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Dominique Browning…a modest and unassuming but powerful book…
—The New York Times
The Skin of the Earth
Land, then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants, and animals. —Aldo Leopold, A Sand County Almanac
We all have a relationship with soil. Some may call it "dirt" and try to launder it out of their children's clothing. Some may use it to grow crops to support their families, some may try to stop it from washing downhill in storms, and some spend their lives studying it and mapping it. Many gardeners think of it mostly when they are removing it from under their fingernails after a day planting bulbs, but anyone who wants to grow plants should know something about it, and surprisingly few do.
When I was a botany student I was not required to know anything about soil science—I learned a bit about soil formation in an ecology class, and that was it. Even today, many professional botanists and even horticulturists know surprisingly little about soil. But think about it: a substantial amount of the biological mass of a plant is in the roots, which need soil to support and nourish the parts aboveground.
Soils are incredibly dynamic—complete ecosystems that are constantly, though sometimes slowly, changing. They have mineral components but are teeming with life, such as fungi, beetles, worms, and gophers. Under most natural conditions they form at a rate nearly equal to that of their erosion, which happens naturally by means of water, wind, and gravity. They are neither permanent nor indestructible.
Skin Deep: Soil Formation and Composition
As soils develop, they usually form a profile of different layers (see figure 1). On top, there might be a relatively thin layer of organic material made up of living, decaying, and decomposed plants and animals. This "O" horizon may be only a few inches thick and is often darker than lower layers. Below that is what soil scientists call the "A" horizon, composed of fine mineral particles (sand, clay, and other inorganic matter) with some accumulated decomposed organic material. Moving deeper are "E," "B," and other horizons, mostly mineral particles of increasing size. Eventually, the soil stops at hard bedrock.
These layers, along with rocks, combine to create what has been appropriately called "the skin of the earth." Like skin, it is rich in variation—of color, moisture, graininess, and so on—and its multitude of types can be classified by these and other characteristics. The United States alone has more than twenty thousand identified soils!
Scientists who study soil formation consider five interrelated factors. The first is the parent material. This usually comprises large igneous rocks, created from volcanic activity; sediments that become solid through intense pressure; or combinations of the two into metamorphic rocks through heat and pressure. The rocks are slowly weathered by physical, biological, and chemical processes to form smaller and smaller particles. Then wind, water, glaciers, and gravity may move these particles long distances, so parent material in a location may not be the only or even the principal component of the soil found there.
The second factor is the climate, primarily precipitation and minimum and maximum temperatures. Wetter climates usually foster more plant growth, which means more organic matter to break up rocks (such as tree roots pushing through concrete) and then decaying to become part of the soil. Water also plays a direct role in weathering parent material. Areas with high precipitation generally have thicker soil layers, especially if the rain falls when the soil is warm, leading to increased chemical weathering as organic matter decomposes and releases organic acids. In colder weather, water can get into crevices in rocks and freeze—exerting a force of 150 tons of pressure per foot and breaking the rocks into smaller pieces—then thaw, releasing the fragments and allowing water to reach deeper into the rock.
Topography is the third factor. Aspect, or the relation of a place to sun and weather, plays a big role in the temperature of a site. In the Northern Hemisphere, north-facing slopes are cooler than south-facing slopes because they get less direct sunlight, while in the Southern Hemisphere the opposite is true. Another slope consideration is the degree of slant, which affects the amount of erosion that will occur through water runoff and gravity: the steeper the slope, the less distinct the layers of the soil profile will be at the top and the wetter the soil will be at the bottom.
The fourth factor is biotic material. In the upper layers of soil there may be up to five tons of living organisms (roots, animals, bacteria, fungi, and so on) per acre. On the surface, plant debris and dead animals decay and provide nutrients, and aboveground vegetation may slow rain as it falls, reducing erosion. Belowground, roots, worms, beetles, and bacteria help break down the decaying material, adding substantial amounts of organic matter to the soil. This organic matter contains large amounts of carbon, which is sequestered underground instead of being released into the atmosphere and contributing to global warming. Roots, worms, and larger digging animals help mix these soil layers and aerate the earth.
The fifth and final critical factor is time. Soils of all ages exist around the world, each in its own stage in the cycle of creation and destruction. As time moves on, all the other factors come into play: large rocks form and erode, local organic matter increases or decreases, and weather takes its toll.
Keeping the Pores Open
Perhaps the most important consideration in soil health is having sufficient pore space between particles to allow water and air to reach plant roots. (Some guidelines suggest that the optimal soil for plant growth has only 5–10 percent organic matter, 40–45 percent mineral particles, and about 25 percent each of water and air.) Roots have important jobs to do, including anchoring the plant, finding and absorbing water and nutrients, storing carbohydrates for later use, and connecting with helpful fungi (described below). Woody roots absorb some water and nutrients, but much of the root's work is done by its more tender ends and hairs, which form on its surface. The roots move through pore space and through larger openings created by earthworms and other soil fauna. If the soil is very compacted, by such things as construction equipment or heavy foot traffic, the roots will not be able to move and capture enough resources or to produce a wide or deep base, affecting their ability to stabilize the plant. Soil compaction is both bad for plant health and difficult to undo: it leads to less animal activity, which means less new pore space creation. Adding a thick layer of wood chip mulch to high-impact areas may reduce compaction.
One measure of compaction is tilth, or the composition of aggregated clusters of mineral grains. The word shares its root with the verb till, to loosen soil manually. Tilling can happen naturally, as through frost heaving or worm or insect movement. Under most circumstances, deep manual tilling is unnecessary and may even destroy soil structure, leading to a long-term loss of pores and the chopping up of integral fungi and soil invertebrates. Deep tilling should only be done for removing lawns or preparing beds for root crops, in soil that is at least 60°F and not too wet. If the soil remains in a ball instead of breaking up when you squeeze it in a fist, give it time to dry.
The Living Soil
Healthy soil is teeming with life. The smallest organisms are the many bacteria that, along with worms and insects, are critical to breaking down organic matter and converting soil nitrogen to forms usable by plants. Larger animals, such as moles, gophers, rabbits, and mountain beavers, live underground in what can become vast burrow systems, creating large, porous spaces. Fungi provide many valuable services, and, of course, there are all those plant roots. Each garden has a veritable city belowground!
The soil biota may affect human health. A 2007 study by Christopher Lowry and Graham Rook suggests a reason why gardening feels so good: a bacterium naturally found in soil, Mycobacterium vaccae, stimulates the human immune system to release serotonin. This hormone is used in antidepressants to increase feelings of well-being. Some scientists even believe that our ever-increasing desire for cleanliness and our distance from farming activities are leading to health problems such as asthma and allergies. Perhaps doctors someday will prescribe gardening for a healthy life.
Cultivating the Fungus among Us
Fungi are found not just in the threadlike networks that hold soils together but in plants themselves. They often attach to the roots, and studies such as those by Regina Redman and her colleagues have found them in stems and leaves, where they increase the plant's cold and salt tolerances. They commonly take the form of arbuscular mycorrhizae (AM), fungal threads that grow branching structures inside the root, where they receive carbohydrates in exchange for protecting the plant against disease and helping it to absorb water and nutrients.
Very little is known about AM fungi in most ecosystems, but many things are implicated in reducing their amount or type: extensive impervious surfaces, excessive disturbances and tilling, topsoil removal, and the use of pesticides and fertilizers. New landscapes have been found to have fewer AM fungi communities than established ones, probably because of the recent soil disturbance during construction. Too much phosphorus (the middle number on the fertilizer package) can also cause problems, by decreasing the ability of AM fungi to colonize plant roots, making them work harder to extract water and nutrients from the soil. Robert Linderman and E. Anne Davis have found that fertilizers in general, even those with less phosphorus, lead to less root colonization.
Mycorrhizal inoculation kits are commercially available to remedy these losses, but smart gardeners will avoid them. These kits can be ineffective—not all plant families use AM fungi—or even cause harm: nonnative AM fungi may change the community structure of native fungi and may replace fungi that local plant species rely on. If you focus on the overall health of your soil and use phosphorus-containing products in moderation, appropriate fungi will find your garden.
I Feel the Earth Move
Time, weather, and the incredible heat and pressure that form large rocks are implacable forces, but their effects on soil are increasingly outstripped by those of living organisms. Humans are perhaps the prime example: our farming procedures can rapidly deplete the soil of nutrients, land clearing can contribute to erosion, and construction practices can compact soils, making them less suitable for plant growth. We think nothing of moving soil to clear foundations for new buildings or even to reshape local topography. The city I live in, Seattle, was built on several hills. In the decades shortly before and after 1900, to flatten out and expand the town, the city leaders moved about fifty million tons of soil from some of the hills into the harbor and other areas! These neighborhoods are still often referred to as the Regrades.
But humans aren't the only large-scale earth movers. Worms turn too. Charles Darwin was fascinated by worms and spent decades studying them. He estimated that they brought more than ten tons of earth to the surface each year on every acre of English land. You might see this process in your own garden, as I have. Late in the winter of the first year I lived in my new house, I was startled to find small mounds of leaves dotted across the ground in my woodland garden. I pushed several aside and in every case found a small hole, each of which held an earthworm. After grabbing the leaves with their mouths and pulling them down the hole, the worms consumed them and defecated the remains, mixing organic layers into the inorganic.
It turned out that my earthworms are actually a widespread European species, Lumbricus terrestris, the very same worms that Darwin studied. Nonnative worms are found all over the world except at the ice caps and in arid lands. A Brazilian worm, Pontoscolex corethrurus, has spread through the tropics, several destructive Asian species in the genus Amynthus have invaded the eastern United States and are moving westward, and the species of worm in my garden—the common nightcrawler known to fishermen and dissected in my seventh grade biology class—has colonized many temperate areas. (Ironically, it is becoming rare in Europe, where two introduced flatworms prey upon it.)
Where native worms already exist, new introductions like these may overconsume local food sources. But perhaps the most serious problems occur where there are no native worms, such as the northern temperate forests of North America, which have not had native worms since before the last glaciations thousands of years ago, as shown in figure 2. There, the worms modify the soil structure, affecting the flora and fauna. Studies in hardwood forests by Cindy Hale and her colleagues at the University of Minnesota show a steep decrease in plant abundance and species diversity along a leading edge of earthworm invasion as the number of earthworms increased. Populations of a rare fern disappeared. The worms eat seeds, which may play a role in decreasing biodiversity, but they do the most damage in greatly altering the forest litter layer, exposing seeds and spores to predation and the desiccation that results from the disturbance of the moist, nutritious soils needed for germination.
Although nonnative worms are responsible for many problems, don't worry too much about the ones you already have. Just follow these few tips to keep them contained. Dispose of worms found in nursery material and never use soil with worms as fill dirt, especially if you live near a wooded area. Worm bins, popular for helping turn food waste into compost, should never be dumped anywhere. Freezing the compost for one to four weeks will kill the worms, but better yet, do not use worms at all: let naturally occurring organisms break down the waste. If you do buy worms to assist in composting, ask your supplier the name of the species and learn more about it—but know that earthworm identification is difficult and species may be sold under incorrect names. The University of Minnesota website listed under Resources can help you learn more.
Using Soil Amendments
Many of us are aware of the importance of healthy soil for healthy plants and are concerned that our soil is inadequate. If it is unable to grow certain plants, we are told that we should improve it—sometimes with amendments, which are things incorporated into the soil profile to increase nutrition, water retention, or drainage. Soil amendments come in organic (plant-derived) and inorganic (generally made of minerals or plastic) forms, but none is perfect.
Before considering soil amendments, you should get an idea of what soil type you have. Avid gardeners would ideally consider the soil at their prospective home before buying it. Libraries and extension agents often have soil maps of the area, so you can see how soil scientists have categorized your neighborhood. These maps are increasingly also found online, usually on local government pages. If your home was built recently, however, your soil type may differ from what the map says it is because construction activities may have altered it with fill dirt from other locations or compacted it.
You might also want to test for key nutrients, especially nitrogen, phosphorus, and potassium, and soil pH (pH, a measure of the acidity or alkalinity of the soil, is important to plants because it affects the decomposition rate of many biotic components and plants' ability to take up various nutrients). Soil that is healthy for most plants has a neutral pH, though many species prefer soil that is slightly acidic or alkaline. Nitrogen is important for foliage growth, phosphorus increases blooming, and potassium encourages healthy roots. These are the three numbers commonly found on fertilizer packages, in this order. You can purchase soil testing kits at garden supply stores, but you can get better results from professional tests for a relatively nominal fee. Check with extension programs to identify some of the companies and organizations that offer this service (such as the University of Massachusetts Amherst, whose soil-test web page is listed under Resources).
Excerpted from The Conscientious Gardener by Sarah Hayden Reichard, Bill Nelson. Copyright © 2011 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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