The six million tonnes of human waste Scotland's population produces every year could soon be used in the quest for food security, according to scientists who warn the soil only has 100 seasons of growing left.

The Sheffield University-based team say society will have to rethink its attitudes to genetically modified (GM) food technology and accept an inevitable role for human excrement in the food chain.

They believe that UK farming's inevitable future will be a combination of GM crops on organic farms fertilised by human waste.

Dr Duncan Cameron, who did his doctoral research at Aberdeen University, is one of the Sheffield team's leaders. He said: "In a time of rapid environmental change we need new ways to intensify sustainable production and protect food crops. "

He said the challenges of modern day agriculture were numerous: climate change, soil degradation, water shortages and growing demand. Phosphorous and nitrogen, essential to the growing process are found in human waste, which the scientists believe could be used more efficiently.

According to the Sheffield team, plants are heavily reliant on fertilisers and many have lost the natural traits that enable them to interact with beneficial bacteria and fungi in the soil.

Alison Johnstone, Green MSP for Lothian and food spokeswoman for the Scottish Greens, said: "This report's focus on organic farming methods is welcome. Reducing our use of chemicals on the land is good for health, carbon emissions, biodiversity and the water sources we use for drinking supplies."