In October 2002 John joined Grain SA as Manager: Production Inputs and in 2004 he became the organisation's Manager: Strategic Development. In June of 2005 he was appointed General Manager of Grain SA. Dr Purchase left Grain SA in September 2007 to become the Chief Executive Officer of the Agricultural Business Chamber (Agbiz). Dr Purchase is also a member of numerous professional societies, such as the South African Society of Crop Production. He was on the Board of this society for eight years, two of which were as President and two as Vice President. He is also still a member of the South African Plant Breeders' Association.