Many of us pay hardly any attention to the ground and simply stride across it unseeing. Yet the soil we tread is what gives us life: as arable land it feeds us, in general it stores water and nutrients, and because it absorbs carbon, it has an impact on the amount of CO2 in the air and in turn on global warming. But our soil is ailing, says Sonia Meller, soil scientist at ETH Zurich: “Intensive agricultural use has worn out the soil in many areas.” That’s why she and agricultural scientist Hélène Iven have developed Digit Soil, the first sensor to measure soil quality on site – quickly and cheaply.
The need for improved and more systematic soil monitoring is painfully urgent. According to the Food and Agriculture Organization of the United Nations (FAO), a quarter of all agricultural land was already highly degraded as of 2011. This means that the soil contains hardly any organic matter, which is where life-giving transformation processes take place. As insects and microorganisms decompose this dead plant and animal matter, using enzymes to break it down into its chemical components, they produce raw materials and nutrients for new plant growth. With advancing degradation, however, soil loses this ecological function and becomes barren. It can no longer offer a habitat for microorganisms, nor can it store water or nutrients. At this point, even fertiliser is no help – the nutrients would simply wash away in the next downpour.
To determine how healthy agricultural soils are today, scientists test them regularly. However, the measurements are time-consuming and expensive, so testing is widely scattered and occurs at lengthy time intervals. Moreover, the soil samples aren’t evaluated on site, but rather in specialised laboratories. “This changes the conditions in the samples and can distort the results,” Meller explains. In addition, various methods are applied in preparing and measuring the samples, so it becomes nearly impossible to compare results from different times and locations.
Meller and Iven aim to change all that with their new portable sensor. Roughly the size of a paperback novel, the small box measures the activity of a host of enzymes responsible for organic material decomposition in the soil. These include phosphates, which break down chemicals containing phosphorous; glucosidases, which split sugar molecules; and proteases, which break down proteins.
The core of the sensor is a square-shaped membrane containing separate fields to which different molecules are applied as substrates for the soil enzymes. A unique feature of this approach is that the substrates contain a chemical component that fluoresces as soon as the enzymes begin acting on it. The sensor’s electronics measure the brightness of this fluorescence, and with it, the level of enzyme activity. All the tester has to do is press the sensor with its membrane into the earth and the results are saved to a USB stick in just a few minutes.
The measurements of enzyme activity provide a basis for conclusions about the soil quality and the effect of agricultural actions, such as tillage or the use of pesticides. Intensive ploughing, for example, eventually leads to lower enzyme activity due to a loss in organic matter. “This is a sign that degradation is beginning,” Iven says.
Nonetheless, processes in the soil are highly complex and it is still difficult to derive direct recommendations for agriculture from the measurements. “We first have to examine more closely how enzyme activity, which we can now measure rapidly and in a standardised way, relates to soil quality,” Meller says, “and to do that, we need a huge amount of data.” Their data collection efforts will soon get a boost from their first customers: several researchers, including some at the Research Institute of Organic Agriculture (FiBL), will begin testing the first version of the Digit Soil sensor this year. This data will then help Meller and Iven to validate their measurement methods and link the findings with processes in the soil.
Ultimately, the two scientists aim to use their rapid test to map how different actions affect the soil. They eventually want to offer the authorities a tool that will aid in providing sustainable support for soil health. In the future, farmers should be able to use the sensor themselves to monitor the quality of their land and the effects of their actions – such as what fertilisers and pesticides they should spray on their fields and which ones they would do better to avoid.
Meller and Iven secured an ETH Pioneer Fellowship for their idea in 2020. At the moment, they are working to make the sensor even more compact. They also hope to have it transmit data wirelessly to a computer or smartphone by the end of 2021.