Jenson Button topped the times during a rain-effected first practice session at the Hockenheimring on Friday. AFP Photo
Schumacher, in the third and possibly last season of his comeback, lost control on bend 12, spun out and hit the tire barrier, ripping off the rear of his car and damaging the left side.
"I simply slid off track because I lost a little concentration: we were talking on the radio and I was altering some settings at the same time," Schumacher said.
He got up and walked away, before returning to watch the marshals haul the wreck away.
The crash brought out the red flag, ending the session with less than four minutes remaining.
Schumacher has won the German GP a record four times and was the last German driver to win his home race, in 2006, his last season with Ferrari. Mercedes is seeking its first win on home soil since 1954.
"The way I ended the second session was obviously not part of the plan and I'm sorry that my mistake has given my guys even more work to do this evening. I think that qualifying will probably be wet tomorrow, so today's weather was useful in that respect," the 43-year-old veteran said.
Heavy showers drenched the afternoon session that produced times more than 10 seconds slower than the first morning practice that was hit by occasional rain.
Jenson Button drove the fastest lap in that session, ahead of McLaren teammate Lewis Hamilton.
With light rain hitting the circuit on and off, Button set the pace with a lap of 1 minute, 16.595 seconds, edging Hamilton by .498 seconds. Maldonado's best lap, by comparison, was 1:27.476.
Championship leader Fernan-do Alonso of Ferrari was third in the morning, ahead of Schumacher. In the afternoon, Schumacher was next-to last in 23rd, while Alonso was 20th.