Soybean oil-coated sand could be a green alternative to plastic mulch

In order to keep water from evaporating from the soil, farmers will often cover the ground around their crop plants with sheets of polyethylene plastic. There could soon be a more eco-friendly alternative, though, in the form of soybean oil-coated sand.

When most people hear the word "mulch," they think of organic material such as bark chips or dead leaves. However, the frequently used plastic sheeting is also a form of mulch. Along with helping to keep the soil moist, it can also reduce weed growth, prevent erosion, and boost the soil temperature by creating a sort of in-ground greenhouse effect.

As is the case with other plastic products, though, the production of the sheeting isn't a very environmentally friendly process. Additionally, once the plastic mulch becomes cracked and torn, it's typically just removed and dumped in a landfill. What's more, tiny particles of it may remain in the soil, potentially even making their way into the harvested crops.

Led by Assoc. Prof. Michael Nicholl, scientists at the University of Nevada in Las Vegas set out to develop a greener alternative that offers similar water-retaining performance.

They started by combining equal volumes of sand and soybean oil, heating the mixture for about an hour, then allowing it to cool. This process caused the oil to partially polymerize, forming a coating around each individual grain of sand. Excess oil was removed by finally washing the coated sand in water, then letting it dry.

In lab tests, a layer of the oil-coated sand was placed on top of various types of soil contained within vertical sections of PVC pipe, after which the soils were saturated with water. It was observed that the water easily flowed through the sand and down into the soil. The challenge, however, lay in keeping the liquid there.

"When water is applied to a soil surface (rain, irrigation) some of it will seep downwards in response to both capillary action and gravity," Nicholl explains. "After water application ends, the soil begins to dry from the top downwards. As the top surface becomes drier, capillary action will cause water from the deeper and wetter soil to rise upwards, where it evaporates at the surface and directly into the atmosphere."

The oil coating helped to keep this from happening, due to its hydrophobic (water-repellant) nature – even though the water still wicked up through the soil, it was essentially "turned back" by the sand, keeping it from evaporating. All told, it was found that as compared to control samples of bare soil, the sand-covered soil had up to 96 percent lower evaporative water loss.

"These findings show that oil-coated sand has the potential to be developed into a sustainable alternative to plastic film mulch," says Nicholl.

A paper on the research was recently published in the Vadose Zone Journal.

Source: American Society of Agronomy