Mulch: The Powerhouse of Conserving Soil Moisture

Giving your plants the right amount of water and watering regularly are not the same thing—at least they don’t have to be. Your plants having the right amount of water simply means that the soil stays evenly moist. The easiest way to keep your soil evenly moist is to conserve its moisture in the first place. Mulching is the best way to conserve soil moisture during the growing season.

Since bare soil loses an incredible amount of water due to evaporation, covering the soil with mulch provides the natural, protective blanket soil needs to thwart the vast majority of this unnecessary water loss. This is why mulch is an absolute powerhouse for conserving water. But mulch’s benefits don’t stop there.

Mulch Epiphanies

I’ve known about the power of mulch my entire life, perhaps more than I wanted to at the time.

During my childhood, my siblings and I regularly mulched a 525-tree orchard with sawdust and spent countless additional hours shoveling and spreading sawdust, straw, wood chips, and the like around thousands of strawberries, blueberry and raspberry bushes, and innumerable other fruits and vegetables of all sorts. Starting with these early experiences with mulch, I regularly noticed the darker color of moist soil under even the lightest sprinkling of mulch, just as I continued to be amazed at how incredibly moist the soil typically is under thick layers of mulch, even during drier times. But, just like my epiphanies about the amount of sun garden plants really need to be truly productive, one time stands out as my strongest object lesson.

We’d been in a drought all summer—you know, one of those summers that has farmers and gardeners regularly remarking, “Can you believe how dry it’s been?”—their constant consternation their own kind of rain dance. The uncovered garden soil around the peppers was powder dry as far as I dared dig into the soil—at least six or eight inches deep. I was fretting and kicking myself for not getting mulch on them sooner—and wondering if I should break my “well, they’re going to have to figure it out for themselves” approach and start the sprinklers. For some reason, I happened to root around under the three or four inches of mulch around the broccoli—just a couple of feet away from the completely dry soil under the peppers. I stopped in shock. The soil was still wet—not moist, but wet—on the surface! 

“O.K., I get it,” I thought,” chuckling in amazement. “It works…but I didn’t know it worked that well!”

Why It Works So Well

The sun’s direct rays heat the soil significantly. This increased heat leads to significantly increased evaporation. Dry air, pushed over the soil surface by wind, similarly, pulls water out of the soil. Together, they evaporate an amazing amount of water from bare soil. And it’s not just rainwater that is lost.

With enough hot, dry weather without replenishing rains, eventually, even the natural capillary action that draws groundwater upward in the soil can’t keep up with the pace of evaporation and the soil becomes increasingly drier to the point of being inhospitable for garden plants.

Mulch, in contrast, sits on the surface, absorbing the sun’s direct heat while only transferring some of it to the soil, and shielding the soil from dry air. Especially good mulches, like plant foliage and straw, transfer very little of their surface temperature to the soil below. Being raised above the ground, plant foliage can’t conduct heat directly to the soil and just dissipates it into the air. Being hollow like polar bear fur, straw also uses air as a buffer, greatly minimizing heating of the soil and, hence, water lost through evaporation. Both are wonderful mulches.

Mulch also follows nature’s example. In nature, we rarely see soil exposed directly to the sun’s rays or dry wind, especially in mature, healthy ecosystems. Instead, the soil is protected by both the leaves of living plants and the dead and decaying remains of plant parts—leaves, stems, branches, etc.—lying on the surface.

Mulching also has several other benefits. Mulch moderating temperature extremes—both high and low—provides a much more hospitable habitat for the soil’s macro- and micro-organisms—from earthworms to the bacteria, actinomycetes, and mycorrhizal fungi responsible for soil’s fertility and friability. Mulching also adds organic matter to your soil, providing the food for all of those amazing soil builders and stored fertility for all of your future crops.