Chris Alenson is from Australia's National Centre for Sustainability and his particular interest is sustainable agriculture. He visited the studio to talk to us about Peak Phosphorus.

Phosphorus is a key component of fertilizers and so is crucial to world food supplies. There are not many places in the world that have phosphorus deposits and in less than 50 years we're likely to run out of it. If we continue along our present path, future generations might find themselves faced with food and national security threats. The good news is that phosphorus cycles in natural systems and many of the serious problems can be avoided by making changes to our waste streams.