with this fabric is that it doesn’t allow organic matter to recycle into the soil. When you put landscaping fabric on your garden, it means your soil doesn’t get to eat anymore. This is definitely not an appropriate mulch, except under pathways and patios.

As we’ve seen, soil needs to be consistently replenished with organic matter, so any of the mulch types we choose have to be composed of organic matter.

Soil is replenished in nature and in our gardens when leaves fall to the ground. Since many of our gardens are low in organic matter anyway, it also happens when we intentionally bring in more leaves, straw, compost and other organic matter to improve the soil.

Putting landscaping fabric in the garden stops all of this and slowly kills the fertility and structure of the soil, and everything living in it.

Landscape fabricThis will look nice and tidy when it’s done, but the soil will never be more than a few inches deep, and will have permanent problems with water and nutrient cycling.

In fact, you should take it out if there’s some in your garden. I’ve done that for many of my clients.


Stones and gravel provide some benefits in that they protect the soil from erosion and decrease evaporation, but they don’t break down into humus and don’t allow organic matter to get down to the soil, so they don’t do much to improve soil health.

That being said, there is an aside here.

If you have certain bigger plants that are special to you, like a new fruit tree, for example, there is a technique called rock mulching that can improve the soil extremely quickly, resulting in amazing plant growth.

Place a few inches of leaves around the root zone of the new tree, which might be a few feet in diameter. Cover those leaves with round stones or flagstones that are small enough so that you can handle them, but big enough to cover some of the leaves, i.e. not pebbles and not 50-pound boulders.

Doing this facilitates the rapid breakdown of the leaves. They may take, for example, eight months to break down normally, but the rock might bring it down to two months or even less.

Whether it is because of the pressure exerted on the leaves, or the prolonged moisture the stone cover provides, or the heat stored by the rocks during the day and radiated back to the soil overnight, or the explosion in the earthworm population under them that often happens, I don’t know, but it works.

The key to success here is doing it a few times. Yes, you have to remove the rocks by hand every couple of months and apply more leaves and place the rocks back by hand. The first time, the leaves may disappear fairly quickly and that will be interesting enough on its own.

But subsequent times, you’ll start to notice a thick layer of worm castings where the leaves once were. This is a bit of work. Not much, but it might take 15 minutes to do a tree.

It’s not a practice most of us will wish to do long term, but we may want to do it for the first growing season to give the trees an incredible boost in organic matter.

Peat Moss and Coir

Peat is organic matter — plants, microbes and even insects and other animals — that has been broken down in wet, anaerobic conditions, generally in marshy areas. In fact, peat bogs can preserve animals extremely well and were used for human sacrifices back in the Iron Age.

There are different kinds of peat moss.

While it’s often thought of as a sterile medium, this isn’t always the case. Some peat is low in microbes and some of it is extremely high in microbes. Some of it is 90% organic matter and some closer to 30%.

While some is high in minerals and some is low, all peat has a high CEC and water-holding capacity.

Sphagnum peat is the most common variety. It’s very acidic, low in nutrients, and high in oils that repel water. Though it holds a lot of water, once it dries out, it needs to be soaked to wet it again. It’s not useful to us a mulch, nor as a soil amendment.

More importantly, peat bogs are unique, vitally important ecosystems for this planet. When they are harvested for their peat unsustainably or to create farmland, these ecosystems are gone, species go extinct, and a huge amount of greenhouse gases are emitted.

Peat bogSlow-growing peat bogs take ages to regenerate, and are among the most threatened ecosystems on the planet.

The bogs are supposed to be restored after harvest, but many aren’t, and some scientists propose that it’s impossible to restore them properly. Shipping peat across the country or the planet isn’t environmentally sustainable.

One could argue the same thing for any fertilizer, but peat is very heavy for the surface area it covers, and for the lack of real benefits it provides.

Coir is ground coconut fiber husk, a by-product of the coconut industry.

It doesn’t have the high CEC of peat, but it also doesn’t have the water and pH problems.

There are sustainability issues with coir, too — do you have coconuts growing where you live? Is the coir coming from them? Most of the coir you see in your garden center is coming from far away. I see no need for shipping this stuff around the world when we have organic matter locally.

Bark Mulch, Wood Chips and Sawdust

Bark mulch and wood chips are some of the most commonly used mulching materials in the garden.

They can be useful or harmful, depending on the source and the use. They can be found in bags or bulk bins at the garden center. They satisfy many of our mulching goals, but unfortunately, they have a couple of potential problems making them one of the mulch types I don’t generally use.

Bark contains oils that repel water. It’s also low in nutrients, so it doesn’t improve soil fertility as much as other mulches.

Conifer bark such as cedar and fir can be high in toxins, as conifers have evolved a strong arsenal of compounds that are their first line of defense against insects and diseases. These chemicals can also cause toxicity problems in the soil.

Wood, including both chips and sawdust, is very high in carbon and very low in nitrogen.

This means microbes have to pull much of the available nitrogen from the surrounding area in order to break down the wood, which can end up causing a nitrogen deficiency in your plants.

If you first include this wood as the carbon source in your compost pile, this situation can be avoided. Otherwise, you may need to bring in a source of nitrogen regularly.

However, if you have shrubs and trees that like a fungal environment, wood chips will promote that, especially if you don’t overwater.

Forest blueberriesCheck out the understorey here! Blueberries and other members of the Ericaceae family (like cranberries and huckleberries) are naturally adapted to forest environments, so they like woody mulch.

Fungi still need water, but not as much as bacteria. If they are coarse wood chips and if you put on just a couple of inches and leave them on the surface without digging them in, the nitrogen shortage may not be a big problem.

Wood chips from the non-bark part of the tree and even sawdust can have some good nutrition in them, so they’re a great addition to composts where they can be properly mixed with nitrogen materials.

As long as they’re from non-treated wood, they can even be used lightly in gardens, especially if mixed with leaves; but on their own, they have the same high carbon problem.

Sawdust breaks down more quickly and can decrease the amount of air and water that enters the soil if you use too much as a mulch.

All of this being said, I often work with what I have and try to use local materials, so if you have access to a large amount of local bark mulch or wood chips at a good price (or free), and you have room to store them, feel free to take them and compost them or pile them and let them compost for a few years.

They can become good organic matter over time.

On top of that, if you already have a good amount of organic matter in your soil, and good humus, wood chips shouldn’t cause any problem.

Also, both bark mulch and wood chips can definitely be used in paths, where I am not trying to create a balanced soil environment. They can do a decent job of controlling weeds there if you apply a few inches and keep this mulch topped up.

Straw, Hay and Grass Clippings

Straw and hay aren’t the most aesthetically pleasing, but they are fairly good mulch types if you can find a source. You may not want to use straw or hay from ryegrass as it has toxins in it, nor from grass that has been sprayed with pesticides, which is common in many countries.

The difference between straw and hay is that straw is just stalks from harvested grain, while hay is finer and has seeds, so hay will often actually produce extra weeds.

You can deal with this by composting it in a hot compost to kill the weed seeds, but make sure you get all the seeds into a hot enough part of the compost pile.

Grass clippings aren’t the best mulch to use in the garden in abundance because they can get so tightly packed together that they inhibit air circulation. Besides, they’re far too important for the soil below your grass plants than to remove them and bring them into the flower or vegetable garden as mulch. They don’t cause thatch or any other lawn problems, but they provide many benefits so please let grass clippings lie in place.

If you have left a little bit too much time between cuts and you simply have too many clippings, add them to the compost pile. Just make sure they’re thoroughly integrated into the pile, because if they are left in a big clump, they may promote anaerobic decomposition.


Of all the mulch types, by far the best is leaves. They do absolutely everything right. That’s why when designing gardens I want to make sure to use plants that make a lot of leaves — not just evergreens — and I want to design the beds to catch all of these leaves too.

Garden treesTrees can bring all sorts of benefits to the garden, in addition to their leaves. Shade, beauty, habitat, water, or even a good place for a second home…

Leaves that fall on the lawn and on non-garden surfaces can be raked into the gardens or mowed and left right on the lawn. If you don’t have enough leaves, your neighbors will usually be happy to give you theirs.

When I worked as an organic gardener on residential properties, I would go to the neighbors of my clients and ask for their leaves too. In many cities, you can rake your leaves to the curb and a big truck will come by to pick them up. But why would you want to give away the best garden mulch ingredient?

Ironically, some organic gardeners do get rid of their leaves and then pay a fortune to buy the leaves back as leaf mold in the spring.

Leaf mold is just leaves that have been slightly decomposed. It’s one of the best mulch types, too, but in most cases, the gardener would have done much better to save the money and keep the leaves in the garden over winter where they can have the benefit of protecting the soil.

If you have a layer of leaves in your garden that’s thick enough, say two to four inches, many weeds will be smothered. You’ll still get some weeds, but they’ll be so easy to pull out that it won’t matter. You can just drop them back on top of the leaves (if they don’t have seeds) to become part of the mulch.

Some people think leaves are not one of the most attractive mulch types for the garden, but is a forest floor unattractive? Is the forest floor covered in two inches of bark?

We’ve been conditioned to think that bark mulch or bare soil is the most aesthetically pleasing, but if you cover your garden in a rainbow of autumn leaves, I think you’ll see it differently, especially now that you know all the benefits they provide. When we remove the leaves, we are breaking nature’s cycle and creating more work for ourselves.

Maple leavesMaple leaves make excellent mulch, and are beautiful as well.

So leaves are the number one best mulch type, but there are exceptions.

In some climates, a thick layer of leaves over the winter may promote such wet conditions that disease is actually increased.

It’s possible to have too many leaves if you have a lot of big trees or if your beds are already covered in groundcovers and you don’t want to totally smother them. In that case, you may just have to compost them or give some away, to a friend or to the city, although I have mulched 12 inches of leaves into some lawns with great success.

Actually, when I was a kid, I recall my dad would pile a bunch of leaves in the back of the pickup truck, head down our rural street to where there were no houses, drop the tailgate, and hit the gas. It was so much fun watching the leaves get caught by the wind and cover the sky like a thousand red and yellow butterflies.

In hindsight, I have no idea why we did this, but it was fun at the time. In fact, I suppose that’s why we did it.

You can also rake some of them into their own pile and moisten them to make leaf mold. It can take a solid year to make a good leaf mold, but it is a beautiful mulch. It’s made largely by facultative anaerobic microbes, similar to the ones in beer and wine, which is why it smells like yeast.

For most of us, too few leaves are the problem, and this is a design issue.

If you find yourself short on leaves, get some plants that produce a lot of leaves. I’m talking about fast-growing annual or perennial plants that get big and provide a lot of leaves in the fall, such as cardoon, rhubarb and ferns.

CardoonCardoon, a close relative of artichokes, has edible stalks and striking silvery foliage.

Now a bit about oak leaves. I’ve never had a problem because oak leaves don’t break down quickly. I’ve always enjoyed that about them because it just means my mulch stays around longer.

And, they don’t acidify the soil to any notable degree, but again, if you have too many, don’t force it. Walnut leaves, on the other hand, aren’t the best mulch because juglone, a plant toxin, does show up in the leaves and can cause some issues.

Seaweed is another incredible mulch that we often think of as leaves, although it’s not technically a plant. It’s a great mulch for people living by the ocean. It often breaks down very quickly, giving you another excuse to visit the beach. There are two main problems I see with using seaweed.

The first is that it may be important to leave most of it on the beach, because it may be food for many organisms.

The second is that there is actually a worldwide shortage of kelp because it’s been overharvested just like fish. It may be that the kelp in your area is abundant, but overall, we’re running out. This information is new to me, but I’m starting to rethink my use of kelp.


Here’s a video I shot in 2013 in the Amazon jungle:

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  • Increasing the organic matter and humus content of our soil is one of our most important goals.
  • Landscaping fabric should only be used on paths, not gardens, and should actually be taken out of gardens if it’s already there.
  • Stones and rocks aren’t a particularly helpful mulch because they do not improve the organic matter content of the soil, but they can be used short-term on top of a layer of leaves around a tree or shrub to hasten decomposition.
  • I don’t tend to use bark mulch or sawdust as mulch because they can cause more problems than benefits, but they can be composted or used as a light mulch in perennial gardens to encourage fungi. I’ve done that with wood chips in my perennial garden.
  • Straw makes a great mulch. Hay is better composted to get rid of weed seeds, and grass clippings are best left on the lawn.
  • Leaves are the best mulch, supplying nutrients, organic matter, a balanced carbon to nitrogen ratio, weed control, a good water-holding capacity, decreased evaporation, homes for critters, and all of the things a good mulch should provide.