The 20-year-old rejected a new contract at St Mary´s and Pompey agreed to pay undisclosed compensation to the Coca-Cola Championship club.
"Martin is a very promising young player who has the ability to go right to the very top of the game," said manager Harry Redknapp.
© Copyright PA Sport Ltd 2007, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: http://gms.to/writeforgms