Plastics used in farming activities are accumulating in agricultural soil worldwide at an alarming rate, said the UN Environment Programme (UNEP) in its latest report.

Plastics are used extensively in agriculture, from plastic-coated seeds to protective wraps used to modify soil temperature and prevent weed growth over crops.

These synthetic materials are also added intentionally to biosolid fertiliser, which is spread on fields, and are used in irrigation tubes, sacks and bottles.

While all these products have helped increase crop yields, there is growing evidence that degraded plastics are contaminating the soil and impacting biodiversity and soil health, said the UN report published Monday.

Also, microplastics, such as the ones used in some fertilisers, are also impacting human health when transferred to people through the food chain.

"There is only a finite amount of agricultural land available," said the report's co-author Professor Elaine Baker from the University of Sydney. "We are starting to understand that the build-up of plastic can have wide-ranging impacts on soil health, biodiversity and productivity, all of which are vital for food security."

Over time, big pieces of plastic can break into shards less than 5mm long and seep into the soil. These microplastics can change the physical structure of the earth underfoot and limit its capacity to hold water. They also can affect plants by reducing root growth and nutrient uptake.

The single-biggest source of microplastic pollution in the soil now is fertilisers produced from organic matter such as manure.

Although these can be cheaper and better for the environment than manufactured fertilisers, the manure is mixed with the same plastic microspheres that are known to be commonly used in certain soaps, shampoos, and makeup products.

While some countries have banned these microspheres, other microplastics continue to enter people's water systems via discarded cigarette filters, tire components, and synthetic clothing fibres.

Progress is being made to improve the biodegradability of polymers used in agricultural products. However, some protective films – used to prevent moisture loss – are now being marketed as fully biodegradable and compostable, which is not always the case.

Bio-based polymers are not necessarily biodegradable, some may be as toxic as fossil fuel-based polymers, and their price is still an issue.

A solution proposed by the report is the "cover crops," which shield the soil and are not meant to be harvested. These nature-based solutions can suppress weeds, counter soil diseases and improve soil fertility, but there are concerns they could reduce yields and increase costs.

"None of these solutions are a magic bullet. Plastic is inexpensive and easy to work with, which makes trying to introduce alternatives a hard sell," Elaine said.

However, the expert recommends governments disincentivise the use of agricultural plastics, following the path of the European Union, which earlier this year restricted certain types of polymers from being used in fertiliser.