How to Treat Desert Soils

There are many misconceptions about the desert. Perhaps one of the biggest misconceptions is that the desert soil is devoid of organisms—that a desert is equal to the beginning of the succession spectrum.

The Conventional View of Ecological Succession.

The Conventional View of Ecological Succession.

As a consequence, people assume that native plants don’t need soil amendments because “that isn’t what they want”. According to some, it’s only the plants from climates with deeper soils that need amendment. People will proceed to plant, digging a hole and stuffing the plant into its new home.

I was once taught this by people I thought knew better than I did. But after almost 30 years of planting and growing and experimenting, planting all sorts of plants, I have come to the conclusion that this just simply isn’t true. Some of what we have learned about soil, and the microorganisms that exist in our soils, has helped us to change our methods. I now think of planting as a form of soil restoration, and it takes work to bring that soil biology back.

Native soils, when they haven’t been trampled, turned, poisoned, dehydrated, and eroded by humans, might not be the same as the soil under the canopies of old growth forest in the Pacific Northwest. But our desert is just as much a climax community of organism diversity. Undisturbed desert soils host matrices of fungus, lichen, bacteria, microarthropods, nematodes, mycorrhizae, and more, like any developed soil system.

More and more rare to see, an undisturbed cryptobiotic crust.

More and more rare to see, an undisturbed cryptobiotic crust.

When we trample these systems, by walking on them, driving on them, bulldozing them, dumping toxic materials in the soil, we kill this biology. Under the best circumstances, without any human effort (or continued disturbance), a thin veneer of biological soil may return in five to seven years. Mature crusts can take 50 years to strengthen. Some lichens and mosses may take hundreds of years to recover.

This is not desert soil, this is disturbed soil

This is not desert soil, this is disturbed soil

When we plant native plants into our landscapes, we need to return some biology to the soil, and we need to think of ourselves as restoring the ecology of the soil, as much as we are trying to restore the flora. The best way to do this in a typical urban or suburban setting is to add compost to the soil profile.

And let’s be clear, we aren’t going to be able to restore the ecology of the soil as it was (that is forever gone). We are working in human timeframes, and our methods of adding biology back to the soil is a sort of microorganism farming—which is essentially what compost aims at doing.

When you add compost (a soil food and inoculation of organisms) to the ground, you are assuring that the new plant is going to get nutrition. It’s not going to be the same as the desert soil that was undisturbed for eons, but it will be a close enough approximation that you will ensure your plant will live a longer, healthier life. And something else that adding compost to the soil does: good compost introduces humic compounds that can lock up toxins like heavy metals, rendering them harmless to the surrounding biology!

Over the years, I have planted native plants with, and without conditioning the soil. In almost every single instance I’ve seen a huge difference. And I see it now when I watch other people plant native plants. I know people who don’t use soil conditioners, and watch their plants sit in a location for years without growing much, or barely surviving, at best.

Something else to remember as well: our region is a complicated matrix of desert, riparian swales, mesquite bosque, and desert grassland, and we are surrounded by what we call the Sky Islands, the mountains that influence our flora, geology, and hydrology. We don’t really live in the sort of desert that most people who don’t live here think of when they hear the word “desert”. We live in an arid, subtropical region located at an ever-shifting ecological crossroads. Our flora reflects this diversity—so, some of our native plants need more enriched soils than others.

Imagine all that has happened to the soil surrounding this building: the surrounding dirt is not native desert soil, it is damaged soil that requires effort to restore. Plants cannot grow in a vacuum—they rely on soil organisms to get their nutrition.

Imagine all that has happened to the soil surrounding this building: the surrounding dirt is not native desert soil, it is damaged soil that requires effort to restore. Plants cannot grow in a vacuum—they rely on soil organisms to get their nutrition.

The concept of compost is certainly in its infancy in our region. Most the research that drives our methods for making compost and amending soils comes from places like Oregon. We can certainly improve upon these methods to develop soil conditioners more appropriate for our region. But the soil conditioners we have available to us now do a good job. And they certainly work better than not adding anything at all.

It’s important also to remember to mulch your plants as well, that is applying a layer of organic material, at the surface around plants, to help shield the soil surface from the sun, and host decomposing insects that break down larger chunks of detritus into smaller pieces. In recent years we have been using wood chips obtained from landscape refuse—trees that were chipped up into small wood chips.

They smell good, keep the dogs and cats from tracking mud into the house, and we’ve noticed a new roster of organisms that came in when we added the mulch: the decomposing insects, but also the ground gleaning birds that eat those insects. Every few years we are replenishing the mulch layer to maintain the nice look and function of the mulch, as the previous years’ layer decomposes into soil.

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Amending soil and mulching are not all that it takes to grow a healthy, native landscape. Learning how to properly grade your landscape, to encourage the rainwater to flow toward plant beds is important. So is properly establishing a plant by applying the water it needs so that its roots can grow outward and become the drought tolerant plant it can be. You don’t get a low water landscape without applying water in the beginning.

And if you feel bad about putting water into your landscape, well, don’t. Of all the things we as humans do to squander the precious resource that is water, putting that water on plants is the least egregious. In fact, watering plants is the ONLY use of water that gives back to nature. When you water a plant, you are feeding birds, butterflies, bees, and a multitude of other organisms that depend on the plant and live in the soil.

Watering plants is a form of ecological restoration.

The water you add to the soil profile is essential to restoring the organisms that reside there. That is where the magic begins. And as your soil matures, organisms that help plants to obtain food and water develop. They also change how the soil holds moisture and nutrients. This can be assisted by continuing to add layers of mulch above, or if you can’t allow organic debris to decompose on the ground, you can add organic foods to the soil.

Plants that are well-nourished are more resistant to disease, and are more drought tolerant. Well-nourished plants also handle the heat and sun better.

If this all sounds like a lot of work, don’t worry, it really isn’t. Most of the work happens upon planting. Any of the followup actions you do will be annual events. And a weekend out of the year is a great price to pay for a native landscape that looks vibrant. We’ve all had plants that seem to sit in the ground and not grow. Most of the time, the reason is nutritional. And now, you know what you can do to prevent that from happening.

Katherine Gierlach