Here’s a question for the season: where does all that mulch go?
Farmers and gardeners across Vermont and New Hampshire have laid down thick carpets of mulch – leaves, hay, straw, newspapers, grass clippings, woodchips – to protect plants and soil from winter’s cold and snow. But a year from now, it will all be gone. Why aren’t our gardens and flower beds knee-deep in mulch after just a few seasons?
The first answer you’re apt to hear is that mulch “breaks down.” There’s an element of truth to this, since freezing, thawing, and soaking, and even punishing winds, can cause mulch to compact or break apart. But if the only thing that happened to mulch was that it broke apart, there’d still be an awful lot of mulch lying around in small pieces. But there isn’t. After a year or two, the stuff is pretty much gone.
Stepping up a notch, some will say that the reason mulch disappears is that it “rots away.” This is a very true, if unhelpfully general, answer. Mulch does indeed rot, but saying so implies that the leaves or hay or straw sit around for awhile, getting wet and slimy, until at some point they dissolve or disappear.
What really happens is a far simpler and more active process: mulch gets eaten. Simple as that. Munched. Digested. Chewed up and spit (or pooped) out. Either way, mulch is actively sought out and consumed by organisms whose main objective in life is to dine on delicacies such as mulch.
Bacteria are the most numerous diners in the soil and, collectively, can account for several tons of the weight of the top 6 inches of an acre of soil. One whole class of bacteria, the heterotrophs, eats mulch directly. The other class, the autotrophs, eats the nutrients made available by the heterotrophs; as part of their eating, autotrophs also take nitrogen out of the atmosphere and fix it in the soil, where it becomes available to plants.
Number two in the diner’s club are microbes called actinomycetes, whose respiration creates that lovely “fresh earth” smell that comes after spading the garden. Actinomycetes are the ultimate soil gourmands, both eating mulch directly and fixing nitrogen at the same time. Actinomycetes have already gone to work on this year’s mulch, despite the cold temperatures that have caused the bacteria to push back from the table.
The third group of digesters is the fungi, the organisms responsible for “rotting” things. These include molds and mushrooms, whose thread-like vegetative parts, called mycelia, can be seen running through good soil. Fungi are at work right now as well, despite the cool weather, which helps explain why this year’s leaves will be good and moldy (and woven with mycelia) by the time the snow melts next spring.
Finally, of course, are animals, from the lowly mite and snow flea to the common ant, from the heard-of but rarely seen nematodes and protozoa to the much-maligned sow bug, slug, and snail. Atop the whole pile of critters are the earthworms, who consume mulch at such a voracious rate that their collected castings can weigh as much as 8 tons per acre after a season of fine dining.
Given this list of soil-dwelling mulch eaters, it’s surprising that we persist in the overall belief that the soil is inert. Pick up a handful of soil, and you’re holding billions of microscopic organisms, plus a few that are readily visible to the naked eye. Soil may not be fully alive in the way that a tree or a person is, but it’s also not dead the way a stone wall is, or even the way a dirt road is.
The difference is not just semantic. Things that “break down”” are inanimate: your car, your house, or the rocks in the stone wall around the garden. To prevent things from breaking down, you “fix” them or “maintain” them. But things that are “eaten” are eaten by somebody or something that is alive, and things that are alive are “cared for,” “tended,” or perhaps even “nurtured” or “fed.” Wouldn’t we treat the soil differently if we were tending it instead of repairing it.
For one thing, we wouldn’t spend so much money buying N-P-K (nitrogen-phosphorous-potassium) fertilizers, which, in human terms, are the equivalent of throwing vitamin pills in a blender and calling it supper. Instead, we’d feed the soil actual food: rough, organic material, such as mulch, that microorganisms in the soil convert into N, P, K, and the other nutrients that soil needs. In the end, mulch isn’t just for frost protection or weed control, as many people assume. It’s also an essential part of a healthy diet for the soil under our feet.