37. Sustainable Soils

Soil, mud, dirt - pretty mundane sounding stuff.  But healthy soil is a critical part of a successful and sustainable garden.  The fact that plants are living things with needs for food, water, etc. is obvious.  What might be surprising is that soil has just as much living complexity, with an incredible array of organisms and complex chemical reactions.  Even though these organisms are out of sight, they are a huge part of a garden ecosystem; without healthy soil, you’ll never have healthy plants.

Most writing on gardening and sustainability naturally emphasizes soil health, including “Nurture the Soil” as one of the 7 main principles of The Bay Friendly Landscape Guidelines.  The key thing to understand about soil is that it is a living “food web” that performs a long list of functions.  These include the “creation of soil structure, storing and cycling of nutrients, protecting plants from pests, improving water infiltration and storage, and filtering out urban pollutants,” (per the Bay Friendly Guidelines).

The Soil Food Web

It is tempting to write a whole article about the soil food web, a fascinating ecology of different organisms with complementary inputs and outputs that come together to form a complex system.   Soil is populated by a huge array of microscopic bacteria and fungi, along with worms and other insects.  A single teaspoon of healthy soil has literally billions of organisms in it.  To state it simply, these organisms are decomposing organic matter like dead leaves and converting them back into food for plants.  They are also creating soil structure that provides pore space for root growth, and the storage of water, oxygen and nutrients for those roots.  One fascinating class of organisms are mycorrhizal fungi which form a critical symbiotic relationship with roots, helping the roots uptake nutrients while being nourished by the root’s byproducts.  Another example is the earthworm which consumes plant debris while tunneling into the soil and leaving behind a tiny tube lined with nutrient rich waste matter that roots love.  The organisms of the soil aren’t going to win any beauty contests, but they’re absolutely essential.

Knowing Your Soil

How do you know if your yard’s soil is healthy or not?  I’ve written previously about the specific case of my backyard soil, describing various tests that can be done and the basics of the soil attributes those tests measure.  So the simple answer is to do some testing, preferably having a professional laboratory analyze some samples.  Generally, if you live in a city, chances are your soil is degraded and compacted and needs some help getting back to a complete, healthy state.  If you live in a rural area, or are building a new house on agricultural land, you probably have healthy, valuable topsoil – “black gold” that should definitely be saved.

Creating Healthy Soil

What should you do to make your garden’s soil healthier?  I have provided a very specific example of the soil preparation for my backyard Native Meadow Experiment.  More generally, when talking about soil, your goal is to literally make it more sustainable – doing things that complete the web of interactions that soil should have to sustain itself over time.  The things you can do mostly involve preserving and/or improving soil structure, organisms and nutrients.  I won’t be able to cover all the possibilities, but three important topics that often come up in my work are:  stockpiling topsoil, avoiding compaction, and adding beneficial organisms.

Stockpiling Topsoil

Scraping up and saving the existing topsoil on a site prior to building construction is a critical component in sustainable landscaping.  It is an added effort and expense that a lot of people resist, but even in urban areas this surface layer of healthy soil is quite different from the subsoil beneath it, which is relatively lifeless.  Only soil testing will tell you how deep the topsoil goes, but it often ranges from 4 to 12 inches deep.   The beneficial organisms and nutrients will survive if piled off to the side and covered with a breathable material while the big construction activities are going on.  If you leave topsoil in place, it will get compacted by the heavy construction equipment, mixed in with construction waste and subsoil and lose most of its value.  Then you may be forced to import topsoil from distant virgin land in big trucks, leading to many bad impacts to the environment.

Avoiding Compaction

It is important to understand that healthy soil is not really a solid thing, it actually consists of a lot of empty void space, ideally around 50%!  This void space is where roots grow, organisms travel, and where water and oxygen are stored for plant uptake.  Compaction of the soil from construction equipment or other heavy traffic will reduce this void space and dimish the soil’s capacity to support the food web.  Certainly soils can recover from a small amount of compaction, but not on the scale that often happens when humans are involved.  Especially with water saturated soil, compaction can create hard to reverse damage.  Nothing’s worse than a bulldozer running over wet clay soil like we have in much of the Bay Area, squeezing out a high percentage of the void space that does not return when it dries.

Aeration of compacted soils often involves some level of stirring it up by tilling, however, if you go too far you can do more harm than good to the soil structure by breaking up stable soil structures and bringing weed seed to the surface.  Tilling can also encourage erosion, not only from rain, but also by wind, so it is critical to protect against this while doing the work.  There are also cover crops that can send roots down into compacted soils and break it up, such as the wild Mustard that you see in between grape vine rows.  These roots can penetrate quite deep, and once they die and decompose they open things up enough to let other organisms get a foothold and allow space for air and water.

Adding Beneficial Organisms

The number one thing you can add to improve your soil is compost.  Ideally you produce your own from food and yard waste, but you can also purchase it.  This compost adds organic matter, but more importantly it is full of those beneficial organisms that can get to work on producing and storing nutrients.  If you have the right bacteria, fungi and other organisms in the soil, they are literally producing fertilizer from organic matter, and you won’t need to add chemical fertilizers.  These beneficial organisms also help prevent plant pests and diseases by providing alternative foods for insects as well as housing predators.  Therefore you avoid the need for toxic pesticides which tend to kill off beneficial insects in addition to the pests.

Incorporating compost into garden soils when you are preparing an area for planting is your starting point; amounts should be based on the specifics of your soil.  You might choose to add even more beneficial organisms to maximize the diversity with an additive like I used in my backyard.  Even after your garden is planted and growing, you should feed the soil with compost by “top-dressing” - this means adding a thin layer around the base of the plants.   This is a far superior “fertilizer” than the chemical fertilizers, which are usually way too concentrated and end up in our streams and rivers causing environmental damage.  If you’re worried about covering up dense areas of low plantings with compost, you can also use compost tea as described in the image on the right.  And don’t forget mulch - mulching the soil surface is critical for preventing erosion and preserving soil moisture, among other things.

Dirty Business

I hope this gives you a little more insight into the dirt under your feet and inspires you to work to enhance this living system in your own garden.  The more I read about the soil food web, the more interesting it gets, so I plan to write about other aspects of this complex topic in the future.  As I’ve mentioned before, Soil Science is a complete profession in its own right and the interactions of different characteristics of soil are very complex and site-specific.  When in doubt, consult a professional for exact recommendations before doing anything to your garden’s soil.

References:

Bay Friendly Gardening Guide and Landscaping Guidelines

Gaia’s Garden – A Guide to Home Scale Permaculture, by Toby Hemenway

Secrets of Great Soil, by Beth Hanson with illustration shown here by Giulio Mencaroni, from Organic Gardening magazine, October/November 2014.

Sustainable Landscape Construction, by J. William Thompson and Kim Sorvig