Leaf Mold? Black Gold!

Leaf compost is simple, free, and will transform you garden soil

If there’s a single ingredient I recommend for almost any soil, it’s the leaves that carpet the ground every fall. I like to use them any way I can – tilling them in fresh in the fall or shredding them to use as mulch. My favorite way to feed my garden with leaves, though, is to set them and forget them, adding them back as a rich, full-diet compost, known as leaf mold.

Leaf mold, the name for compost made entirely of leaves, has a number of advantages above and beyond regular compost. As a soil conditioner, it is unparalleled. Adding the spongy material left after leaves decompose literally fluffs the soil, increasing space for water and air and creating terrific habitat conditions for soil organisms. In fact, researchers have found that leaf mold can increase soil water retention by up to 50% to promote living soil activity, improve drought resilience, and increase plant production. Additionally, soil organisms thrive on the organic food provided by leaves and leaf mold. A fungally-decomposed material, leaf mold increases the diversity and abundance of soil life, a fact verified by the prolificacy of earthworms in leaf-amended soils. With all the increased activity, soil structure improves dramatically, over a few short seasons.

Meanwhile, leaf mold has great effects on soil chemistry as well. With a near neutral carbon:nitrogen ratio, it won’t tie up plant nutrients as it decomposes. With their extensive roots, trees also draw up abundant minerals from deep rock and soil, which are transferred to leaves rich in calcium, magnesium, phosphorous, and potassium. Likely because of this, leaf mold can have a slight liming effect on the soil, as it improves structure, drainage, nutrient content, and aeration.

The benefits are so great, and leaves so abundant and free, that I use leaf mold as the foundation for all of my garden amending. To do so is unbelievable simple.

Step 1. Rake leaves.

If done as soon as possible after they fall, leaves retain their high mineral content to add nutrients to the soil. I like to rake a big pile of material onto a large tarp. Dragging the tarp by its edges, I can move a huge volume of this lightweight material.

Step 2. Build a pile.

Because most leaves have the optimal C:N ratio for composting, it’s not necessary to add additional compost materials or fertilizers to the pile. Simply build a pile out of leaves with the minimum 3′ x 3′ compost pile dimensions needed to generate heat. Wet and mix the pile, then step away. Covering with a tarp will also help conserve moisture as it decomposes. The leaves will basically compost themselves. To speed up the process, shred the leaves first, periodically wet and mix the pile, or add a little bit of organic nitrogen.

As an alternative, leaf mold can be made in garbage bags. Fill the bags with leaves. Add a little water. Shake the bag to distribute moisture and stab a few holes for aeration. Set in the shade and come back in 6 months to a year. Simple. The advantage of bags is that they can be stacked to save space.

Step 3. Feed the soil.

Because it’s a cold composting process, the completely finished leaf mold can take some time. But if you have the space, I can’t think of a better, or easier, way to use it to build garden soil. A completely crumbly, finished leaf mold won’t look like leaves at all – just rich, black humus. Depending on moisture and temperature, this final product can take anywhere from six months to two years. Once you get your piles going, though, you’ll always have a continuously cycle of finished black gold. In a pinch, I’ll even use it before it’s completely finished.
Using leaf mold is equally simple. Mix it into the top six inches of soil with bed preparatioin in the fall or spring, or use it as a thick mulch on perennial or no-till garden beds. Because it is composed entirely of leaves, leaf mold, unique among composts, is free of weeds, pests, and disease.

Leaf mold is also great for container gardener, and can be mixed into raised beds to regenerate and renew nutrients and organic matter. If sieved through a half inch screen, it can also be used as a replacement for peat moss in potting soil mixes. (It’s best to avoid peat moss at all costs, as it is a non-renewable mined from peat bogs that are globally important for storing carbon and couter-acting climate change.)