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News last update:19 Nov 2020

Self-watering soil to reduce water usage in agriculture

Engineers at the University of Texas have created a new type of soil that can pull water from the air and distribute it to plants. According to the university, this self-watering soil could transform farming.

This new type of self-watering soil could expand the map of farmable land around the globe to previously inhospiatble places. According to the University of Texas it also reduces water use in agriculture at a time of growing droughts.

Super-moisture-absorbent gels

The “atmospheric water irrigation system” as developed by the engineers uses super-moisture-absorbent gels to capture water from the air. When the soil is heated to a certain temperature, the gels release the water, making it available to plants. When the soil distributes water, some of it goes back into the air, increasing humidity and making it easier to continue the harvesting cycle.

“Enabling free-standing agriculture in areas where it’s hard to build up irrigation and power systems is crucial to liberating crop farming from the complex water supply chain as resources become increasingly scarce,” said Guihua Yu, associate professor of materials science in the university’s Walker Department of Mechanical Engineering.

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The gels in the soil pull water out of the air during cooler, more humid periods at night. Solar heat during the day activates the water-containing gels to release their contents into soil. - Photo: University of Texas
The gels in the soil pull water out of the air during cooler, more humid periods at night. Solar heat during the day activates the water-containing gels to release their contents into soil. - Photo: University of Texas

3-4 grams of water per gram of soil

Each gram of soil can extract approximately 3-4 grams of water. Depending on the crops, approximately 0.1 to 1 kilogram of the soil can provide enough water to irrigate about a square meter of farmland, say the engineers.

The gels in the soil pull water out of the air during cooler, more humid periods at night. Solar heat during the day activates the water-containing gels to release their contents into soil.

Experiments

The team ran experiments on the roof of the Cockrell School’s Engineering Teaching Center building at UT Austin to test the soil. They found that the hydrogel soil was able to retain water better than sandy soils found in dry areas, and it needed far less water to grow plants.

During a four-week experiment, the team found that its soil retained approximately 40% of the water quantity it started with. In contrast, the sandy soil had only 20% of its water left after just one week.

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The super-moisture-absorbent gels, left dry, right wet. - Photo: University of Texas.
The super-moisture-absorbent gels, left dry, right wet. - Photo: University of Texas.

Radishes

In another experiment, the team planted radishes in both types of soil. The radishes in the hydrogel soil all survived a 14-day period without any irrigation beyond an initial round to make sure the plants took hold. Radishes in the sandy soil were irrigated several times during the first four days of the experiment. None of the radishes in the sandy soil survived more than two days after the initial irrigation period.

“Most soil is good enough to support the growth of plants,” said Fei Zhao, a postdoctoral researcher in Yu’s research group who led the study with Xingyi Zhou and Panpan Zhang. “It’s the water that is the main limitation, so that is why we wanted to develop a soil that can harvest water from the ambient air.”

The water-harvesting soil is the first big application of technology that Yu’s group has been working on for more than two years.

‘Gels commercially available for agricultural production in the future’

According to Guihua Yu, associate professor of materials science in the university’s Walker Department of Mechanical Engineering, the gels gels can be commercially available for agricultural production in the future. “Currently, the main components of the gel including the water-capturing hygroscopic polymer (polypyrrole) and the thermo-responsive polymer (poly(N-isopropylacrylamide)) are still relatively expensive, as they are not yet commercially scaled for production. However, we believe the gels can be commercially available for agricultural production in the future with better industrial scale synthesis as well as further optimising gel materials and fabrication procedures. For example, exploring very low-cost raw materials can decrease the production cost; simplifying fabrication process can allow more scalable production of materials.“
Professor Yu says application of the gels is straightforward. “The gels can be easily mixed with any regular soil as the soil enhancer at a certain ratio for crop planting. The common agricultural mixer can be used for better mixing and uniform distribution of the gels in the soil.“
Professor Yu also says that testing shows the gels do not need to be re-appplied over time. “We found that the gels could work well in our testing of at least over a month when doing our project. We believe that if they are not degraded by bacteria, they can always work and there is no need to re-apply them.“
Currently, the gels have been tested with crops such as radish, peas and lettuce, professor Yu says. “The gels can provide sufficient water for their germination and growth. We believe it can be used to irrigate different kinds of crops with various water requirement by further enhancing water uptake of the gels and tailoring the ratio between the soil and the gels.”

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