Smart electronic soil sensors could enable farmers to deliver tailored doses of water to their crops, maximising food production while saving water.

Efficient water use is a key challenge for farmers challenged with feeding a growing global population in the face of climate change. In response, researchers in Saudi Arabia have developed a rapid, sensitive soil moisture sensor that can show significant variations in the texture and water storage capacity of different types of soil.

At the heart of the sensor is a metal-organic framework (MOF) with a very high affinity for water. MOFs are highly porous synthetic materials with a cage-like internal structure that can be tailored to host specific small molecules, including water.

“Irrigation management can help improve crop quality, decrease agricultural costs and preserve water,” says Mohamed Eddaoudi, who led the research at King Abdullah University of Science & Technology (KAUST).

“Highly sensitive and selective soil-moisture sensors offer the potential to improve the water management process,” his colleague Khaled Salama adds.

“With their modular porous structure and easy functionalisation, MOFs are excellent candidates for sensing applications,” says Osama Shekhah, a research scientist in Eddaoudi’s team. “MOF thin films have already been incorporated into electronic devices, paving the way for their translation to real-world use.”

The MOFs in the study were selected based on their hydrolytic stability, water capacity and water uptake. The team coated them MOFs onto an inexpensive electrode microsensor that can be fabricated by inkjet printing or laser etching.

When this sensor was inserted into moist soil, air in the MOF was displaced by water, altering its electrical capacitance, a process that can be detected and measured.

The six most common types of soil for agriculture are sand, clay, silt, chalk, peat and loam. Each MOF device was tested in clay and loamy-sand soil types, which can show significant differences in texture and water-holding capacity.

“We are now designing and developing a portable prototype MOF-based soil moisture sensor that can be easily used for control experiments in real-world, in-field measurements,” Eddaoudi says.

Looking into the future, Salama adds, “We anticipate that MOF-based soil-moisture sensors will advance the next-generation soil-moisture sensor technology, offering automated and precise irrigation systems."