Recently, I read an article about the Landgrab, a temporary urban farm, installed in the middle of the Shenzhen/Hong Kong Biennale of Architecture/Urbanism Congress. This unique landscape installation had vegetables, cereals, fruit and pasture, which are basic food sources for Chinese families. The architects of Landgrab described it as a reminder to tell people where and how their food is created. The idea of urban farms is not something new. However, the huge contrast that this pastoral mini countryside in the middle of the most dense, contemporary, high-technology downtown of Shenzhen City is just so amazing.
Then, I began to wonder, how many people can urban farms actually feed? How effective is scattered farming? Does it make sense to build community gardens and farms at the city level?
To answer these questions, I made a few assumptions.
- Assumption 1
Everyone is vegetarian and will eat sweet potatoes for calories only, as sweet potatoes are the highest caloric dense food. According to this link, sweet potato land provides 70,000 kcal/ha/day. With fairly primitive technologies, one square kilometer of land can feed 2350 people on a 2000 kcal/day diet of pure sweet potatoes.
- Assumption 2
Everyone on average eats a 2000 kcal diet. Even though the numbers will be different depending on where you are, and also the population age distribution. Places with more young kids probably need less calories per person.
- Assumption 3
One (1) person of labor is needed to work on 1 acre of sweet potato farm land. The number of people needed to work on sweet potato farms differs according to different scenarios. The assumption of one person per acre is used after reading this discussion. One person per acre, which is 1 person per 0.004 square-kilometer, which is 247 people per 1 square-kilometer. Combined with Assumption 1, 247 out of 2350 people (10.5% of the population) need to work on the urban farm.
Using these assumptions, let’s look at three different places and see much land we actually need to grow sweet potatoes.
The first place I would like to introduce is Manhattan, as it first comes to my mind when thinking of American cities with higher density.
In 2019, Manhattan Borough had a land area of 22.82 mi²/59.1 km². The population is 1.629 million. Therefore, 693 square kilometers of sweet potato land is needed to feed the residents. It is 11.7 times of the size of Manhattan.
When we put this on the map, we can see that all of Brooklyn Borough, Queens Borough, and part of Hempstead will need to be converted to sweet potato land in order to feed Manhattan.
Now let’s look at a smaller scale in Manhattan, for example, Milan Condominium on East 55th Street. This condo has 32 stories and 118 units, and the land area is 0.001 km². Most of the units are 2-bedroom units. Therefore, I assume the total resident number is 250. The population density would be 250,000 people per km².
For this condo, we need 0.1 km²/27 acre of sweet potato land. When we put it on the map, we can see that six blocks are needed to grow sweet potatoes.
The second place I used as a sample is Macon, Georgia. Macon is a small city in central Georgia. In 2019, the population of Macon is 153,159. The land area of Macon is 255.2 mi²/661 km². Therefore, Macon needs 65.17 km² of sweet potato land, which is only 10% of the area.
Let’s look at a typical block of single family houses in Macon. This block’s land area is 16.24 acre/ 0.066 km². There are 62 units. The average household size of Macon from American Community Survey is about 3. Therefore, let’s assume there are 186 people living here. The population density would be 2,818 people per square kilometer.
We need 19.5 acre / 0.079 km² of sweet potato land to serve this block. The undeveloped green space on the west of this block would be sufficient to grow enough sweet potatoes.
For the third place, I went back to Shenzhen, where the story started. In 2017, Shenzhen had 12.53 million people, and the land area was 792 mi²/2051.27 km² . For this amount of people, Shenzhen needs 5332 km² of sweet potato land. It is roughly 2.6 times the city limit area. For this amount of farmland, 1.3 million people will need to work on the farm.
Let’s look at a typical residential community in Shenzhen. This is 宝能 (Baoneng) V-House, a mixed-use development consisted on 8 high-rise buildings, commercials, apartments, and landscape. More information of this community can be found here (in Chinese). The land area of this development is 13.59 acre / 0.055 km². There are roughly 1,133 units and 3,000 residents based on limited information available. The population density of this community is 54,545 people per km² .
In order to feed this community, we need 1.27 km² or 314 acre of sweet potato land. When we put it on the map, we can see that the entire Shenzhen International Garden and Flower Expo Park and surrounding blocks would be converted to farmland under this scenario.
As a conclusion, we can see that for places with lower population density (like Macon, Georgia), urban farms might work pretty well. However, for places with higher density (like Shenzhen or Manhattan), there isn’t enough space for locally grown food, despite everyone eats sweet potatoes.
There’s an article on Science 2.0: Urban Agriculture? Only 1 Percent Of Seattle Residents Could Eat Locally Even With All Viable Space In Use. The researchers of this study uses remote sensing technologies to summarize land area that could be converted to farmland, which includes backyard gardens. The result is that only 4% of the Seattle population will be fed if these lands are used to grow kales. In reality, the number would be more like 1% of the population. As stated in this article,
A city can grow tomatoes, kale and lettuce but it gets more complicated once you factor in other necessary proteins, fats and carbohydrates that often travel from across the county and world.
Organic agriculture for the masses is a popular myth but urban agriculture is a plain old pipe dream.
Lastly, I would like to embed the video introduciton of Landgrab here as I found it very interesting.