Compost vs. Peat Moss: When Do You Use Each One?

by Matt Gibson and Erin Marissa Russell

Peat moss and compost are two all-natural soil amendments that are commonly used by modern gardeners to adjust the physical structure of the soil and to help improve fertility and increase nutrient levels. Each of these soil amendments has different properties and should be used when there is a specific need for the ingredients while creating a balance in your property’s soil and rejuvenating it for upcoming growing seasons.

Though compost is widely available commercially, many gardeners around the world process and create their own compost by recycling garden, lawn, and kitchen scraps and allowing them to decompose in a controlled environment. Peat moss, on the other hand, is mined from bogs in Canada and in the northern US using surface harvesting. Organic gardeners use a variety of different soil amendments to help them get the most out of their soil. Read on to learn more about compost and peat moss.

Improving Your Soil Quality

In order to improve the quality of your soil, you need to understand what you are working with. Soil building and soil enhancement are important tasks that help you improve your soil, but you need to know what your soil is lacking before you can choose the correct amendments that you want to use to balance and reinvigorate your soil.

The best way to figure out what kind of soil that you have is to have the soil tested by your local extension service or by a company that offers soil testing services. Alternatively, you can purchase a home soil test kit and test it yourself. After you know your soil’s salinity and pH level, you will have a better idea of which amendments you will need to add to balance your soil to optimal levels for the upcoming growing season and beyond. Most labs will also provide recommendations on what amendments to use to and what quantities of each are needed to help balance out your soil efficiently. 

Differences Between Compost and Peat Moss

Compost is relatively cheap to purchase and it’s also free to make your own using kitchen and garden scraps, though it does take some time to decompose and mature. Compost is cheaper than peat moss, as peat must be harvested, packaged, and transported. Peat’s composition is pretty consistent and uniform, whereas compost is composed of many different sources, including leaves, kitchen scraps, wood, decaying plant matter, and more, so it varies in quality more so than peat moss. 

Compost that is made mostly from plant products tends to be low in salt, but salinity levels can vary in compost depending on what the compost is made from. Peat moss, on the other hand, is very low in salt content and is often used to reduce salt levels in garden soils. Compost pH is usually neutral or alkaline, while peat moss pH levels are quite acidic, usually in the range of 4.4 When large amounts of peat moss are added to your soil, lime should also be mixed in to help neutralize the pH level.

Compost is rich in nutrients, contains many helpful microorganisms, and can be an excellent mulch when added as a top layer to your garden beds. Peat moss is not incredibly fertile but it does have some nutrients and microorganisms. Peat moss is not a good mulch as it has such a high water retention that it can keep water from passing to the soil below. Also, when it dries out, it can simply blow away in the wind.

Compost can compact and sometimes contains weed seeds, unlike peat. Because peat moss does not compact, it can provide excellent soil aeration for multiple years. Both compost and peat moss hold water well, but peat moss has better moisture retention levels, especially when added to rocky or sandy soils.

All About Compost

Composting, as a gardening technique, has been around for centuries. Adding decomposing organic matter to your soil is the most natural way to replenish nutrients and fertilize the topsoil and beyond. Because it takes multiple years for topsoil to naturally replenish itself, gardeners and farmers have used compost instinctually to assist the replenishment process to help the soil produce valuable crops year after year.

These days, composting options have a repackaged modern appeal. Homes, corporations, small businesses, and communities have worked together to create regular composting systems. Composting bins are available everywhere you look thanks to the efforts of environmental activists and their composting awareness campaigns.

Home composting systems only require a small bin or container in the kitchen area where food scraps are collected. It’s always a good idea to break down your kitchen scraps as much as possible by chopping or shredding them before dumping them into the container.

Add a thin layer of soil and a small amount of water to the mixture intermittently to help speed up the decomposition process. Don’t add too much water at one time, however, as compost materials do not have the ability to absorb much water at once, so it is essential that water is added slowly over the period of several weeks. Otherwise, the whole mixture may likely turn into a gross, stinky mush. Keep in mind that the process of decomposition produces its own liquids as well, so it doesn’t take much water to keep the bin moist, but not watery. If the mixture starts to become too soggy, add more scraps and soil to balance it out.

Composting in a contained environment is very different from composting methods of old. Original composting was simple. Farmers just threw out their potato peelings and other scraps into the garden and let their pigs do the rest. The pigs would eat the scraps, digest them, and then spread the manure over their crop fields.

Though the methods of today’s composting and the composting of the past vary greatly, the end result is pretty much the same. Nutrients are processed and transferred from decaying organic matter into usable garden soil. When the soil is fully fertilized, it is believed to have absorbed the full potential of nitrogen and other essential nutrients found in healthy soil.

Uses For Compost

Commonly called black gold, compost uses a process known as cation exchange capacity to improve the structure of the soil and help it to better hold nutrients. Compost also promotes the presence of earthworms which significantly improve the soil quality. Earthworms enrich the soil by digesting and processing decomposing organic matter and producing nutrient-rich castings, as well as helping the aeration of the soil by burrowing through it.

Compost can be used as a backfill when planting shrubs, trees, and perennials. It is perfect for establishing planting beds and lawns. It can also serve well as a mulch for landscape plantings and gardens, and as a side dressing for vegetables or for erosion control.

All About Peat Moss

Peat moss is a decomposed fibrous material which forms when mosses and other living materials decompose within peat bogs. The most important difference between peat moss and the compost that garders create in compost bins in the backyard is that peat moss is composed primarily of moss, and the decomposition occurs without the presence of air, which significantly slows the process of decomposition. While you can create quality compost in less than a year’s time, it can take several millennia for peat moss to form. Peat bogs gain less than a millimeter in depth every year. As the growth rate of peat moss is so slow, peat moss isn’t considered to be a renewable resource.

The majority of the peat moss that is used in the United States comes from bogs in Canada. Even though the mining of peat moss is strictly regulated, groups like the International Peat Society highlight the significant controversy surrounding peat moss harvesting. While only 0.02 percent of the reserves are available for harvest, the mining process releases massive amounts of carbon into the atmosphere, and the bogs continue to spill out excess carbon long after the mining is completed.

Because of the environmental concerns, many gardeners feel guilty about using peat moss in their gardening projects. People with strong feelings on either side of the issue each can make a strong case on the ethics of using peat moss in the garden, but in the end, it’s up to you to decide whether the environmental effects overpower the benefits to your garden that peat moss provides. As a compromise, perhaps consider using peat moss as sparingly as possible in garden projects, using it only on a case-by-case basis, for projects such as starting seeds or making your own potting mix. For larger projects, such as amending garden soil, use compost instead.

In the garden, peat moss is a great choice as a growth starter for plants, fruits, flowers, and vegetables and it meets organic farming requirements as well. Peat moss is commonly used as a soilless alternative for starting seedlings in a greenhouse setting due to its ability to retain moisture for lengthy periods of time. Peat moss is also widely used as an alternative for household plants because of the safety of the ingredient mix and its ability to retain water for long periods. It is able to absorb water quickly and store it efficiently until it is needed.

In regions that receive large amounts of rainwater, peat moss is an ideal candidate for outdoor use. It works well for household gardening as well because plants can be watered just once per week, or often even less if a fair amount of water is given with each watering.

The majority of peat moss mixes are made from a base of decomposed sphagnum moss. This base is often mixed with other organic materials and other mosses and then blended together and sifted prior to packaging and selling. Peat moss ranges in texture from rough, resembling clumpy dirt, to a medium similar to chopped up tree bark.

Peat moss smells like organic matter, but is no more pungent than a compost bin with the lid removed. It’s safe for indoor and outdoor use and is not toxic to humans, animals, or the environment. Peat moss is ready to use fresh from the package, unlike composting materials, which can take months to be ready to use in your garden beds.

Uses For Peat Moss

Peat moss is most commonly used as a soil amendment. It is also often used as an ingredient in planting mixes and potting soils. Some gardeners even add peat moss to their compost as a carbon source, as well as a bulking agent. As peat moss is sterile, its presence in soils can help minimize disease issues. Peat moss should never be used as a mulch because it can blow away in the wind if it becomes dry and its fibrous structure and water retention capability can steal moisture from the underlying soil.

Home gardeners use peat moss as a soil amendment or as an ingredient in potting soil. Its acidic pH level makes it perfect for acid loving plants such as camellias and blueberries. For plants that like a more alkaline soil, compost is a much better option. As peat moss doesn’t break down readily or compact, just one application can last for several years before needing to be replaced. Peat also doesn’t contain harmful microorganisms or pesky weed seeds that may be present in poorly made compost.

In smaller amounts, peat moss is an important component of most store-bought potting soils and seed starting mediums as it holds several times its weight in moisture and releases the water to the plants roots as its needed. Unlike other mediums which allow nutrients to be leached out of the soil with rain or irrigation, peat moss holds onto nutrients so that they are not rinsed out of the soil with ease. Peat moss, by itself, does not make a good potting medium, but must be mixed with other ingredients. Potting soils should be comprised of one third to two thirds peat moss at most.

Oftentimes, peat moss is referred to as sphagnum peat moss as much of the decomposed material in a peat bog comes from sphagnum moss which commonly grows on the top of the bog. Sphagnum moss and sphagnum peat moss, however, are two distinctly different materials. Sphagnum moss is comprised of long, fibrous strands of plant material, and is often used by florists to line the insides of wire baskets, or as a decorative filler for potted plant compositions.

Compost Vs. Peat Moss

Compost and peat moss are both common soil amendments that are each added to soils to improve them in different ways. When deciding which ingredient to add to your garden beds, here are some important differences to consider.

Peat moss is a bit more expensive than compost, especially if you are making your own compost, or purchasing it from a local source. Compost can also be cheaper when purchased in bulk. The peat moss that we buy is generally sourced from Canada, so harvests must be shipped from there, which increases the cost exponentially.

If nutrient content is what you are after, peat moss has very little to offer in that regard. Compost is much better for replenishing nutrients in your soil. However, compost is not an alternative to fertilizer. Fertilizer is much higher in nutrient content than compost, and will still be needed no matter what soil amendments you use to help keep nutrient levels high enough to allow your plants to thrive.

The main nutritional value that comes from using compost is from its effect on soils and the microorganisms that are contained within the soil. Peat moss increases the cation exchange capacity of the soil, helping it to hold on to nutrients for extended periods, like moisture, releasing them when needed to help your plants thrive. 

Peat moss has a low pH (4.4), so if you use a lot of it, lime should be added as well to balance out the acidity. Plants that thrive in acidic soils (called ericaceous plants) such as blueberries and rhododendrons, love growing in peat, but most plants tend to prefer soils that are closer to neutral in pH range. Compost typically has a neutral pH level or slightly alkaline leaning.

Peat moss can last for years in soils, as it doesn’t compact and provides ample aeration and water retention. Compost has a tendency to compact, as well as lose its nutritional value over time, so it should be replenished on a yearly basis. Both mediums hold water, though peat moss tends to hold water more efficiently. This makes it a great choice for amending sandy or rocky soils that tend to lose moisture, as well as nutrients, too quickly.

Peat moss can be hard to get wet at first, and once it dries out, it can be very hard to re-wet. Composts tend to vary depending on the contents of the compost mix, on how easy they are to re-wet. Once wet, both these materials hold water very well and release it to the roots of your plants over time. If peat moss gets too dry, you can moisten it in a plastic bag overnight. Using warm water will help speed up the process, as it is absorbed much easier and quicker than cold water. You can also add a few drops of detergent to the water to act as a surfactant, which will help speed up the re-wetting process.

Composts have many variables in their composition, which depend on the sources used to create the compost. Peat moss, on the other hand, has a uniform composition. Composts may also contain some contaminants, depending on source materials. Peat moss contains very few microorganisms, while composts are rich in microorganisms. More often than not, these microorganisms are beneficial, and can improve the soil drastically, by helping to aerate it and by replenishing nutrient levels.  

Peat moss contains no weed seeds. Composts which are prepared properly should not contain weed seeds, but this is not always the case. If the compost was treated well, it would reach high enough temperatures to kill any weed seeds during the decomposition process. If it remained properly covered during the preparation, that would prevent any weed seeds from blowing in. Sometimes, however, inexperienced gardeners may compost weedy plants, which could lead to contamination of weed seeds. To test out your compost, put some into a pot and water it. After a couple of weeks, you should be able to tell if any weeds have begun to germinate. It’s a good idea to test out compost before adding it to your garden beds to make sure you are not growing weeds instead of your desired plants.

Peat moss contains no disease suppressing qualities, compost, on the other hand, contains microorganisms, which may suppress some disease-causing pathogens. Peat moss is a natural resource, obtained by harvesting peat bogs. Peat moss is usually obtained from surface harvesting. Due to modern environmental regulations, most of this is done after conducting environmental impact analysis research, which has led to more renewable and sustainable harvesting techniques. Composts, alternatively, nearly always use recycled organic matter. 

Peat moss should never be used as a mulch, while composts are great for using as a mulch or side-dressing around garden plants. The only downside to using compost as mulch is that unless it is applied in thick layers, composts won’t suppress many perennial weeds. Peat moss should never be used as mulch as it will dry out soils by absorbing water from them, not allowing it to penetrate beneath the mulch layer. Also, once dry, it may blow off the surface.

A solution to the differences between peat moss and compost is to use both in combination in order to reap the benefits of each medium. Some gardeners incorporate peat moss and compost when planting, then top dress perennial plants with compost in subsequent years. Peat moss decreases the tendency of compost to compact, which may extend the life of compost dramatically.

Alternatives and Combinations

Peat moss and compost, when used in combination, have shown very positive results for many gardeners attempting to enrich their soils. Using a combination of the two amendments provides gardeners with a way to reap the benefits of both products. 

In addition to compost and peat moss, there are many other methods for treating and amending soil in the hopes of providing a better medium to increase plant productivity. Aside from other soil amendments, there are alternative methods to replenishing healthy soil. Some experienced farmers and gardeners use the fallow field theory, giving their land, or garden beds, a periodic rest, typically once every seven years, to allow the land to naturally replenish itself. The idea behind this theory is that the ground knows how to heal itself and needs to give, as well as receive. By allowing your land to rest occasionally, instead of placing a consistent demand on your soil, you allow the land to heal and renew itself for future harvests. 

By choosing to not plant in a particular area for a year, gardeners and farmers allow the land time to regenerate nutrient levels. During the fallow year, farmers and gardeners can also apply treatments such as soil amendments, fertilizers, and other additives, to give their crops a boost for the next planting season by enriching their topsoil.

Field or soil aeration is another popular method for treating and improving your fields or garden beds. Using a mini tiller, you can turn the topsoil over in rows, bringing to the surface a healthy layer of soil with unused nutrients. The used layer of soil is turned underneath the top layer, where it can begin to regenerate while not disturbing the new plants.

Which is Better?

After a review of the information provided in this article, you should be able to better make the right decision for amending your soil using compost, peat moss, or a combination of the two, when needed. The compost versus peat moss debate is only one of the concerns you may face when willing your crops to grow into plentiful harvests. Compost is better for certain tasks and peat moss is better for others. Amending your soil is all about balance and knowing what your soil is lacking beforehand.

Common Questions and Answers About Compost Versus Peat Moss

Are peat moss and compost the same thing?

Peat moss and compost are not the same thing. Peat moss is a natural product that’s formed as layers of moss grow over one another. (Peat moss is the bottom layer.) Compost is made as everyday waste materials decompose into nutrient-rich soil. Peat moss is sterile, has an acidic pH, and is not high in nutrients or microorganisms. Compost is high in both nutrients and microorganisms and has either a neutral or slightly alkaline pH.

Does peat moss loosen soil?

Adding peat moss to heavy or compacted soil is a good way to loosen and aerate it for gardening. Peat moss also improves soil’s ability to drain well, and in sandy soils, adding peat moss helps the soil retain water and make moisture available to plants.

Does peat moss make good compost?

You can use peat moss when setting up a new compost pile or add it to an existing compost heap as a brown, or carbon, material. It will help to balance out green materials that are heavy in nitrogen. If you’re using peat moss to start a new compost pile, spread a layer of eight inches of peat moss, then cover it with a few inches of soil from the garden or finished compost. In existing compost piles, you can spread a layer of peat moss over the pile when the bottom layers begin to decompose, then use a fork or spade to mix the peat moss into the compost. If your compost heap has begun to smell, that usually means the compost has too much nitrogen, and balancing it out with carbon-rich peat moss will help keep the smell at bay.

Does peat moss make soil acidic?

Peat moss is rather acidic, with a pH level that’s usually around 4.4. Most soils are much more alkaline than peat moss, so adding peat moss to your soil will increase its acidity.

Does peat moss raise pH?

Because peat moss has a pH level around 4.4 and soil usually has a pH level between 4 and 8.5, most of the time adding peat moss to soil will raise the pH level, not lower it. If you aren’t sure of your soil’s pH level, refer to our article How to Test pH In Your Soil.

How does peat moss improve soil?

Peat moss improves soil because it does not compact over time, so it loosens soil and aerates it. Adding peat moss to soil also helps increase the soil’s capacity for drainage. In sandy soils, incorporating peat moss will help the soil to retain water and make moisture available for plants.

How long does it take for peat moss to break down?

It takes several years for peat moss to break down and decompose, making it a long-term amendment to your soil. Compare the several years it takes for peat moss to decompose to the single year it takes for compost to decompose.

How much compost do I add to clay soil?

Add a layer of one or two inches of compost over the area of clay soil you wish to amend, then mix the compost into the soil. If you’re working with sandy soil instead of clay soil, use a layer of three inches instead of one or two.

Should I mix peat moss with soil?

In almost all cases, peat moss should be mixed with soil before it’s used in gardening. Soak your peat moss in water before mixing it with soil to hydrate it. Let it soak for a few minutes, then stir and add more water.  Your peat moss is well hydrated when a drop or two of water comes out when you squeeze a handful. You do not want to add so much water that it streams out when squeezed. Then add a layer of two or three inches of peat moss over the area of soil you wish to amend. Mix the peat moss down into the top 12 inches of soil.

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