by Roberta Fabbrocino

If your area has a temperate or continental climate, you probably get to enjoy four different seasons every year. The changing of leaves is perhaps the clearest indicator that the season is changing along with temperature changes. Seeing bare trees in winter, flowering branches in spring, lush greenery in summer, and warm-toned in autumn it’s the beauty of the cycle of seasons, but did you know that this perennial change is vital for soil health? Dead leaves, in fact, a primary ingredient of humus, a dark amorphous type of soil organic matter (SOM) which name sounds a lot like the Middle Eastern dish, but it’s actually made of leaf litter and decaying organic matter. The presence of humus is beneficial for soil health, as it helps the soil to retain moisture and stimulates the formation of an adequate soil structure. So if you have a garden or a backyard, you may want to keep your leaf rake in the shack more often. Let’s see why! 

Messier gardens, emptier landfills 

According to the United States Environmental Protection Agency, the generation of yard trimmings including leaves, grass, brush, and tree trimmings from residential, institutional, and commercial sources in municipal solid waste (MSW) amounted to 35.4 million tons in 2018, making up 12.1% of the total MSW generation. In the same year, public and private solid waste programs sent an estimated 10.5 million tons of yard trimmings, representing 7.2% of all MSW to landfills. But why is that a problem? When organic matter, such as dead leaves, decomposes in anaerobic conditions in landfills emits methane, a greenhouse gas. Its anthropogenic emissions are also generated during the production and transport of oil, coal, natural gas, and animal farming. In 2020, the total methane emissions amounted to about 570 million tonnes (Mt). Human activities generated 60% of said emissions. Like the other greenhouse gasses (carbon dioxide, nitrous oxide, and ozone), methane traps the heat causing the temperatures on our planet to rise, and it is about 28 times more potent than carbon dioxide at warming the Earth over a 100-year period. 

Leaves: soil’s favorite food 

The forest floor may seem like a quiet, static place to human eyes, but if you look closer, you’ll see that it is as lively as it gets. Leaf litter or litterfall provides a habitat for a plethora of organisms that depend on it for shelter or forage. The list includes plants like the Hyacinthoides non-scripta or common bluebell, amphibians like caecilians and salamanders, birds, and even large mammals such as deers and elks. Smaller organisms whose presence is crucial for soil health also eat plant detritus: earthworms. They aid soil aeration, infiltration, nutrient cycling, structure, plant growth, and water movement. Earthworms and other decomposers such as amoeba, cryptostigmata, insect larvae, millipedes, mollusks, nematodes, oribatid mites, pot worms, rotifer, springtails, tardigrades, and woodlice convert the carbon structures of fresh residues into carbon dioxide (CO2) and water (H2O), and plant nutrients. The surrounding plants absorb these substances allowing the cycle to continue.

Keeping plants warm

When the temperatures drop, dry leaves form a natural barrier that protects the roots from the chilly breeze, keeping them freezing and thawing in cold areas. As an organic winter mulch, dry leaves also help retain moisture in the soil while letting enough rainwater seep down to the soil surface and allowing air to reach the ground.

Enough is as good as a feast

As we have learned, a thin layer of dry leaves is beneficial for the soil, but a thick layer may do more harm than good. It could keep air and water from penetrating the ground, potentially exposing the roots to fungal diseases and making them susceptible to rot. If you have an outdoor space where fallen leaves tend to pile up, you can safely compost the excess leaves at home.