A quarter of farmland is highly degraded, according to the first report into the state of the world's land resources

The United Nations has completed the first global assessment of the state of the planet's land resources, finding in a report that a quarter of all farmland is highly degraded and warning the trend must be reversed if the world's growing population is to be fed.

The UN Food and Agriculture Organisation (FAO) estimates that farmers will have to produce 70% more food by 2050 to meet the needs of the world's expected 9-billion-strong population. That amounts to 1bn tonnes more wheat, rice and other cereals and 200m more tonnes of beef and other livestock.

But as it is, most available farmland is already being farmed, and in ways that decrease its productivity through practices that lead to soil erosion and wasting of water.

This means that to meet the world's future food needs, a major "sustainable intensification" of agricultural productivity on existing farmland will be necessary, the FAO said in its report, State of the World's Land and Water Resources for Food and Agriculture.

The report was released on Monday, as delegates from around the world meet in Durban, South Africa, for a two-week UN climate change conference aimed at breaking the deadlock on how to curb emissions of carbon dioxide and other pollutants.

The report found that climate change coupled with poor farming practices had contributed to a decrease in productivity of the world's farmland following the boom years of the "green revolution", when crop yields soared thanks to new technologies, pesticides and the introduction of high-yield crops.

Thanks to the green revolution, the world's cropland grew by just 12% but food productivity increased by 150% between 1961 and 2009.

But the UN report found that rates of growth had been slowing down in many areas and today were only half of what they were at the peak of the green revolution.

It found that 25% of the world's farmland was now "highly degraded" with soil erosion, water degradation and biodiversity loss. Another 8% was moderately degraded, while 36% was stable or slightly degraded and 10% was ranked as "improving".

The rest of the Earth's surface is either bare or covered by inland water bodies.

In western Europe, highly intensive agriculture has led to pollution of soil and aquifers and a resulting loss of biodiversity. In the highlands of the Himalayas, the Andes, the Ethiopian plateau and southern Africa, soil erosion has been coupled with an increased intensity of floods. In rice-based food systems of south-east and eastern Asia, land has been abandoned thanks in part to its loss of cultural value.

The report found that water around the world was becoming ever more scarce and salinated, while groundwater was becoming more polluted by agricultural runoff and other toxins.

In order to meet the world's water needs in 2050, irrigation must become more efficient because most systems perform well below their capacity, the FAO said.

The agency called for new farming practices such as integrated irrigation and fish-farm systems, as well as overall investment in agricultural development.

The investment deemed necessary until 2050 is $1tn (£642bn) for irrigation water management for developing countries alone, with another $160bn for soil conservation and flood control.