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Soil gel tests

Self-watering soil could transform farming

Image credit: University of Texas at Austin

A new type of soil created by engineers at The University of Texas at Austin can pull water from the air and distribute it to plants, potentially expanding the map of farmable land around the globe to previously inhospitable places and reducing water use in agriculture at a time of growing droughts.

The team's atmospheric water irrigation system, details of which have been published in the journal ACS Materials Letters, 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 Walker Department of Mechanical Engineering.

Each gram of soil can extract approximately 3-4g of water. Depending on the crops, approximately 0.1-1kg of the soil can provide enough water to irrigate approximately one square metre of farmland.

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.

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 per cent of the water quantity it started with. In contrast, the sandy soil had only 20 per cent of its water left after just one week.

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. Last year, the team developed the capability to use gel-polymer hybrid materials that work like "super sponges," extracting large amounts of water from the ambient air, cleaning it and quickly releasing it using solar energy.

The researchers envision several other applications of the technology. It could potentially be used for cooling solar panels and data centres and also to expand access to drinking water, either through individual systems for households or larger systems for large groups of people, such as workers or soldiers.

The ability to grow more food on previously poor-quality or inhospitable land will become increasingly important as the world's population continues to grow. There is already considerable pressure on good farming land.

There are also environmental pressures at play. Major shifts in land use and food consumption are known to be necessary for the UK to hit its 2050 goals. Many UK farmers are already shifting their farming operations towards the goal of becoming climate neutral by 2035, with many farmers reporting that sustainability is one of their top priorities. The double whammy of the Covid-19 pandemic and the pending conclusion of the Brexit transition have both acted as catalysts for new ways of thinking.

These changes followed the UK government's Agriculture Bill, announced at the start of 2020, which will reward farmers for protecting wildlife and tackling climate change, as examples of work done for ‘public goods’. The new Bill was necessary, given the UK's imminent move away from the current EU subsidy system.

Genetic engineering techniques could also become necessary and play an important role in ensuring the future of global food security. Many countries worldwide are facing the double burden of hunger and undernutrition alongside overweight and obesity, with one in three people across the globe currently suffering from some form of malnutrition.

Meanwhile, researchers at the University of Exeter have calculated that global warming of 2°C would lead to about 230 billion tonnes of carbon being released from the world's existing soil.

Global soils contain two to three times more carbon than the atmosphere and higher temperatures speed up decomposition, reducing the amount of time carbon spends in the soil (known as "soil carbon turnover").

The new international research study, led by the University of Exeter and published in the journal Nature Communications, reveals the sensitivity of soil carbon turnover to global warming and subsequently halves uncertainty about this in future climate change projections.

The estimated 230 billion tonnes of carbon released at 2°C warming (above pre-industrial levels) would be more than four times the total emissions from China and more than double the emissions from the USA over the last 100 years.

"Our study rules out the most extreme projections, but nonetheless suggests substantial soil carbon losses due to climate change at only 2°C warming and this doesn't even include losses of deeper permafrost carbon," said co-author Dr Sarah Chadburn, of the University of Exeter.

This effect on the world's soil is so-called "positive feedback" - when climate change causes knock-on effects that contribute to further climate change.

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