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Salt is damaging crop-growing soil around the world and it could lead to a shortage of food by 2050.

That's according to a UN study that says 7.7 square miles (19.9 square km) of irrigated land, an area the size of Manhattan, has been lost every day for 20 years due to salt degradation.

And they say this could mean lower crop yields and even health problems in future.

A study by Canadian-based researchers at the UN University says that salt degradation is ruining crop yields around the world. Salt degradation is caused by improper drainage in irrigation systems. In areas such as the Pakistan, this can reduce rice yields (stock image shown) by 69 per cent

The study was conducted by the UN University's Canadian-based Institute for Water, Environment and Health.

They say today an area the size of France is affected by salt degradation - about 62 million hectares (20 per cent) of the world's irrigated lands - up from 45 million hectares in the early 1990s.

Salt degradation occurs in arid and semi-arid regions where rainfall is too low to maintain regular movement of rainwater through the soil and where irrigation is practiced without a natural or artificial drainage system.

If irrigation is not practiced with appropriate drainage management this can trigger the accumulation of salts, in turn reducing the productivity of soil.

The cost of salt degradation is estimated at around £275 ($441), which means it is about £17 billion ($27.3 billion) for the past year.

The effects of salt degradation are numerous. In India's Indo-Gangetic Basin, for example, salf-affected lands where cotton is grown could have a yield up to 63 per cent less than healthy land.

In the Indus Basin in Pakistan, meanwhile, the crop yield for rice was found to be up to 69 per cent lower.

And the issue is also a problem for Western countries with the economic impact of salt degradation in the Colorado River Basin in the US estimated at £470 million ($750 million).

The researchers warn that, if left untended, the problem could lead to food shortage issues towards the middle of this century.

Salt degradation occurs when the salt content (salinity) in soil becomes too high. While it can be caused by natural processes, it can also be caused by artificial processes like irrigation. Without a proper drainage system, salts will be left behind after plants use water and begin to accumulate (stock image shown)

'To feed the world's anticipated nine billion people by 2050, and with little new productive land available, it's a case of all lands needed on deck,' says principal author Dr Manzoor Qadir, Assistant Director of Water and Human Development at the UN University's Canadian-based Institute for Water, Environment and Health.

'We can't afford not to restore the productivity of salt-affected lands.'

Dr Zafar Adeel, Director of UNU-INWEH, notes the UN Food and Agriculture Organization projects a need to produce 70 per cent more food by 2050, including a 50 per cent rise in annual cereal production to about three billion tonnes.

'Each week the world loses an area larger than Manhattan to salt-degradation,' he explains.

'A large portion of the affected areas in developing countries have seen investments made in irrigation and drainage but the infrastructure is not properly maintained or managed.

'Efforts to restore those lands to full productivity are essential as world population and food needs grow, especially in the developing world.'

All hope is not lost, though. The researchers say there are a number of techniques that can be used to halt the issue of salt degradation, and even reverse the process.

These include tree planting, deep plowing, cultivation of salt-tolerant varieties of crops, mixing harvested plant residues into topsoil and digging a drain or deep ditch around the salt-affected land.

The study, Economics of Salt-induced Land Degradation and Restoration, was published in the UN Sustainable Development journal Natural Resources Forum.