Building Soils for Better Crops

Ch 22. Soils for Urban Farms, Gardens and Green Spaces

SARE Outreach
Fred Magdoff, Harold van Es | 2021 | 394 pages

From New York City to Chicago, Venezuela to Lima, … rooftop gardens and urban vegetable patches are growing fresh food close to the people.

National Geographic

When most people think about where food is grown, their vision is usually of farms, large and small, in rural regions. The majority of these farms have been in agriculture for decades or longer, and they have never been used for significant residential, commercial or industrial purposes in their past. But in towns and cities around the country, there is a rapidly growing interest in urban food production, from school and community gardens to nonprofit and commercial urban farms. Similarly, urban green spaces, street trees and backyard gardens provide important relief from dense urbanized environments and have proven to be important to city dwellers’ overall wellbeing. 

Managing soils on urban farms and green spaces is in some ways similar to managing them on rural farms. For example, there is a need to provide adequate water and nutrients to the soil, and to ensure that the pH is balanced, just as with rural agricultural soils. Another similarity is that a main source of soil degradation in urban areas is compaction from lost organic matter and traffic (construction activities, vehicles, pedestrians, etc.).

However, in other ways, managing urban soils is quite different. Urban lands often have gone through any number of residential, commercial or industrial uses in the past, and this land-use history presents unique challenges to the aspiring urban farmer or gardener. Because of their history, urban soils intended for food production often start off in poor shape: they are usually compacted and with low organic matter content, low nutrient availability, and low biological activity and diversity. But unlike soils on rural farms, contamination by toxic compounds is one of the greatest challenges facing urban food growers, and it must be addressed before food can be safely grown and marketed in local communities. This chapter explores the primary challenges you are likely to encounter when preparing urban soils for food production, and it outlines strategies for making these soils both productive and safe to human health. Also, we will discuss challenges with establishing and maintaining urban green infrastructure like parks, street trees and ornamental gardens.

urban construction impacts soil
Figure 22.1. A history of construction activity at an urban site oftentimes results in compacted soil that contains both debris and contaminants. Photo by Francisco Andreotti.

Common Challenges with Urban Soils

Typically, the first challenges you are likely to find with urban soils are compaction, the presence of concrete, construction materials and other trash, and the presence of toxic compounds. The basic causes of compaction in urban settings are very similar to those discussed in Chapter 6, such as traffic from heavy vehicles. However, in urban settings, it is oftentimes construction activity rather than the use of farm equipment that causes compaction and soil degradation. Because construction jobs are often done on tight schedules, the compaction potential of working on wet soils is likely to be ignored. Also, construction regularly involves either removing topsoil or adding fill to build up the ground level, along with the use of very heavy equipment (Figure 22.1). All of this results in disturbed, compacted soils low in organic matter and biological activity (Figure 22.2) In addition, construction debris and chemical waste materials left behind in many cases become part of the soil matrix, frequently raising the pH (because concrete contains lime).

There are many kinds of toxic compounds that can be present in urban soils, and they can come from a variety of sources depending on the location and land-use history of a property (Figure 22.3). Addressing the presence of toxic compounds is critical not only because urban farms and gardens produce food for human consumption, but also because urban operations tend to emphasize educational programming. If members of the community are going to visit an urban farm on a regular basis, especially children, it is essential to resolve any problems related to toxic compounds in the soil.

While all of these problems are solvable, their solutions might prove time consuming and expensive, depending on their severity. For example, the opportunity to use forms of tillage to reduce compaction may be limited in urban settings due to unique aspects of urban farming, such as the presence of underground utilities, a lack of space for heavy equipment or the cost. Therefore, if you are thinking about growing food or developing green spaces on an urban property, you should carefully evaluate the condition of its soils first and develop a plan for resolving any problems you identify.

Soil Contamination

Soil contamination is much more prevalent in urban areas than in agricultural ones. In urban soils, lead is the most common contaminant to pay attention to. It is prevalent due to its long-time use in gasoline (banned since 1989 in the United States) and paint (banned since 1978 for residential use). But there is a wide range of other contaminants from current and past land uses that could pose problems, such as petroleum products and legacy pesticides (lead arsenate, copper sulfate, etc.) (Table 22.1). Cases that are of special concerns include former industrial sites, areas along major roads, recent construction sites, waste disposal sites and junkyards. In some cases, contaminants can end up on a property from distant sources through atmospheric deposition (the process by which particles and gases in the air, such as those that come from tailpipe emissions, settle on the ground or in bodies of water). 

People are exposed to soil contaminants through different possible pathways:

  • Ingesting soil. The risk is greatest when the soil is left bare, especially with chemicals that are concentrated at the soil surface. This is especially of concern with children because they like to play in soil and may put dirty hands in their mouths. 
  • Breathing volatiles and dust. When winds or human activity sweep up bare contaminated soil, contaminants may enter the lungs and become absorbed into the body. Chemicals that stay at the surface are most susceptible to wind erosion. Fine soil particles themselves can also damage the respiratory system. Again, children are at greater risk of inhaling contaminated dust because of their behavior.
  • Eating food grown on contaminated soil. The food that is grown at a contaminated site can expose people to toxic compounds in two ways: either contaminated soil finds its way onto a vegetable that is eaten without being properly washed or peeled, or the crop absorbs contaminants through its roots. Also, food crops grown using pesticides may contain residues of these chemicals and can expose people when they are eaten.
  • Exposure through the skin. Skin is generally an effective barrier against contaminants, but in extreme cases a person may be impacted through rashes or blisters. Pesticide contaminants can also pass through the skin.

Some contaminants are highly adsorbed by soil particles, especially when the soil is around neutral pH. These contaminants typically remain close to the soil surface, although over time they may mix slightly into the soil due to biological activity or any form of digging or tilling. In the case of lead, the risk of exposure from contact with contaminated soil is significantly higher than from a crop that has absorbed the metal from the soil. This is because plants absorb minimal amounts of lead, especially when pH is neutral. You are much more likely to expose yourself to lead from dirty hands, breathing in dust, or from produce that isn’t adequately cleaned. However, lead can accumulate in roots, so growing root crops in lead-contaminated soils should be avoided.

Other contaminants include organic compounds like industrial solvents, pesticides and petroleum products. Industrial solvents like trichloroethylene (TCE) move readily through the soil and can reach groundwater. Some pesticides can remain in the soil for many years and slowly percolate into groundwater. Over time, certain organic compounds are degraded by microorganisms in the soil. Petroleum products tend to stay near the surface. 

Obviously, contaminants that stay at the surface pose a larger risk of human exposure, especially if they also suppress vegetation and are therefore more prone to wind and water erosion (Table 22.2). But in that case the contaminants can also be more readily removed by scraping the top layer of soil (and replacing it with good topsoil or compost). Contaminants that readily leach to groundwater may pose a problem through drinking water. Again, the highest risks are with children. They are also more sensitive to toxic contaminants than adults.

Table 22.1.  Common Contaminants in Urban Soils Based on Previous Land Use
Land UseCommon Contaminants
Agriculture, green spaceNitrate, pesticides/herbicides
Car wash, parking lots, road and maintenance depots, vehicle servicesMetals, PAHs*, petroleum products, lead paint, PCB* caulks, solvents
Dry cleaningSolvents
Existing commercial or industrial building structuresAsbestos, petroleum products, lead paint, PCB caulks, solvents
JunkyardsMetals, petroleum products, solvents, sulfate
Machine shops and metal worksMetals, petroleum products, solvents, surfactants
Residential areas; streets; buildings with lead-based paint; where coal, oil, gas or garbage was burnedMetals, including lead, PAHs, petroleum products, creosote, salt
Stormwater drains and retention basinsMetals, pathogens, pesticides/herbicides, petroleum products, sodium, solvents
Underground and aboveground storage tanksPesticides/herbicides, petroleum products, solvents
Wood preservingMetals, petroleum products, phenols, solvents, sulfate
Chemical manufacture, clandestine dumping, hazardous material storage and transfer, industrial lagoons and pits, railroad tracks and yards, research labsFluoride, metals, nitrate, pathogens, petroleum products, phenols, radioactivity, sodium, solvents, sulfate
*Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic chemicals produced when coal, oil, gas, wood and garbage are burned. Caulks containing harmful polychlorinated biphenyls (PCB) were used in schools and other buildings that were renovated or constructed from approximately 1950–1979.
Source: Boulding and Ginn (2004)
Table 22.2
Health and Environmental Effects of Common Soil Contaminants in Urban and Industrial Areas
Contaminant TypeExamplesComments
MetalsCadmium, zinc, nickel, lead, arsenic, mercuryAdsorbed by soil at the surface unless physically incorporated. Sometimes a gas. Affect the central nervous system and mental capacity with long-term effects.
Radioactive MaterialsRadon, uranium, plutonium, cesium, strontiumMostly soil adsorbed or gaseous. Degrade over long time periods. Acute toxicity in high doses; cancer.
Industrial solvents Chlorinated organics like PCE, TCE, DCECan leach to groundwater or be volatile. Slowly decompose in soil. Affect the central nervous system and mental capacity.
Petroleum productsBenzene, toluene, ethylbenzene, xylene, kerosene, gasoline, dieselRisk from drinking water and inhalation from volatilized product. Irritation; affect the central nervous system and mental capacity.
SaltsSodium chlorideCause sodic soil conditions, aggregate breakdown and compaction.
Agricultural inputsNitrates, pesticides/herbicidesImpaire water quality. Irritation; affect the central nervous system and are associated with cancer.
Other organic and inorganic pollutantsPCB, asbestos, drugs and antibioticsAssociated with cancer; sometimes acute toxicity and central nervous system. Affect aquatic biology and drug resistance.

Testing for Soil Contaminants


When evaluating a plot of land for its suitability for urban farming or gardening, the first step is to research its history. Try talking to the property owner, and use the internet, public library, city hall or tax assessor’s office to seek records that would reveal past uses. Useful records include old aerial photos, maps, permits and tax records. Also, visit the site to see whether potential sources of contamination are nearby, such as old houses with peeling paint or a highway. Both cases could mean a high level of lead in the soil. Generally, a site that has a long history as a green space or residential property will have fewer problems than one with a commercial or industrial past (Table 22.1).

After you learn what you can about the property’s history, consult with your state environmental agency, local health department or local Cooperative Extension office to determine the kinds of tests you should perform to accurately assess the condition of the soil. Also, while there are interim guidelines published by the U.S. Environmental Protection Agency (EPA, Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices, 2011), there are no established federal rules for what soil contaminant levels are considered safe for urban agriculture, so you should work with these qualified professionals to interpret the results of tests and make a plan to recondition the soil. At a minimum, the EPA recommends that urban soils be tested for pH, percent organic matter, nutrients, micronutrients and metals, including lead. Soil testing is described in detail in Chapter 21.

When testing for potential contaminants, you may need to collect samples separately for each contaminant you want to test for, and your sampling procedure for each may vary. For example, you may collect samples at different depths depending on the suspected contaminant (a heavy metal near the surface versus a solvent that may have leached into the soil), or the intended use of different sections of the property (a play area versus a growing area). In addition, contaminants may have been buried in the past.

The distribution of contaminants can be unpredictable, so testing in many locations in the plot may be required. Sections of a property that have obvious signs of potential problems may require separate testing procedures. These can include areas next to old buildings with peeling paint (a higher risk of lead), patches of bare ground where vegetation would otherwise be expected (a sign of compaction or concentrations of toxic compounds), or near stormwater drainage features (which could be bringing petroleum-based compounds, pesticides or other chemicals onto the property from the surrounding neighborhood). Note that the presence of lead in the soil rarely causes physical damage to plants. On the other hand, other metals, such as copper, zinc and nickel can be phytotoxic at high concentrations.

A thorough site assessment of a property should also take into account other conditions that could affect its viability for urban farming or gardening, such as slope and drainage patterns, the presence of aboveground and belowground utility lines, or existing unwanted structures, including possibly the buried foundations of previous buildings. (In the United States, visit www.call811.com, or call 811 to get information on buried utility lines before starting any digging project.)

SEEK OUT ADDITIONAL RESOURCES

Due to the potential risk to human health of farming on contaminated soils, it is advised to work with environmental consultants and local Extension specialists with expertise in urban soils when assessing whether to use a site. Depending on the severity of its problems, it can be expensive to assess and clean up a site. The EPA Brownfields Program (www.epa.gov/brownfields) offers grants to state, local and tribal governments, as well as to nonprofits, for these purposes, and might be an option when one or more urban farms seek to partner with a local municipality to clean multiple sites at once. The USDA’s Urban Agriculture Toolkit provides information on how to start an urban farming operation and identifies technical and financial resources that might be available to help with each step.

Further reading on the risks and recommended approaches to site assessment, soil testing and soil management is readily available through state Extension offices and federal agencies, such as:

  • Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices (EPA)
  • Evaluation of Urban Soils: Suitability for Green Infrastructure or Urban Agriculture (EPA)
  • Gardening on Brownfields series (www.gardeningonbrownfields.org, Kansas State University)
  • Gardening on Lead Contaminated Soils (Kansas State University)
  • Soils in Urban Agriculture: Testing, Remediation and Best Management Practices … (University of California)
  • Minimizing Risks of Soil Contaminants in Urban Gardens (North Carolina State University)

Urban Agriculture and Soil Contamination: An Introduction to Urban Gardening (University of Louisville)

Soil Reconditioning Strategies


Once you have an understanding of the specific problems associated with a particular urban property, decide on the most appropriate reconditioning (improvement) strategies (Table 22.3). Most make the decision to pursue mitigation (coping) versus removal strategies at this point. Using excavators and trucks to remove contaminated soil is an expensive and extreme option that may be required for highly contaminated sites, and regulations on excavated soil with contaminants make the whole process difficult as well as costly.

Again, improving the soil so that it is safe for food production and for the community will take time and could prove costly. Before you begin, you should have a plan in place that accounts for this time and cost.

Table 22.3 Typical Reconditioning Techniques for Degraded Soils
TechniquePhysicalChemicalBiological
Soil removalX
RakingX
Tillage and subsoilingX
DrainageX
Soil amendments and additives*XXX
RecyclersX
Cover cropsX
MulchXXX
*Examples can include manufactured additives to improve soil structure (physical), commercial fertilizers and composts.

Practices to improve urban soils fall into physical, chemical and biological categories, just as they do in any agricultural setting. In urban situations, the strategies outlined here should generally be considered and used in that order, from physical to chemical to biological.

Physical practices can provide immediate solutions to compaction, poor drainage or the presence of toxic materials in the soil, but they’re not necessarily easy. If contaminant levels are modest or concentrated near the surface, scraping only the top layer of soil and replacing it with quality topsoil might be more feasible. Additionally, a thin layer of contaminated topsoil can be diluted by using tillage or subsoiling to mix it with soil deeper down. This will also alleviate existing compaction problems. If compaction is the primary concern with an urban soil as opposed to contamination, removal is not a recommended approach but amelioration in place makes more sense. Other physical practices include removing old structures and trash, and raking the soil to either level it or to remove old construction debris and trash near the soil surface.

Figure 22.4. Huerta del Valle, a four-acre urban farm that serves low income communities in Ontario, CA, uses organic waste from a local food distributor to produce compost on site. Photo by Lance Cheung, USDA.

Depending on soil test results, you will probably need amendments to alter nutrient and mineral levels, or pH. Phosphorus binds to lead, making it less dangerous over time, so be sure to use phosphorus fertilizers if the soil tests indicate a deficiency. Mineral amendments, such as lime or dolomite, may help with poor drainage or to stabilize pH.

Compost, cover crops and other organic amendments are usually required before producing any crops to increase organic matter, improve soil structure and promote soil biological activity, and they should be used each growing season to maintain soil health. Like tillage, mixing in compost will further dilute toxic compounds. Also, organic matter binds some contaminants, making them less available to plants. Compost is readily available in urban areas, but be sure to use only high-quality compost from reliable sources and pay special attention to finding a supply that is itself free of contaminants and weeds. Local restaurants, cafes, arborists and municipal compost piles are common sources (Figure 22.4). The use of cover crops is discussed in Chapter 10, and compost is discussed in Chapter 13.

Mulches, including living mulches, can be used to suppress weeds and reduce erosion. When soil contamination is a concern, mulches have the added benefit of acting as a barrier that reduces contact with contaminated soil. They can also reduce the splashing of soil onto crops.

MAINTAINING HEALTHY SOILS

Even after an urban farm or garden has been put into production, good soil management remains critical. Since most urban farms are under continuous, intense production during the growing season, soils can lose fertility quickly and need to be replenished. The best ways to maintain soil health in urban systems are the same as in rural ones. They are described in detail elsewhere in this book, including:

  • Cover crops (Chapter 10)
  • Crop rotation (Chapter 11)
  • Composting (Chapter 13)

Rather than try to improve a property’s soil, many urban farmers and gardeners opt to build raised beds instead, filling them with a mix of imported topsoil and compost. Again, make sure the topsoil and compost you plan to use is free of toxic materials before buying it. Placing a layer of landscaping fabric on the soil surface before adding the new soil for the raised beds helps to limit roots reaching the original soil. A fabric barrier also lessens soil in the bed mixing over time with the buried soil deeper in the ground through biological activity.

Urban Green Infrastructure

We have discussed concerns related to urban soils in the context of crops and food production. But natural areas, parks and ornamental gardens are also highly treasured by residents and visitors. Similarly, yards and gardens are small areas of relief from the urban bustle and are cherished by city dwellers. Urban areas also have a lot of food waste, tree leaves and tree trimmings that can be turned into compost or mulch and used to improve the soil—done at the municipal scale or in home backyards. Under ideal conditions even pet waste can be safely composted. (While cities also generate a lot of sewage sludge at wastewater treatment plants, there are often concerns with contamination by industrial and household products that keep it from being used to grow food.)

With the de-industrialization of many cities, urban renewal projects frequently involve the redevelopment of former manufacturing and transportation sites into housing and office developments, or urban parks. Care needs to be taken to study the nature of the previous land uses and the associated possible contamination as we discussed earlier in this chapter.

Remediation

Similar to establishing urban farms, the development of green spaces needs to consider different options. Most green spaces involve perennial plants, and much of the soil health considerations need to be addressed up front. Generally you want the soil to support attractive vegetation at a low maintenance cost. This requires good drainage, high waterholding capacity, good rooting and low weed and disease pressure. This is usually accomplished through the same practices we discussed earlier: loosening compact soil, adding compost and fertilizer, balancing soil pH and mulching.

Except in extreme cases of contamination when the soil may need to be removed, landscaped areas can generally have poor soil buried by trucking in good top soil. Or the soil that is there can simply be improved with amendments. Burying soil in place is often sufficient for the remediation of industrial or built sites that contain various debris materials. Building raised beds or berms is a common approach in urban gardens, both to address poor soil quality and to improve drainage. Placing a layer of landscaping fabric on the soil surface before adding the imported soil for the raised beds helps to limit roots reaching the original soil and lessens mixing of the imported material with the surface layer.  

When the soil is compacted or has low organic matter, and when there is little chemical contamination or other waste materials in the soil, the best option is probably to improve what is there through mixing and adding organic materials. The physical, chemical and biological quality of the soil can then be improved by applying and incorporating compost using excavators or bucket loaders. The so-called “scoop-and-dump method” works well when there is no existing vegetation on the site (Figure 22.5). If there are trees and other large plants that need to be saved, an air spader (a device that blows soil away with high air pressure) may be used to gradually remove the compacted soil around existing roots and then replace it with healthy soil.

Special Soil Mixes and Street Trees

Plants in pots, planter boxes and green roofs require clean soil mixes that allow for excellent drainage (because of the low gravitational drainage potential due to shallow depth), high water- and nutrient-holding capacity, and low weight. Soil for rooftop gardens needs to be light enough so that it doesn’t overburden the roof, and heavy enough that it anchors the plants and won’t be dislocated by wind or water.  Soil mixes are typically combinations of special minerals like vermiculite clay (treated by heating), perlite (expanded volcanic rock particles) and organic materials like peat moss, compost or biochar. These manufactured soil materials have favorable physical, biological and chemical characteristics with low density, but they generally cost more than traditional soils. Containers for growing plants need to have holes in the bottom to allow for water drainage to avoid saturated conditions when watering.

Street trees are valuable assets to a neighborhood because they moderate the microclimate and improve the aesthetics. Special challenges exist with trees in sidewalks and parking lots. Unlike those in parks, cemeteries or green strips along boulevards, street trees are growing in a paved environment. The pavement substrate (the soil material immediately underneath) is often highly compacted in order to meet the bearing capacity standards to support the sidewalk pavement plus the additional loads from possible emergency vehicles. Oftentimes the tree roots grow big and break or tilt the sidewalks, thereby creating a health hazard and liability for the municipality. They also frequently die prematurely due to the highly constricted rooting environment combined with salt, and heat and moisture stresses. 

Street trees therefore create a dilemma between the engineering requirements for a strong pavement that supports high loads (requiring a compacted substrate), and the need for a healthy rooting environment for the trees. One solution is the so-called gap-gradedsoils that can meet both objectives (Figure 22.6). Such materials contain only particles of certain sizes, with some sizes deliberately left out in order to ensure that there will be good amounts of pore space. This soil material commonly uses large, uniform stones as a skeleton matrix that can support high loads from the pavement, while allowing large pores for tree root protrusion under the pavement. These pores are partially filled with high quality soil material to support the tree functions. On golf courses and other greens, similar gap-graded soil materials are applied (typically sand, with certain sizes omitted) to better support foot traffic while still maintaining healthy turf growth.

Other Construction Concerns

Compaction is common with any type of activity that involves soil disturbance, digging and construction equipment, and it affects both rural and urban areas. This is less of a problem if the area is subsequently paved over, like a parking lot in front of a new store. But compaction may have a long-term negative impact if the area will be revegetated or used again for crop production or green infrastructure.

It’s important to understand and to pay attention to the ways that construction equipment can cause compaction if such equipment is needed when preparing a site for urban farming or landscaping. Oftentimes construction jobs are done without regard for the high compaction potential with wet soil. Also, when digging work is done (for example when installing a pipeline, a drain system or a septic system) the fertile topsoil is commonly mixed with subsoil and the site ends up with poor soil at the surface after filling the holes. Therefore, good construction work should follow some principles:

  • When construction vehicles are involved near the site, limit traffic patterns to controlled lanes. If possible, cover traffic lanes with metal plates or geotextile fabric under gravel to spread the loads from the vehicles.
  • Avoid traffic and construction when the soils are wet and highly susceptible to compaction.
  • When digging, first remove the fertile topsoil layer and stockpile it separately before digging deeper into the soil to install the items (cables, pipes, etc.). Then refill the subsoil first and loosen it with rippers. Finally, reapply the topsoil material and avoid further compaction (Figure 22.7).

Generally, urban areas experience increased runoff as a result of sealed surfaces. Roofs, streets, parking lots and other types of development have high potential for runoff and discharge of undesirable contaminants, like oils from leaking cars. Urban stormwater programs aim to contain or slow the direct discharge to water courses through water retention systems. These can often be incorporated into landscaping features of green infrastructure. Notably, swales allow for extended infiltration times and settling of sediment, and gravel covered drain systems (French drains) diverge runoff away from structures (Figure 22.8). Stormwater mitigation practices are generally required by state law for large site developments, and design manuals are available to help developers comply.

Chapter 22 Summary

Contamination and compaction of soil are common problems in urban areas and must be addressed before putting urban land into food production. The most significant issue to identify and resolve is the risk of exposing farmworkers and community members to soil that is contaminated with toxic compounds. The most common contaminant in urban and suburban areas is lead, and ingestion of contaminated soil is the most common pathway of exposure. Working with environmental experts to carefully assess the site and its land-use history, along with testing the soil, will help you evaluate the risks and determine if it’s feasible to use the site for urban food production. Similarly, green spaces in urban areas may also be impacted by contamination or compaction issues. The strategies for improving degraded, contaminated soils include physical (such as soil removal), chemical (such as altering pH) and biological (such as adding composts) practices. Remediation, or excavating large amounts of contaminated soil and replacing it with clean soil, can be expensive and is usually reserved for only the most contaminated sites. Burying contaminated soils with healthy soil material may be a more economical option. In-place mixing of organic materials and subsequent mulching and use of appropriate plantings are often good options for green spaces and gardens.

A Case Study, City Slicker Farms

Oakland, California

When City Slicker Farms moved into its new location in West Oakland, a 1.4-acre site that was once a paint factory, the nonprofit urban farm faced the challenge of rebuilding the soil from the ground up. 

While the soil went through a remediation process, City Slicker still needed to bring in new soil for the entire site. “Because this is topsoil that’s coming in and it’s being brought in big loads, the soil structure was very poor,” says Julie Pavuk, director of urban garden education. It appears that the soil also came from different sources, she adds, as soil textures vary throughout the farm.

Dealing with a new soil wasn’t unfamiliar to the organization, whose mission has been to empower community members to meet the basic need for healthy food by creating organic, sustainable and high-yield urban and backyard farms. Since its founding in 2001, City Slicker Farms has built more than 300 community and backyard gardens out of raised planter boxes. The reason they use raised beds is two-fold: community members who may not be physically able to do in-ground gardening can still participate, and they can install gardens in places where there may not be natural soil, such as parking lots. Over the years, they discovered that not all soil is fit for raised bed production. At times they had to shovel soil back out because it was too compacted, Pavuk says. It took some time to determine that a sandy loam soil called “Local Hero Veggie Mix” from a local company, American Soil and Stone, was the best fit for their planter boxes because of its structure and nutrients.

The main issue they had to address at their new location, the West Oakland Farm Park, was soil compaction. “Some of the initial challenges were just literally being able to dig in and create enough space so that the plant roots could actually grow and go down as far as they needed to be to avoid becoming stunted,” Pavuk says. To prepare the soil for production, City Slicker Farms implemented the biointensive methods of double-digging and layering in a lot of compost—residential green waste provided by Waste Management. The manual labor paid off. “Those methods really work to help us address some of those things like soil structure and make sure we’re adding a lot of nutrients back into the soil,” she says. “Just yesterday, I was out digging in some of the beds, and I was surprised at how easy it was compared to how it had been in that particular space earlier.” 

Rebuilding soil was also a better challenge to deal with than the one they faced before: land impermanence. Before purchasing the brownfield that would become the Farm Park, thanks to a $4 million grant from California’s Proposition 84, City Slicker Farms operated on empty sites through temporary arrangements. They were at risk of losing their spaces at any time. Pavuk recalls one day they got the news they had one week to move out from one of their sites. “We salvaged what we could from it, and the food was distributed, but we lost one of our big production spaces, and it happened very quickly,” she says. This made the organization even more aware of the food insecurity the neighborhood faced and kicked off the process of owning their own space. 

Designed in partnership with the community, the West Oakland Farm Park is not only an urban farm but also a much-needed green space and community hub where people can visit to relax, learn and play. It features a greenhouse, nursery, orchard, vegetable and herb gardens that the Farm Park staff and volunteers use for food production, a chicken coop, beehives, a demonstration kitchen, an outdoor classroom, a playground, and 28 plots for community members to garden themselves. Like the backyard gardens, the community plots have raised planter boxes to make gardening more accessible to the community, while the rest of the crop production is in-ground. 

City Slicker Farms moved into the site in 2016, and it opened to the public that summer. All of the food grown at the Farm Park goes to community members who lack access to healthy food or are experiencing food insecurity. While the farm has been providing food to those participating in their gardens program, they are moving to a “community fridge” model. They’ll distribute their food through free refrigerators that an organization called Town Fridge has set up in public spaces around Oakland, allowing anyone to access free food and drinks anytime.

With a better soil structure now in place, the farm is moving away from biointensive methods and is now looking at how they can correct deficiencies to grow even healthier and more nutrient-dense foods. Farm manager Eric Telmer started with soil testing to create a fertilization plan to address some of their plants’ stunting and yellowing. He found that the soil is low in calcium and sulfur but very high in magnesium and potassium. To bring the soil into balance, he’s been applying an oyster shell flour as a substitute for hi-cal lime, as well as gypsum and CalPhos. 

They rely on composting and cover crops for nitrogen. Their compost comes from three sources: compost created onsite from crop residue, such as faba bean cover crops and other organic matter sources, which they usually layer with either manure from their chicken coops or with donated horse manure; worm castings from their worm bins, where they feed the worms food scraps and burlap; and city compost. To kill weed seeds and pathogenic organisms, City Slicker does hot composting. The middle of the compost pile needs to reach at least 130°F for a certain number of days, depending on how big the compost pile is, and they turn it to ensure every part of the pile reaches the center.

For cover cropping, faba beans are the farm’s first choice because of their ability to produce nitrogen and to grow quickly. The farm will cut the beans just below the soil level after they’ve flowered but before they’ve set seed. This kills the plant while leaving the roots and nodules to continue providing nitrogen. The tops are then either used as mulch, added to compost or served as feed in their chicken coop.

The faba beans also add diversity to their rotation. While the Farm Park grows a variety of crops, including tomatoes, cucumbers, squashes, peppers, beans, radishes, eggplants, bok choy, carrots and peas, its rotation is heavy in brassicas like collard greens, mustards, kale and swiss chard. The faba beans appear to be helping to control pest issues on the brassicas, particularly aphids. Pavuk explains that the aphids will attack the faba beans, but soon after, ladybugs will appear and eat those aphids. This cycle helps keep these beneficial insects in the Farm Park to deal with aphids on brassicas and elsewhere.

Since the West Oakland Farm Park is located in an industrial area, there weren’t a lot of plant communities that attracted beneficial insects. To address that, City Slicker built insectary strips filled with plants like chamomile and bachelor buttons at the headrows of their beds to serve as a “beneficial insect oasis,” Pavuk says. “We’re looking to hopefully prevent some of our pest problems by growing much healthier plants and by increasing the amount of habitat we have for our beneficial insects, so that we’ll be able to use more of those biological controls as part of our pest management strategy.” The Farm Park also has its beehives to provide the dual benefit of pollination and honey production. In the four years they’ve been on the site, Pavuk has seen more native bee species and other pollinators show up, like hummingbirds and butterflies.

But one of the biggest indicators that their soil health practices have set them on the right path are earthworms, which they didn’t have when they first started production on the site. “Their presence to me is an indicator that our soil is improving, and they’re helping to improve it,” Pavuk explains. “That first year was so hard, in part because the soil needed so much work, but also we didn’t have the diversities of insects and creatures. The next year was amazing because then the other things started to come and the soil was improving; all of it was happening at the same time in concert.”

The crops that are crucial to their mission of providing healthy food to the community reflect that change. “The plants are thriving in ways they simply weren’t initially,” Pavuk says.

Chapter 22 Sources

Bassuk, N., B.R. Denig, T. Haffner, J. Grabosky and P. Trowbridge. 2015. CU-Structural Soil®: A Comprehensive Guide. Cornell University. http://www.structuralsoil.com/.

Boulding, R. and J.S. Ginn. 2004. Practical Handbook of Soil, Vadose Zone, and Ground-water Contamination: Assessment, Prevention, and Remediation. Lewis: Boca Raton, FL.

Gugino, B.K., Idowu, O.J., Schindelbeck, R.R., van Es, H.M., Wolfe, D.W., Thies, J.E., et al. 2007. Cornell Soil Health Assessment Training Manual (Version 1.2). Cornell University: Geneva, NY.

New York State Department of Environmental conservation. 2015. Stormwater Management Design Manual.  https://www.dec.ny.gov/docs/water_pdf/swdm2015cover.pdf.

Schwartz Sax, M., N. Bassuk, H.M. van Es and D. Rakow. 2017. Long-Term Remediation of Compacted Urban Soils by Physical Fracturing and Incorporation of Compost. Urban Forestry and Urban Greening. doi:10.1016/j.ufug.2017.03.023.

Soil Science Society of America. 2015. Soil Contaminants. https://www.soils.org/discover-soils/soils-in-the-city/soil-contaminants.

U.S. Environmental Protection Agency. 2011. Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices. U.S. Environmental Protection Agency. 2011. Evaluation of Urban Soils: Suitability for Green Infrastructure or Urban Agriculture. EPA publication No. 905R1103.