I love the fall. The return of cooler weather drives the mosquitoes and deer flies out of the New England woods so I can hike the old logging roads without suffering the barrage of bites that make summertime walks miserable. I love the clear, slanting sunlight of autumn afternoons, whose rays seem pitched at the perfect angle to backlight the trees’ vivid foliage.

One autumn scene that depresses me, however, is the sight of my neighbors (some of them, anyway) raking leaves out to the curb so that the town truck can vacuum them up and haul them away. By doing so, my neighbors are short-circuiting a natural cycle and gradually impoverishing the soil in their yards.

Why is this? Arguably, the single most important constituent of the soil—from a gardener’s perspective, anyway—is the organic matter. This consists of both composting plant material and the products of that process, the dark, decay-resistant material horticulturists call humus. The composting material releases plant nutrients as it decays and acts like a sponge to absorb and retain moisture. The humus, meanwhile, acts as a glue to stick the soil’s mineral particles (sand, silt, and clay) together into crumbs. This, in turn, makes the soil loose and crumbly so that air, rainwater, and plant roots penetrate it more easily.

The point to remember is that both the raw organic matter and the humus do decay (although the humus decays more slowly), and both must be replenished on a regular basis if the soil is to remain healthy and productive. The rates of decay vary with the climate: Here in New England, humus can persist for decades, but in places where the climate is hot and humid, the need for replenishment is more constant.

When I lived and gardened in central Texas, for example, I used to dig a couple of inches of spent mushroom compost into my vegetable beds every spring, and by the end of the growing season (late fall), the soil would have returned to a dusty, mineral silt, seemingly completely deficient in any organic content.

In nature, as the humus in a soil decays, it is replaced by the decomposition of debris—leaves, stems, dead roots—deposited by the plants growing on the soil. This creates a self-regulating cycle: The more nutrients the plants take from the decaying humus, the faster they grow and the more debris they deposit (to create humus at a greater rate).

Short-circuiting this process by removing all the debris means that you must either resort to synthetic fertilizers and extra irrigation, or replenish the humus with materials such as sphagnum peat or manure brought in from off site. Both come at an environmental cost. Trucking in manure consumes fossil fuels. The importation of peat from northern bogs requires energy for harvesting and drying the material as well as for hauling it down to market. The damage done to the bogs adversely affects one of Nature’s greatest carbon sinks, resulting in a release of huge quantities of CO2 into the atmosphere, accelerating the process of global climate change.

The easiest way to reduce this portion of your carbon footprint is to let your garden return to the natural cycle of organic matter replenishment. Keep your leaves on-site and use them either as a source of compost or as a mulch. In my next post, I’ll describe a garden that makes particularly good use of this material.