Image copyright AFP Image caption The sugar-cane harvest in India is at risk from salt damage

About 2,000 hectares of fertile land are lost each day due to damage caused by salt, according to a UN analysis.

The total area now affected is equivalent to the size of France - 62 million hectares - which has increased from 45 million 20 years ago.

Salt degradation occurs in areas of dry irrigated land with little rainfall and where there is no natural drainage.

The report is published in UN journal Natural Resources Forum.

It suggests tree planting, deep ploughing and the production of salt-tolerant crops. It also proposes digging drains or ditches around the affected land.

These methods would be expensive but the authors say the cost of inaction would be worse. They estimate the global cost to be $27.3bn (£16.9bn).

"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," said lead author Manzoor Qadir from the UN University's Institute for Water, Environment and Health (UNU-INWEH).

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

Growing food needs

Many regions across 75 countries are affected, including the Aral Sea Basin in Central Asia, the Indo-Gangetic Basin in India and the Yellow River Basin in China, causing an impact on many important crop yields.

Image copyright Spl Image caption A Dutch farmer says potatoes could be grown in salt-affected land around the world

In India, for example, wheat, rice, sugar-cane and cotton production are all at risk - crops that are vital for livelihoods. In the Colorado River Basin reports have shown that damage from salt could cost the US up to $750m (£465m) per year.

Co-author of the work, Zafar Adeel, also of UNU-INWEH, said: "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."

The authors estimate that food production will need to increase by 70% by 2050 to feed the world's growing population.

One of the methods proposed to combat the damage has already been developed by a Dutch farmer. Marc van Rijsselberghe said he had produced potatoes which can be irrigated with salt water.

He told the BBC's Farming today programme that his crops could reduce pressure on fresh-water resources and said he had already harvested 50 tonnes of saline-tolerant potatoes this year.

He said his potatoes could now be grown on 300 million hectares of land all over the world.

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