Section C: Urban Agriculture in the Agroecosystem

• Projected Outcomes
• Background / Lessons
  • Introduction
  • The Ground Beneath Our Feet
  • Ecological question 1: What are the nutrient and water flows in the system?
  • Ecological question 2: What are the sources and sinks of pollutants in the system?
  • Ecological question 3: What are the interactions of living organisms in the system?
  • Ecological question 4: What are the energy flows in the system?
• Activity
• Additional Reading

Projected outcomes:

1. Students will learn about different types of soils and growing systems in urban agriculture.
2. Students will learn about some key sustainable practices for urban crop production.
3. Students will learn about pollutants in urban agriculture and urban environments.
4. Students will learn about key interactions of living organisms in urban agriculture.
5. Students will learn about energy use in urban agriculture.

Background / Lessons:

Introduction

Like natural ecosystems, agroecosystems are characterized by nutrient flows and cycles, energy flows, and the interactions of living organisms with each other and the physical environment. However, agroecosystems differ from natural ecosystems in two key ways.

• First, we expect them to export biological goods for our use.
• Second, we deliberately manipulate them to produce those goods in abundance.

These two human requirements of agroecosystems in turn affect their key ecological processes. Sustainable urban agriculture seeks to work with urban ecosystem processes to achieve production goals.

The urban environment presents different ecological challenges than the rural environment. Growing space is limited. Urban gardens and farms typically grow a variety of vegetables and sometimes fruits in a small space, requiring specialized growing methods for crops to be both productive and sustainable. In addition, urban ecosystems present unique environmental challenges to growing food. Sustainable urban agriculture operations address these external issues while working to achieve production goals. Each type of urban agriculture has ecological sustainability strengths and weaknesses that will be addressed in this section.

This section begins with a quick look at the role of soil in the urban agroecosystem. It then encourages students to think about the ecology of sustainable urban production by posing four ecological questions.

1. What are the water and nutrient flows in the system?
2. What are the sources and sinks of pollutants in the system?
3. What are the interactions of living organisms in the system?
4. What are the energy flows in the system?

The Ground Beneath Our Feet

Growing media in urban agriculture ranges from traditional soils in ground-based urban gardens and farms to hydroponic farming, which involves no soil at all. These growing systems have different structures and requirements, but they all serve the same mechanical functions. Firstly, they provide a substrate for plant roots to grow in. They allow water to drain or circulate so plant roots have access to oxygen, but also retain enough water for plant use. Finally, they store and provide nutrients essential for plant growth.

Traditional Soils

Rooftop Soils

Rooftop soils are designed to be lightweight and to drain rapidly to minimize stress on a roof’s load-bearing capacity. Rooftop farms use a variety of soils, including commercial potting mixes, commercial green roof mixes, and experimental mixes. To meet weight limitations and drainage requirements, commercial potting and green roof mixes combine organic material (e.g. compost, peat) with lightweight materials such as vermiculite and perlite (highly porous mineral compounds), coconut coir (a coconut husk byproduct), biochar (a partially combusted, porous organic solid), and expanded shale and slate (heat-treated rocks, making them more porous). The only commercial product designed specifically for rooftop farming, Rooflite, is a blend of compost, spent mushroom media, and expanded shale. Rooftop vegetable farming is a relatively new practice. Experimentation is ongoing to find soils that meet the special requirements of rooftops, while fulfilling the nutrient needs of crop production.

For an in-depth resource on rooftop farm soils, see this Cornell University publication by Harada, Yoshiki et al.

Hydroponic Media

Hydroponic systems grow plants in nutrient rich water solution rather than soil. Plants are suspended in net pots or other containers with slotted sides that anchor the plants and allow water to circulate around the roots. Sometimes, the pots are filled with substrates that support the roots. Common substrates include perlite, pumice, hydroton (porous expanded clay pellets), gravel, coconut coir, and Rockwool (a superheated rock and chalk product that’s spun like wool). The individual pots are suspended in a larger container that holds the solution. In a passive system, space is left between the base of the plant and the water reservoir so roots can get oxygen needed for respiration. Active systems use water-circulating equipment and air pumps to oxygenate the water. For more information on hydroponic growing systems and management, see this University of Minnesota Extension website.

Ecological question 1: What are the nutrient and water flows in the system?

In a sustainable system, nutrients are recycled and if outside inputs are required, they are obtained from renewable sources. The three macronutrients for crops are nitrogen, phosphorus, and potassium. Urban growers can benefit from using local sources of organic material to return these nutrients to the soil. Catching stormwater runoff from impervious surfaces can be a renewable source of water in urban areas.

Ground-Based Urban Agriculture

The following methods help recycle or conserve water and nutrients in the home garden or urban farm:

• Mulch – Mulch is a layer of material used to cover the bare soil around growing plants. Urban areas have an abundance of mulching material, including lawn clippings, bark chips, leaves, cardboard, and old newspapers. Mulching reduces water loss by keeping soil temperature lower, prevents weeds from germinating by blocking out sunlight, and improves soil health by adding organic matter into the soil. Using lawn clippings and dry leaves as mulch reduces the amount of yard waste in urban landfills.
• Compost – Composting returns nutrients and organic material back to the soil and supports soil organisms, which in turn improve aeration. Composting in urban areas recycles food and yard waste that might otherwise end up in landfills and produce methane. (See additional discussion of compost and activities, Module IV Section C.)
• Soil testing – Regular soil testing with a professional lab informs a grower of plant available nutrient levels. With this information, growers can customize fertilizer and amendment applications for their soil’s specific needs, reducing fertilizer overuse and runoff.
• Cover Crops – Cover crops are seeded following the harvest of the normal crop. Densely growing plants, such as oats, winter rye, buckwheat, white clover, and crimson clover make good cover crops because they leave little bare soil, reducing erosion. Incorporating cover crops into the soil at the start of the next growing season adds organic matter and recycles nutrients. Furthermore, legume cover crops have a beneficial relationship with bacteria that convert atmospheric nitrogen into a plant-usable form of nitrogen (fix nitrogen) and can act as a non-synthetic N fertilizer.
• Growing Plants with Different Root Structures – Plants have diverse methods for mining water and nutrients from the soil. Deeper rooted vegetables like tomatoes, parsnips, and melons, retrieve water and nutrients from a different layer of soil than more shallow-rooted plants such as lettuce. The roots of shrubs and trees reach even deeper in the soil profile. The nutrients their roots extract are incorporated into their tissues; and using their fallen leaves as compost and mulch recycle nutrients from deep in the soil profile to the top for shallow-rooted crop plants. By growing a variety of crops, farmers maximize the retrieval of water and nutrients available in the soil.
• Growing Methods that Reduce Water Loss from Weeds and Evaporation – By using dense crop spacing, growers on a small mixed vegetable garden or farm can reduce water loss from weed growth and evaporation from bare soil. This method is particularly suitable for leafy vegetables like lettuce, kale, spinach, and other greens. Another source of water loss is stormwater runoff and evaporation from pathways. Covering pathways with mulch can reduce weed growth and evaporation water loss.
• Succession Planting – Succession planting utilizes valuable garden space through the entirety of the growing season by planting new crops in an area following the maturation and harvest of the first crop. This increases productivity by maximizing the use of the garden space available. Additionally, keeping the soil covered throughout the growing season reduces water loss from exposed ground.
• Plant Interactions/Associations – Plants in an urban garden or farm compete for space, water, sunlight, and nutrients. But plants can also interact in beneficial ways that increase nutrient availability, plant health, and water conservation. For example, certain vegetable crops in the legume family, such as peas, fava beans, and garden beans, fix nitrogen. These crops can be used in succession or as companion plantings to increase the productivity of heavy nitrogen users like lettuce, corn, or potatoes. Similarly to cover cropping methods, using a living mulch like red or white clover between vegetable rows not only suppresses weeds but adds nitrogen to the soil. Of note, clovers can also be aggressive weeds that compete with crops. The shade of vining plants that spread out across the ground like squash or cucumbers grown underneath tall plants like corn or okra can reduce weed growth and evaporation.
• Water Catchment Systems – Sustainable urban growers can conserve water by using rain barrels, cisterns, and other water catchment systems to store rainwater that would otherwise run off impermeable surfaces. This water can later be used to irrigate crops.

Compacted soils can be a big challenge in urban areas. In compacted soils, water and nutrients are less available for plant uptake. Highly compacted soils can even result in stunted root growth. While soil compaction is very difficult to reverse, a focus on soil health can improve conditions over time. The sustainable practices outlined above that add organic matter to the soil, like mulching, composting, and cover cropping are good practices to reduce soil compaction. Organic matter additions support soil biology, such as earthworms and plant roots, that can break apart and aerate compacted soil. In cases of severe compaction, urban growers can create raised beds with compost and/or imported soil.

Rooftop Farms

Rooftop farms must deal with several unique challenges in comparison to ground-based farms. Rooftop soils are engineered to be lightweight and drain quickly. Fast draining can cause nutrient leaching from the soil. In addition, rooftop farms have several compounding factors that cause hot and dry conditions for plants. In comparison to ground ecosystems, rooftops have very shallow soil depths. This means they hold less water and heat up faster than ground soils. Further contributing to high evaporation rates, rooftop farms must also cope with windier conditions than the typical ground-based farm, with wind speeds doubling for every 10 stories of building height. Adding to this challenge, vegetable crops are less drought tolerant than many ornamental plants typically grown on green roofs. 

Rooftop farmers employ several techniques to combat these extra challenges. First, growers tend towards crops that are more adapted to intense heat and sun, like peppers and tomatoes, rather than trying to grow cool season crops like spinach and peas. Similarly to ground-based farms, the use of mulch, compost and cover crops can help reduce water loss. However, the type of mulch must be carefully considered because some types are likely to blow away with greater wind speeds. Ongoing research seeks to improve the water retention of rooftop soils while maintaining its lightweight nature. Finally, to limit wind-damage to crops, growers may provide extra support to stabilize plants.

Despite the water-saving techniques outlined above, extra irrigation is nearly always needed on rooftop farms. Generally, drip irrigation systems are the most efficient way to provide water to crops. This water usually comes from municipal water sources, which is a challenge to the sustainability of these systems. Practitioners are seeking ways to recover runoff from stormwater and irrigation. One model is working with building engineers to catch runoff in cisterns, which can then be recycled for further use on the roof. See the Brooklyn Grange case study for more information.

See Q2 for a further discussion of nutrients on green roofs.

Vertical (Hydroponic) Farms

A significant advantage of vertical farming is its low water use. Hydroponic growing can significantly reduce water use in comparison to conventional agriculture, particularly in dry climates. Soil-less mediums are efficient at delivering nutrient-rich water to the roots of plants. Furthermore, vertical farms are typically indoors, so less water is lost to evaporation. Finally, the nutrient-rich water is recycled within the system. The amount of water saved depends on the management and efficiency of the farm. New technologies employ sensors and software to optimize the amount and timing of watering.

While vertical farms may reduce total water use, they usually rely on municipal water sources for their initial water inputs. Because they are generally indoors, no rainwater (a renewable resource) goes towards nourishing the plants. The outside water and nutrient inputs reduce the sustainability of vertical farms. See this interview with Dr. Kai-Shu Ling and Dr. James Atland for further discussion of the benefits and challenges of vertical farming.

Ecological question 2: What are the sources and sinks of pollutants in the system?

A pollutant is a contaminant that is damaging to human health or the environment. It is important to note that whether something is a pollutant depends on context. For example, soil particles are a critical resource for growing food. However, a soil particle washed away is called sediment – a pollutant that can damage aquatic communities. Additionally, the concentration of a pollutant matters for biological health. Some pollutants, such as nitrogen, are beneficial at the right level.

A pollution source is where a pollutant comes from. A sink is where the pollutant ends up. Urban agriculture exists within a larger urban environment that can be large sources of pollutants. Sustainable urban agriculture seeks to not add to these pollutants; and can even be a sink or reduce the source of some urban pollutants.

Pollutant Sources in the Urban Ecosystem

Ground-Based Urban Agriculture

Ground-based urban agriculture uses sustainable practices to avoid being a pollution source, instead seeking to reduce urban pollution. With careful planning and alternative methods, ground-based urban farming can reduce pesticide and fertilizer use and prevent soil erosion.

Weeds can be controlled in a variety of ways in urban agriculture besides herbicide inputs. The best way to prevent weed growth is reducing the amount of bare soil available for weed germination. Keeping the soil covered also reduces soil erosion, another pollution source. Heavy mulching, succession planting (see Q1), inter-planting, and cover crops (see Q1) reduce bare soil throughout the growing season. Inter-planting involves planting a variety of crops in the same space. Generally, early season crops such as radishes and lettuce are planted around late season crops like broccoli and peppers. The leafy early season crops keep the soil covered while the late season crops grow and mature. Weed suppression can also be achieved by utilizing the property of allelopathy. Allelopathy is trait by which an organism produces one or more biochemicals that affect the growth and reproduction of other organisms. In this case, two types of cover crops—winter rye and oats—produce allelopathic chemicals that suppress weeds, including lamb’s quarters, purslane, ragweed, and crabgrass. Finally, when the occasional weed does pop up, urban farms employ hand weeding and hoeing.

Sustainable urban growers control disease and pests through a variety of means to reduce pesticide use. Close monitoring and knowledge of the life cycles of diseases and pests allows a grower to anticipate problems and make adjustments to management practices. This technique, called Integrated Pest Management (IPM), employs a variety of sustainable techniques to minimize pest problems.

IPM Handout

Although crop rotation is difficult in small, urban spaces it can still be used to control problems such as fungal diseases that overwinter in the soil. If a large-scale problem occurs, it may be necessary to avoid growing the problem crop or crops in the same crop family for at least a year. In addition, growing rambling or vining plants on vertical supports such as trellises results in better air circulation so leaves dry out faster after rainfall or watering. This makes the vegetables less prone to developing problems from moisture-loving fungi like rust or powdery mildew.

Sustainably managed ground-based urban farms are smaller sources of pollutants than many turf grass areas, reducing harmful inputs related to turf grass management. Furthermore, active soil management can reduce heavy soil compaction typical of urban areas and therefore allow for more stormwater infiltration in cities. Finally, urban farming can utilize municipal sources of food and yard waste as compost or mulch, resulting in a much more sustainable use of these wastes. Interestingly, leaf collection on urban streets can reduce nutrients in urban stormwater and serve as a source of compost or mulch.

Rooftop Farms

Sustainable rooftop farms employ similar growing methods to ground-based agriculture to reduce being a source of pollution. Compost, mulch, and cover crops reduce soil erosion and outside fertilizer inputs. Similar techniques are also used for weed and pest control. However, rooftop farms must reach a delicate balance of providing crops with sufficient nutrients, while not contaminating runoff with these nutrients.

While numbers vary based on climate and type of green roof, green roofs are shown to reduce overall amount of stormwater runoff in comparison to conventional roofs. Thus, green roofs are a promising way to reduce the total volume of stormwater runoff in a city. On the other hand, vegetable crops have greater nutrient requirements than many drought-tolerant plants—such as sedums and grasses—typically used in ornamental green roof designs. This means fertilizer inputs are usually required for rooftop farms. This presents a challenge as excess nitrogen and phosphorus in green roof substrate is likely to contaminate runoff, adding to the existing problem of nutrient-rich runoff in urban areas. Current research is seeking to address these problems, for example, this study from a partnership between Brooklyn Grange and SARE.

Vertical (Hydroponic) Farms

Vertical farms recycle the nutrient-rich water used to grow plants so there is little release of nitrogen, phosphorus, and other nutrients into the municipal water system. Since vertical farms are indoors, there’s minimal pest and disease problems, so crops can typically grow without pesticides. While vertical farms do not produce many pollutants, they also do not act as a sink for any urban pollutants.

Ecological question 3: What are the interactions of living organisms in the system?

Urban agriculture is often limited by space constraints. While this may decrease the volume of produce grown at any one site, it does have some advantages. The smaller scale of urban agriculture operations often allows growers to take advantage of beneficial plant interactions that may not be feasible on a larger scale farm.

Farmers can use vegetable plants as “trap” crops, “masking” crops, or plants that attract beneficial predatory insects. For example, if Blue Hubbard squash is grown in the vicinity of other varieties of squash and zucchini, it will serve as a trap crop that will lure squash bugs and squash vine borers away from the preferred squash crops. Plants in the allium family, including garlic, onions, and scallions, act as a “mask” crop by emitting volatile chemicals that make it difficult for pests to locate other crops. Some herbs and many wildflowers can be grown in or around the garden to host beneficial pollinating or predatory insects.

Planting a variety of crops and wildflowers has biodiversity benefits too. While there are many reasons for declining insect populations, scientists have pinpointed pesticides, urbanization and habitat destruction, and monoculture crops as big factors. Urban agriculture can help combat these sources of insect decline. By increasing plant diversity and creating a more structurally complex green space in comparison to turf grass, urban agriculture can serve as habitat for insects, birds, vertebrates, and soil invertebrates. Not only does planting a variety of crops and native pollinator flowers promote biodiversity, but having pollinating insects around can help increase crop yields for many fruits and vegetables.

See these University of Minnesota Extension and University of Arizona Extension publications for more on companion planting.

Activity 1: Plant Associations and Interactions

Ecological question 4: What are the energy flows in the system?

One significant advantage of all urban farming practices is that food is produced right next to a big population of people. This means very little energy is involved in transporting food from where it is grown to where it is consumed.

In general, sustainable energy practices in urban agriculture include:

• Limiting use of synthetic fertilizers and pesticides, which have high manufacturing energy costs.
• Using energy from renewable sources such as wind or solar to power on-site refrigerators for storing produce.
• Minimizing the use of fuel-powered equipment, such as tractors.
• Using equipment that is properly maintained.

Ground-Based Urban Farms

Because of the smaller scale, many urban farms use more manual labor than rural farms. This reduces overall energy costs because tractors and other farm equipment run on energy from fossil fuels.

Rooftop Farms

Rooftop farms have both energy costs and savings. Rooftop farm construction requires a significant amount of energy to manufacture and install a roof membrane, root barrier, geotextile filter fabric, drainage plates, and the specialized soil media required for rooftop growing. In addition, oftentimes extra infrastructure is needed in a building to support the extra weight of a rooftop farm.

On the other side, rooftop farms can reduce the urban heat island effect and fossil fuel use associated with cooling buildings. Hard surfaces in cities, such as buildings and roads, absorb heat from the sun, causing cities to be warmer than the surrounding countryside. This is called the urban heat island effect. Rooftops contribute to the urban heat island effect because they tend to be dark in color, causing them to absorb much more heat than they reflect. Conventional roof surfaces can reach temperatures of 150°F or more during the summer, in turn transferring this energy to the interior of buildings. This increases the energy demand for cooling these buildings.

Green roofs can help reduce the urban heat island effect and conserve energy. The vegetated surface of green roofs shades buildings and reflects energy rather than absorbing it. Furthermore, evapotranspiration (the combined effect of evaporation from the substrate and plant transpiration) cools surrounding air by transferring the energy into the evaporated water rather than the air. Green roofs with deeper soils and wide roof coverage, like rooftop farms, can provide more cooling benefits than green roofs with shallower soils or smaller roof coverage. See this EPA publication on green roofs for more information on how green roofs can help mitigate the urban heat island effect.

Vertical (Hydroponic) Farms

The major energy disadvantage of vertical farms is they don’t use natural sunlight as the source of plant energy. Growing plants vertically in stacked systems requires artificial light sources, which can be costly and a huge use of energy. Vertical farming also requires energy for water-circulating equipment, air pumps to oxygenate the water, and humidity control through ventilation, heating, and air conditioning (HVAC) systems. Finally, vertical farming uses fertilizers from outside inputs, which can take a lot of energy to manufacture.

Additional Reading on Companion Planting and Urban Biodiversity

• Walliser, Jessica. (2020) Plant Partners, Science-Based Companion Planting for the Vegetable Garden. Storey Publishing.
  • Book on beneficial plant interactions
• Readel, Ann. (March 28, 2022) In Wisconsin: Stowing Mowers, Pleasing Bees. New York Times.
• Buckley, Cara. (December 3, 2021) Meet an Ecologist Who Works for God (And Against Lawns). New York Times.