It’s All About the Soil

The Ranch site, once a gravel parking lot, has been transformed into a thriving ecosystem. The secret? Mulch! Photo by Kyra Saegusa.

The Ranch site, once a gravel parking lot, has been transformed into a thriving ecosystem. The secret? Mulch! Photo by Kyra Saegusa.

As a research horticulturist and coordinator of the Huntington Ranch Garden, I spend my days on this half-acre experimental site working to create a balance between a productive garden, a livable space, and a wildish ecosystem. Part of the work I do is researching varieties of vegetables, herbs, and fruit trees that thrive with little care and less water in the hot and arid climate of Southern California.

The key to growing edibles in a time of drought is healthy soil. In the same way that our bodies depend on millions of microscopic organisms to stay functional and in balance, healthy soil is host to a microscopic world that gives it—and the plants that grow in it—the necessary building blocks for a healthy ecosystem. The organisms in the soil create and maintain the pathways through which water and nutrients travel. Without happy soil life, you don’t have soil—you just have dirt.

An ideal mulch is a mixture of woods in different sizes and textures that will break down at an uneven rate. Photo by Lisa Blackburn.

An ideal mulch is a mixture of woods in different sizes and textures that will break down at an uneven rate. Photo by Lisa Blackburn.

There really is only one basic rule for building healthy soil biology in your garden: keep your soil covered. MULCH! And when I say mulch, I don’t mean thin layers of pretty, unnaturally colored redwood bark or other decorative options, such as lava rocks, that you get at the local garden store. I’m talking about four to six inches of the good stuff. An ideal mulch is a mixture of woods in different sizes and textures that will break down at an uneven rate, not only providing long coverage, but also creating habitat and food for the decomposers in your soil: worms, fungi, and bacteria, among others. Diverse, clean mulch can be purchased and delivered in bulk from local soil yards. Avoid using mulch made of strongly scented woods—such as cedar, redwood, pine, and eucalyptus—until their scents have faded, indicating that their volatile oils have broken down. The oils contained in these woods can inhibit soil life.

Ranch apprentice Ellen Herra arranges sheets of overlapping cardboard as the first step in sheet mulching. Photo by Lisa Blackburn.

Ranch apprentice Ellen Herra arranges sheets of overlapping cardboard as the first step in sheet mulching. Photo by Lisa Blackburn.

Seven years ago, the Ranch site was nothing but a gravel parking lot for construction crews building the Chinese Garden. Since then, we’ve transformed the place into a glorious and flourishing ecosystem. What’s our secret? Seven years of sheet mulching. There are a few variations to this technique, but the one we use at the Ranch is amazingly simple and is the quickest shortcut to producing good soil. Here are the steps:

  1. Make sure your soil is moist. (You don’t even have to weed, unless you have Bermuda grass.)
  2. Place thoroughly wet cardboard over your soil, having first removed all non-biodegradable tape. (Use only matte cardboard, not the shiny type.) Double layers of cardboard are best, with as much overlap as possible to ensure that all the soil is adequately covered. It is very important that the cardboard be thoroughly wet to ensure moisture in this layer.
  3. Spread four to six inches of damp mulch on top and water it deeply.
Beneficial fungi spread through the mulch layers, slowly breaking them down. Photo by Kyra Saegusa.

Beneficial fungi spread through the mulch layers, slowly breaking them down. Photo by Kyra Saegusa.

Soil life loves to inhabit and eat the cardboard. Beneficial fungi use the corrugation as housing, forming webs throughout the small tunnels as the cardboard decomposes. Good bacteria go wild over the glues in the cardboard, and worms and macroarthropods—such as roly-polys, earwigs, and centipedes—hunt prey as they break down the material. In six months or so, depending on the weather and moisture level, the cardboard will have disintegrated, and a layer of rich compost, teeming with soil life, will have replaced it.

Even if you aren’t interested in growing your own food, healthy soil is the number one way to conserve water in your Southern California garden. Happy and healthy biology in your soil helps retain moisture and cut down on the need to water.

In roughly six months, sheet mulching produces rich compost, teeming with soil life. Photo by Lisa Blackburn.

In roughly six months, sheet mulching produces rich compost, teeming with soil life. Photo by Lisa Blackburn.

If you’re interested in learning more about creating healthy soil, you can attend a workshop, “Paving the Way for an Organic Planet: Growing with Healthy Soil Biology,” cohosted by The Huntington and the Rodale Institute, a world-renowned leader in organic gardening. The workshop runs June 6-7 and is open to the public with advance registration.

Visitors to The Huntington who would like to explore the Ranch Garden can self-tour the site (follow the signs from the Brody Botanical Center) and talk to staff during a monthly Open House on the fourth Saturday of every month. (Confirm dates and times on The Huntington’s online calendar.) A visit to the Ranch is a great way to pick up ideas and inspiration. Advanced gardeners can learn even more by signing up for workshops in our Ecosystem-Based Gardening Series.

Boysenberries thrive in the Ranch Garden’s rich soil. Photo by Lisa Blackburn.

Boysenberries thrive in the Ranch Garden’s rich soil. Photo by Lisa Blackburn.

Additional resources:
Learn how to make your soil richer with microbes from the Soil Doctor, Doug Weatherbee.
You can purchase quality mulch in bulk from Soil and Sod Depot: 818-686-6445.

Related content on Verso:
Harvest Time on the Ranch (Nov. 5, 2014)

Kyra Saegusa is Ranch coordinator and research horticulturalist at The Huntington.

7 thoughts on “It’s All About the Soil

  1. Can you use sheet mulching where there is winter? If it takes 6 months to produce compost, that means we have to start mulching in winter, maybe November or January.

    • In areas outside of Southern California, the sheet mulching technique works even faster due to more dependable rainfall, especially in spring and summer when the heat and water activate the soil biology and it really starts working on overdrive. The added moisture breaks down the cardboard faster, making it more readily available to the decomposing organisms in the soil. Please remember that the rate at which the cardboard and mulch breaks down can be extremely variable depending on weather and moisture conditions. In an area with cold, wet winters if you were to sheet mulch your yard in fall, by late spring ideally the area would be ready to plant with the cardboard and first couple inches of mulch having broken down. If your ground/cardboard freezes, the biological processes will start again only after it thaws, but the freeze and thawing will help make the cardboard more easily accessible to the decomposers. Also, if your soil is decent enough to plant in and you are sheet mulching for soil coverage or weed suppression, you can make a hole in the mulch and cardboard layer and directly plant into the soil while it passively builds soil. To see whether your soil is ready, dig into the mulch every three months to check whether the cardboard has broken down. Sheet mulching is a passive soil building technique and is meant to be used over long periods of time.

  2. Thank you so much for this information! One question: I got a truckload of mulch delivered to my yard. It had LOTS of eucalyptus pods. Will the seeds germinate in the mulch? Do I now have to watch out for seedlings as they sprout to pull them all up to keep a thicket of trees from growing? I’m not concerned about the allelopathetic nature of eucalyptus on my other plants.
    Appreciate the help!
    best,
    Kathy Williamson

    • Many eucalyptus tree seeds need fire to germinate, so depending on the type in your mulch, you may not need to worry about seedlings. I warn people not to accept mulch from a tree trimming company unless you can trust them to deliver “clean” mulch, meaning no weed seeds, diseased trimmings or other detritus. Make sure that you establish this criteria before accepting any mulch onto your property. Or buy mulch from a reputable source.

      • Hi Kyra,
        Thanks, that was very helpful. Just a few more questions. Once the cardboard, and 4 – 6 inches of mulch is down, how often should the area be watered? We thought we had soaked it thoroughly, about two weeks after it was all finished, but the water did not penetrate through the mulch layer, all the way to the cardboard.
        Second question: Can I speed up the process, if in addition to the cardboard and the 4-6 of mulch, we roll out clear plastic sheeting on top of all of it to also solarize the area?
        Lastly, we got large iridescent green beetles that fly, in one of the loads of mulch. How bad are they for the garden & the new drought-tolerant plants we’ll be putting in during Sept.?
        thanks!
        Kathy

        • Hi Kathy,

          Thanks for your message. The key is to thoroughly water the ground, then the cardboard, and then the mulch. Sometimes the top layer of mulch will dry out but the cardboard and soil will remain moist. If they dry out, watering it can hasten the process but do not overwater or you can create anaerobic conditions. The rate at which it breaks down depends on your specific conditions. I don’t recommend using plastic because its solarizing effects can harm your microbiology. Also, the plastic may dry it out faster between waterings. As for the beetles, in their larval stage they’re the big white grubs that help break down organic material into compost. You want them in your garden. The adult beetles are only interested in your fruit and veggies once they are ripe and they won’t bother your drought-tolerant plants.

          Kyra

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