The zoo beneath our feet: We’re only beginning to understand soil’s hidden world

The gardener has a long, touchy-feely relationship with the soil. As every good cultivator knows, you assess the earth by holding it. Is it dark and crumbly, is there an earthworm or beetle in there, is it moist, and when you smell it, are you getting that pleasant earthy aroma?

All these signs are reassuring, and have been through the ages, but they are mere indicators of something much greater and infinitely mysterious: a hidden universe beneath our feet.
This cosmos is only now revealing itself as a result of scientific discoveries based on better microscopic imaging and DNA analysis. There is much still to learn, but it boils down to this: Plants nurture a whole world of creatures in the soil that in return feed and protect the plants, including and especially trees. It is a subterranean community that includes worms, insects, mites, other arthropods you’ve never heard of, amoebas, and fellow protozoa. The dominant organisms are bacteria and fungi. All these players work together, sometimes by eating one another.

The awareness of this biosphere should change the way gardeners think about cultivating plants and heighten everyone’s understanding of the natural world. In other words, don’t ever call it “dirt” again.

The sheer vitality of it is mind-bending: A teaspoon of good loam may contain a billion bacteria, yards of fungal strands, several thousand protozoas and a few dozen nematodes, according to Jeff Lowenfels, a garden writer based in Anchorage and co-author of “Teaming With Microbes.”
This is, basically, how it works: Plants manufacture carbohydrates through photosynthesis, but not just for themselves. They release some of their carbon sugars into the soil, which causes the bacteria and fungi to show up to feed. The bacteria crowd around the root zone, and the fungi form vast networks of interlocking strands that often link one plant to another. The bacteria convert nitrogen and other nutrients into forms the plants can use, often by getting devoured by other microbes.

The fungal strands, the mycelium, effectively increase the root mass of its host plant by as much as a thousand times and transport a bevy of goodies to the host plants, including phosphorus, copper, calcium and zinc. There is also evidence that trees use this network to send signals to one another if, say, leaf-eating pests have arrived. In his Ted Talk, mycologist Paul Stamets referred to mycelium as “Earth’s natural Internet.”

Although some plant (and human) diseases are caused by soil-borne fungi and bacteria, most of these microbes are beneficial and keep the bad ones in check. The organisms assist in other ways, by increasing the size of soil particles, which improves the ability of the soil to hold water and air. Even in the middle of a city, the subterranean world is thriving.

Scientists took almost 600 soil samples from across New York’s Central Park and discovered a surprising diversity and richness. They identified more than 120,000 types of bacteria and more than 40,000 species of fungi, protozoa and arthropods. Among the unexpected findings: The microbial species were the same, more or less, as those found in parts of the world with dramatically different flora and climates from New York’s, including Antarctic cold deserts, tropical forests and grasslands.

There was a strong association between the diverse organisms in each sample. “Unravelling these relationships will be critical to building a more integrated understanding of below-ground ecology,” the researchers wrote in a paper published by the journal for the British Royal Society. “Our work highlights that most of the diversity found in soil remains undescribed.”

Enough is known, however, to create a 21st-century subset of farming known as regenerative agriculture. The farmers have discovered that if you foster this biosphere, you don’t need expensive fertilizers because the microbes repay the plants with nutrients. They also, for obvious reasons, avoid pesticides that would kill this soil life.

The farmers do as little soil digging as possible because traditional tillage destroys the fungal networks and the desirable soil structure. Cover crops keep the soil life happy between growing seasons.

Advocates of this low-impact farming say it can restore soil carbon lost by the historic conversion of forest and prairie to farmland and help to mitigate greenhouse gases. In the 1990s, an Agricultural Research Service scientist in Beltsville, Sara F. Wright, discovered a sticky coating to fungal threads named glomalin that, it turns out, is a major reservoir for carbon.

Lowenfels says it’s also time for gardeners to adopt practices that nurture the soil biosphere. To say he thinks deeply about this subterranean world is an understatement. In addition to “Teaming With Microbes,” he has written “Teaming With Nutrients.” His latest title is “Teaming With Fungi,” which dwells on the type of fungi that directly associate with plant roots. They are known as mycorrhizal fungi, and he’s a big fan of adding them to his plants when they are installed, either as a spray or in powdered form available from the garden center. “It works. My tomato plants are bigger than the control, they’ve got more fruit on them, the plants are so healthy,” he told me. “My carrots are unbelievable this year.”

Some gardeners turn to compost tea to build soil microbes. This is made by aerating sugars, compost and humic acids in non-chlorinated water and then spraying the brew on plants and soil. Others are not convinced that this is needed, though everyone agrees that the way to foster the soil food web is to top-dress growing beds and lawns with organic matter such as shredded leaves or finished compost.
James Nardi, a biologist at the University of Illinois in Urbana, offers this advice: “Work with your fellow non-human gardeners. I never use synthetic fertilizers, and I never use pesticides.” Nardi’s 2007 book, “Life in the Soil,” remains an excellent introduction to the subject.

In the fall, he mixes horse manure with fallen leaves, shreds the mixture and applies it as a mulch to his growing beds. “In the spring, I have this lovely, spongy soil,” he said. Lowenfels shreds autumn leaves on his lawn and lets the biosphere use them over the winter. The organic gardener’s mantra has never seemed more appropriate. Feed the soil, not the plant.

● Earthworms: Earthworms (and other worms) play an important role in the hidden biosphere. Most worm species in the garden were imported by Old World settlers, and some worms in certain regions have caused a problem by processing organic matter too efficiently. The latest culprit is a creature called the Japanese crazy worm (Amynthas agrestis), which multiplies like, er, crazy and damages the soil structure through mass feeding. It is long established in parts of the Southeast but has spread recently to Wisconsin and Illinois, where it is causing problems. But the European earthworms familiar to most gardeners are helpful.
Worms provide critical assistance to smaller organisms by breaking down and incorporating leaves into the soil, so all may eat. Worm castings are rich in nutrients, including calcium, nitrogen, phosphorus and potassium. The most famous observer of earthworms, Charles Darwin, estimated that they could add as much as 40 tons of casts per acre annually.

● Insects: Thousands of insects (and spiders) live in a patch of soil. Some are considered pests by humans — Japanese beetle grubs, termites and weevils, for example — but others are beloved or at least beguiling and include the larvae of lightning bugs and cicadas. Dung beetles convert animal waste into humus, a service we take for granted. Ants are the most abundant soil insect. Although some species are pests or nurture pests such as aphids, ants with their highly organized colonies are essential members of the soil biosphere. They assist in the conversion of litter to humus, move and mix large quantities of soil, and spread the seed of bulbs and other desirable plants.

● Other arthropods: The more conspicuous of these include millipedes and centipedes, as well as woodlice. Millipedes feed on plant debris and microbes; centipedes eat other arthropods. Woodlice, or sowbugs, are crustaceans that like soft plant debris and make quick work of green plant material and newly fallen leaves.

One of the most abundant, but barely visible, arthropods in the soil are springtails. They are named for a tail-like structure that allows them to jump when threatened. As many as a billion or more can live in an acre of soil. Depending on species, they cycle plant debris or feed on fungi, algae or other springtails.

Mites are generally regarded by gardeners as pests, and some are — sucking sap from plants and spreading disease. But the soil houses an immense community of non-pest species that are essential to the cycle of life. Half the known species of mites live in the soil, where they feed on decaying plant litter. Nardi writes that they “set the stage for smaller decomposers like bacteria and fungi to free most of the energy and nutrients stored in those leaves.” Some mites are predatory and attack nematodes and other small creatures.

● Nematodes: Nematodes are tiny wormlike creatures that have traditionally been viewed in agriculture as serious pests that harm plants by feeding on their roots. More recently, the view of nematodes has become more nuanced because some species are now commonly used (and purchased) as predators of garden pests such as slugs, vine weevils and white grubs, to name a few. In truth, the world of nematodes is much greater and can only be imagined. Experts believe there may be close to a million species, of which only a fraction have been described scientifically.
Some nematodes eat soil bacteria and fungi, while others prefer to consume other soil arthropods and protozoa. Their value to the garden is in converting nitrogen into a form that plants can use.

● Protozoa: Protozoa are microscopic creatures that live in vast numbers in the film of water between soil particles. The most well-known is the amoeba, but these microbes come in several forms, including species that move with a single flagellum or with hair like cilia.
They are the major predator of bacteria, and in consuming them they release nitrogen and other nutrients to plants. Protozoa, in turn, are eaten by nematodes and other small arthropods.

● Bacteria: Historically, bacteria have been associated with germs. Some of the nastiest human diseases — anthrax, typhoid, tuberculosis and syphilis, for example — are the result of bacterial infections. But we have come to know too that our guts are full of beneficial bacteria and essential to our health.

The soil is the same way — the bad actors are outnumbered and usually outwitted by the good ones. Healthy soil is loaded with bacteria, and because they’re not very mobile, they tend to hang out in vast numbers on and around the roots of plants, a zone known as the rhizosphere. There can be as much as 100 times more bacteria around plant roots than elsewhere in the soil, and with good reason. The plants feed them carbon sugars. The microbes give back nitrogen.

● Fungi: Fungi break down organic matter, which is why you will see mycelium strands in compost piles and under leaf litter. Two basic forms of fungi form a symbiotic relationship with plants. One exists in proximity to root tips and associates with hardwood trees and conifers. The other penetrates the cell wall of the roots and is found in plants of the domestic landscape — flowers, shrubs, grasses and vegetables.

The fungi grow tiny, fragile strands called hyphae. They are a tenth the thickness of human hair, but there are so many of them that they form a vast network, effectively extending the reach and efficiency of plant roots. In her book “The Soil Will Save Us,” science writer Kristin Ohlson says there can be as much as 320 miles of hyphae in a cubic foot of soil. At least 80 percent of the plants on Earth connect to these fungal partners.

“Gardeners need to know this stuff,” Lowenfels said. “A thinking gardener is a better gardener.”

https://www.washingtonpost.com/lifestyle/home/the-zoo-beneath-our-feet-were-only-beginning-to-understand-soils-hidden-world

Reconsidering the Underworld of Urban Soils

by Laura Solano

Look down. If you are in a city or large town, below you is a vast network of hidden systems that support your life: pipes that carry natural gas, potable water, stormwater, sewage, and communications wires. These pipes rarely come to mind, but we agree that their operation is for the common good, that survival is not possible without them, and that armies of workers should keep them running. Surrounding those pipes are soils that are equally critical to our existence but to which we give much less attention. If we truly understood the delicacy of soil as a dynamic living system integral to the health of our towns and cities, our neighborhoods and families, we would be more cautious about how it is perceived, treated, and protected. Healthy soil performs important functions such as sequestering CO₂, mitigating stormwater runoff, supporting plant life, and sustaining the microbial populations that form the basis for all living things. So essential and complex are the conditions for soils in more developed areas that a new branch of science has arisen and is now being intensively pursued: the science of urban soils.

The Challenges of Urban Soils

Urban soils are are naturally-occurring soils that have been disturbed by development in a way that affects their functioning and properties. Urban soils are distinguished by a number of similar features: Their horizons (the natural vertical order of soils) have become jumbled by excavation. This makes urban soil horizons confoundingly diverse; one layer may be hospitable, but adjacent layers may not be, creating abrupt changes that can cause impermeable interfaces. Soil structure (the balance of solids and pores) has been crushed out of existence by mechanical compaction that chokes off water and air exchange. Organic matter (the source of plant nutrients) is low or missing from lack of replenishment, and this imbalances the soil biological community (bacteria, fungi, nematodes, arthropods, earthworms, insects, and more). Soil volumes that are important for plant health decrease because of interruptions from urban debris such as construction waste and rocks. Finally, the predominance of pavement separates soils from natural inputs such as nutrient-rich leaf litter, and this separation causes the nutrient cycling system to slow or shut down.

In the urban environment, soils are likely to be sealed off from the agents that build healthy soil—including wind, precipitation, ice, temperature, gravity, and mineralization—which frequently have been replaced by anthropogenic processes detrimental to soil functioning. Urban soils often become defined by human activities and land use histories at a particular location rather than by the continuum of geologic processes. This disrupted order makes urban soils particularly challenging to analyze, manage, and construct.

Urban Soils in the Service of Stormwater Management

Urban soils have the potential to be an important partner in stormwater management, use, and protection. The Natural Resources Conservation Service has recognized that soils with good infiltration and permeability can significantly reduce stormwater runoff rates and volumes that might otherwise overwhelm and impair the performance of the chain of water bodies that sustain our water supplies and the ecosystems that are necessary for healthy living [1]. Good infiltration reduces runoff by letting water soak into soils before it builds up to damaging volumes and velocities that would erode topsoil and carry both silts and pollutants to waterways. Permeability influences how quickly absorbed water drains through soil to useful depths for plants and recharge. Water that reaches root zones reduces irrigation needs. Some soil can filter toxic compounds or excess nutrients by holding them, degrading them, or otherwise making them unavailable. All of these benefits are feasible when soil has adequate pore space, which is only possible when soil’s natural physical texture and structure have been preserved or created.

Over-compaction of soils is one of the greatest deterrents to implementing best practices for stormwater management, because crushed particles minimize pore space and prevent water and air from moving through. In a study by the University of Florida, soil compaction from construction vehicles reduced infiltration by 70 to 90% [2].  This is perilously close to impermeable pavement.While people recognize that reducing pavement is the primary way to improve stormwater management, few see the same connection with soil. It is not enough to substitute pavement with plant beds if nothing has been done to prevent construction compaction. Without a soil management plan that includes practices for dealing with compaction before, during, and after development, urban soils will continue to become, plot by plot, a decommissioned resource in stormwater management.

 A Partnership between Urban Soils and Vegetation

The greatest positive effect of healthy urban soil is most evident in plants, the workhorses of the environment that clean the air, absorb CO₂, abate high temperatures, support wildlife, slow stormwater runoff, and keep erosion in check. In recent years, there has been resurgence in support for increasing the vegetation and tree cover in American cities. We are well aware of the positive ecological, social,[3]  and economic value of plants for individual properties, community open space, and urban regions [4]. Ecologically, a single large tree in the city is said to be ten to twenty times more beneficial to the environment than a single tree in the forest [5]. Yet the health of urban trees is declining at a rapid rate. A recent study by the U.S. Forest Service looked at twenty cities and found that they are losing tree canopy cover on average by 3% per year [6]. While this loss may seem small, over time the cumulative effects are severe. For example, Washington, D.C. lost 64% of its acreage-coverage from 1973 to 1999 at an average annual rate of 2.5% [7]. Still we continue to ignore the most basic need of trees: healthy soils. On most urban sites, fertile topsoil is absent, plant roots are restricted, air and water movement is suppressed, and nutrients cannot be exchanged. All this puts plants at an extreme disadvantage. The evidence of poor soil is all around, telegraphed by unhealthy plants. So if trees are to become “beautiful utilities” as urban tree expert Henry Arnold [8] suggests, then soil must also be treated in projects as an essential utility: analyzed, engineered, budgeted, scrutinized, and maintained.

Advocating for Soil

There are sound economic reasons to invest in good soil. As one of the core infrastructural materials in every urban landscape project, soil needs only to be tended more carefully to make it a viable component of stormwater management. Using soils to store and retain water as part of the stormwater management system can reduce costs for piping, drainage structures, runoff storage tanks, irrigation systems, and infrastructure maintenance and can provide more flexibility in design, since hard systems can add horizontal and vertical complexity that limits design options. Plants (especially street trees) with well-functioning soil are more able to start and sustain the nutrient cycling system without big infusions of maintenance after establishment and in maturity. When they do get maintenance, they are more likely to respond. Healthy soils beget trees that live longer and grow bigger, enabling them to cast more shade, and absorb more CO₂, and runoff. Even asphalt benefits from healthy trees, since shade improves its performance and durability [8]. Last, trees in good quality soil are far less prone to infection and pests, virtually eliminating the need for chemical treatments [9]. Investing in soil is critical for the long-term health of urban trees and by extension for the success of sustainable landscape projects and green infrastructure programs.

Why then do urban soils get so little attention when they are such a critical part of our environmental infrastructure and, ultimately, of human well-being? Some of the unawareness stems from societal and governmental ignorance. While keeping water and air usable is an unquestioned necessity, few people have the same association with city soils. For the most part, urban soil is considered mysterious, complex, and costly. Design professionals have an important role to play in dispelling unwarranted concerns and helping solve tangible problems: They should lead the way, project by project, educating their clients, agencies, and others about the need for healthy soil. Before that happens, designers must step up their own soil education. My interactions with colleagues suggest a dearth of understanding of basic soil science and the need for soil management in landscape projects. Often other landscape architects reach out for soil advice only when something has gone wrong. Designers do however have a thirst for this information as is shown by the increasing number of packed sessions in soil education at the annual meeting of the American Society of Landscape Architects (ASLA), the professions’ largest organization. Perhaps the neglect is also due to the fact that soil is not yet a hip topic; it has no visual presence. For many, design attention is reserved for visual effects; the hidden, infrastructural elements of landscape have long been considered the domain of engineers and scientists.

In my work as a landscape architect at Michael Van Valkenburgh Associates, soil discussions begin early, sometimes in the concept phase and always by schematic design. Soil is always an item on the design checklist. Just as all practitioners request surveys to locate utility lines, we request USDA soil tests to understand what we have to work with. Partnering with soil scientists, we have learned to interpret laboratory tests so we can ask the right questions and frame discussions. We keep up with developments in soil science (biology is the big topic now), often consult allied professionals, incorporate quality control practices into our specifications, and closely monitor sourcing, blending, and installing of soils during construction. We consider soil rigorously, as we do any other product or system in our projects.
I don’t mean to imply that assuring good soil is obvious or easy; it is neither, even for a firm that has been attempting it for twenty years. Every project brings unique soil challenges and clients with different agendas. The client may be unfamiliar with non-traditional stormwater approaches and therefore reluctant to consider soil-dependent systems. Brownfield properties often have contaminated soil or no useful soil at all. In other kinds of properties, existing soils could be reused if amended, but space may be too limited to manage soil-blending operations. Sometimes soil chemistry is limiting. For example: elevated pH from concrete or limestone rubble can interrupt nutrient exchange and narrow plant selection; high salinity in soil near tidal waters wreaks havoc on water uptake and cellular structure in plants. Local contractors often have no experience with installing designed soils. In my experience, construction managers show little tolerance for any aspect of landscape construction that is dynamic, an inherent characteristic of soil in particular and landscapes in general. Unless we have a repeat client, the process of educating, convincing, and making monetary tradeoffs to get good soil starts anew on every project. Sometimes we battle the sins of others’ projects in which someone tried but failed to improve soils. Projects with unsuccessful or difficult soil processes often produce rumors that the landscape architect specified unrealistic soils that cost too much and slowed the schedule, even if the problem was caused by the laxity of a member outside the design team.

Repositioning Soil as Infrastructure

How can we begin a campaign for good urban soil? We can start by talking with city hall, one of the biggest makers of landscapes and planters of trees, about the importance of soil. How many of the thousands of landscapes planted every year include soil improvement? Atlanta, Detroit, Denver, Los Angeles, and many other cities have tree-planting programs. Ambitious past and current mayors like Richard Daley and Michael Bloomberg launched campaigns to plant a million trees. Despite current commitments to increasing urban vegetation through tree planting, under current practices the mortality rate for young street trees is shockingly high: Some studies have found that over twenty-five percent of newly planted trees die within two years of installation [10,11], wasting already strained public funds and leaving behind a depressing reminder of failed nature. Wouldn’t it be more strategic to forgo planning one million trees in poor soil and instead plant 500,000 trees in good soil? [12]

To be stewards of urban soils, we need to ask pointed questions early in and throughout projects and insist on satisfactory answers that ensure positive long-term results for stormwater and planting. When zoning requires developers to add or replace trees, we need to ask for more than in-kind caliper inches and to require a soil management plan. When contractors install soil, they need to treat it like the valuable commodity it is or bear the cost of remediation. State and municipal specifications (which are used by contractors defensively instead of proactively) already define which dirt is suitable for backfill—why not extend this thinking to include requirements for the type, procurement, handling, and installation of planting soil? Landscape architects and anyone else who works with the landscape need to heed these too. Such guidelines should not be overly technical or onerous. Plant species should be matched to soil conditions, especially its pH and water supply. Trees should be planted at the right elevation to expose the root flare so soil doesn’t suffocate the tree. Adequate soil volume (800 to 1400 cubic feet per tree) and shared root space to encourage root spread should be provided [13]. Soils should be arranged to mimic the horizons in nature in which the top is rich in nutrients, the middle has the correct structure to encourage root growth, and the bottom is drainable. To resist compaction and maintain water and air exchange, soils higher in medium-to-coarse sands (rather than easily compactable fine sands and loam) should be used, and limits on density should then be set. Wet or frozen soils should not be moved or installed. To promote water and air exchange, rootball zones in tree pits should be exposed and at least half of the surface area of a plant bed should be left open, or a simple aeration system should be installed. Well-aged compost should be used to to provide 5% to 10% organics to the top layer of soils. And last, utilities should be placed at least three feet from trees.

There are more technical elements and specifications to consider, especially for sites with no soils, but as Stuart Shillaber the superintendent of horticulture at Boston’s Rose Fitzgerald Kennedy Greenway Conservancy advised me recently about introducing organic maintenance, “be happy when someone can implement 60% of the program. The rest will come when clients see results.”

The installation and upkeep of our existing “hard” utility systems requires substantial public and private investment. Creating well-functioning soils would not require large funds from the public since this work can be achieved project by project.  Upkeep of hard utilities is costly and disruptive; not so for soils that can be tended several times a year with substantially less trouble. The ASLA estimates that every year nearly 4.6 million acres are affected by public and private landscape projects [14]. Making headway on a quarter to third of that amount would start a revolution.

The Future of Urban Soils

From my vantage point, prospects for improving urban soils are good. In my thirty-year career as a landscape architect, there has never been a time of greater interest, research, and resources for managing urban soils and as many successfully constructed projects using urban soils. Advocacy for higher quality soil is rising nearly forty years after Dr. Phil Craul (professor emeritus at SUNY) and his colleagues started the field studies on urban soils that led to his 1992 publishing of the seminal Urban Soil in Landscape Design. Today, the USDA’s National Resources Conservation Service has substantial mapping, literature, and research on urban soils [15]. Ted Hartsig, a division chair of the Soil Science Society of America, tells me that the organization recently formed an urban soils division and committee whose focus is issues of urban soils including morphology and classification, the relationship of chemicals and nutrient quality, physics, biology, and structure, as well as the restoration and management of these soils. Urban soils studies are proliferating at public and private universities like Johns Hopkins and Kansas State.

Most critically, the public is starting to understand at the personal level of their gardens that the old adage “better to put a $5 tree in a $50 hole than to put a $50 tree in a $5 hole” is correct. Remember that until professionals and individuals teamed together to demand action, climate change was downplayed. Landscape architects and other professionals must play a part, whether through projects, lobbying our government, writing articles, lecturing, self-education, or speaking up in any propitious situation. We can be plausible leaders in the discussion to invest in another underworld utility, the first that is purely for the public good.

References

[1]  Soil Quality Information Sheet. Soil Quality Indicators: Infiltration”, Natural Resources Conservation Service, USDA, January 1998 http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_019144.pdf

[2]  J.H. Gregory, M.D. Dukes, P.H. Jones, and G.L. Miller, “Effects of urban soil compaction on infiltration rate,” Journal of Soil and Water Conservation, Volume 61, Number 3. http://abe.ufl.edu/mdukes/pdf/stormwater/Gregor-et-%20al-JSWC-compaction-article.pdf

[3]  Geoffrey H. Donovan, David T. Butry, Yvonne L. Michael, ScD, Jeffrey P. Prestemon, Andrew M. Liebhold, Demetrios Gatziolis, Megan Y. Mao, American Journal of Preventive Medicine, “The Relationship Between Trees and Human Health: Evidence from the Spread of the Emerald Ash Borer,” Volume 44, Issue 2, pp. 139–45, http://www.ajpmonline.org/webfiles/images/journals/amepre/AMEPRE_3662-stamped_Jan_8.pdf

[4]  “Statistics on the Economic Value of Trees,” Conservation Montgomery, http://conservationmontgomery.org/resources2.html

[5]  “Study: Nations urban forests losing ground; New Orleans, Albuquerque, Houston losing Trees.” News Release, USDA Forest Service, February 23, 2012, http://www.fs.fed.us/news/2012/releases/02/urban-forests.shtml

[6]  Stephen C. Fehr, “Mayor Working To Keep It Green; Williams Pleads For More Trees,” Washington Post, November 17, 1999, http://caseytrees.org/wp-content/uploads/2012/02/02.01.1999-original-article-washingtonpost.pdf

[7]  Henry Arnold, “Sustainable Trees for Sustainable Cities,” Arnoldia, Volume 53, Number 3, 1993, http://arnoldia.arboretum.harvard.edu/pdf/articles/1993-53-3-sustainable-trees-for-sustainable-cities.pdf

[8]  E. Gregory McPherson and Jules Muchnick, “Effects of Street Tree Shade on Asphalt Concrete Pavement Performance,” Journal of Arboriculture, Volume 31, Number 6, November 2005, 303, http://www.fs.fed.us/psw/publications/mcpherson/psw_2005_mcpherson001_joa_1105.pdf

[9]  “Basics of Organic Maintenance”, UMass Extension, Center for Agriculture, http://www.extension.org/pages/62978/basics-of-organic-landscape-maintenance

[10]  “New Research Survey Suggests Urban Trees are On the Decline,” Public Radio International, March 16, 2012, http://www.pri.org/stories/science/environment/new-research-survey-suggests-urban-trees-are-on-the-decline-8967.html

[11]  Jacqueline W.T. Lu, Erika S. Svendsen,
Lindsay K. Campbell, Jennifer Greenfeld, Jessie Braden, Kristen L. King, and Nancy Falxa-Raymond, “Biological, Social, and Urban Design Factors Affecting Young Street Tree Mortality in New York City,” City and the Environment, Volume 3, Issue 1, 2010, http://digitalcommons.lmu.edu/cgi/viewcontent.cgi?article=1069&context=cate

[12]  “As City Plants Trees, Some Say a Million Are Too Many,” The New York Times, October 18, 2011, http://www.nytimes.com/2011/10/19/nyregion/new-york-planting-a-million-treestoo-many-some-say.html?pagewanted=all

[13]  James Urban, Up by Roots: Healthy Soils and Trees in the Built Environment, International Society of Arboriculture, 2008

[14]  “What is Landscape Architecture?” American Society for Landscape Architects, http://www.asla.org/nonmembers/LicPac99.htm

[15]  Soil Quality Information Sheets, Soil Quality Institute in cooperation with the National Soil Survey Center, NRCS, USDA; and the National Soil Tilth Laboratory, Agricultural Research Service, USDA, http://soils.usda.gov/sqi/publications/publications.html#utn

Suggested Reading

Timothy A. and Philip J. Craul, Soil Design Protocols for Landscape Architects and Contractors, Jon Wiley & Sons, 2006.

James Urban, Up by Roots: Healthy Soils and Trees in the Built Environment, International Society of Arboriculture, 2008

“Standards for Organic Land Care, Practices for the Design and Maintenance of Ecological Landscapes”, NOFA Organic Land Care Program publication, Northeast Organic Farmer’s Association, 2011. http://www.organiclandcare.net/sites/default/files/upload/standards2011.pdf

“Landscape Performance Series: Benefits Toolkit, Fast Facts Library, Scholarly Works”, Landscape Architecture Foundation, http://lafoundation.org/research/landscape-performance-series

http://landscapeurbanism.com/article/reconsidering-the-underworld-of-urban-soils/

The Performative Ground: Rediscovering The Deep Section

Deep Urbanism is a reading of the city that acknowledges the complex ecological and biogeochemical processes taking place above, below and within the urban ground. In the city, nothing can simply be placed on the surface; the composition of the urban ground requires that structures inevitably extend deep into a complex mix of disturbed soil horizons, construction rubble, pipes, subways, utilities. The most innocent-looking walkway may sit on 30-foot piles driven deep into silty soil; small hillocks might be braced with highly engineered geotextiles. A simple meadow may require the complete reconstruction of a “natural” soil profile; a sunken garden may require drainage infrastructure and thousands of pounds of concrete to keep the water table at bay. Much of the action of landscape is deep underground, buried in space or time.

Read more: http://landscapeurbanism.com/article/the-performative-ground/">