* Overproduction, overgrazing, deforestation damaging soil
* FAO estimates 25 pct of agricultural land is highly degraded
* Arable land from China to Sub-Saharan Africa already lost
* Rising population, more meat consumption increase land use
By Nigel Hunt and Sarah McFarlane
LONDON, July 16 (Reuters) - The challenge of ensuring future food security as populations grow and diets change has its roots in soil, but the increasing degradation of the earth's thin skin is threatening to push up food prices and increase deforestation.
While the worries about peaking oil production have been eased by fresh sources released by hydraulic fracturing, concern about the depletion of the vital resource of soil is moving centre stage.
"We know far more about the amount of oil there is globally and how long those stocks will last than we know about how much soil there is," said John Crawford, Director of the Sustainable Systems Programme in Rothamsted Research in England.
"Under business as usual, the current soils that are in agricultural production will yield about 30 percent less than they would do otherwise by around 2050."
Surging food consumption has led to more intensive production, overgrazing and deforestation, all of which can strip soil of vital nutrients and beneficial micro-organisms, reduce its ability to hold water and make it more vulnerable to erosion.
Such factors, exacerbated by climate change, can ultimately lead to desertification, which in parts of China is partly blamed for the yellow dust storms that can cause hazardous pollution in Asia, sometimes even severe enough to cross the Pacific Ocean and reduce visibility in the western United States.
Arable land in areas varying from the United States and Sub-Saharan Africa, to the Middle East and Northern China has already been lost due to soil degradation.
The United Nations' Food and Agriculture Organization (FAO) has estimated that 25 percent of agricultural land is highly degraded, while a further 8 percent is moderately degraded.
Crawford said the degradation of soil could in theory lead to more land being bought into agricultural production, which would deal a serious blow to efforts to stem climate change, since clearing forests for farmland leads to a heavy net increase in greenhouse gases.
"If we keep treating our soil the way we do, we will have to convert about 70 percent of the earth's surface into agriculture to meet demand for food by 2050 (from about 40 percent now)," Crawford said.
That is in part because there will be many more mouths to feed. The United Nations has projected that global population will reach 9.6 billion by 2050, up from 7.2 billion last year.
Emerging nations are also embracing Western diets that include more consumption of meat, which will add further to the strain on agricultural resources.
Crawford also noted that moderately degraded soil could only store about half the amount of water of good soil, adding to pressure on limited water resources.
"We need to find ways of pricing the true cost into food, including the environmental cost of soil degradation," Crawford added.
Food security became a hot topic after record high grain prices in 2008 marked the start of a period of volatility.
Agricultural markets are still unstable, after near-record prices in 2012 prompted increased production, which led to surpluses.
Prices have since fallen back on the rebound in production and global stocks, with decent harvests expected in several major grain producers including the United States this year, but there's a risk of complacency on the long-term outlook.
"We are trying to make sure when we talk about food security we talk about healthy soil. The link has been missing to some extent," said Moujahed Achouri, Director of the FAO's Land and Water Division.
"We do believe there that now there is momentum (to tackle the soil problem)."
Price pressure and ultimately margin pressure can lead to farmers taking shortcuts to achieve something in the short term at the expense of the long term, said Nicholas Lodge, managing partner at Clarity, a Gulf-based agricultural investment firm.
"You can really have a harmful impact on soil in as little as one season," said Lodge.
"If you happen to have damaged the soil and you're losing the top soil, it's not then an easy matter to repair that situation or replace that soil."
One of the main drivers of soil degradation has been the trend towards less diversity in agriculture.
"In a lot of agriculture it has become a monoculture, so you just don't get the diversity of plants that are necessary for healthy soil, and often the agricultural practices are all about mining the soil rather than managing it," said Tim Hornibrook, head of Macquarie Agricultural Funds Management Limited.
Vietnam is one example of a country where there has been an increased focus on one crop with a huge surplus of robusta coffee grown to export to the global market.
The U.S. Department of Agriculture also estimates that corn will be harvested on around 177 million hectares this year, a rise of around 65 percent over the last 50 years.
"Farming with monocultures leads to decreased productivity," Hornibrook said.
Excessive use of fertilisers can also cause damage to soil, at times altering its acidity or salinity in ways that reduce microbial activity and therefore ultimately plant growth.
More education in the farming sector on how to conserve soils, along with better use of technology, is expected to help tackle the problem.
"Technology which can help includes imagery which allows you to do soil mapping of what mineral and nutrients are in the soil and applying fertiliser according to the requirement of each individual area of the farm," said Hornibrook, adding that investment was challenging as the sector was fragmented and capital starved.
"The issue doesn't get addressed without capital. Investing in your soil costs money and therefore the ultimate way to incentivise farmers to do it is higher food prices."
But higher prices alone won't encourage consumption patterns that provide a healthy balance for both people and soil.
"Consumers make choices largely on price, farmers make decisions largely on profit," Crawford said, adding there was no clear incentive to encourage behaviour that benefited health or the environment.
"We need to try and encourage better diets from a health and environment point of view."
(Editing by Veronica Brown and Will Waterman)