Environment

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

Soybean oil-coated sand could ...
Cylinders of PVC pipe packed with uncoated sand (left) and polymerized soybean oil-coated sand (right)
Cylinders of PVC pipe packed with uncoated sand (left) and polymerized soybean oil-coated sand (right)
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Cylinders of PVC pipe packed with uncoated sand (left) and polymerized soybean oil-coated sand (right)
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Cylinders of PVC pipe packed with uncoated sand (left) and polymerized soybean oil-coated sand (right)

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

5 comments
Spud Murphy
Here in Australia, all the cattle farmers create silage by wrapping big bales in plastic. A small percentage gets recycled, but most gets burned off in the fields, despite that being illegal and despite much of the wrap being PVC, which generates extremely toxic materials like dioxins when burned. The Department of Primary Industries just looks the other way - I know, because I have reported it several times and the response is always "talk to your council, they enforce pollution laws" - they have no interest in forcing farmers to change their ways, despite having the power to do so.

Anyway, plastic wrap and sheet is used in all forms of farming and is an environmental nightmare, yet very little is done about it because it is put into the too hard basket. So anything that can replace it is a great thing indeed. Sadly, ideas like this take forever to make their way into the mindset of Aussie farmers, who are generally quite a backward bunch.
piperTom
I don't understand how this can work in practice. Does the farmer plow the sand into the soil after the season? Is there some way to pick up the sand? Or does the farmer never plow again after putting sand on top?
ADVENTUREMUFFINffin
I'm with PiperTom. How much sand needs to be used. What is the application limit (how many years). Lots of unanswered questions that should be vetted before publication.
Martin Yale
As questioned - how would you apply this and get rid of it - something such as this - which for the Australian is actually an Australian invention and would be applied in a similar way - https://www.bioplasticsmagazine.com/en/news/meldungen/14112017-Leaf-enters-licence-agreement-for-biodegradable-coating.php
Don Duncan
1. "Weeds" are unwanted, but not unnecessary. They are indigenous, adapted to the soil, and store water, build up soil fertility, add organic material. 2. The ignorant mismanagement of soil is the main problem, not some mistake of nature. If the farmer respected nature and had the humility, the dedication to careful study of the natural processes in place, it wouldn't be necessary to buy artificial 'fixes" such as chemical fertilizers, pesticides, herbicides, plastic mulch. THEY DON'T WORK (long run).
Intellectual laziness leads to taking advice from an ignorant sales/businessperson only interested in selling.
Read: "The One Straw Revolution", by Masanobu Fukuoka.