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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 above ground.

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 terrestis, 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 Additional 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 Additional Resources).

Once you understand the fertility and texture of your soil you are in a position to plan your garden. If you want to grow rhododendrons in heavy, alkaline soil, your first impulse might be to rush out to buy amendments to create a more acidic soil. But there are several things you should consider first.

There is a limit to how much you can do to improve your soil. The "skin of the earth" is complex, and manipulating it is usually about as easy and successful as permanently changing your own skin. Digging in amendments to a depth sufficient for root growth, for example, is expensive and difficult to accomplish on a garden-size scale, especially to the rooting depths of trees and woody plants, such as rhododendrons or oak trees, which have root systems that grow to from two to thirty feet deep and perhaps ten feet wide, depending on the species. Thoroughly mixing the amendments with the native soil is also challenging, and if you do not combine them thoroughly, you risk creating an interface of different soil textures. Roots often do not move easily through soils with different textures, which limits their growth, rendering them unable to fully support the plant or adequately forage for water and nutrients. Also, as previously noted, you run the risk of destroying pore space if you add the amendments when the soil is wet.

To avoid the temptation of soil amendments and fertilizers, garden within the normal ranges of pH and organic matter of your native soil type. Work with your soil, not against it: when planting containerized plants, break up the root ball, removing as much of the potted soil as possible, spread the roots, and backfill the hole with the soil that came out of it, top-dressing with an organic mulch. The mulch will slowly decompose, adding all the nutrients the plant should need.

One of the most common soils people want to amend is heavy clay with poor drainage and aeration. To improve it, 25-50 percent of the soil volume must be amended. It may be possible to do this to the rooting level of perennials in a small area, but not to that of woody plants. If you are determined to amend clay, use only composted leaves and pea gravel. Adding sand will only make the situation worse.

If you are growing herbaceous plants or container gardening, it is all right to use amendments. Just remember that you generally need only 5-10 percent organic matter, so don't overdo it. If you're making a perennial or vegetable bed, dig amendments in at least a foot deep, and ideally deeper. Many vegetable gardeners prefer creating a raised bed and filling it with purchased topsoil.

In most situations, amendments only introduce nutrients that quickly leach out of the soil. Their collection and production can also harm the environment, so if you must use them, select sustainable materials. In the following pages I discuss several organic and inorganic options.

Organic Amendments

Commonly used organic amendments include straw, grass clippings, compost, manure, wood chips, sawdust, and peat (Sphagnum) moss. The most highly recommended amendments are sustainable, meaning that they can be replaced at a rate at least equal to their removal. For instance, straw and grass clippings are produced by the removal of aboveground plant parts. The roots can then regenerate the aboveground growth, or the plants and more seeds can be reseeded into the harvest area for yet another crop. Compost is decomposed weeds or clippings from plants that would otherwise be thrown out. As for manure ... well, there is no danger of it being produced in insufficient amounts. Just make sure that the manure you use-whether steer, chicken, horse, or zoo animal-is matured to eliminate ammonia and other harmful chemicals that can chemically burn your plants. Sawdust and wood chips, added to increase the permeability of soils to air and water, are usually by-products of timber milling. With the rise of sustainable forestry practices, these products are also increasingly sustainable. However, some horticulturists think that bark and sawdust, especially as mulch, do not improve aeration very much and may even adversely affect it by forming dense surface layers. Wood chips, however, do not form those layers. The use of peat moss is hotly debated and is discussed below.

Compost and manure, in particular, are used to increase soil fertility because they are high in nitrogen. (Biosolids, or treated sewage sludge, are not appropriate for home use.) This element is an important nutrient for plant growth, and it is sometimes necessary to add more to the soil, especially if the soil doesn't get enough from fallen leaves. (The microorganisms that decompose organic matter also require nitrogen-but the amount deposited by precipitation is usually enough.) Before you add nitrogen, however, test to determine if your soil needs more. If it gets too much, some might end up in streams and lakes (see the following chapter). And more nitrogen might mean more pests: vast numbers of studies, including a 1984 paper from the noted entomologist Thomas White, have shown that plant-eating insects, which also need nitrogen to grow, are attracted to plants with higher nitrogen levels. Compost or manure will also be more effective used as a mulch than as an amendment, because the nitrogen will leach more slowly into the soil when it is irrigated.

Sphagnum Peat Moss

If you have indoor or outdoor potted plants, chances are good you use a commercial potting soil mix. And if you do, chances are good it has peat: so-called soil-less mixes rely heavily on this material. In addition to peat, potting mixes usually contain compost, soil, and sand-a general mix has about equal parts of the three key ingredients, though specialty mixes often vary the proportions-plus other ingredients, perhaps including blood and bone meal, for added nutrients. (If you would like to make your own, see the Organic Gardening website under Additional Resources and remember to use one of the recommended alternatives to peat.) Peat is popular with potting soil companies because it holds moisture well, meaning gardeners have to water less. It also is good for starting seeds because it seems to foster less of the damping-off fungi that plague seedlings, especially when they are warm and wet. Its acidity is also good for growth in many plants. However, there are several good alternatives to peat, summarized in table 1, and many reasons not to use it. Suggest to the management at your local garden center that they provide mixes with peat alternatives.

[Table 1: Alternatives to Peat]

Peat is produced worldwide in wetlands that are important for a number of reasons (as described in the following chapter). Peat bogs exist in 180 countries, appearing on every continent but Antarctica, in both tropical and temperate areas-especially colder temperate areas. They cover at least 2.5 million square miles in total. The Convention on Biological Diversity, an international treaty with 168 signing nations, has noted that it is important to minimize the degradation and to promote the restoration of peat bogs. In 2002 the Ramsar Convention on Wetlands, developed by an international group of scientists in 1971 to protect such regions and now with 159 signing nations, established a committee to monitor progress on the implementation of its Guidelines for Global Action on Peatlands. There are meetings, which include scientists as well as industry representatives, every four years to assess the status of peat across the globe.

Peat bogs are dominated by Sphagnum moss, whose many species are all found in wetlands. Sphagnum has an interesting growth pattern: its stems and strands may be quite long-but they are alive only in the top inch or so. The lower, dead part is shaded and often submerged in water. Because peat favors waterlogged, acidic, and low-oxygen conditions, it grows very slowly, but the mats of dead vegetation may be one to several yards thick. The lowest parts of the stems may have been in place for centuries!

Much of the current mining of peat is for horticulture, but in some parts of the world, such as Ireland, it is an important traditional fuel source still in use, and in Scotland distillers use peat to give scotch its lovely smoky flavor. The process of obtaining peat moss can be quite lengthy. Ditches are dug to drain 95 percent of the water from the bog, which can take a year or more. The peat is then cut or vacuumed out, destroying bogs that may have been developing for centuries, and further dried and ground before being packaged and sold.

There are many reasons to preserve peat bogs. Some bogs may be thousands of years old, and they are important records of human and plant history because their acidic, low-oxygen conditions are excellent for preserving biological materials. In 1967, Canadian botanists were able to germinate ten-thousand-year-old lupine seeds found in a Canadian peat bog! In a more gruesome case, in 1950 two Danish men found a corpse while cutting peat to burn for fuel. Thinking they had found a murder victim because of a noose around its neck, they notified the police, who, baffled by the case, turned to an archaeology professor. The so-called Tollund Man (named after the village where he was found) was a murder victim, all right. However, he was two thousand years old, so the perpetrator was long dead. The typical peat bog conditions had preserved his soft tissues so perfectly that the features of his face, including wrinkles on his forehead and stubbly beard on his chin, were clearly distinguishable.

Peat bogs are also home to many living creatures. While bogs around the world have many things in common, such as species of Sphagnum and acidic, low-oxygen conditions, each has a unique collection of biota: the communities of plants and animals that live in bogs are not the same everywhere. Bog biota can also be unusual. For example, carnivorous plants, those curiosities of the plant world, are frequently found in peat bogs. They have evolved the ability to trap insects (despite the exciting name, they rarely snare larger animals) because peat bogs are so low in other sources of nutrients. These plants trap their prey with sticky hairs, pitchers of poison, or leaves that snap shut before the plant chemically dissolves the decaying bodies, absorbing their nutrients. Such plants usually have very limited ranges. The decrease in peat bogs is likely to harm them and some of the other species that live in these isolated, specialized habitats.

Humans depend on peat bogs too-even from afar. In recent years peat bogs have been recognized as a huge sink, or repository, of carbon. It has been estimated that northern latitude peat bogs hold 455 billion metric tons of carbon, which is slightly less than the amount estimated to be in all other living organisms. There is mounting evidence that large amounts of atmospheric carbon dioxide (CO2), released through the burning of fossil fuels, lead to increased temperatures worldwide and have consequences in every part of this planet. The peat bogs' capacity to capture and hold carbon is critical, and there is debate about how much impact mining them will have on climate change. Defenders of peat mining, for instance, claim that the huge expanses in areas too remote to mine still sequester millions of tons of carbon each year. They also claim that peat bogs produce methane, a greenhouse gas like CO2, which would mean that active, or growing, peat bogs are positive contributors to global warming. Environmentalists counter that CO2 emissions are increased by draining the bogs prior to mining, which increases decomposition of peat and other organic components . Most independent scientists agree with environmentalists that the carbon-holding capabilities of peat bogs are important in preventing climate change, and they further say that the bogs preferred for mining produce relatively little methane. The Intergovernmental Panel on Climate Change has determined that when peat bogs are destroyed, the harm from increases in other greenhouse gases, such as carbon dioxide and nitrous oxide, far outweighs the benefits from the decrease in methane. (See chapter 7 for more on climate change.)

Why can't we just regrow the bogs after a harvest? This happens to some extent, but because Sphagnum grows so slowly, abandoned peat bog communities do not grow back quickly, even when the ditches are removed and the water system is restored, and peat is not replenished at anywhere near the rate at which it is mined. Susan Jonsson-Ninniss and a colleague found that although a peat mine had unassisted growth soon after it was abandoned, after two decades its community had only about 51 percent of the species found in the undisturbed parts of the bog. Perhaps more disturbing, researchers noted that the regenerated area had significantly more trees than the undisturbed parts, especially birches, suggesting that mining peat from bogs fundamentally alters their structure.

Can we do anything to help the bogs grow back? Ecological restoration works to help disturbed ecosystems return to their original state more quickly. Line Rochefort and colleagues collected a small amount of four Sphagnum species from undisturbed bogs. They sowed both live whole strands and shredded bits across exposed dead peat and found that the size of the moss fragments did not affect peat regeneration. A layer of no more than about three-quarters of an inch was most effective. If it was thinner, they grew mostly Sphagnum; if thicker, they got mostly vascular plants. The restoration sites with a protective mulch of straw and some additional phosphorus also grew better. The study proved that bogs can grow faster than by natural regeneration, thus increasing the sustainability of peat mining. However, these experiments lasted just five years, so we have little information about the effects of such restoration efforts over time.


Humus from Alaska is promoted as a source of compost and is sold in parts of the United States. Like peat, it takes thousands of years to form. While it may be necessary to remove some prior to building construction, an increase in demand will increase the probability that it is mined (like peat) in nonconstruction areas, so use other forms of sustainably and locally produced compost instead.


Sometimes an idea catches on like wildfire before much thought or research supports it. So it is with biochar, a fancy charcoal that is sweeping the agricultural and horticultural worlds with amazing speed. It is essentially charcoal that is made by burning wood at a low temperature for a long period. As wood burns, some of its carbon compounds become gases and some remain as solids, such as cellulose and lignin. Biochar's slow burn means about 25-35 percent of the original dry mass remains. According to Professor Sally Brown at the University of Washington, however, biochar is less effective as a soil amendment than compost because it does not attract soil microbes, which improve soil structure and water retention and help the soil hold important nutrients such as nitrogen and phosphorus. Char is effective at holding carbon in the soil, but Dr. Brown points out that char does not really help plants grow, and compost excels at that-and fast-growing plants sequester carbon very well!

Inorganic Amendments

Inorganic amendments are used to alter soil pH and to change soil structure. As with organic amendments, you should look for products from sustainable, environmentally safe sources. Wood ash is sometimes used for acidification, but its content and pH vary, and its use is not advised. Ground limestone makes soil less acidic while adding calcium and magnesium. Limestone quarries may or may not be friendly to the environment, mostly depending on how local political agencies regulate them, so you may wish to avoid using limestone unless you can investigate the source. Finely crushed oyster shells might be a better solution: like limestone, they can be applied to increase the soil pH and are a rich source of calcium and magnesium, and they are sustainable as long as the shellfish are harvested sustainably.

A few other amendments are used primarily to improve soil structure. Gravel, perlite (a silica mineral), and sand are quarried or mined and incorporated to increase drainage. The same caution about quarries applies. Note that the sand should be of low salinity to avoid injury to plants-look for horticultural sand.

Some gardeners attempt to improve the water-holding capacity of their soil, especially in containers, with vermiculite. Vermiculite is a naturally occurring mineral that is mined, then heated to extreme temperatures, which makes it swell. Vermiculite has been used for many purposes and is often included in soil mixes. Most of the vermiculite mines that have been tested have deposits of asbestos, fibrous mineral crystals that can cause severe lung damage and lung cancer. Unfortunately, it doesn't stay in the mines. The U.S. Environmental Protection Agency tested sixteen commercially available potting mixes and found detectable levels of asbestos in five, while one released airborne fibers with simulated use in a lab. Consumers have no way of knowing which bags contain asbestos, so the EPA recommends that buyers use the products outdoors, thoroughly wet them (to prevent airborne fibers), and be careful about bringing clothes indoors afterward.

It's easy enough to avoid these risks: forgo the use of vermiculite and instead grow plants suited to the precipitation of your climate, employing mulches and good soil practices to improve soil moisture.

Using Mulch

Any substance that is laid on top of the ground can be considered mulch: bark, wood chips, rocks, recycled glass, cardboard, newspaper. Even some plants and ground covers are used as living, or green, mulch. Each type of mulch has unique properties and production, with implications for conservation. Like amendments, mulches can be organic or inorganic. Organic mulches may provide some nutrition to the soil, while inorganic ones may increase soil temperatures and are usually long-lasting. Given that there are practically an infinite number of things that can be used as mulch, with new ones introduced all the time, I will cover only the most common types, summarized in table 2. Use mulches that are produced locally to reduce their carbon footprint (see the chapter on climate change for more information).

[Table 2: Mulch Comparisons]

Unlike with soil amendments, there are many good reasons to use mulch. One of the most common is to increase the retention of moisture. Exposed soil is subject to heat and wind, which increase evaporation. A protective layer of mulch can shield water from their effects. Most mulches will hold some water, making precipitation a little slower to reach the soil, but the retention benefits result in an overall increase in moisture. In general, organic mulches retain soil moisture better, with the exception of gravel, which is also effective. Not surprisingly, living mulches do not increase soil moisture, because they use the water themselves (although some, such as kinnikinnick and creeping rosemary, are efficient water users).

Mulch can decrease the germination of weeds, which also compete with landscape plants for available water. Weeds can increase the loss of water from the soil by up to 25 percent. They may also steal sunlight if they cover or are higher than your landscape plants. Existing weeds may be stressed by a mulch covering, but it is most effective to remove them before applying mulch. I have managed to tame amazing infestations by weeding an area and then directly applying at least three inches of bark or wood mulch.

Many people use mulch for temperature control. In hotter regions, organic mulches can lower soil temperature as much as 50°F! They can raise the temperature, too: a protective layer can prevent roots from freezing. Just be sure not to bunch the mulch around the stem, because a lack of air circulation and trapped moisture can lead to rot. Inorganic mulches, such as stones, store and conduct heat and are sometimes used to increase the temperature around heat-loving plants. Mulches can also help the soil heat up earlier than bare ground in the spring, giving plants an early boost, and plastic sheet mulches, especially clear plastic, can kill weeds by raising the soil temperature, a process called solarization.

According to Dr. Linda Chalker-Scott, known to many as the Mulch Queen and the author of a definitive review of mulches, the word mulch is derived from the Germanic term molsh, which means soft or spongy. It is therefore not surprising to learn that mulches are a great way to prevent soil compaction. Just a small layer of mulch can help reduce the impact of feet, equipment, vehicles, and even pounding rain. (If you using mulch to protect tree roots, lay it as far as the branches extend.) Mulch can help prevent surface-soil erosion by cushioning the impact of rainfall. However, living or any other mulch cannot stabilize slopes-this requires more deeply rooted plants such as trees.

Organic Mulches

The most common organic mulch is probably bark chips, from conifers or hardwood species. It is a by-product of timber milling-the bark is stripped off the felled tree, then chopped up and sold in bags or in bulk. Because this is waste material, as long as the trees are harvested in a sustainable way, the product is fully sustainable. Some barks, such as from fir trees, decompose quickly, requiring more frequent application, and many will give you terrible splinters, so wear gloves when working with them. Bark tissue is waxy, so the chips might not absorb and rerelease water into the soil. Some bark mulch may also come from trees held in salt water before milling and may be very saline.

Related to bark are wood chips. They are sometimes made from recycled wood products, such as old pallets, or are a side product of industry. In areas where paper is produced, some trees are chipped to produce pulp. The larger chunks may be sold as mulch or for playground surfaces. If you purchase bulk wood chips, which is how they are usually sold, from a supplier, you should ask where they are from and how the trees are harvested. A bulk supplier likely buys directly from the mill and should know or can ask.

In most places wood chips do not result in a substantial loss of forest trees. In fact, mulch from urban arborists, made from tree and other plant trimmings, would end up in landfills if not used. Many arborists, looking for a place to dump their chips without paying a fee, will bring them to your house for free or at a low cost. (Another free source of mulch: if you need to remove a tree from your property, have it chipped and left in place.) Arborist mulch is also good because it usually includes pieces of varying sizes, which prevents it from becoming packed down to form a barely penetrable layer, something that occasionally happens with bark. Be sure, however, that the source of the wood is local: some plant pests have been transported in wood products into previously uninfected areas.

There are, however, places where wood chips are harvested unsustainably. In the southeastern United States, forests of bald cypress (Taxodium distichum) have been destroyed to make mulch. This mulch is reputed to be rot- and termite-resistant, but environmentalists claim that the older trees with those properties are long gone and younger trees do not have the same properties. These coastal forests provide important ecosystem services such as buffering against powerful storms like Hurricane Katrina, which devastated New Orleans in 2005. It is estimated that twenty thousand acres are logged each year in that state alone, mostly for mulch. Make sure you know where your mulch comes from.

Another concern with wood chips and bark is that they can deplete soil nutrients, especially nitrogen. Woody mulches do not have enough nitrogen to support the microbes that decompose their carbon, so the microbes must take some nitrogen from the soil. But while there might be a slight zone of nitrogen deficiency at the soil/mulch interface, studies such as those by Greenly and Rakow in 1995 and Pickering and Shepherd in 2000 have shown that this does not cause much harm. Shallow-rooted annuals or herbaceous perennials might feel the greatest impact, as will gardens where the woody material is incorporated into the soil as an amendment. If you are concerned about a nitrogen deficiency in your garden, put down a thin layer of compost before mulching with wood products and avoid very fresh chips.

In nut-growing regions, crushed shells-from pecans, filberts, and macadamias, for example-are sold as mulch. To reduce carbon emissions in transport, only use nutshells if you live in an area where nuts are grown and/or processed. The shells are very attractive and long lasting but also expensive compared to other mulches. Most organic mulches are highly effective at a depth of at least three inches, so if you don't want to pay for that many shells, you might use fewer as the decorative topping on a less expensive mulch. To appeal to another sense, use cocoa hulls, left over from chocolate production. A deep, rich brown, they smell like chocolate for at least a few weeks after application-which may or may not be a good thing! And if you own a dog, be careful: there is some concern that they are attracted to the hulls, which contain theobromine, a chemical toxic to dogs.

So-called fertile mulches are usually a mixture of bark or wood chips, which break down slowly, with nutritious but short-lasting materials such as compost or manure. (You can mulch with pure compost, either your own or purchased, but it will break down quickly. See the chapter on recycling for more information.) They are sometimes sold under deceptively delicious names, such as "chicken and chips," which consists of wood chips and manure from chickens and other animals. Some gardeners swear by them, but it is difficult to generalize about them because different manufacturers use different formulas. If you are curious, ask other area gardeners if they use them and which local suppliers they recommend. Ask the suppliers what is in their mulch-it will generally be components already discussed, enabling you to judge their sustainability.

Some people use a thick layer of local vegetable matter as mulch. Most gardeners can find pine needles, grass clippings, leaves, and small stems in their own yards. These can work very well and provide some nutrients to the soil, but they will likely need to be reapplied at least annually. If you want to use leaves or stems in particular, investing a bit of money in an electric shredder/chipper will pay off in a short time. Grass clippings are best left on the lawn to replenish the soil with nitrogen naturally. If you live in an agricultural area, you may have access to baled straw-made of barley, oats, or wheat stems-following the harvest. Such fields used to be burned, but this is now banned in most places to preserve air quality, so using the straw helps with a waste problem. Just be aware that hay may contain many weed species.

There is some worry that mulches-and organic ones in particular-may harbor rats and other pests. In general, particulate mulch will not provide a good habitat, but some living mulches, such as dense vines, do offer