More than half of the world’s population lives in an urban area, so it’s no surprise cities are a key contributor to climate change. Urban soils may be a key place to test new sustainable solutions to make cities more resilient to climate change and reduce their carbon footprint, according to researchers from Columbia University, who developed a smart city Plant Spike to help major cities measure and monitor urban soil health.

Urban soils have the potential to provide substantial ecosystem services when properly monitored. Soil plays a role in facilitating temperature regulation because wetter, denser soils hold more heat than drier soils. Healthy soils can also contribute to air quality, and they provide recreational greenspaces.

“By collecting data on temperature, light exposure, and moisture of urban soil, we can start to form solutions to improve existing structures,” said Caroline Yu, a researcher at Columbia University’s engineering school. This would allow cities to improve drainage systems and alter soil mixture contents to mitigate the effects of climate change. Many urban areas also have an opportunity to restore soils that are currently marked as brownfield sites.

While there are a variety of studies underway to monitor urban soil, they are seldom executed on a large scale. In fact, according to Yu, current urban soil solutions consist of researchers walking to each individual tree pit to measure and gather sensor data. Not only is this costly, but it’s extremely time-consuming given that soil systems cover a high proportion of land and multiple indicators need to be examined to effectively study soil health. As such, methods like intrusive sampling or manual laboratory testing make it difficult to collect the data needed to better manage soil systems and develop sustainable models. To find holistic and data-driven solutions, urban planners need to measure soil from large areas simultaneously.

To help bring new scalable solutions into the fold, Yu and a research team from Columbia University developed the Plant Spike—a system that can be implanted in subsurface locations across major cities to remotely monitor and measure urban soil health.

As figure 1 illustrates, the low-powered system uses a low-cost, small form-factor printed circuit board (PCB) to transmit dynamic sensor data through Bluetooth Low Energy (BLE) to a client device. Designed to withstand major temperature shifts, the PCB includes temperature and light sensors to collect data, as well as CapSense technology on the base of the board to measure humidity and soil moisture of the surrounding environment.

Plant Spike

Figure 1: Plant Spike custom PCB design; front and back

Notably, Plant Spike is the first sensor network that incorporates BLE transmit-only connectivity in conjunction with moisture, temperature, and luminance sensors on a small form-factor PCB. The BLE platform serves as the wireless communication network to allow for peripheral and central device connections. This gives the team the ability to gather data from a computer peripheral or input-output device and when within range of a tree pit with a smartphone. Figure 2 outlines how data is transmitted through the BLE device.

Figure 2: Plant Spike system and communication configuration showing a BLE module in transmit-only mode sending dynamic sensor data to a gateway device

In New York, more than 690,000 trees span 130,000 blocks of concrete sidewalks, so it’s clear that the urban environment can be difficult to monitor, especially if done manually.

To test its functionality, the team implanted Plant Spike in a laboratory-based urban soil testbed on Columbia University’s Morningside Campus in New York City, and in various unguarded and guarded tree pits across the city, as illustrated in figure 3. The sensor testbed was built to study urban soil samples in a lab environment and to compare the results to soils with Plant Spikes across the city.

Figure 3: Indoor testbed with Plant Spike (left) and unguarded (center) and guarded (right) tree pits in New York City

Once implanted into the tree pit soil, the team could start monitoring and collecting data in real-time through a smartphone app, as shown in figure 4. While the actual data needed to be processed in a lab, the smartphone app provides a future path for integrating Plant Spike into a connected smart city application.

Figure 4: A smartphone application to connect to Plant Spike module via BLE

Overall, the research team found Plant Spike not only required fewer resources, but the data it collected could play an essential role in evaluating plant growth, biodiversity, water storage, and nutrients and contaminant filtering. Additionally, its low cost makes it an attractive option for municipal maintenance teams and even for everyday citizens interested in monitoring their tree pits.

For future cases, the team is already working on improving Plant Spike’s capabilities so that it can be scaled up and implanted in many city-wide locations.

“We are very excited to be designing a second version of Plant Spike that has a larger transmission range and enables an additional sensing mode for chemical stress indicators,” said Yu. “Our goal is to keep each node below the $10 price point so that researchers and companies can use Plant Spike in scaled urban soil sensing solutions.”

For more information on smart cities, visit the IEEE Xplore digital library.