How Does Leaf Litter Decompose?
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How does leaf litter decompose? Fallen leaves break down mainly through the work of soil microbes. Fungi and bacteria break down the leaf material, and the nutrients the leaves contain are gradually released into the soil in forms that plants can take up. Moisture, oxygen and the nitrogen content of the leaves all affect how quickly this happens. This article explains that mechanism using two Rutgers Cooperative Extension fact sheets, one on leaves in the landscape and one on backyard leaf composting.
Quick answer
- Microbes do the work. Rutgers says “Beneficial microbes, like fungi and bacteria, break down leaf organic matter and convert the nutrients into forms that plants can absorb.”
- Nutrients go back to the soil. According to the same fact sheet, “As leaves decompose, essential nutrients are returned to the soil naturally.”
- Moisture affects the pace. The composting fact sheet says “Inadequate moisture reduces microbial activity, while excessive water may cause anaerobic conditions.”
- Dead leaves can be low in nitrogen. The composting fact sheet notes “Dead leaves lack adequate nitrogen for rapid decomposition.”
What leaf litter is
Leaf litter is the layer of fallen leaves on the ground. Rutgers defines the term this way: “Leaf litter is the common and scientific term used to describe fallen and decomposing leaves.” The fact sheet adds that “leaves are not garbage” and that leaf litter “nourishes soil and sustains life.” The definition includes the word “decomposing,” which tells you the material is already breaking down when it is called litter.
The process is biological. The Rutgers landscape fact sheet describes soil organisms that have long been part of the cycle: “Countless species of beneficial soil organisms also evolved with leaf drop, decomposing leaves and cycling nutrients for plant uptake while sequestering massive amounts of carbon.” The sentence names the organisms, the leaves they work on, and the result, which is nutrients that plants can take up again.
The microbes that break leaves down
The main agents of decomposition are microorganisms. The landscape fact sheet names fungi and bacteria directly. It states that “Beneficial microbes, like fungi and bacteria, break down leaf organic matter and convert the nutrients into forms that plants can absorb.” This sentence describes two steps. The organisms break down the organic matter, and they change the nutrients in that matter into forms that roots can take in.
The fact sheet’s figure caption shows fungi on fallen leaves. It describes “Last year’s oak leaves slowly decomposing with the help of small pinwheel mushrooms.” The caption shows the leaves changing over time, but it does not give a timeline, so the speed of the process has to be read from the mechanism rather than from a number.
Composting sources describe the same biology from the pile side. The Rutgers composting fact sheet says that “This process involves primarily the microbial decomposition of organic matter.” It also explains that “Raw organic materials can be converted into compost by microorganisms.” The fact sheet adds that leaves carry their own organisms, because “leaves, themselves, contain a multitude of microorganisms.” For that reason the fact sheet says that “Available commercial activators or starters definitely are not needed.”
How nutrients return to the soil
Decomposition is also a nutrient cycle. The landscape fact sheet says “As leaves decompose, essential nutrients are returned to the soil naturally.” The fact sheet compares this to store-bought inputs and states that “Chemical fertilization and store-bought mulches are no match for all the benefits that leaves provide to soil and plant life.” The benefit it describes comes from the leaves breaking down in place rather than from an added product.
The fact sheet also links decomposition to soil structure. It says “As leaves break down, they help improve soil structure by increasing its porosity.” It adds that “This allows roots to penetrate more easily and enhances air and water movement through the soil.” The same fact sheet says that the process “also improves water holding capacity.” These are the physical results of leaf breakdown that sit alongside the chemical release of nutrients.
Moisture, oxygen and nitrogen
Three conditions set the pace of decomposition, and the composting fact sheet describes each one. The first is moisture. Rutgers says “The composting pile must be kept moist, but not soggy, for proper decomposition.” It gives the reason: “Inadequate moisture reduces microbial activity, while excessive water may cause anaerobic conditions.” Too little water slows the microbes, and too much water can leave them without the oxygen they need.
The second condition is oxygen. The composting fact sheet says that “The main objectives of turning are to shift materials from the outer parts of the pile closer to the center for better decomposition, and to incorporate oxygen.” It also warns that “If the pile is too large, the interior will not obtain the oxygen needed for adequate, odor-free decomposition.” A pile that is too big can therefore decompose poorly in its center even when it holds plenty of material.
The third condition is nitrogen. The composting fact sheet explains: “Dead leaves lack adequate nitrogen for rapid decomposition.” It then says that a high-nitrogen fertilizer added to the pile “may speed up decomposition,” and it adds that “while it is generally unnecessary to add fertilizer,” a faster result and a product with more nutrients can be obtained with added nitrogen. The fact sheet therefore treats nitrogen as a factor in speed, and as something a pile can usually do without.
Temperature and turning in a pile
When leaves are composted in a pile, temperature becomes a visible sign of activity. The composting fact sheet says “As microorganisms decompose organic matter, temperatures within the pile increase.” It describes the center of a working pile this way: “Within a few weeks after starting, the pile should be hot in the center. Heating generally indicates that the pile is decomposing properly.” The same fact sheet says that a failure to heat “may be caused by too little or too much water, improper aeration, packing too tightly, or a pile that is too small.”
Turning helps the process along. The fact sheet says “The pile should be periodically turned or mixed.” It recommends that “During warm weather, turn the pile once a month,” and it advises against frequent turning in cool weather because “it allows too much heat to escape.” The fact sheet also says the outer material should move toward the center so that the leaves decompose better and take in oxygen.
Volume is another sign of progress. The fact sheet says “As leaves decompose, they should shrink to less than one-half of their original volume.” This gives a simple check on how far the leaves have broken down.
What happens when leaves stay on the ground
Leaves left in place go through the same breakdown on the surface of the soil. The landscape fact sheet says “Soil moisture retention reduces evaporation and decreases the need for irrigation,” and that “Insulation provided by leaf litter keeps soil cooler in the summer and warmer in the winter.” These are effects of the leaf layer on the soil beneath it.
The fact sheet also warns about too much leaf material in a bed. It says “Be careful not to put too many leaves in garden beds, because just like over-mulching, it can smother plant roots and prevent sufficient air exchange for soil microbes.” This is the oxygen problem again, seen in a bed rather than a pile. A thin layer leaves room for air and for the microbes. A thick layer can block them.
Shredding changes the habitat as well. The landscape fact sheet notes that “shredded leaves reduce habitat value for beneficial insects, so this should be avoided when possible.” The sources in this article do not give a measured decomposition speed for shredded leaves compared with whole leaves, so this article does not state one. Our guide on how to make compost from leaves covers the practical side of a leaf pile.
Carbon and the wider cycle
Decomposing leaves also store carbon. The landscape fact sheet says “Decomposing leaf litter stores carbon in the soil, which helps fight climate change.” The statement is about carbon kept in the soil, and the fact sheet presents it as a benefit of leaving leaves where they fall. It does not give a carbon figure for a given yard, so this article does not supply one.
The fact sheet also discusses what happens when leaves are removed. It says that “Transporting collected leaves to composting facilities, landfills, or other disposal sites requires trucks that typically run on diesel or gasoline.” It also notes that if leaves end up in a landfill, they “can produce methane as they decompose anaerobically.” The comparison shows that where leaves decompose affects what gets released, even though the basic breakdown is the same.
For readers who want another use for the same material, our guide on how to make mulch from leaves and grass describes a different way to put leaves to work in the yard.
How we chose
We chose two non-retail sources, both from Rutgers Cooperative Extension. One is a landscape fact sheet on leaf litter (FS1369) and one is a fact sheet on backyard leaf composting (FS074). Both were fetched directly with curl and checked for the quoted statements in this article. The two pages cover the same subject from different angles, which is why they support each other on decomposition, moisture, oxygen and nitrogen. We did not use retail pages. The quotes come from the text of the two fact sheets.
What we could not verify
The sources explain the mechanism but do not give a decomposition rate for a particular leaf type. The landscape fact sheet mentions “oak leaves” in a figure caption, and the composting fact sheet does not give a number of weeks for a leaf pile to finish. Neither source states how much nitrogen a given leaf type contains.
What the sources do not say
The two Rutgers pages do not name the individual fungal or bacterial species that break down leaves in a given yard. They do not give a time for leaves to decompose on the ground. The composting fact sheet gives a pile-size range and a turning schedule, but it does not convert those into a decomposition rate. Where a figure is missing from the quoted text, this article leaves it out.
Frequently asked questions
What microbes break down leaf litter?
Rutgers names fungi and bacteria. Its landscape fact sheet says “Beneficial microbes, like fungi and bacteria, break down leaf organic matter and convert the nutrients into forms that plants can absorb.” The fact sheet does not list other groups.
Do fallen leaves feed the soil?
Yes, according to the Rutgers fact sheet. It says “As leaves decompose, essential nutrients are returned to the soil naturally,” and it describes the leaves as a source of nutrients that plants later absorb.
Why does a pile of leaves decompose slowly?
The composting fact sheet gives several reasons. It says “Dead leaves lack adequate nitrogen for rapid decomposition,” and it notes that a pile needs moisture “but not soggy” and enough oxygen. A pile that is too large does not get oxygen into its interior.
Should leaves be turned while they decompose?
The composting fact sheet recommends turning. It says “The pile should be periodically turned or mixed,” and it advises turning “once a month” during warm weather. It warns against frequent turning in cool weather because “it allows too much heat to escape.”
Is it harmful to leave leaves in a garden bed?
The landscape fact sheet warns about too much leaf material. It says “Be careful not to put too many leaves in garden beds, because just like over-mulching, it can smother plant roots and prevent sufficient air exchange for soil microbes.” A thinner layer leaves room for air and for the microbes that decompose the leaves.
