Martinson, the Nationals' No. 20 prospect, hit .268 with 12 home runs, 53 RBIs and an .858 OPS in 73 games at Potomac, where he also played the second half of last season. The 24-year-old was a fifth-round pick in the 2010 First-Year Player Draft.
Johnson hit .267 with seven home runs, 28 RBIs and an .803 OPS in 53 games for Harrisburg, showing increased power. The 27-year-old spent much of last season with Syracuse and hit .231 with no homers.
Tom Schad is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.