Every plant owner must come across peat and humus to improve their soil.
But what’s the difference between the two? Which should you use in your gardening projects?
Peat (or peat moss) is semi-decomposed sphagnum moss. Being highly water-retentive and naturally acidic, peat is a popular choice for propagation, amending soil pH, and improving soil aeration. On the other hand, humus (including peat humus) is highly decomposed, nutrient-rich organic matter and is great for improving soil fertility and mulching.
In this article, we will make an in-depth comparison between these two commonly used soil amendments and how they are best used in the garden.
Is it peat or peat moss? Humus or peat humus? Peat moss or Sphagnum moss? It can be confusing as these terms are often used interchangeably in some contexts but not in others.
The differences are very simple. You only need to understand the 3 layers of a peat bog, where peat, humus, and sphagnum moss come from.
Peat is basically dead and decomposed plant materials that are deposited and accumulated under a top layer of live vegetation.
Peat moss is a type of peat. Since wetland vegetation is mostly mosses (mostly Sphagnum moss) and grass-like sedge, Peat is basically dead Sphagnum moss.
Apart from peat moss, there are also coco peat (which is the decomposed fibers of coconut husks), sedge peat (which is formed from decomposed sedge), etc.
To be more specific, peat can be further divided into “peat moss” (partially decayed sphagnum moss) in the middle layer of a bog and “peat humus” (highly decomposed sphagnum moss) which gets pushed to the bottom layer of a bog.
Note that when used in contrast with sphagnum moss which is a live plant, “peat” is often used as a general term to refer to dead moss, which includes both “peat moss” and “peat humus”.
1.2 Humus vs. Peat Humus
Peat humus is a type of humus (pronounced as “hue-mus”, not to be confused with the Middle Eastern dish “hummus”).
Both are highly decomposed organic matter. While humus is decomposed from a variety of organic matter such as kitchen waste, leaves, grass clippings, bark, insects, etc, peat humus is decomposed from mostly Sphagnum moss.
Humus of organic matter generally takes up to a few months to fully decompose. But, peat humus can take hundreds, if not thousands, of years of decomposition.
In terms of the terminology, when people compare “peat” with “humus”, they can be comparing “peat moss” (partially decayed moss) with “peat humus” (highly decayed moss).
But they can also be comparing peat moss with regular humus decomposed from general organic matter. This is because peat humus, as a non-renewable natural resource, is hard to find or is even prohibited for horticultural use in some countries. Even though some products name themselves “peat humus”, it’s mostly composted wood fibers or is basically peat moss which is partially decomposed.
Peat moss has the same dark brown color as soil. However, it’s not as heavy as soil and has a spongy texture that crumbles into fine visible fibers of Sphagnum moss.
On the other hand, humus (or peat humus) which is highly decomposed is much darker, in fact almost black in color, and has a finer texture than peat moss. It typically also has a smell like wet soil.
Peat (or peat moss) comes from the bottom layer of wetland or bogs. It is harvested by temporarily draining water from peat bogs and is then cut from bogs in “bricks”, dried, screened, compressed, and packaged for distribution.
Humus is mature compost and is obtained from decomposed organic matter. Unlike peat, consisting mostly of sphagnum moss, compost comprises a wide range of organic matter, like seeds, leaves, fruit peels, kitchen scraps, and wood shavings.
Both peat (particularly sphagnum peat moss) and humus have high water absorption and retention capabilities because of high porosity. They can hold up to 10 times their own weight in water which is double what mineral soil can hold.
Peat tends to retain more water than humus, especially when added to sandy soil.
However, peat can become hydrophobic, or water-repellant once they are dry.
Peat moss is not suitable for improving soil fertility because it contains very little, if any, plant nutrients. You will need to apply fertilizer when using peat moss.
On the other hand, humus or finished compost is rich in Nitrogen (6%) and also small amounts of Phosphorous and Sulfur.
Peat and Humus are two different soil amendments. And although they’re sometimes used for similar applications, each one is more suited to a particular task than the other.
6.1 For starting seeds
Both peat and humus are water retentive, but humus may contain pathogens and fungi which can affect the success of seed germination.
Peat is the best medium for starting seeds as it holds moisture and slowly releases it. This prevents tender seedlings from drying out during germination and helps speed up rooting. Peat is also sterile, so there’s little chance of infection.
Peats that are sold in pellets also help reduce transplant shock and root damage. The dry pellets can rise into the shape of a self-contained pot as the peat is held together by fine netting. You can put 2 seeds in each pellet. When roots emerge through the porous walls of a pellet, you can transplant the seedling together with the peat without damaging the tender roots.
Peat pellets for starting seeds
6.2 For propagation
Peat is a better choice for propagating cuttings, as it is sterile and retains moisture.
Use a mixture of two-part potting soil and one-part peat for propagation.
6.3 For improving aeration
Both peat and humus can be used to aerate the soil due to their high porosity, but peat tends to do it better. That is because peat is light and fibrous and doesn’t compact. It can keep the soil aerated for years. However, one should also take the higher cost of peat into the account.
Finished compost contains many organisms, including earthworms, bugs, and other organisms. As the organisms eat, reproduce, and excrete, they burrow through the soil, creating paths for air to enter the soil. But within a year or so, the compost will likely become compacted.
Loose peat moss
Compared to other growing media, humus (including peat humus) is hands down the best choice for improving soil fertility because it contains humic acid with very high “cation exchange capacity”, namely the capacity of the soil to attract positively charged plant minerals, such as Calcium, Magnesium, Potassium, and to prevent nutrients in the soil from being leached away during watering.
Also, they are rich in nutrients that are readily available for immediate uptake by plants.
Regular humus (from kitchen waste, leaves, tree branches, etc.) is also much more affordable than peat, making it the best choice for improving soil fertility.
Peat also has a high (though not as high) cation exchange capacity to attract and hold plant nutrients but it itself has little to no nutrients. It is also more expensive than humus.
According to a study (Puustjarvi & Robertson, 1975), the cation exchange capacity of highly decomposed black peat is 160 cmol(+)/kg, which is higher than Sphagnum sedge peat (110 cmol(+)/kg).
Another study by the University of Florida shows that humus of other organic matter can have a CEC of even up to 300 cmol(+) /kg. In comparison, clay soil kaolinite only has a CEC of 3 – 15 cmol(+)/kg.
Peat is a popular choice for amending soil acidity for acid-loving plants, such as berries, azaleas, camellias, hydrangeas, cucumbers, beans, tomatoes, carrots, etc.
Peat is naturally acidic because of the anaerobic decomposition of peat bogs which are often waterlogged with little or no oxygen.
Peat has a pH of 3.5-4.1 (for untreated, raw peat harvested from a peat bog) and a pH of 4 to 4.5 (for peat after treatment).
However, more and more people are looking for peat-free alternatives such as ericaceous compost which also has a low pH, due to environmental concerns and the increasingly high cost of peat.
Humus, on the other hand, is less acidic, with a pH of between 5 to 7.
Peat is rather expensive for use on a lawn of any size. Additionally, it contains no nutrients to make the grass greener.
Humus compost is the better option as it’s less expensive and contains nutrients to make your lawn green. Spread ½ or ¼ of compost over the grass, and leave it there. Rain, human activity, and soil organisms will quickly work it into the soil.
Peat is not suitable for mulching because it absorbs all the water, preventing it from infiltrating the soil.
Humus is a good choice for mulching as it allows water to infiltrate while also reducing evaporation. But you’ll need to use a thick layer if you also want to prevent weeds.
Flower beds are better served with compost. It is cheap, easier to re-wet when dry, and provides nutrients to the plants.
Simply work 2–3 inches of compost into the flower bed, and you’re good to go.
The best approach for tomatoes and vegetable gardens is to mix both Peat and compost into the soil. Peat will make the soil slightly acidic, and compost will provide nutrients.
The mixture should contain two-parts soil, one-part peat moss, and one-part compost.
Peat can hold a lot of water, but it becomes hydrophobic and difficult to re-wet when dry. It is best to use compost as it’s easier to re-wet when dry and contains nutrients for healthy growth.
Mix 2 to 3 inches of well-finished compost with potting soil for indoor pots.
Acid-loving plants like tomatoes and berries and moisture-loving plants like spinach and celery do well in peat-based soils. But you will need to apply fertilizer continuously to keep the plants healthy.
It is not sustainable to harvest peat from bogs because it can take hundreds if not thousands of years to form. Although only a small percentage of peat has been harvested in the world, it has been harvested at a rate faster than it can produce.
Research has shown that harvested bogs can only return to an ecologically balanced system in only 5 to 20 years because peat forms at a rate of only an inch every 15 to 25 years.
Countries like the United Kingdom have resolved to end the sale of peat for gardening by 2024.
Because of that, peat is more expensive than humus, and more and more people are using coco coir or coco peat as an alternative to peat moss.
In contrast, humus or finished compost is sustainable, as it can be produced within months, using different organic materials.
Peat and humus are both decomposed organic matter with different pH, water retention, and nutrient content.
Being highly water-retentive, peat is best for germinating seeds and seedlings, and propagating cuttings. Its low pH value also makes it suitable for growing acid-loving plants
Humus, on the other hand, is best for mulching, improving soil fertility for its high mineral content. Its fast decomposition time and cheap production cost also makes it a great candidate for use in flower beds and lawns
Puustjarvi, V. & R.A. Robertson (1975). Physical and chemical properties, p. 23–38. In: D.W. Robinson and J.G.D. Lamb (eds.). Peat in horticulture. Academic Press, London, UK.
Peat Moss vs. Peat Humus – Knowledgebase Question – Garden.org. (n.d.). Garden.Org.
Perry, L. (n.d.). What is Peat Moss? University of Vermont.
What’s the Difference Between Spagmoss and Peat Moss? The Environment. (2020, November 30). Besgrow – At the Root of Healthier Plants.
Sphagnum Moss vs. Peat Moss. (2014, July 27). Garden.Org.
Mira, M. (2021, February 11). Peat Moss: Benefits and Disadvantages. Sunday Gardener.
National Geographic Society. (2012, October 9). bog.
Nutrient Management. University of Florida.
Photo credits:Ragesoss, CC BY-SA 3.0, via Wikimedia Commons
Michael Graham / Peat Hagg, Borrowdale Moss