Salt's poisonous effect is growing threat to crop yields

As global warming increases the world’s arid areas, scientists warn that restoring productivity to salt-affected agricultural land will be essential to feed an expanding population.

by Tim Radford, Climate News Network, November 1, 2014

LONDON − Salt is poisoning around 2,000 hectares of irrigated farm land every day – and has been doing so for the last 20 years, according to new research.

Think of an area about the size of 3,000 football fields that can no longer be used to produce food each day. And then remember that the global population actually grows by around 200,000 people every day.

Manzoor Qadir, senior research fellow at the United Nations University’s Institute for Water, Environment and Health, and colleagues report in the journal Natural Resources Forum that an area of farmland the size of France – 62 million hectares – has been affected by the build-up of salts in irrigated soil. This is one-fifth of all irrigated land.

“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 Dr Qadir. “We can’t afford not to restore the productivity of salt-affected lands.”

Ancient hazard

Salts degradation is an ancient hazard in arid and semi-arid lands, where groundwater is pumped from aquifers below the bedrock and used to grow crops.

Evaporation and transpiration leave precipitated salts around the roots of each crop and – since there is no fresh rainwater to wash away the salts − sooner or later the levels build up to intolerable scales, and the land becomes increasingly unproductive.

The UN Food and Agriculture Organisation warns that to feed the projected 2050 population, farmers will need to grow 70% more food. Cereal production alone will have to increase by 50%, to a total of three billion tonnes a year. But, each week, the world loses an area of land the size of Manhattan to salt degradation, thanks to poor soil management, bad drainage and other problems.

The researchers, from Canada, Jordan, Pakistan and Sri Lanka, based their estimates on more than 20 studies in the last two decades in Australia, India, Pakistan, Spain, Central Asia and the US.

They also totted up the estimated economic losses: more than $27 billion a year.

In the Indo-Gangetic basin of India, the build-up of soil salts could reduce wheat harvests by 40%, and cotton by more than 60%.

Employment losses could be as much as 50-80 man days per hectare, and human health problems could be between 20% and 40% greater because of the effect. Animal health problems could increase by anywhere between 15% and 50%.

In the Indus basin in Pakistan, the average overall wheat grain loss has been put at 32%, and the average rice yield has fallen by 48%.

The worst affected regions of the world are the Aral Sea basin in Central Asia, the Indo-Gangetic basin in India, the Indus Basin in Pakistan, the Yellow River basin in China, the Euphrates basin in Syria and Iraq, the Murray-Darling in Australia, and the San Joaquin Valley in the US.

The researchers warn that their calculations concern only crop-yield losses.

Degraded lands

“However, the crop yields from irrigated areas not affected by salinisation have increased since 1990 due to factors such as improved crop varieties, efficient on-farm practices, better fertilizer use, and efficient water management practices,” they say.

“Consequently, there may be larger gaps in crop yields harvested from salt-affected and non-affected areas under similar agro-ecosystems, suggesting an underestimation of the economic cost of salt-induced land degradation.”

“These costs are expected to be even higher when other cost components − such as infrastructure deterioration (including roads, railways, and buildings), losses in property values of farms with degraded land, and the social cost of farm businesses − are taken into consideration.

“In addition, there could be additional environmental costs associated with salt-affected degraded lands as these lands emit more greenhouse gases, thus contributing to global warming.”

Some yield could be recovered. For example, farmers could irrigate more sparingly, plough deeply, dig drains, plant trees, select salt-tolerant crops, and dig in the stubble and plant waste.

An essay in the journal Trends in Plant Sciences also notes that around three hectares of farmland are lost every minute.

But plant science itself could help. Sergey Shabala, professor of crop physiology and plant nutrition at the University of Tasmania in Australia,  points out that millions of years of evolution have already devised a possible answer.

He says: “We should learn from nature and do what halophytes, or naturally salt-loving plants, are doing: taking up salt but depositing it in a safe place – external balloon-like structures called salt-bladders.”

Over-riding problem

New approaches to plant breeding could certainly provide part of the solution. The over-riding problem, however, is that water is already being used on a prodigal scale, in a globally-warming world in which some regions are in any case predicted to become even more arid.

Nine-tenths of the Aral Sea – once the world’s fourth largest lake − in Central Asia is now a sandy desert. The dust blown from it has salted half of Uzbekistan’s soil, and 70% of Turkmenistan has become desert, according to a report in the journal Nature.

But the cotton and wheat farmers in the republics that border the Aral Sea are among the highest users of water in the world. A Turkmen, on average, consumes four times the water used by a US citizen, and 13 times that of a Chinese one.

And although the Western hemisphere is in the grip of a calamitous and sustained drought, the real problem, according to Marcia McNutt, the former director of the US Geological Survey, and now the editor-in-chief of Science magazine, is that underground aquifers in the south-western US have been emptied for irrigation at such a rate that the contours on the land itself have started to change.

Californian mountains have risen up to 15 millimetres because of the water loss.

“It is high time we started managing our precious water supplies in harmony with the laws of nature,” she warns.