Many organic fertilisers being applied to gardens and farms contain tiny fragments of plastic, according to a new study.

Widely considered a problem affecting the oceans, this work suggests microplastics may actually be far more pervasive.

Having entered the soil, the scientists behind the study have warned these tiny fragments could end up in the food we eat.

The production of organic fertilisers is generally considered environmentally friendly as it involves recycling food waste from households and other sources to make useful products that can be used to grow more food.

However, contamination of the waste used to produce these fertilisers – which are used by gardeners and farmers alike – means tiny microplastics are making their way into the soil.

“One example is people use plastic bags and then put everything together in the bin, and then this is entering the waste treatment plant and ending up in the fertilisers,” Professor Ruth Freitag, one of the study’s authors, told The Independent.

Professor Freitag, who is based at the University of Bayreuth in Germany, said this is a particular problem in her country due to its progressive composting policy.

Close to 12 million tons of food and garden waste ends up being either composted or sent to biogas fuel plants each year in Germany, due to legal requirements concerning the disposal of organic waste.

By sampling fertilisers produced at different kinds of waste treatment plants, Professor Freitag and her colleagues were able to determine the extent of the contamination problem.

They found the samples had varying amounts of tiny fragments, fibres and spheres of plastics, all smaller than five millimetres, depending on the care with which the compost had been processed.

Plastic waste across the world: in pictures Show all 15 1 /15 Plastic waste across the world: in pictures Plastic waste across the world: in pictures A father and son on a makeshift boat made from styrofoam paddle through a garbage filled river as they collect plastic bottles that they can sell in junkshops in Manila. The father and son team earn some three US dollars a day retrieving recyclables from the river. AFP/Getty Plastic waste across the world: in pictures A composite image of items found on the shore of the Thames Estuary in Rainham, Kent. Tons of plastic and other waste lines areas along the Thames Estuary shoreline, an important feeding ground for wading birds and other marine wildlife. Getty Images Plastic waste across the world: in pictures Children collect plastic water bottles among the garbage washed ashore at the Manila Bay. According to the United Nations Environment Programme, at current rates of pollution, there will likely be more plastic in the sea than fish by 2050. AFP/Getty Plastic waste across the world: in pictures Plastics and other detritus line the shore of the Thames Estuary. In December 2017 Britain joined the other 193 UN countries and signed up to a resolution to help eliminate marine litter and microplastics in the sea. It is estimated that about eight million metric tons of plastic find their way into the world's oceans every year. Once in the Ocean plastic can take hundreds of years to degrade, all the while breaking down into smaller and smaller 'microplastics,' which can be consumed by marine animals, and find their way into the human food chain. Getty Plastic waste across the world: in pictures A dump site in Manila in 2013. The Philippines financial capital banned disposable plastic shopping bags and styrofoam food containers, as part of escalating efforts across the nation's capital to curb rubbish that exacerbates deadly flooding. AFP/Getty Images Plastic waste across the world: in pictures Children swims in the sea full of garbage in North Jakarta, Indonesia. Getty Plastic waste across the world: in pictures An Indian woman holds a jar filled with Yamuna river water polluted with froth and toxic foam to be used for rituals at the river bank in New Delhi, India. The Yamuna River, like all other holy rivers in India, has been massively polluted for decades now. The river that originates in a glacier in the pristine and unpolluted Himalayas, and flows through Haryana, Delhi and Uttar Pradesh before merging with the Ganges River in Allahabad, once used to be the lifeline of the Indian capital. Currently, it is no more than a large, open sewer that is choking with industrial and domestic discharge that includes plastic, flowers and debris and has virtually no aquatic life. EPA Plastic waste across the world: in pictures Plastic waste is washed up on South Troon beach in Scotland. Recent reports by scientists have confirmed, plastics dumped in the world oceans are reaching a dangerous level with micro plastic particles now being found inside filter feeding animals and amongst sand grains on our beaches. Getty Plastic waste across the world: in pictures Children collect plastic to be sold and recycled, in a polluted river in suburban Manila. The city's trash disposal agency traps solid waste floating down waterways that was thrown into the water by residents of slums along riverbanks upstream. AFP/Getty Plastic waste across the world: in pictures View of the Carpayo Beach in La Punta, Callao, some 15 km of Lima. In 2013, the NGO VIDA labeled the Carpayo Beach as the most polluted in the country - 40 tons of trash on each 500m2. AFP/Getty Plastic waste across the world: in pictures Trash from Kamilo Beach in Hawaii. Gabriella Levine/Flickr Plastic waste across the world: in pictures A scavenger collects plastic cups for recycling in a river covered with rubbish near Pluit dam in Jakarta. Reuters Plastic waste across the world: in pictures Rubbish fills Omoa beach in Honduras. Floating masses of garbage offshore from some of the Caribbean's pristine beaches are testimony to a vast and growing problem of plastic pollution heedlessly dumped in our oceans, locals, activists and experts say. AFP/Getty Plastic waste across the world: in pictures A man climbs down to a garbage filled river in Manila. Plastic rubbish will outweigh fish in the oceans by 2050 unless the world takes drastic action to recycle the material, a report warned in 2016. AFP/Getty Plastic waste across the world: in pictures Garbage on East Beach, Henderson Island (Pitcairn Islands), in the south Pacific Ocean. The uninhabited island has been found to have the world's highest density of waste plastic, with more than 3,500 additional pieces of litter washing ashore daily at just one of its beaches. EPA

Their results were published in the journal Science Advances.

Though concentrations in the samples were fairly low, with a maximum of around 150 microplastic particles per kilogram, from here the researchers say it is likely the particles will enter the food chain.

Ultimately, they could even end up being consumed by humans.

A major problem with research into microplastics, according to Professor Freitag, is that so little is known about them.

Almost all research into these tiny pollutants has focused on their presence in the sea, and the threat they pose to marine life.

The UK has banned the manufacture of microbeads – tiny pieces of plastic added to some cosmetic products – but these are low-hanging fruit when considering the wider problem of microplastic pollution.

The origin of most microplastics in the environment remains a mystery.

“The problem is microplastics are not made in the ocean,” said Professor Freitag.

“It is of course quite nice and safe to investigate it there because nobody is responsible – it’s just somewhere in the ocean.

“Most of it is manmade and created on land, but we don’t know very much about these pathways.”

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While Germany has been particularly forward-thinking in its treatment of organic waste, Professor Freitag said it is likely the conversion of organic waste into fertiliser will become more widespread everywhere – making this issue all the more relevant.

“Burning organic waste is not a very efficient procedure and putting it into landfill cannot really continue, so in the end it is going in that direction,” she said.

If this is the case, it will be vital to take a cautionary approach that attempts to keep the plastic pollution to a minimum.

A key point from the study was the finding that different kinds of waste treatment plant produce vastly different concentrations of plastic in the fertilisers.