He was a prime mover in its Food for Life campaign, and also worked with TV chef Jamie Oliver to improve school meals.
He had been policy director for 18 years following earlier work as a Labour government minister from 1974-79, at the Departments of Environment, Industry, and Northern Ireland (covering education and health).
He had also been president or chair of several conservation NGOs, director of Greenpeace UK (1985-2000), and chaired Greenpeace Japan for six years until 2001.
A statement from the Soil Association said: “Peter has been such an important, charismatic figure in the environment and organic movements throughout his lifetime.
“Alongside his Soil Association role, he ran his family’s 890-acre organic farm in Norfolk, with cattle, sheep and arable crops.
“He was an extraordinary man with incredible knowledge across a wide range of subjects, ranging from beef farming to sustainable cosmetics.
“A true campaigner all his days, he will be greatly missed by all his Soil Association colleagues.”
Among other roles he was also a member of the BBC's Rural Affairs Committee; the Department of Education's School Lunches Review Panel; the Department of Environment, Food and Rural Affairs’ Rural Climate Change Forum and the Organic Action Plan Group.
He worked as a special lecturer in biological sciences at Nottingham University from 1984 until 2002, and was on the board of the EU £12m research project 'Quality Low Input Food' from 2004-2009. He also worked as an environmental consultant.