Leaves left on the ground really become soil?

Don’t they just get moldy?

Yes, the leaves do become part of the soil. And, yes, “mold” can be involved in the process, but most of the time, that’s a very good mold to have around your yard. Let’s take a look at how this works.

Each fall, nature gives your yard a “windfall” of leaves and plant litter. It truly is nature’s bountiful gift, but you may not realize it. You might even think it’s a nuisance!

Most plant litter (there are always exceptions in science and nature!) has the potential to become nutrients and rich soil for your garden or lawn. Unhealthy plants – say leaves with powdery mildew or other diseases – of course should be removed. But don’t lose this windfall of nutrients and potential soil by “withdrawing it from the bank” too early!

How does this work?

In fall, the leaves of deciduous trees turn vibrant hues of red, yellow, and orange. They swirl to the ground, covering your grass. Many annual plants die and wither. This honors nature’s Law of Return: plants use up nutrients during the growing season, and give them back when the season is over. Even evergreens drop some needles, and their cones.

Oftentimes, homeowners or uninformed lawn care professionals think this cover of leaves and dying plants is unsightly. Or, they fear the plant litter may ruin their lush green grass or garden area. They go into “fall mode”: rake up the leaves, pull out dying annuals, bag and ship them off. But this process takes a piece of our land’s natural fertility along with it. Year after year, we deplete this natural source of nutrients and soil, until we need fertilizers or compost to get our lawns green and our garden growing robustly.
How does nature use up this plant litter and turn it into soil? Let’s look at the “decomposer food web.”

1. Invertebrates, such as earthworms, beetle larvae, millipedes, mites, slugs and snails, that live in the soil shred plant materials into smaller and smaller pieces, increasing the surface area on which soil bacteria and fungi can prey. Mulching the litter with your mower helps speed this process along – but in natural areas like forests, nature does all the work!
2. Next up is something you might call mold. Scientists call it fungi. Fungi can “send out” filamentous threads, called hyphae, that operate much like plant roots. These hyphae release acids and enzymes necessary to break down dead plant material. This makes nutrients available to plants to sustain their own growth. You may have seen this whitish “mold” under leaves and thought poorly of it. It’s quite hard-working, and adds a lot to your soil.
3. As the litterfall is consumed by the decomposer food web, water and inorganic nutrients (i.e. nitrogen and phosphorus) are released into the soil, where they can be taken up again by plants to foster new growth.

So, do yourself a favor this spring (and next fall) and leave the leaves. Don’t leave them in big piles that will suffocate your grass, but mow them over to form a mulch. Not only will this replenish your soil’s fertility, but will help suppress weeds and retain moisture to boot.

In your garden area, of course, move the plant litter aside for some of the newer plants, but keep most of the soil covered. That “mold” you see on the leaves is fine. You may want to cover the dead plants with a new layer of mulch.

One thing you don’t want to do is to use decomposing leaf litter in a newly dug hole to replace compost. The difference between finished compost and decaying leaf litter is huge in the world of soil biology and plant life. The decaying leaves actually take up a lot of energy, whereas compost is ready to give back fully. Also, the biology of decomposition means that it needs some oxygen to work best – and by burying, you’re disrupting nature’s cycle.

If the aesthetics of leaves and plant litter are too much to bear, start a compost pile with your leaves and grass clippings, so you can use it later in your garden! It’s a win-win-win: save time raking, improve your soil, and reduce greenhouse gas emissions associated with leaf blowers, green waste removal, and methane release from landfills.

By Jessica Chiartas, UC Davis

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