September 2023 - Sally Brown Research Library & Commentary
Provided for consideration to MABA members by
Sally Brown, PhD., University of Washington
Are we there yet?
I’m writing this more than a month before Biofest but it will likely land in your inbox just before the festivities begin. The theme this year is ‘Back to the Future’ and in a way – this library is a spin off on that theme. Here I’m using that theme to look at how much food we can grow in cities and how that is impacted by how much of the nutrients in cities are recycled. This library also features three papers by yours truly, the work only possible because of the cooperation of a number of programs and funding from King County and NW Biosolids.
Many of us live in cities. When people think of cities they think of concrete and asphalt. Dirt and slime. Garbage in smelly mountains. And rats. At least that is the less optimistic take on them. But they can also be centers of culture. Great places to go out for dinner. They can have wonderful parks and pools. And they are dense in critical resources, like nitrogen and phosphorus. They also have backyards and these days, community gardens. In other words, places to grow food. This leads to the question ‘How sustainable are our cities’ or how close to the future are we?
The first article An urban agro-ecosystem: the example of nineteenth-century Paris is the ‘back’ part of the Back to the Future theme. Way back in the day, horses transported people around the City of Light. Likely back then the lights were gas powered - not electric.
This article details how food was grown in Paris in the mid 1800s. Enough was grown to supply each Parisian with 50 kg of fresh vegetables each year. High intensity systems produced 3-6 crops per year, with about half of the cultivation happening under glass. The soil consisted of horse manure. Loading rates of the manure ranged from 300 to over 1000 tons per hectare EVERY YEAR. The heat released by the manure also helped farmers to grow throughout the year. The author attempts to account for the N, P and K produced and used and to do energy calculations for the process. He notes that the productivity in Paris in the mid 1800s was higher than or equal to any commercial farms operating today. Using the horse manure turned a potential pollution problem into a resource.
That was then, this is now. Article #2 How Much Food Can We Grow in Urban Areas? Food Production and Crop Yields of Urban Agriculture: A Meta-Analysis looks at the literature to see how close we can get to Paris 150 years ago. The authors start with the usual litany of challenges facing food production. They note that food grown in urban areas, in both ‘green’ and ‘grey’ spaces. Green means soil and grey means alternative (hydroponic, rooftop, office buildings etc). Current estimates suggest that between 5-10% of the legumes, tubers and vegetables eaten are currently grown in cities. They categorized urban food production by type of crop and included vegetables and fruits (I’m OK with that) and also cereals and oil seed crops. You don’t see too many wheat fields in office buildings. For all categories (except sugar crops primary), yield of crops in urban areas was higher than in conventional agriculture.
On a vegetable to vegetable level, cucumbers and tomatoes grown in urban areas were the big winners. While the authors talk about different types of growing systems and compare indoor versus outdoor yields, there is no mention of horse manure or biosolids and the potential impact of residuals use on urban systems.
Article #3 Quantifying use in a community garden program with extensive resource provision to gardeners is the first of an urban trilogy. Here we surveyed gardens in the Harvest Pierce County program where any garden that wants it gets an annual supply of Tagro.
Gardens are also provided with soil testing, irrigation, materials for pathways and to build raised beds. Harvest Pierce County is a relatively new program. If urban agriculture is to succeed, it is important for people to show up. This was a first step in seeing if that was the case. We measured the size and number of plots and ranked them by how well used they were.
It turns out that these gardeners were generally serious. Plots were better used if they were bigger, but overall use ranked over 2.25 on a scale of 0 to 3. They also like their Tagro. Over the three year period that we looked at, each garden got about two Tagro deliveries.
Does that Tagro make a difference? The answer to that question is given in the fourth article. Article #4 How does your garden grow? Impact of residuals-based amendments on urban soil health, vegetable yield and nutritional density reports on a study we did with multiple field sites and multiple amendments. We grew vegetables with vermicompost and Bokashi (fermented food waste) at the prison garden in Monroe. We grew with GroCo at the Renton Treatment plant and with Tagro in Tacoma. Each set of plots also had a fertilizer control. We also grew kale with all of the above in the greenhouse.
They say that a picture is worth a thousand words. Below you can see Toby, the first author of the paper in front of the control treatment and the Tagro treatment.
In disturbed urban soil (the Tacoma control) nothing worked except the organic amendments. In healthy urban soils - Renton and Monroe - some amendments did better than the fertilizer and others did worse. Amendment recipes make a difference.
The final paper in the library Steps to circularity: Impact of resource recovery and urban agriculture in Seattle and Tacoma, Washington is an attempt to see how close we are to the future. The future here being a city that takes advantage of the value of ‘waste’ to grow their own. We looked at recovery rates for the nutrients in food scraps and wastewater for Seattle and Tacoma. Both cities are national leaders in biosolids and Seattle is a leader in food scrap collection and composting. Both cities have terrific community garden programs. For a description of Tacoma’s see article #3. Seattle has the P Patch program - one of the oldest in the nation
(https://www.seattle.gov/neighborhoods/p-patch-gardening).
The standard that we used for nutrient recovery was 6.6 kg of N and 1.1 kg P per year. Those are the quantities of each nutrient needed to grow the crops (excluding meat and dairy) per person per year. The good news is – that is just about what we produce in #1 and #2. The bad news is, particularly for #1- the vast majority of it is released in the effluent. The figure below taken directly from the paper shows the nutrient flows in Tacoma. You can see that most of the N goes to the sound and not the soil.
We used the data from paper #4 and #3 to estimate how much of the vegetables we eat each year could be grown in community gardens. Here the answer is more encouraging. With Tagro or with vermicompost growing just on the land in the Harvest Pierce County Program - everyone in Tacoma could share several kale Caesars each year. To feed the entire city a portion of kale a day for a year- you would need close to 15,000 hectares if you just added fertilizer. Add vermicompost or Tagro and that land requirement goes down to 300 hectares or less.
This last article is also the subject of several columns in Biocycle
(https://www.biocycle.net/connections-nitrogen-and-phosphorus-in-urban-waste/)
We are not quite in Paris of the 1850s yet but there is a way back to the future. Come say hello at Biofest and we can have a toast to the future!
Stay tuned for additional information on the MABA November 7, 2023 webinar, which will include speakers from the W 4170 group.
Sally Brown is a Research Associate Professor at the University of Washington, and she is also a columnist and editorial board member for BioCycle magazine.
Do you have information or research to share with MABA members? Looking for other research focus or ideas?
Contact Mary Firestone at [email protected] or 845-901-7905. |
