Nobody describes fall better than Albert Camus: “Autumn is a second spring, when every leaf is a flower.” He’s right. Especially this fall. The colours are intense. Sadly once the leaves drop from the trees, many of us shuffle them into recycling bags for city pickup. But recycling those leaves in our gardens can improve our soil.
In nature — in woodlands and forests — trees get most of their soil nutrients from decomposed leaves & twigs (litterfall). Litterfall is the main way nutrients return to the soil as well as the main source of soil organic matter. Why not use Nature’s time proven nutrient recycling and soil improver in our gardens instead of removing them from our garden?
How does leaves improve our soil?
Leaves feed soil organisms, from earthworms to bacteria.
Many of these soil critters are actively involved in nutrient cycling and soil structure improvement of the soil, which means food and a good home for plant roots!
Leaves are a source of humus.
When the leaves breakdown completely it becomes humus. The complex organic molecules in humus acts as glue, binding and holding soil particles together, which improves soil structure.
Decomposing leaves improves soil’s CEC.
Humus has a high cation exchange capacity (CEC). A high CEC increases the ability of soil to hold onto nutrients.
There’s a couple of ways we can use our leaves: straight away as mulch, throw into the compost and finally let the leaves rot down to a leaf mold.
Since leaves lose nutrients and pick up contaminants the longer they stay on the ground, gather the leaves as soon as possible after they fall from the tree. Shredding the leaves with a mower or a string trimmer/weed whacker not only ensures quicker decomposition, but it also reduces leaves matting.
Recycling leaves: Mulch
The easiest option for recycling leaves is to use them as mulch. I use leaves in my clay soil perennial bed, which has over the years improved my clay soil and kept my perennials growing healthy, strong and floriferous.
Colorado State University Extension suggests using leaf mulch of 4 to 6 ins. (10 to 15 cm). The leaves act as an insulator, keeping the roots from freezing in the winter. And as the leaf mulch breaks down, it improves the soil.
Recycling leaves: Composting
Leaves have high carbon to nitrogen ratio — ranging from 30 to 50 — causing them to break down slowly. Leaves are used as the “browns” in the compost. For speedy decomposition in the compost bin, add a similar amount of “greens” (e.g. grass clippings).
Believe it or not, there has been a fair amount of thought and scientific studies conducted in the esoteric field of leaf decomposition! Cornelissen’s 1996 Journal of Ecology paper states that leaf decomposition depends of the physical and chemical properties of the leaf include:
High concentration of lignin and tannins
High lignin : nitrogen ratio
High C : N ratio
Leaves physical toughness
Physical barriers like leaf hairs, spines or wax.
All this extra stuff (i.e. lignin, tannins and spines) not only protects the leaves from being eaten or attacked, but it also happens to slow leaf decomposition as well!
It’s all very well and good, but how does that help us, the gardener?
Don’t use leaves that fall off the tree brown (e.g. oak leaves). Brown leaves usually indicate high levels of lignin and tannins. Glossy leaves, leaves with spines or hair will also decompose slowly. These are best used on leaf mold piles or for long-lasting mulch. On the other end of the spectrum, leaves that fall down when they are green decompose extremely fast.
The Compost Gardener shares information on the best/worst leaves to use in the compost:
Leaves lower in lignin (& higher in calcium and nitrogen)
Ash, cherry, elm, linden, maple, poplar and willow break down in about a year.
Leaves higher in lignin (& lower in calcium and nitrogen)
Beech, birch, hornbeam, oak, sweet chestnut, magnolia and holly need two or more years usually to breakdown.
And if you don’t know your tree names, you can follow The Compost Gardener’s catchy leaf-composting phrases:
Green Leaves – some trees shed green leaves. These can be added in moderate amounts.
Red or Yellow Leaves – These can be used in small amounts.
Brown Leaves – Should be avoided but are good for leaf mold.
Recycling leaves: leaf mold
Leaf mold. No, it isn’t a type of fungus growing on leaves! Leaf mold is the result of decomposing leaves for 6 to 12 months. Leaf mold is very much like compost, but instead of using many types of organic matter, only leaves are used. Leaf mold is easy to make.
If you have a small yard, gather the leaves into a plastic bag (with a few slits for air). Moisten the leaves and let it sit overwinter
If you have a bigger yard you can either pile it up or put into a wire bin (6 ft by 3’), add water and let the leaves decompose
Not many gardeners know this, but leaf mold is far superior to compost as a soil conditioner!
Leaf mold increases the water retention of soils. According to some university studies, the addition of leaf mold increased water retention in soils by over 50%.
Leaf mold also improves soil structure and provides a fantastic habitat for soil life, including earthworms and beneficial bacteria.
The only drawback to leaf mold is that it’s low in nutrients, so you will still need to add organic fertilizers to the soil if you need to increase soil fertility.
Since recycling leaves reduce our reliance on outside sources of soil amendments, it’s a sustainable method to improve our soil! It. No matter how we use leaves in our garden —mulch, compost or leaf mold— recycled leaves improve the soil in our garden by adding organic matter.
Colorado State University Extension. Mulches for Home Grounds
Cornelissen, JHC An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types. Journal of Ecology V84 No4 Aug 1996
North Carolina Cooperative Extension. Making and Using Leaf Mold
The Compost Gardener. Composting Leaves – A Worthwhile Challenge