The crop protection industry might be working hard to make pest management more sustainable, but researchers are warning its efforts are seeing arable farms add to a wider problem: Plastic pollution.
Plastic soil mulches and crop covers are widely seen in arable farming, helping to protect crops from damaging pests, weeds and inclement weather, and preserving soil moisture.
The preference for covering up is no surprise; in the hierarchy of pest management, the first priority is always to try and prevent pests reaching crops in the first place so there’s less need to apply products – either chemical or biological – later.
Plastic is affordable, durable and easily moulded and stretched, so its use makes sense in this context. Yet the short-term advantages could bring serious long-term costs, say experts.
Macro or micro, plastics are everywhere
A study in Californian strawberry fields found large quantities of plastic fragments (macroplastics) in fields where plastic mulches were used.
Scientists said those fragments are linked to negative effects such as reduced soil moisture and less available nitrogen. And the more fragments that were found, the worse the consequences.
“The plastic mulch provides benefits, but at the expense of long-term soil quality. It’s difficult and expensive to remove these particles from the soil, so once they are there they can stay there indefinitely,” Dr. Ekta Tiwari, who led the work at California Polytechnic State University, told an audience at the Goldschmidt geochemistry conference.
Previous work at Wageningen University and Research in the Netherlands uncovered a “cocktail of microplastics” on fields where both plastic mulches and compost had been previously used.
The researchers linked the use of mulches to higher levels of the tiny plastic particles in soil, and compost applications to increased concentrations in water.
Plastics from broken crop nets, widely used to cover certain crops during critical times of pest pressure, have also been widely found in the sediments of water bodies.
A commonality between much research done to date in this area is that polyethylene is the degraded plastic most often found on farms, whether in small or large pieces. But now that the biggest problem is underlined, what’s the solution?
Material matters
There appears to be no shortage of suitable materials to make the necessary shift away from plastics, so long as economies of scale can be achieved. Research and development picture is varied, from early concepts to older ideas that have already been used on a small scale for years.
Indian researchers have recently made the case for biodegradable starch blends to be the basis of an anti-plastic movement in a range of agricultural settings.
While they acknowledged that starch on its own is brittle and attracts water, the incorporation of blends, surface modifications and nanomaterials has the potential to make them highly attractive, they say.
Fabric made of cellulose – the compound which provides plants with their structure – has shown potential as a mulching material in tests, keeping out pests, absorbing water and allowing slow release of fertilisers.
Other plant-based agricultural fabric materials include jute, which releases nutrients during decomposition to aid plant growth and is already widely-available for gardeners and smallholders, and ramie, a plant fibre from the nettle family, which has shown promising results in experiments looking at its effects on the root environment.
Novel polymers, new applications
Growing demand for alternatives has also driven considerable amounts of research into biodegradable polymers over the last few years, with some impressive results.
Courgettes grown using a thin biodegradable film covering the soil were of higher quality than those using a thicker, polyethylene mulch.
Meanwhile a number of different films performed comparably to polyethylene in a study using strawberries, with Bio-Flex, made from PLA, a type of polyester made from the sugars of various crops, proving the best of the bunch.
It may well be the case that soil coverings of the future will even be applied differently; less laid, more sprayed.
In an experiment with cotton crops, a spray-on formulation of a novel fully-biodegradable polymer called SBM-Transpirational, designed by research teams at CSIRO in Australia, provided multiple benefits over a plastic mulch that degrades somewhat, but crucially not at the molecular level. Cotton grown with the novel mulch had improved yield, use of water and soil temperature.
“It’s applied via high pressure spray direct to the soil, so almost like a paint but the polymer it’s actually a biodegradable polyurethane that forms a film over the soil that helps suppress weeds and preserve moisture,” explained Dr Stuart Gordon, who leads the project for CSIRO.
“We can pigment [the polymer] any colour you like… if it’s black, then it helps with seed germination and works well in cooler temperatures or we can pigment it white or silver to reflect heat; whatever is required.”
The formulation is still being tweaked by the team, but more good results have been seen in tomato production, among a number of other crops. The concept may be “only a couple of years from commercial success,” said Gordon, but further investment is needed to scale the project up.
Change will come
Of course, the new wave of technologies are not perfect. A biodegradable product has much shorter field life than conventional plastics and – at present – the performance of novel materials and structures, though often impressive, isn’t quite as predictable as ‘business as usual’.
At a purely economic level, the price of biodegradable alternatives still has some way to go if plastics are to be forced out of agricultural practices.
But with the United Nations warning about an “alarming” accumulation of plastics in global soils, there is clearly the impetus for change.
Academic energy is there, finance is slowly following suite, and if scaled up and tailored to farmers’ needs, there seem few reasons why the industry as large won’t buy into the idea.
“I think that uptake will come. You just need a few lead growers to have enough access to the product for others to follow,” added Gordon.
“We know how that works, and we see it often. It’s almost the golden rule with agriculture.
“As soon as somebody starts up something that’s a little bit more productive, the others jump in very quickly afterwards.”
