Ongoing soil management is critical to the success of any organic farming operation, and food production at urban sites presents a host of soil management challenges. Our business, Seattle Urban Farm Company, has created gardens in many unique locations, including restaurant rooftops, school parking lots, vacant urban lots, and suburban backyards. Every urban site comes with distinct soil-related challenges, and the solutions are often different than those applied at more traditional agricultural plots.
If you are an urban farmer, you might find a location with decent quality parent soil, but more likely you’ll be starting with compacted construction backfill, pure sand, an asphalt rooftop, a paved parking lot, or potentially contaminated soil. Due to the limited space available in an urban setting, opportunities for producing fertility on-site are usually limited, and you may need to rely heavily on purchased soil and compost.
In this article, we’ll share our experiences with soil management when growing crops in a wide range of urban settings. While most of our farms are not certified organic, we manage all of them using organic methods. We’ll start by covering some basics on dealing with soil contamination and buying soil and amendments, and then we’ll discuss two general scenarios: using in-ground beds with actual soil, and growing in raised beds or containers using manufactured growing media and/or potting soil.
Below: This small urban farm just outside of downtown Redmond, Washington, uses purchased compost and cover cropping to maintain fertility. All photos by Hilary Dahl.
Dealing with contaminated soil
Soil contamination can be a major concern when producing food in urban areas. Generally, the greatest concerns are arsenic and lead. It’s unlikely that vegetable crops will absorb lead or arsenic when grown in soils with high organic matter levels, adequate pH (above 6.5), and an appropriate watering regime. However, contaminated dust will get on your hands and body while working in the garden and then can be easily ingested; and dust will settle on the crops and can be ingested whenever produce isn’t properly washed.
Lead is primarily a problem in areas adjacent to houses or buildings constructed prior to 1978 due to lead paint leaching (lead paint was outlawed for residential use in 1978), and within a few feet of established busy roadways (exhaust from historical use of leaded gasoline). Arsenic can be an issue in areas with a history of manufacturing (especially smelting), and underneath structures built with pressure treated lumber prior to 2004 (the arsenic component of treated lumber was voluntarily outlawed by the industry in late 2003).
If you are planning to grow food in areas with a likelihood of contamination, we strongly recommend testing your soil before moving ahead with your urban farm. Our favorite agricultural soil testing labs are at the University of Massachusetts (https://soiltest.umass.edu/), which offers a lead test in many of their standard soil testing options, and A&L Labs (http://www.al-labs-eastern.com/agricultural.aspx), which offers specific tests for lead, arsenic, and many other heavy metals. You can also get a soil test through a local environmental lab, which may be familiar with sources and contaminants particular to your city. The benefit of using an ag soil lab is that you can also get pH, nutrient levels, and crop production recommendations along with the test for contaminants.
If your test shows high levels of arsenic or lead, you have several options. One is to physically remove the contaminated soil by hand or with an excavator. Be aware that disposing of the contaminated soil will likely be subject to regulations specific to your locality, so we suggest checking with local government first if you’re going this route. Also, be sure to use appropriate personal protective equipment to keep you and your employees safe. A second option is to isolate the contaminated soil from your growing operation. This can be done by covering the area with a thick layer of organic mulch or an impermeable barrier. You can then set up constructed raised beds or containers on top of the barrier and import new soil to fill them. This will keep soil dust off your hands, feet, and the produce. Obviously, the level of contamination will dictate your actions- if you find high levels of contamination and/or are unsure of the best method to deal with the situation, consult with a professional environmental lab or consider exploring alternative sites for your urban farm.
Below: This terrace-level garden grows salad greens, shiso and chives for Seattle restaurant Sushi Kappo Tamura. The plastic containers are only four inches deep so nutrient management in the rooftop growing media is critical.
Compost and soil
Urban farmers may not have the option to produce fertility on site through animal manures or cover cropping, and often need a large amount of new soil when first setting up an operation. If you’re looking for a supplier, start by checking with area landscapers to see where they buy their amendments. If you can’t find quality compost or soil locally, you may need to order it in from an operation that specializes in compost for food production. Below are issues to be aware of when sourcing soil:
Commercial Compost: The quality of purchased compost in urban areas can vary greatly. Some “compost” available from landscape suppliers has a large amount of bark or sawdust in it, and will tie up nitrogen if incorporated into the soil (such compost is more appropriate for mulching ornamentals or trees). Some commercial compost contains biosolids (composted human waste), which may be an issue for a certified organic operation. Commercial compost can also have high levels of soluble salts or heavy metals. Ask your supplier what ingredients they use in their compost and if they test their compost. A reputable operation should know what’s in their product and be able to supply you with lab test results. If not, be wary! Take your own sample and have a lab analyze it before purchasing. It’s rare that commercially produced bulk compost will supply enough nutrients and minerals to grow quality vegetables on its own, so be prepared to use additional organic fertilizer and mineral amendments after adding compost.
Soil Mixes: Many suppliers sell pre-mixed soil blends than can be very useful for urban farmers. When initially amending existing soil, soil mixes can be easier to work with and create a better initial structure (large applications of compost can create a clumpy soil that doesn’t allow direct seeded crops to germinate easily). If you are growing in a constructed raised bed on top of existing soil, a pre-made soil mix is often the best option. These mixes are usually made from commercial compost, screened topsoil, sand, and other ingredients. As with commercial compost, quality can vary greatly. Go out and check the soil at the landscaping yard before buying: does it feel like a texturally balanced agricultural soil, or is it filled with clumps of organic matter and chunks of bark or wood chips? Ask the supplier about their ingredients and how they make the soil. Ask if they test the soil and if not, take a sample and send it to a lab yourself.
Potting Soil/Rooftop Growing Media: If you’re growing in containers or constructed beds on top of an impermeable surface, potting soil or rooftop growing media are the best growing mediums. Topsoil and pre-made soil mixes typically do not have enough aggregate to maintain good structure in this type of situation. Potting soil contains perlite, vermiculite, clay shards, or a healthy sand fraction to help maintain structure and drainage in a container environment. Rooftop growing media is similar to potting soil, but is usually contains a more highly engineered blend of ingredients to reduce weight and for maximum drainage. Check with local landscapers, especially those that specialize in rooftop gardens, for suppliers. As with compost, ask the supplier about their ingredients and methods, and don’t hesitate to send a sample into a lab for testing before you commit to buying. Growing media can sometimes contain wetting agents or other synthetic ingredients. If you are a certified organic operation, be sure your growing media meets organic standards before purchasing.
In-Ground Urban Farming
In many urban farming situations, you’ll find the best course of action is to grow directly in the existing soil at the site. This is true for many backyard urban gardeners, farmers growing for market on multiple small urban or suburban plots, or growing on larger leased plots in or near a municipality. Soil management at these sites will be similar to that for agricultural soils, so we won’t spend a lot of time on it here. The major difference is that you’ll likely be starting with extremely poor soil, and it may not be desirable to slowly build your soil over time. You’ll need to be producing quality crops right away, so it’s worth an initial investment in soil and compost to get your soil up to par before you start farming.
Start by improving the structure. If the soil is highly compacted, small mechanical equipment like a rototiller may not be able to penetrate the ground. On a small scale, loosening things by hand with a pick or spading fork may be your best option. On a larger scale, heavy excavating equipment may be necessary.
Once you have things loosened up, add a large amount of purchased soil mix or compost to bring organic matter levels up and create a workable structure for food production. A 6-8” layer on top of the existing soil is a good starting amount. Generally, we’ve found that a soil mix is better than straight compost for this initial application because it creates a better texture for seed germination and incorporates more evenly into the existing soil.
If you want to avoid working existing soil, you can build up by creating raised beds on top of the soil and filling them with purchased soil. If the existing soil is not contaminated, you do not need a barrier between it and the new soil. Over time, your crops will root into the subsoil and gradually improve it.
Once you have good soil structure in place, it can be managed just like any other agricultural soil. Test your soil and balance pH, nutrient, and mineral levels as directed. Even if you’ve already tested the soil for contamination before getting started, we recommend testing again after adding significant amounts of new soil and/or compost because the entire soil makeup will be different from what you started with. Regular applications of compost and/or cover cropping can be used to maintain fertility over time.
Below: This rooftop farm produces vegetables for Bastille Restaurant in Seattle’s Ballard neighborhood. Beds are 12 inches deep, lined with landscaping fabric and filled with bulk potting soil. Shade covers keep tempteratures cool for salad greens and keep birds away from produce.
Growing on an impermeable surface
Urban farmers may find themselves presented with an impermeable surface to grow on: a rooftop, an old parking lot, or contaminated parent soil that cannot be safely grown in. In these situations, growing in raised beds or containers may be your only option. Using hydroponics or aquaponics may also be excellent choices, but are beyond the scope of this article (for more on hydroponics, see Andrew Mefferd’s article in the February 2016 issue of Growing For Market).
Growing in containers is quite different from growing in the ground on agricultural soils because soil biology and earthworm activity is limited, and nutrients can be quickly leached from the soil. Using potting soil or rooftop growing media is essential for growing in containers. The additional aggregates help the soil media to maintain structure and drain effectively in the absence of typical soil life. Using a standard soil mix or topsoil in containers will result in severe compaction and poor drainage.
Proper drainage is critical when growing on an impermeable surface to ensure your soil does not become a perpetually sodden mass. Using appropriate growing media is the first step for good drainage. Additionally, make sure water is able to pass from underneath your containers via drain holes or gaps at the container’s base. Lining raised beds with heavy landscape fabric will keep your media in place but still let water pass through. Also, make sure water will move away from your containers once it drains out. A 1-2% slope on the impermeable surface will help ensure water can easily move towards a drain or edge of the hard surface.
In general, the larger the volume of soil you have to work with (the bigger the container/deeper the raised bed), the better. For raised beds, 16 to 18 inches of depth is ideal, but 8-12 inches will work well. If site constraints do not allow for this much soil (the depth of rooftop growing media is often limited due to the structural loading limitations of the building), you can work with as little as 4-6 inches of soil for shallow rooted crops like salad greens. The smaller the volume of soil, the more frequently you’ll need to irrigate and supply additional fertility and the more prone your crops will be to stress and disease.
When growing in potting soil, a pre-planting application of organic fertilizer is essential for baseline crop fertility. Be sure to follow the manufacturer’s application recommendations, as excessive application rates will result in high soluble salts in the soil (this is detrimental to plant growth). Generally, a single application of granular fertilizer will carry short-season crops such as salad greens to the point of harvest. Longer season/heavy feeding crops such as tomatoes will generally require additional fertility every 1-3 weeks. This can be accomplished by side dressing with granular fertilizer, foliar feeding, or with liquid fertilizer watered on through a hose-end sprayer or injected into the irrigation system.
Because container gardens are prone to leaching, regular soil testing is extremely important to maintain a balanced soil. Once a year is a good baseline frequency for testing, but 2-3 times a year may be necessary depending on your crops and soil volume. Levels to keep a particularly close eye on when growing food in containers are pH, nitrogen, potassium, boron, and sulfur (all can be affected by leaching).
As in many agricultural soils, pH levels will drop (become more acidic) in your growing media over time due to cation nutrients leaching from the soil. Agricultural lime works well for raising pH back to appropriate levels for crops (in the 6-7 range). Remember that lime will take a few months to change the pH, so apply it at the start of the off-season if possible. If your growing media is too alkaline (pH above 7; we’ve found this to be an issue occasionally), you can use elemental sulfur to drop the level back down.
Nitrogen and potassium levels can be maintained by using a balanced organic fertilizer as detailed above. Organic sources of fertility generally release slowly into the growing media, so can be less prone to leaching than conventional fertilizers. However, if you do find a deficiency, blood meal or liquid fish are good sources of quickly available nitrogen, and sulfate of potash works well to quickly restore potassium levels. Sulfate of potash is also a good source of easily available sulfur, as is gypsum and epsom salts.
Boron is another finicky mineral when growing in containers. Some boron can be supplied to crops by foliar feeding with liquid kelp or mixing kelp meal into the soil, but you may still experience a deficiency over time. Solubor or Borax dissolved in water can return levels to normal. Use caution when supplementing boron; if overapplied, it quickly becomes toxic to plants.
Just like growing food in agricultural soil, growing media needs regular maintenance to be productive. As you harvest crops, clear, and replant, nutrient levels and soil volume slowly drop over time. Adding a small amount of top quality compost to the soil once or twice a year will help combat this. Also, you will occasionally need to add more media to refill your containers/beds. We have found that is not necessary to completely replace growing media on a regular basis unless it’s fully shot through with plant roots (as is the case when growing tomatoes in 5 gallon size containers). Cover cropping is not recommended for building soil in growing media as decomposition is extremely slow due to limited soil biology and it takes valuable space out of production for a long period of time.
Applications of aerobic compost tea, commercial mycorrhizal fungi, or other beneficial bacterial inoculants may be helpful in maintaining a level of soil biology and reducing disease. If you are a certified organic operation, check with your certifier before applying as aerobic teas may be prohibited. We have generally found that using fungal and bacterial inoculants is not necessary if we’re vigilant about maintaining appropriate mineral and nutrient levels.
If you are not growing year-round, physically covering your soil in the off-season to prevent leaching is a good idea. Impermeable barriers such as tarps or black plastic work, as do organic mulches such as straw. If using organic mulch, be sure to rake it off before replanting in the spring (it will tie up nitrogen if turned into the soil).
Managing soil in an urban setting has some distinct challenges compared to traditional agricultural soil management. The main keys to success are avoiding contaminated soil, choosing appropriate soil and amendments to get started with, and managing mineral and nutrient levels into the future. Putting significant time, effort, and research into your soil development will pay off in improved yields and reduced pest and disease problems.
Brad Halm is the co-founder and owner of the Seattle Urban Farm Company, and is co-author of two books: Food Grown Right, In Your Backyard and High-Yield Vegetable Gardening.