What Is Soil? A Close Look at the Ground Beneath our Food

Soil, Um… Rocks?

by Christina De La Rocha

If you like to eat food, you ought to spare a few thoughts for soil. If only because our lives depend on it, we owe it to soil to know at least a little something about it. We should also stop treating it like dirt. None of us land dwellers would be here if soil wasn’t such a fine place for plants to grow and didn’t serve as the basis for almost all land–based ecosystems, even ones as aquatic as moors and bogs.

What Is Soil?

But what is even soil and how is it not just dirt?

The short answer is that soil is what happens when air, water, and biology get to work weathering rocks and the minerals that make rocks up. Given time—and it can take thousands of years—what emerges out of that activity, plus the life, growth, death, and decay of the organisms that come to live in the soil as it develops, is a layered construction of soil. These layers, containing mixtures of minerals and organic matter, serve as a good host for life. Not just the plant life that digs its roots down into the soil, but also the tiny animals, fungi, archaea, bacteria, and protists that make the soil their home. If you want to get philosophical about it, you could wonder if these organisms have constructed their own niche by pitching in to create and mature a soil. Or you could go back to the beginning of this paragraph and view soil simply as the emergent property that arises when air, water, biology, and rocks interact with each other at the Earth’s surface.

But of course, there’s a little bit of both things going on.

What Is Soil Made Of?

According to the Soil Science Society of America, an association of more than 6,000 soil scientists, environmental scientists, farmers, and other enthusiasts of that basic matrix underpinning most land–based ecosystems, soil is a mixture of minerals, organic matter, air, and water.

Air and water you already know well.

Minerals are the various different types of crystals—like quartz, feldspar, plagioclase, and calcite—that make up rocks. Each of the nearly 6,000 recognized rock–forming minerals has its own particular chemical composition and crystal structure that give it a specific set of physical and chemical properties. This is the first hint that, because soils can be made from such a wide range of starting material and in such a wide range of mixtures, there are probably a lot of wildly different types of soil on Earth.

Closeup of rock minerals: jagged pinks and blacks
Mineral crystals in granite. The pink crystals are feldspar, the clear crystals are quartz, and the black crystals are probably biotite or hornblende. Photo, by Randolph Black, is in the public domain.

Organic matter refers to the materials that make up the non–mineral parts of living things—all the stuff made of carbohydrates, lipids, and proteins, to put it roughly. This shouldn’t be confused with the use of organic to refer to a certified standard for the production of crops and livestock without, for instance, the use of artificial nutrients and petrochemical fertilizers. Organic matter in soils comes in three basic flavors: still alive, freshly dead and easily digestible, and refractory (meaning that it’s super hard to break down). Unsurprisingly, the non–living organic matter in soils consists of a lot of cellulose and lignin, those complex, slightly crystalline, woody polysaccharides that are the hard to digest structural materials of land plants. Slightly less unsurprising, if only because you might think the answer should be lignin (because trees), cellulose is the most abundant organic polymer on Earth.

The Materials That Make Up Soil Also Have to Be Weathered

Another defining feature of soils is that their constituents have been at least partially weathered. This can involve physical weathering, like erosion; chemical weathering, like oxidation or hydrolysis; or biological weathering, which is when life itself does the physical or chemical weathering. You almost don’t need to include this in the definition of soil, since you’d be hard pressed to find a mixture of minerals and organic matter out there in the world that hadn’t been weathered. But it’s useful to flag that weathering is such a core part of the creation of soil. Given the wide range of conditions and organisms on Earth, it’s also our second big hint that soil is a very particular creation of localities, time spans, climates, organisms, and materials.

It also says that soils are not static. They’re constantly forming and constantly changing as creatures burrow through them, plants take up nutrients and exude carbon dioxide and other waste products, as rain percolates through them or, during the dry seasons, they dry out. Minerals are constantly being broken down to produce clay minerals and solutes. And air is constantly—slowly—diffusing in and out of the pore space in the soil profile, helping to keep things oxygenated.

How to Make Soil

Step 1: The Parent Layer, or C Layer

Step 1 in making a soil is breaking down rocks into mineral grains sand–sized or smaller. This is where weathering first comes in. The breakdown of the rock happens through erosion, or when freeze–thaw cycles break up rocks, or via the grinding action of glaciers as they traverse over rock. Even biology, with its plant roots and burrowing animals, can get in there into cracks and wedge them apart.

Such production of the broken up material needed to create a soil can happen right there where the soil will form or it can happen as many as thousands of miles away. After all, some soils have formed from the rock flour left behind by retreating glaciers or from the particulates deposited by the wind.

Once you have a layer of grains somewhere suitable for starting to form soil, you have the layer of soil known as the C layer, or parent layer of soil. It’s not soil yet, though! But it is the starting point.

Step 2: The Topsoil, or A Layer

The next layer to develop is the A layer, or topsoil. As plants and soil organisms move in and, aided by air and water, get increasingly to work on the materials of the parent layer, they increasingly alter it. Roots break the parent material further up and exude carbon dioxide and organic acids that start dissolving or chemically altering the mineral grains in the parent layer. Soil solutions pick up solutes. Clay minerals begin to form. Soil organisms, including archaea, bacteria, fungi, protists, and animals from microscopic to merely small, move in. As the plants and other organisms live, excrete waste, and die, organic matter begins to accumulate and then to decay, releasing nutrients into the soil. Eventually, this topsoil develops into a luscious mix of minerals and organic matter that is full of nutrients and a wonderful place for plants to thrive.

Congratulations! Now you have a soil, even if, with just two layers, parent and topsoil, it’s still quite immature.

Step 3: Adding Layers E and B, the Elucidated Layer and the Subsoil

As time goes by in a soil, layers can develop between the topsoil and the parent layer. These are known as the E and the B layers of soil.

As water percolates down through the soil, it leaches out some of the clays, organic matter, and minerals, leaving behind grains of more inert minerals like quartz. As a result, the bottom of the topsoil layer can become notably depleted in the materials being leached out of the topsoil, creating the E or elucidated layer at the base of the topsoil.

Not all soils develop an E layer. In any case, as the leachates wash out of the topsoil, they accumulate in the B layer, which is also known as the subsoil. The subsoil contains less organic matter than the topsoil above it and biology is far less active here. The subsoil also tends to be reddish because it accumulates iron oxides.

At this point, the subsoil (and possibly also the E layer) is between the parent material (the C layer) that served as the source material for the soil and the topsoil (the A layer) that is at the surface.

The O Layer and the R Layer

Lastly, although we don’t mean this chronologically, many soils will have an O layer and/or an R layer.

The O layer is an organic–rich layer that can develop on top of the topsoil. It consists of things like leaf litter and other decaying plant matter. It’s the one layer of soil that is not predominated by minerals. But not all soils will develop this humic layer on top of their topsoil.

The R layer, meanwhile, is the bedrock underneath the soil. Because the parent layer can be blown in on the wind, deposited by glaciers, or transported there by a number of other means, the bedrock underneath a soil does not need to be the source of the material for the parent layer that served as the source of minerals for the soil. Sometimes the bedrock layer is a very, very long way down below the soil (where I live, there’s several tens of meters of clay you have to get through before you reach solid rock).

How Soil Is Not Just Dirt

If you dig down into a soil, creating an undisturbed vertical wall that allows you to look at the soil’s profile, you’ll immediately see these layers, also known as horizons, since they’re horizontal. These horizons are a non-negotiable feature of soils.

Diagram showing a cutout section of soil and the various layers, or horizons, from top (ground leverl) to bottom (bedrock)
Where to find the various possible layers in soil.
Source: (https://commons.wikimedia.org/wiki/File:Soil_Horizons_Diagram.jpg) shared under the Creative Commons Attribution–Share Alike 4.0 International License.]

Without these horizons, you just have dirt, which is no longer soil because it is just something someone dug up and dumped somewhere. Dirt is out of context, unstructured, and divorced from its origin. Dirt isn’t a finely developed ecosystem hosting biogeochemical cycling between representatives of a wide variety of the tree of life, water, minerals, nonliving organic matter, and air. Dirt is just… dirt, where some of this stuff might happen, but only if it’s lucky.

Types of Soil

Because soil matters so much to the survival of the human race, each nation tends to have a set of categories by which it classifies its soil. One widely recognized classification system is the USDA Soil Taxonomy, which recognizes 12 different main types of soil and dozens of subtypes of soil into which thousands of individual soils can be categorized.

Part of this enthusiasm for ordering soils into types is because it’s fascinating how parent materials, local climates, and local biology combine to form so many different individual soils and how commonalities result in soils that can be grouped together into these 12 categories. The rest of it is because recognizing the different soil types helps us better understand the ecology of natural areas and helps us optimize our farming and gardening practices.

To give you some of the highlights of the major USDA categories of soil:

Immature Soils

Entisols

The most abundant type of soil on Earth are the entisols, which are basically all soils that are so immature, they barely even have a layer of topsoil to their name. You can find them in every sort of place where soils can accumulate. Currently, they cover about 18% of Earth’s ice–free land area (which is all the land that isn’t trapped under the Greenland Ice Sheet, the Antarctic Ice Sheet, or a glacier).

Inceptisols

The second most abundant type of soil on Earth, the most fabulously named inceptisols, are also on the immature side of things.

Although they’re further along in their development than the entisols, inceptisols have not yet accumulated much in the way of clays, iron oxides, or organic matter in their topsoil and the other horizons in the soil are very poorly developed. Inceptisols tend to occur in subhumid to humid climates in mountainous regions, on steep slopes or on young surfaces. It can be possible to grow crops on them and use them as pasture, but, when we use them, it’s mostly they’re used for things like forestry. About 15% of Earth’s ice–free land area hosts inceptisols.

What Is Soil? Side profile of soil, with fungal roots visible at the bottom while the layers transition through different colurs of earth
Profile through a brown earth soil, a type of inceptisol, forming on glacial deposits in the Netherlands.

Mature Soils

In terms of more mature soils, this is where the where they’re located, what they’re made from, and what kind of climate they’re subjected to become more important in determining the type of soil they are.

Gelisols

Take, for instance, the gelisols which are generally found at higher altitudes and/or latitudes. Because they occur in such cold climates, gelisols contain permafrost within a meter or two of their surface. In regions that are warm enough for the upper portions of the soil to thaw during the warmer seasons, the materials in this upper portion have been clearly shuffled around by cycles of ice formation and melting.

As you might imagine, gelisols are already falling victim to global warming. This is bad for gelisols and bad for us for, as gelisols thaw or lose their permafrost, they release copious quantities of methane, a powerful greenhouse gas, into the air, driving further global warming. Any structures, like buildings or roads, built on these frozen soils are also in for an unhappy time when these gelisols lose their permafrost, and therefore status as gelisols, to a warming world.

Soil profile of a gelisol.
Soil profile of a gelisol.

Histosols

Histosols stand in total contrast to the gelisols. By definition, histosols don’t contain permafrost. Forming in bogs, moors, fens, and peatlands where organic debris piles up faster than it decomposes, what histosols do contain is copious quantities of organic matter.

They’re a great storehouse for carbon that could otherwise exist in the atmosphere as a greenhouse gas. We ought to protect them for this reason, as well as in order to protect the ecosystems they support. Instead, we have a long and very bad habit of destroying histosols by harvesting peat for potting soil or to burn for heating.

Two men digging peat bricks on the moor for use in heating
Cutting peat from a histosol on the Orkney Islands to dry out and then burn for heating. Photo, by Derek Mayes, has been used under the Creative Commons Attibution–Share Alike 2.0 Generic License.]

Vertisols

Vertisols are a super fun type of soil because they contain clays that shrink when they dry out and expand when they get wet. Don’t build a house here! Or anything else. On the other hand, they’re fertile soils, with a propensity to get waterlogged at their surface.

From the looks of things, I live on a vertisol… or at least there was vertisol under the house before they dug it out and replaced it with sand that was more stable to build on. Even so, every week we have new cracks in the paster on the walls of our house as the ground expands and contracts beneath the sand that it’s built on.

And the garden, oh the garden! In wet years the soil is so waterlogged the lawn is like a slip ‘n slide and all the potatoes get blight. And in dry years, the cracks, which form hexagons at the surface, seem to reach down at least halfway toward the center of the Earth.

Soil profile cutout with measuring tape showing depth
Cracks in a vertisol during the dry season. Photo, by Harã Hayder, is available for use under the Creative Commons Attribution–Share Alike 4.0 International License.]

Alfisols

But the soil I grew up on was probably an alfisol. These are highly weathered soils that are great for growing things in. For instance, California’s state soil, the San Joaquin soil, which is found throughout California’s great Central Valley and supports a lot of the state’s prolific agricultural endeavors, is an alfisol.

The San Joaquin soil’s topsoil is a reddish loamy layer—meaning it’s made up of a mixture of sand, silt, and clay—that’s rich in organic matter. The layer under that is similar, but lacks the organic matter. Underneath that is a reddish–brown clay or clay–loam layer that exhibits a sort of pillar like structure. Because there’s so much clay in this layer, it inhibits water and plant roots from extending deeper into the soil. Beneath this, at a depth of half meter to a full meter is a hardpan or duripan layer made of clays cemented by silica.

Soil profile cutout in a very dry landscape, cars visible in distance
Clear to see is the duripan layer of, not an alfisol in this case, but an aridisol, in Arizona.

This duripan layer is pretty typical of alfisols (as well as in aridisols, which form in intensely dry climates). Plant roots and water definitely don’t penetrate through this layer of the San Joaquin soil, which can extend down another one half to full meter. Meanwhile, however, the San Joaquin soil has been so heavily modified by tilling, intentional destruction of the duripan, and addition of fertilizers and pesticides, it’s not really clear that there’s much actual San Joaquin soil left!

Learn more about The Twelve Orders of Soil Taxonomy at the USDA’s Natural Resources Conservation Service.

Three Cheers for Soil

I could go on like this for another six soil types, but I will refrain and say instead three cheers for the sheer diversity and utter importance of what is an unintended consequence of climate, air, and water meeting rocks and minerals meeting biology. The next time you’re out in the countryside, in a garden, or even in the grocery store, spare a thought for the wonder that is SOIL! Right now Earth is the only planet in the solar system that actually has any. All the other planets just have dirt or, if you want to get technical about it, regolith.

There is something quite fabulous about the thought that the moment there’s a place where physical, chemical, and biological weathering can get to work on rocks and their minerals, the slow process of developing soil will start. Even more fabulous is that the various layers will develop themselves, without any intentional intervention from anyone. You’d even be able to predict what type of soil would develop of its own accord if you knew enough about the parent material and the local climate.

There is a certain peace and satisfaction to be found in that fact that even unintended consequences can unfurl in ways that are reasonably repeatable and, once you get to the bottom of them, make perfect sense.

Leave a Comment