26 October 2021

Low cost technology helps African farmers to measure carbon in soil

In Malawi, a $400 calibrated reflectometer has allowed to record the carbon concentration in soil. A key step for better managing agricultural land

by Matteo Cavallito


Carbon content is crucial for soil productivity. Its accurate determination, moreover, is the starting point for a proper land management. Which means selecting the right nutrients and irrigation strategies. And, more in general, making the right decisions in calibrating all the factors that contribute to agricultural yield increase. This process, however, can be very expensive and unaffordable for smallholders. In order to solve the problem, a team of researchers has tried to develop an alternative low cost method. And the results, released in recent months, look very promising.

Chasing carbon? Just follow the light

The investigation, conducted in Sub-Saharan Africa by a group of scientists led by Soil Science Society of America researcher and Michigan State University professor Sieglinde Snapp, tested the efficacy of a simple and cheap device: a $400 portable reflectometer. This instrument measures the reflectance of a surface. The result, when properly calibrated with other parameters, can also show some chemical and physical properties of a substance. Including carbon content.

Mappa dei siti di studio nel Malawi centrale e meridionale. L'asse a destra indica la scala di concentrazione del carbonio nello strato superficiale del suolo (fino a 20 cm di profondità). In rosso i confini dei distretti studiati. I poligoni blu indicano i villaggi campionati. Dai villaggi evidenziati in blu scuro provengono i campioni analizzati nei test di convalida. Immagine: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Map of study sites in central and southern Malawi. The base layer is AfSIS estimated organic C in the top 20 cm of soil. Districts studied are labeled and outlined in red. Blue polygons denote villages sampled, dark blue villages were reserved as a validation dataset. Image: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

“The reflectometer predicted carbon levels precisely

“Selected study sites were located across sixteen villages in central and southern Malawi,” the authors say. Data collected by the reflectometer on hundreds of soil samples were placed in relation to other parameters such as “ambient temperature, moisture, barometric pressure, and global positioning system.” Later, numbers were compared with the amount of carbon recorded through the so-called combustion method, a proven system for calculating the concentration of the element in the soil.

The test was successful: “We found that the reflectometer predicted soil carbon levels precisely,” explained Sieglinde Snapp. “It gave sufficient accuracy to inform soil management practices. What is unique about this handheld sensor is that the provides the data directly in the field, in the absence of a good phone connection.”

A step forward on the road to food security

In Sub-Saharan Africa, the researchers say, the implications of the study are especially relevant. “With minimal training, extension staff can use the reflectometer to carry out assessment of soil carbon in real time with farmers in their field,” says Snapp. “This represents a significant step forward in improving agronomic management in data-poor locations. Access to such immediate and locally relevant soil data can empower Malawian farmers to make more informed management decisions based on their unique contexts.”

And what’s more, the use of the reflectometer allow to “to predict fertilizer responsiveness based on a C threshold.” Which is really important since “soils in this region vary greatly in characteristics and their fertility is highly sensitive to management,” Snapp says. For farms operating on degraded land with limited resources restoring productivity is “a major policy goal” on the road to food security.