Sheet Mulching (Lasagna Gardening) – Does It Harm Soil?

Sheet mulching, also called lasagna method or lasagna gardening, is a popular technique for creating a new garden. Some people even use it as an annual mulching system to keep weeds down.

Lots of people claim success with the technique because it does kill grass and weed plants. Others claim that sheet mulching reduces the oxygen levels in the soil, thereby harming the biology in the soil. Which side of the argument is correct? Should you be using sheet mulching in the garden?

What is Sheet Mulching?

In its simplest form, sheet mulching is a process where a compostable material like newspaper or cardboard is placed on the ground. This is then covered with soil or other organic material. After a period of time, the paper, a term I will use to refer to both newspaper and cardboard, kills the underlying grass and weeds and then decomposes. The process prepares an area for a new garden.

Many people claim that the result is a vary friable soil, full of earthworms.

The process can also be more involved where several layers of organic material and paper are layered, one on top of the other, and that is where the term lasagna gardening comes from, but many use the term when even a single sheet of paper is used.

Does Sheet Mulching Work?

Many thousands of gardeners have used it and have reported good results. It is also a common technique of permaculturists.

Exactly how do we define the word “works”?

A main use of lasagna gardening is to kill the existing grass and weeds so that the area can be turned into a garden. Newspaper and cardboard certainly keep light from the plants they cover and over time most of them die. The exception might be tough rhizomatous plants like bindweed and Canada thistle, which can grow long distances underground to reach light, but for the most part, the underlying plants are killed. Using this definition the method does work.

The paper and various organic matter does eventually get incorporated into the soil, thereby increasing the soil organic matter. This also works.

Most gardeners are quite happy with the outcome of sheet mulching. So at least on some level it works.

Concerns With Sheet Mulching

Gardeners look at things they can easily see; dead grass and decomposing organic matter. They have no way to see problems with the biology in the soil, also called the soil biota.

Some people are concerned that the paper reduces the exchange of gases, namely oxygen (O2) and carbon dioxide (CO2), between the soil and the air.

All soil biota, including plant roots, need to breath in O2 and exhale CO2. When the exchange of gases between the soil and air is reduced, CO2 builds up in the soil, suffocating the soil biota and plants. The concern is that the paper is indirectly killing all kinds of living organisms in the soil. But since gardeners can’t see this life, they don’t know if it is harming the soil.

Does Sheet Mulching Reduce Oxygen in Soil?

The answer is almost certainly yes, but that does not tell us very much. The amount of reduction is what is critical. What we need to know is, does sheet mulching reduce the oxygen level enough so that it harms soil biota?

Unfortunately, nobody has studied this problem to provide an answer, but a recent study (discussed below) does provide some insight..

In the absence of scientific data we can apply reason to the problem, and use the observations of thousands of gardeners.

Oxygen Exchange Study

A recent study from Washington State University looked at the exchange of gases between soil and air, when various mulches were applied to the soil surface. They found that, “typical mulches (wood chips, cardboard, landscape fabric, polyethylene film) … have strikingly different effects on gas movement between the soil and atmosphere. While mulches reduce CO2 and O2 diffusion across the soil-atmosphere interface, the reduction in diffusion was not sufficient to cause significant changes in CO2 and O2 concentrations in experimental mesocosms compared to bare soil, with the exception of polyethylene mulch film, where CO2 concentrations in soil increased significantly compared to no mulch, and O2 concentrations decreased to 16%. ”

Cardboard sheet mulching did not significantly affect the O2 and CO2 levels in soil.

It is important to note that (a) this was done in a lab using relatively small containers, (b) no plants were growing in the soil, (c) a seal was made between the edge of the mulch sheet and the container and (d) there were no measurements of microbial activities. Although the information is interesting, the data can’t be extrapolated to a garden situation.

The garden is dealing with much larger sheets of paper. The sheets are normally overlapped and water holds them together, in effect making one large sheet that covers the bed.

In the garden there are edge effects. Air moves from outside of the covered area, to areas under the sheet. This was prevented in the above experiment. The experiment did not take into account any lateral gas movement in the soil.

Although the study does not tell us much about a garden scenario it does suggest there is no problem with oxygen levels under sheet mulching.

Long Term Use of Sheet Mulching

Washington State University reports that long term use of paper as a mulch in orchards, produced higher yields and faster tree growth than controls and was comparable to some organic mulches. Any changes in oxygen levels certainly did not have a detrimental effect.

Carbon Dioxide Around Roots

What are the normal gas levels in soil?

From the book, Soil Science for Gardeners, “The CO2 level in air is 0.04% and in soil it normally ranges from 0.3 to 5%. Due to the high level of microbes, the level in the rhizosphere can be as high as 20%.”

Both roots and microbes thrive in the rhizosphere, the area around the roots, and yet the CO2 levels are very high. Would sheet mulching ever raise the levels to 20%? Probably not; especially in light of the above study which showed no significant changes.

Earth Worms Under Sheet Mulching

Users of sheet mulching report that they find many earthworms in the new soil and this is taken as proof that the method is good for soil.

A strong opponent of sheet mulching, Dr. Linda Chalker-Scott, makes the claim that it is the lack of oxygen caused by the paper, which causes the worms to come to the surface where gardeners can easily see them.

That argument is not very sound. If the sheet mulching caused a drop in oxygen levels which in turn caused the worms to surface, you would expect this to happen soon after the paper was applied to the soil. What the gardeners are reporting is seeing lots of worms many months later as the paper is decomposing.

Also consider this. Most gardeners cover relatively small beds; mostly less than 6 feet wide. Surely an earthworm that is finding the oxygen levels to be too low, would be able to crawl 3 feet to the edge of the sheet to get some air? On a rainy day you see them crawl a long way on a wet driveway.

A much more likely explanation can be gleamed by understanding earthworm behavior. When soil is dry they tend to burrow deeper in order to keep moist. When soil is moist, they move closer to the surface. Sheet mulching traps moisture near the surface of the soil, right next to where the grass and weeds are dying, creating a worm heaven. Lots of moisture and plenty of food.

I suspect sheet mulching does not produce a lot of extra worms. It just produces an environment that attracts them and where gardeners can easily see them.

Plant Health Under Sheet Mulching

You can get all sciency and measure gasses but what we really care about is plant growth. The data here is clear. Many, many gardeners report success with this system. It grows great plants.

That does not mean they are healthier under sheet mulching, only that they grow well enough to satisfy gardeners.

My Opinion

Until we have more real science, we can only use the data we have. It does kill grass and most weeds. The paper does decompose over time. Plants do grow well in the resulting soil.

Does sheet mulching reduce the movement of water and air? Yes, but we don’t know to what extent. It does reduce the movement of water, but it also decreases evaporation which may, at least in part, compensate for less water reaching the soil. It can be easily augmented with watering if required.

Was the soil and soil biota harmed? Probably. It is almost certain there is some impact on the microbial community.

But ….. I suspect this has limited impact in a typical garden where the beds are narrow. The biggest change will be in the center of the bed with less effect near the edges.

The other important question that needs to be asked, is how long does this effect last? Once the paper decomposes, the soil will revert back to its former communities because all of the soil around the area has been unaffected. Microbes from these areas will quickly move back into the new bed.

The other thing to keep in mind is that you are converting an area from grass and weeds, into one where you grow other plants. You will probably add some organic mulch, compost, or fertilizer and maybe all three. All of this activity will also change the microbe community. Is this effect any less than the one from sheet mulching? Maybe not.

Almost all gardening activities will change the microbe community. Something as simple as adding a new plant will have an effect at the rhizosphere level. As gardeners, we would be naive to think we don’t change the soil biota when we garden.

Let’s answer the original question. Does sheet mulching (lasagna gardening) harm the soil?

Based on the data we have, it has a limited short term effect, that does not harm the soil or soil biota significantly. It is unlikely to have a long term effect.

Limit the Effect of Lasagna Gardening

It is a good idea to limit any effects this might have but doing the following.

• Use newspaper instead of cardboard. It kills weeds just as well and decomposes much faster. Cardboard is also more likely to attract termites.
• Us this method to start a new garden. Don’t use it as an annual practice for managing your weeds. A wood chip mulch is better for soil and controls weeds just as well.