The pilot stated that he was spreading "Urea" on a cotton crop. Prior to commencing a procedure turn the aircraft overflew a stand of trees. When the aircraft was about three quarters of the way through the turn, the pilot felt the aircraft begin to stall. He lowered the nose and increased power, but the aircraft descended into the trees. It continued to fall between the trees and slid for 70 metres before coming to rest. The pilot stated that during a previous landing he had experienced a willy-willy which had originated from the stand of trees. He was conscious of downdrafts near the trees. During previous turns he had flown well past the trees before commencing a turn. On this occasion he feels that the aircraft was affected by a downdraft.