Reimagining sand: A new tool against drought

The Anasazi, a once-flourishing tribe in the American Southwest, lived on bounties of corn, squash and beans. In 1276 A.D., however, a long, unforgiving drought made agriculture untenable, forcing them to migrate away from their settlements. Droughts, even the catastrophic ones, are not figments of the past. With changing weather patterns, droughts are predicted to become more frequent and intense, thereby exacerbating their impact on global agricultural systems.

To support this effort, researchers at Texas A&M University have developed an innovative solution by chemically modifying sand into a highly hydrophobic material. When injected into soil just below the roots, the modified sand forms a hydrophobic layer that prevents the soil from draining quickly. 

“In recent years, drought has become a bigger concern, particularly in southern states like Texas, where temperatures soar in the summer months,” said Dr. Mustafa Akbulut, professor in the Artie McFerrin Department of Chemical Engineering. “One of the most important uses of water is irrigation, so we need clever management strategies to enhance water use efficiency. Our current research investigates the application of modified sand in enhancing water retention in dry soils.”

The details of the study can be found in ACS Omega.

According to the United Nations for Disaster and Risk Reduction, the number and duration of droughts have risen by 29% since 2000. Although sophisticated irrigation methods are available today, droughts can impose heavy strains on irrigation water availability. Rising temperatures exacerbate droughts through increased evaporation, which dries out soil and water sources. In these conditions, improving the water-holding capabilities of soils, particularly those that are porous, becomes necessary. 

In their research, Texas A&M engineers modified the surface chemistry of sand. To create hydrophobic soil, they exploited the natural surface chemistry of silica, soil’s fundamental building block. Particles of silica are abundantly studded with hydroxyl groups that are highly reactive sites. By chemically binding the hydroxyl groups on silica with compounds called organosilane, the researchers created a modified, hydrophobic sand. 

“We only changed the surface chemistry of sand by using an extremely tiny layer, less than two nanometers,” said Akbulut. “For context, our hair is around 10 micrometers, so a thousand times thinner than our hair.”   

They reasoned that a layer of this hydrophobic sand placed below regular topsoil will prevent water from draining quickly, allowing the soil to retain moisture for longer durations. To test their hypothesis, the researchers tried to mimic typical agricultural conditions for growing tomato plants. In plastic cups, they filled the lower layers with normal sand and the top layers with dry soil. In some cups, they included modified sand at the very bottom. Tomato seeds were then sown in each cup. The researchers irrigated each sample daily and collected drained water from each sample to measure water retention and the impact of hydrophobic sand on plant growth.

Upon analysis, they found samples with hydrophobic soil had less drainage of irrigation water compared to controls. They observed a direct positive correlation between the enhanced water retention caused by hydrophobic sand and the growth of tomato seedlings. Specifically, seedlings cultivated with the hydrophobic sand bed exhibited approximately twice the growth stature compared with those grown in unmodified soil.

"With our method, we envision using an injection system to modify a two-to-three-inch-thick layer of soil just below the root level," said Akbulut. "By targeting the sub-root layer, we can enhance water retention while minimizing changes to the soil chemistry in the root zone. Our modified sand is chemically stable, and the injection technology, when developed, will be minimally invasive."

Ultimately, the need for this kind of technology will be most useful for porous soils, like sandy soils that require frequent irrigation and fertilization due to poor nutrient and water retention.

Other contributors to this research include Yashwanth Arcot, Ramya Srinivas and Minchen Mu from the chemical engineering department and Drs. Mahshad Maghoumi and Luis Cisneros-Zevallos, both in the Department of Horticultural Sciences at the Texas A&M College of Agriculture and Life Sciences. Cisneros-Zevallos also holds appointments with Texas A&M AgriLife Research and the Institute for Advancing Health Through Agriculture. 

This work was partly funded by the U.S. Department of Agriculture (USDA) and by the USDA National Institute of Food and Agriculture through the Specialty Crop Research Initiative.