Composting Paper And Cardboard – What You Need To Know

Originally published March 26, 2022.

One of the challenges many aspiring composters face is where to obtain materials to build their compost piles. While everyone can save kitchen scraps, and there are many sources of free green material, some people have difficulty sourcing a steady stream of brown material.

I’ve been an organic gardener for over 10 years, making my own compost and building my soil with a goal of not adding any harmful chemicals or elements. Although nearly every garden reference says to use cardboard and newspaper in your compost, I wanted to understand for myself what the risks were. Because of this I was inspired to perform a lot of heavy research into what chemicals are in cardboard and newspaper, and what the risk of using it as an ingredient in compost.

I often refer people to use cardboard or newspaper as it can generally be found without too much trouble, and is free. Cardboard in particular is readily available as it can be obtained from dumpsters and recycling areas of big-box and grocery stores. However, I often get asked if it is safe to use cardboard or newspaper in composting or for smothering grass to make a new garden – in particular are the glues and inks safe to use?

Well, in this article I’m going to go deep into all the nitty gritty details of cardboard and newspaper – the good, the bad, and the ugly. I’m going list out the potential risks people bring up in regards to both cardboard and newspaper and try to address them in the most fact based way possible.

*RELATED ==> See our comprehensive list of 101 items you can compost here

I’m going to start by just how cardboard and newspaper are made, and the risks associated with their base components. Then, we will discuss types of pollutants and chemicals we need to be concerned about. Following this, we can dive into what is deemed safe by environmental regulations for compost (yes, there are limits on the compost you buy). And finally, we will compare some peer reviewed studies that examined how many pollutants are in cardboard and newspaper.

But , if you are just looking for the short answer – aka TLDR – here it is:

It is safe to compost brown corrugated and bleached white cardboard, without printing or coatings. It is safe to compost black & white newspaper that have no color on the entire sheet (including graphics). These ingredients in a compost pile will not add significant amounts of heavy metal pollutants, and the compost can still be considered organic.

Sources – USDA, Organic Materials Review Institute, Canadian Compost Standards. [1][2][5]

Peer reviewed studies have shown that the trace element/metal concentrations of compost with cardboard/newspaper are below maximum allowable limits of Canadian/EU regulations for compost land applications, if they are uncoated and have no or only black ink. [3][4][5] And, one should avoid long-term exposure to trace metals, as their accumulation in the body can have adverse health effects. [6]

One may wish avoid composting any form of cardboard/newspaper that is glossy or uses colored ink, as they will contain somewhat higher levels of heavy metals. [7] There is difficulty determining trace metal concentrations in inks, as their formulations are proprietary, and can thus only be determined via testing (which generally costs several hundred dollars).

Now, don’t let this information dissuade you from starting a compost pile. Compost made with proper ingredients will give your soil loads of benefits!

Ok – sorry for the long intro, but buckle up! For those interested in the details of composting cardboard/paper, let’s get started. Here is the digital table of contents –

• How paper is made
  • White or Bleached paper
  • Can you compost paper with ink?
• How cardboard is made
  • Composting cardboard glue
  • Packing tape
• Is it safe to compost ink?
• What pollutants can occur in compost from paper products?
• What levels are allowed to be considered ‘safe’?
• Natural levels of heavy metals in soil
• How much pollutants in compost come from Cardboard and Newspaper
• Is using cardboard and paper in the garden ‘organic’?

How paper is made

When it comes to paper, it all begins as trees. Wood is de-barked and processed into pulp, and then refined into paper. Pulp is made by pulverizing wood chips, then heating/boiling it and exposing it to sodium hydroxide and sulfide sulfide to dissolve the lignin in the wood, which bind the cellulose fibers together. [8] From here it can be pressed to it’s desired thickness of paper. [9]

White or bleached paper

White paper, like what comes out of printers, junk mail, and newspaper is generally bleached white. This is done with chlorine, and the reaction produces a dangerous chemical called dioxin. Dixon is a potent carcinogen even at low levels. [10]

But, dioxin is present in many common paper products such as napkins, paper towels, and coffee filters. Dioxins in coffee filters has long been studied, in fact a very thorough study was completed way back in 1989 concluded that there was no significant risk of dioxin exposure when using bleached coffee filters. [11] This makes complete sense, as coffee filters have been in use for decades, we would have seen any ill effects by now. [12]

Furthermore, Aerobic Composting was found to remove approximately 80% of dioxin from dioxin contaminated soil. [13]

So, at the base level the components of regular brown or bleached paper are not much different than wood. But, what matters is when ink or dyes are added. Or, in the case of newspaper, if a glossy finish is applied. The glossy finish on paper or colored dyes and inks contain significant amounts of heavy metals, which we will discuss in detail later.

Can you compost paper with ink?

You can compost paper with black ink without any risk of contaminating your compost, provided there is no colored ink or coating on the paper. If the paper contains any colored writing or graphics or glossy coating, then it should not be composted as the colors or coatings will contain heavy metals.

How cardboard is made

In regards to cardboard, I’m going to be focusing on regular, corrugated cardboard, as it is the most commonly used for shipping/boxing. Below is a simple cross-section of standard corrugated cardboard:

At the most basic level, corrugated cardboard consists of two flat faces separated by a wavy flute. Both the faces and the flutes are made from pressed pulp following the same pulping process as described for paper in the previous section.

The flat faces are generally made from “kraft paper“, which itself is made from long fibers of softwood trees. [14] While the ‘corrugated’ flute is made from short fibers of hardwood trees. Now, this is the most basic type of cardboard. Different flute wave forms, or multiple flat faces can be made to provide extra strength, etc.

From here, the cardboard can have text or labels printed on it. It can have wax or glossy coatings applied. And it will eventually get folded into a box, taped up, and shipped to your house!

What chemicals are in cardboard or paper?

Typically there are around two hundred chemicals that are used in the manufacture ore paper or cardboard.[23] At a base level, these chemicals do not contain heavy metal pollutants and will be decomposed via the composting process and won’t pose any risk to people or animals.

Composting glue from cardboard

A corn-starch based glue, which is applied at high temperatures is used used to hold it all together. Being corn-starch based, and in small amounts, it is unlikely that it would survive the composting process. Other adhesives used in cardboard manufacturing are derived from natural polymers in vegetable roots [15] [16]. So, the glues being derived from plants, are certainly safe to compost.

Combining all this information, cardboard is typically 95-99% cellulose wood pulp and 1-4% starch based glue. [17] So, it is no surprise that cardboard is highly recommended as a ‘brown’ material in compost.

Below is a short video detailing out the manufacturing process quite well:

So, when it comes to base cardboard, we are able to determine that the face, fluting, and glue are all plant based. We know that at the base level, kraft paper, the fluting, and the cornstarch-based glue should breakdown organically without any pollution above what is naturally within the wood fibers. Although there is a chance that certain pollutants can be added later.

Is the tape used on cardboard boxes compostable?

Most shipping boxes that arrive at homes are packed used in clear packing tape or a different, kraft-paper based tape. The clear plastic packing tape comes in one of two kinds, a petroleum based tape or a clear, compostable tape (yes, compostable) made from bio-plastic with starch based adhesive.

But as we will see, just because something is compostable doesn’t mean it is not without risk. I have not located any pollutant data on clear, compostable tape. Furthermore I am unaware of any method to identify it on a box (unless it was labeled as such) as opposed to clear petroleum based packing tape. Thus, I would suggest that you don’t try to compost any clear packing tape unless you know the source!

The other kind of tape you may encounter is water-activated and the base material is kraft-paper. This tape is compostable, and the adhesive is starch based, similar to what is in cardboard. You may also recognize this tape as what Amazon Prime uses in their packages. However, the colored inks used on Amazon packing tape may be a source of heavy metals (more on this later).

Is it safe to compost ink on paper or cardboard?

When it comes to the printing on a newspaper or cardboard box, it is more difficult to determine the composition. Inks are proprietary, and their formulas are not shared publicly. But, as a general rule, cardboard and newspaper with only black ink can be used as an ingredient in compost, or mulch for organic farms. The main restriction is that no glossy, or colored inks are allowed. Also, the cardboard cannot be waxed or treated with fungicides. [1] [2]

There is plenty data showing that colored inks and dyes contain heavy metals.[cardboard ink metal] Although, it is difficult to determine their concentrations outside of actual testing, as the formulas for the inks are proprietary and not readily shared. None the less (Elmas, et al 2018) found that the inks used in packaging contributed to most of the heavy metals present.

In my own compost piles at home, I try to avoid colored inks. Even when it comes to composting egg cartons, I will only compost the bottom half of the carton as it has no printing on it. [UPDATE DEC 2023]- I no longer compost egg cartons at all since I learned they were made from recycled newspaper, that often contains colored inks).

Pollutants in compost

When we think of pollution, compost doesn’t come to mind. In fact most of us associate ‘compost’ with organic, green gardening, which we should. However, that doesn’t mean there won’t be pollutants in compost you purchase, or even make yourself. It is more of a matter of how much is there…

When one thinks of the term “pollution”, industrial smokestacks spewing black smoke and soot or nasty harmful pesticide and chemical residues often come to mind. But, in regards to compost, even if synthetic chemicals make it to compost, they won’t necessarily survive the composting process. As anything chemically organic (even if synthetically made) can be broken down by microorganisms in the pile given enough time. *However, this doesn’t mean you should start composting herbicide treated grass clippings- in fact, you shouldn’t.

But, there is one type of pollutant that cannot be broken down – heavy metals. Heavy metals are base elements, and cannot be decomposed further. Sure, they may be bound to other items or themselves in a safe way, but the fact is – they will remain. You can find them on the periodic table – these aren’t decomposing further.

What heavy metals get into compost

The heavy metals that one may encounter in compost (from municipal solid waste) are arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu, mercury (Hg), molybdenum (Mo), nickel (Ni), lead (Pb), selenium (Se), and zinc (Zn). Now, some of these are beneficial to plants and animals, but accumulating large amounts of trace metals over time can lead to harmful effects for humans and the environment. [6]

Compost that is made from municipal mixed solid waste will contain certain levels of heavy metals. This is due to their presence in trash and other items. Backyard composters should have no problem minimizing the amount of these trace elements in their compost pile, as you can control what goes into the pile. And numerous studies have shown that regular yard/kitchen waste has almost no detectable amounts of trace elements/metals. The main contribution would be if you used manure (from the feedstock to the animals), and possibly newspaper/cardboard with colored inks or glossy coating. But, we should discuss how much is ‘ok’ before moving on.

Trace metal limits in compost

When investigating acceptable limits on compost (for land application), I examined standards from United States, EU, and Canadian. The United States standards for trace metals in land application of bio solids are much less restrictive than both EU & Canada. [18] [19] This means that the allowable pollutant levels for land application of bio solids in the US are much higher (by order of magnitude) than our neighbors to the North. Since Canada has very clear regulations that are quite strict I will use their standards as a baseline of acceptable metal concentrations.

*If interested, the specific metal concentration limits in US standards that have the greatest deviation with Canada and EU countries were cadmium, chromium, copper, lead, nickel and zinc (see page 15 of this report). [19] The US allows more metals than EU/Canada by orders of magnitude.

The government of Canada publishes and regulates amount of trace metals in compost standards depending on application [5]. Based on the amount of heavy metals present will give a rating of AA, A, or B. Category AA & A are classified as “very good compost” with no usage restrictions. [20] While Category B can still be used as an organic soil conditioner, but not anywhere that regular and frequent contact with humans would occur. It is typically used at mine reclamation sites, reforestation but not in any agricultural capacity unless subject to further Canadian government regulations.

Item	Column 1: Metal	Column 2: Category AA Compost (mg/kg dry weight)	Column 3: Category A Compost (mg/kg dry weight)	Column 4: Category B Compost (mg/kg dry weight)
1.	Arsenic	13	13	75
2.	Cadmium	3	3	20
3.	Chromium	210	210	1060
4.	Cobalt	34	34	150
5.	Copper	100	400	760
6.	Lead	150	150	500
7.	Mercury	0.8	0.8	5
8.	Molybdenum	5	5	20
9.	Nickel	62	62	180
10.	Selenium	2	2	14
11.	Zinc	500	700	1850

So, if we have government regulations allowing human contact with category AA & A compost, then we can use those as our baseline acceptable limits.

I’ve written about compost many times, and always have at least one pile composting in my backyard. And there is no shortage of materials listing what can or cannot be composted. When it comes to carbon-rich ‘brown’ materials, cardboard and newspaper are two of the most frequent suggestions and are accessible by almost anyone. So, now we need to examine their risks and discuss them.

Natural levels of heavy metals in soil

Like so many things in life, heavy metals can be a bit complicated. Your soil will have some amount of heavy metals already present in them. Furthermore, anything you bring to your garden – be it plants, mulch, or compost will add heavy metals to the soil. Still, it is fool hardy to just mindlessly ignore any potential long term risk of accumulating heavy metals in our bodies. And if we can reasonably minimize those risks, we should.

What about natural amounts of metals in soil?

The heavy metals that we are concerned about in pollution such as mercury, lead, and others are all throughout our environment. Your soil in your own backyard will actually contain some. Certain studies have been conducted that show base lead (Pb) concentration levels in residential areas can range from 35-755 ppm. [21] Older industrial areas will have higher concentrations than newer, more rural areas.

So, knowing that metal concentrations can vary wildly, we should take a look at what limits have been established by organic government standards in the next section.

How much pollutants in compost come from Cardboard and Newspaper

I need to start this section by stating the amount of pollutants in compost can truly only be determined via testing. This can be done by many State University laboratories at fees that usually run several hundred dollars. Although I obviously had the idea of testing my own compost, obtaining this kind of information is expensive for a small mom & pop blog like ourselves!

But, we are in luck. There have been some peer-reviewed studies that have measured the amount of pollutants in municipal compost, and where the pollutants came from. So, not only can we see real world results from regular municipal composts, but we can better inform ourselves what to avoid composting to keep our compost piles more organic.

As a side note, the studies are fascinating, as you can really get a feel for where these pollutants come from. I definitely gained a more thorough understanding of dangers of landfills and proper disposal. I had always tried to recycle electronics, but little did I realize how much pollution their waste gives off compared to other forms of trash. Ok sorry for my little detour – back to cardboard and newspaper.

A 1992 study of municipal compost from Cape May New Jersey [3] and a 1995 study of municipal waste in Switzerland [4] analyzed trace metal concentration in waste ingredients. It must be noted that while extraordinarily thorough, these studies focused on many different waste categories as they relate to compost, and classified cardboard and newspaper in somewhat generic terms. Also, there is no indication whether the cardboard had been stripped of labels/tape, had glossy coatings, or printing (black or color). So, the metal concentrations of regular pure brown cardboard or plain black/white newspaper may be lower than reported here.

None the less, the results (which can be viewed in the table below in ppm) will summarize the metal concentrations from both the 1992 New Jeresey study, and the 1995 Swiss study of compost pollutants based on ‘ingredient’ into the compost, and compare them to the Canadian compost standards (Catagory A) we discussed earlier. Note that I have converted units from the papers to be in a ppm.

Item	Column 1: Metal	Cardboard – NJ Compost	Newspaper – NJ Compost	Cardboard – Swiss	Newspaper – Swiss	Canadian Limts (Cat. A)
1.	Arsenic	0.3	0.2	NA	NA	13
2.	Cadmium	ND	ND	2.2	2.5	3
3.	Chromium	ND	ND	NA	NA	210
4.	Cobalt	NA	NA	NA	NA	34
5.	Copper	17	25	84.3	12	400
6.	Lead	24	ND	43	23	150
7.	Mercury	0.22	0.41	0	0	0.8
8.	Molybdenum	NA	NA	NA	NA	5
9.	Nickel	8	ND	NA	NA	62
10.	Selenium	NA	NA	NA	NA	2
11.	Zinc	73	83	60	41	700

So, we can see from the above results, that for all pollutants that were measured, they all fell below the limits set by Canadian compost for land application without restriction. This confirms that conventional advice that newspaper and cardboard are safe additions to any compost pile.

Now, there were some trace metals that were not tested in the 1992 & 1995 studies [3][4]. But when we combine these numbers with the information on cardboard/paper manufacturing, and the allowable materials for use on a organic farm, we can conclude that they are probably safe.

Conclusion – composting plain paper and cardboard is safe

Plain paper with no glossy coatings, no colors or dyes, no plastic stickers is safe to compost. And regular brown or bleached cardboard without colored ink, wax coating, and without glossy finish is safe to compost. This is per government standards and guidelines, as we have seen in our breakdown of how these items are manufactured.

However, even still, adding anything to your garden will add heavy metals. They are ubiquitous and are already present in the soil. The question comes, how much will adding compost made from cardboard or paper add?

Well, the lead concentration in newspaper and cardboard range from 20-40 ppm, while the residential survey in Canada had concentrations as high as 750ppm! So, you would need to add a whole lot of cardboard to approach those naturally occurring levels!

Is using cardboard or paper in the garden organic?

Since we have established that regular, brown corrugated cardboard will provide metal concentrations that are quite low, we know it should be safe for gardening. As it would take an extraordinarily long time to accumulate heavy metals in the garden from adding cardboard. Furthermore, heavy metals are naturally in the soils, with higher concentrations around old industrialized areas.

That being said, the USDA, Canadian government, and the Organic Materials Review Institute will permit the use of plain brown cardboard and black and white newspaper to be used as mulch for organic farms. But- the cardboard and newspaper cannot contain any color printing or dyes. And this applies to the entire newspaper sheet. If any color ink/image exists on a sheet, that sheet cannot be used.[1] [2] Also, the cardboard and paper must be made from recycled materials.

How about using cardboard for sheet mulching?

Cardboard can be used for sheet mulching to construct a garden, and be considered organic. Again, this is based not only on the OMRI, USDA, and Government of Canada, but on the data we have just been reviewing. Just make sure it is plain brown cardboard and does not have any coatings, dyes, or colored inks.

Related ==> See our guide & video to sheet mulching here.

What about composting Amazon Boxes?

Amazon boxes can be composted. The cardboard boxes that Amazon uses are plain, uncoated corrugated cardboard. While they will have printing on them, it will be black and not colored. However, as with any box received you should remove any shipping labels and plastic tape.

Furthermore, the ‘Amazon Prime’ tape, while it is kraft-paper based and compostable, it obviously contains colored inks, as it is a black and blue color. So, if one wants to be organic, you should take steps to not compost the tape.

Amazon doesn’t coat their boxes with any chemicals

Also – in researching this article I came across reports of Amazon spraying boxes with chemicals. Contrary to what some commenters on the internet may say, Amazon does not spray their boxes with anything. Posts were circulating on social media in 2020 stating that they sprayed chemicals on their boxes. However, Amazon conducted an interview with Snopes stating that they do not spray any chemical on their boxes. [22]

Ok! I think I’ve written enough about cardboard for one day! BTW – if you happen to print this article out, do so in Black & White color and then COMPOST IT!

Read more on composting here