Unique soil has ability to pull water from air

U.S. researchers design self-watering soil to harvest atmospheric moisture as a new source of fresh water for irrigation

A new type of soil that uses absorbent gels to capture water straight from the air and distribute it to plants has been created by engineers at the University of Texas at Austin.

The gels absorb water droplets from cool, humid air at night. Then, when the soil is heated by sunlight, the gels release the water, making it available to plants. As the water moves through the soil, some of it evaporates, which increases humidity.

“Agricultural irrigation systems rely on complex water and energy supply chains, which are expensive, inconvenient, and inaccessible to remote regions,” said Guihua Yu, associate professor of materials science and engineering in the university’s Walker Department of Mechanical Engineering.

“These challenges motivated us to explore atmospheric water, which is abundant and widespread all over the world.”

Yu said atmospheric water can be used regardless of geographical or climatic conditions.

His team designed a self-watering soil to harvest atmospheric water as a new fresh-water resource for irrigation.

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 the soil.

“The super-moisture-absorbent gel (we called SMAG) is a solid, jelly-like, soft material,” said Yu. “The original shape of synthesized gel is determined by the mould, and it can be cut to small pieces or ground to particles if needed. The designed self-watering soil is composed of uniformly mixed micro-sized particles of the gel and sandy soil. The size and shape of the gels can be easily tailored depending on requirements of different types of soils.”

The gel is composed of temperature-responsive polymer, which transitions from its ability to retain water to releasing it at around 35 C. The water is released to the soil when the daytime temperature is higher than that temperature point.

The temperature at which water is released can be modified to suit a broader temperature range, Yu said.

During a four-week test, the team found the hydrogel soil retained about 40 percent of the water quantity it started with. In contrast, sandy soil in the study had only 20 percent of its water left after just one week.

In a separate experiment, the team planted radishes in both types of soil. The radishes in the hydrogel soil all survived a 14-day period without irrigation beyond an initial watering to ensure the plants became established. However, 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.

Yu said that soils mixed with the gels are able to use atmospheric water as long as the gels can be exposed to the moist nighttime air. The size of the gels can be tailored according to the water need and different types of soils to have better contact and improved water-holding capacity, permeability, and good aeration for better plant growth.

“The weight ratio of the gel and sandy soil in the self-watering soil is about 1:3,” he said. “The amount of the gels can be changed for various water requirements according to the local humidity, temperature, solar intensity and different plants.”

Each gram of soil can extract about three to four grams of water. Depending on the crops, 0.1 to one kilogram of the soil can provide enough water to irrigate about a square metre of farmland.

“For SMAG-soil for sustainable agriculture, we only tried the germination and growth of several vegetables in the self-watering soil, such as radish, peas and lettuce,” he said.

“The designed soil can be certainly used to reduce water use for agricultural irrigation and enhance water utilization efficiency. But the water uptake of the gels should be further enhanced to satisfy water requirements of different kinds of crops in different locations and enable atmospheric water irrigation without extra water supply.”

The development of the SMAG-soil is still in the research stage and not ready for commercial development.

“The cost of chemical ingredients for our current materials involved in this self-watering soil is relatively high since the main components of the gel including the water-capturing polymer (PPyCl) and the thermo-responsive polymer (PNIPAM) are still relatively expensive because they lack commercial-scale synthesis and production,” said Yu.

“Low-cost raw materials should be used to reduce the production cost. Moreover, the material fabrication and agricultural application are still in the stage of lab-scale assessment, which need further development to be commercially available in the near future.”

The study was published in ACS Materials Letters.

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