An Introduction to Backyard Soil
Shortly after entering the profession of landscape architecture, I began to have conversations about soil. This seemed like a worthwhile subject, since plants are one of the primary materials in design and they obviously needed something to be planted in. It took me several years to realize that I had no idea what I was talking about, and that this was not an uncommon problem. In the design field, "soil" is a word tossed around by landscape architects to imbue a scientific argument to their work, but it lacks any sort of intellectual support. Most horticulturalists and garden shops only know enough to recommend a product. Soil scientists know so much as to be unhelpful. The following is my attempt to introduce the basic concepts relevant to a homeowner who wants to have an educated conversation with a landscape professional.
Why Soil Is Important
Soil is a fundamental component of life on Earth, but popularly misunderstood in terms of it importance. This misunderstanding arises out of a particular tension between its basic accessibility and the amount of information surrounding it. This is a series of posts intended to walk you through the intellectual portion step-by-step, and only focus on the practical, everyday tasks that a person would do inside a residential parcel.
My assumption here is that most readers have smaller (under 2 acres) parcels with urban/anthropogenic soil, and are interested in amending and improving the soil on their property. If you have a larger parcel, you may have tracts of undisturbed soil and are more likely interested in soil conservation. The principles involved for understanding the general structure and behavior of soil are the same, but the methods of conservation are less about managing the soil directly and more about managing the surrounding environmental factors, which is not the purpose of this series of essays. If you don't consider yourself a land steward, at least consider that anyone you bring onto your property, for the purpose of helping you manage outdoor spaces, is going to mention "soil" at some point. A general understanding of what soil is will give you a foundation to assess a potential landscape contractor's knowledge, and engage in more advanced discussions about how to specifically approach your project, which will ultimately generate more value.
Learning about soil should not feel overwhelming. It can be acknowledged as extremely complex, and approached with rigor and discipline, but mastery of the knowledge of your local soil should feel attainable, and it shouldn't require technical language. While I will introduce technical terms, my intent is to connect them to the common ones you already use every day.
Why Soil is Confusing
The trouble with taking an interest in soil is the ubiquity of the term in relation to its application. If you consult with someone about your soil type, you may get 8-10 different answers. You might have hydric soil. Or acidic soil. Or sandy soil. This isn't anyone's fault. It results from the fact that soil is intrinsic to our everyday experience and has therefore been studied from a variety of angles. All of these terms describe a dimension of soil, but are not a complete picture of a soil type.
The discipline of soil science addresses this issue. The Natural Resources Commission of the USDA maintains an extensive catalog of soil types, known as a series. But it is not a helpful resource for the curious layperson, as the relationships between the hierarchies of classification and the terms used are not intuitive. For instance, if you have the resources to hire a qualified soil scientist, they might definitively identify it as "fine-loamy, mixed, semiactive, thermic Aquic Hapludults," or as Altavista. This requires another several hours of translation. Most folks will only remember that dirt is confusing.
Soil classification follows a six-tier hierarchy starting with the Order as the most general and moving down to the Series as the most specific. However, where most other classification systems follow an accessible hierarchy, soil taxonomy is more obscure. As an example, here is an introductory summary from the description of a local soil that occurs in the Piedmont region of the Southeast where I live:
> The Pacolet series consists of very deep, well drained, moderately permeable soils that formed in residuum weathered mostly from felsic igneous and metamorphic rocks of the Piedmont uplands. Slopes commonly are 15 to 25 percent but range from 2 to 60 percent.
This is a decent description, particularly if you know what felsic igneous rock is (I don't), but doesn't actually tell us much in what we can do with our soil. In that case, it may be possible to draw some inferences based on the taxonomic classification:
> Fine, kaolinitic, thermic Typic Kanhapludults
Obviously, this is less helpful. Maybe if we look at the "Range of Characteristics" we'll find what we're looking for:
> The Bt horizon is at least 10 to 24 inches thick and extends to a depth of 18 to 30 inches. Depth to a lithic contact is more than 60 inches. The soil is very strongly acid to slightly acid in the A horizon, and very strongly acid to moderately acid throughout the rest of the profile. Content of rock fragments, dominantly gravel, ranges from 0 to 35 percent in the A and E horizons, and 0 to 15 percent in the Bt horizon. Most pedons have few to common flakes of mica in the solum, and few to many in the C horizon.
Alas, no joy. My point here is that if you are taking an interest in your soil conditions, have narrowed down the possibilities to the specific soil type on your property, and are going to the most authoritative source, the official description provided by the USDA will not be helpful unless you were paying attention in your earth sciences classes in college. Being able to read a soil series description is a useful skill, but it isn't necessary for the purpose of building soil in your garden. That requires a little common sense and a willingness to think like a plant. The content we will cover here introduces soil kinesthetically. I suggest grabbing a handful and reading along, or doing so shortly afterward.
The very, very basics
Let's start with a scientific grounding. Soil scientists define soil as:
> The surface mineral and/or organic layer of the earth that has experienced some degree of physical, biological and chemical weathering.
We're not deep into technical terms yet, but this definition contains a number of terms that need to be unpacked in order build a good practical understanding. To master the basics, we'll focus on the content of soil (mineral/organic), how it occurs in layers, and the process of weathering.
Soil Content - Clumps & Voids
Soil consists of four parts, the mineral and organic contents mentioned above, plus air and water. We can generalize this even further by grouping the minerals and organic contents into clumps, and the water and air into voids.
A simpler way of thinking about soil.
The clumps are the solid materials and the voids are filled by fluids. The clumps provide nutrients to plants, and the voids provide oxygen, carbon dioxide, and water. The size of the clumps and clump/void ratio define the soil's texture, and loosely this determines the level and range of biological activity in the soil, affecting which plants, animals and microbes are able to survive in it. The science-y word for a clump is _pedon_.
We can ask a lot of good questions with just this basic framework: Are there more clumps than voids? Are the voids larger than the clumps? Are clumps mostly one material, or a combination? What do the clumps feel like? Are they dry? Are they wet? Do they have an odor? If we pour water over the soil does it drain? Does it pool?
A sketch of a pedon. The soil tends to break at its respective horizons. The mineral and organic content fuses together and forms the voids to hold water and gas.
Asking these types of basic questions help us understand what the soil needs.
Weathering
Weather is arguably the most obvious thing on the planet, weathering is less noticeable. I think of this as the microscopic effects of macroscopic processes. Any time two or more objects interact with each other, tiny pieces break off. This is weathering. With regard to soil, there are 3 types: physical, chemical, and biological. We won't go deeply into how weathering or other soil-forming processes occur yet, other than to acknowledge that they happen. The clumps mentioned above are composed of small fragments of rocks, leaves, sand, sticks, bugs, and other detritus, and these got small over the course of weathering.
Consider a thunderstorm, a recognizable form of weather. A large volume of water sheets off a roof and into a gutter. As it flows out, it pushes pebbles and small rocks along with it, which chip off pieces with every tumble. This is physical weathering. The rain contains small amounts of carbonic acid (pure rainfall is slightly acidic, which reacts with the minerals in those same rocks and alters their composition. This is chemical weathering. In the same storm, water infiltrates the soil and reaches the roots of a nearby tree. As the tree takes in water, its roots grow around these same rocks and break them into smaller pieces. This is biological weathering. Note that biological weathering generally involves physical or chemical weathering when examined in detail. It is included as its own process because it tends to indicate the extent to which a particular soil type supports life.
Soil Horizons
All these broken pieces get mixed into the soil, form clumps, and continue to experience weathering in smaller and more complex ways. Each large scale weather event and the activity of plants and animals moves these clumps around differently, but over time these clumps will form similar characteristics and become horizons. These are the noticeable layers of difference in a cross-section of soil.
An example of soil horizons in North Texas.
Plant people are really only concerned with the top two layers: the O-horizon, which is basically the layer of leaves that fall off trees, and the A-horizon, which is the layer we dig into, often called topsoil. Most processes conducive to plant growth occur in the A-horizon. This is where most plant roots grow, with the exception of larger shrubs and trees that penetrate into the B-horizon. When we add mulch or top-dressing to a flower bed, we are creating or adding to the O-horizon. It's easier to call these the root layer and organic layer, or the topsoil and mulch layer.
Conclusion
As described above, soil generally consists of a set of basic elements, clumps and voids, that occur in visible layers, soil horizons. These form over time through a set of very simple processes that interact in complex ways. Attending to this creative tension between simplicity and complexity is the key to improving soil. One doesn't need an academic background to understand it, but one does need to cultivate a sense of curiosity and an awareness that each additional piece of information is going to open new doors to knowledge. For the urban and suburban land steward, my advice is to work toward becoming intimately familiar with the intricacies of the soil inside your property line, bearing in mind that what you observe specifically only applies to your parcel and possibly your immediate neighbors, but what you observe generally will apply to almost everyone within your climate or ecoregion.
Let's also clear up a common misconception, there is a difference between soil and dirt. Soil is the result and a component of complex and ongoing ecological processes, dirt is just bits of rocks and dust. Soil has points of intervention, dirt does not. The typical homeowner can't really change the weather patterns that dictate their soil type, but you can increase the biological content, adjust the density, adjust the topography, and select plants that will facilitate healthy soil consistent with the ecological parameters of your region. The posts that follow will cover these strategies in greater detail.
Additional Resources
Basics of soil from the Soil Science Society of America
Basics on mineral content from American Geosciences Institute
Soil horizons from Wikipedia
