BY TONI GINNETTI Staff Reporter

Sean Marshall knows all about on-the-job training.

"That's what a lot of it is,'' the Cubs rookie starter said, "but this is the best place to learn.''

For Marshall, 23, schooling was expected to come at Class AAA Iowa this season after a relatively strong 2005 split between Class A Daytona (4-4, 2.74 ERA in 12 starts) and Class AA West Tenn (0-1, 2.52 in four starts).

After pitching a total of 94 innings in his third pro season, the 6-7 left-hander would have been on a good track had he headed to Iowa. But a dazzling spring training and a Cubs rotation badly in need of arms combined for the opportunity of a lifetime for the former sixth-round draft pick from Virginia Commonwealth.

So far the positives have outweighed the negatives for Marshall, who will be on the mound tonight in St. Louis.

"I'll pick up things each time I go out there,'' he said this week. "Like my last time out [Saturday vs. Atlanta], I learned about using the inside of the plate more instead of putting the ball over the plate. But you don't want to walk guys because I know it always seems that leadoff walk scores.''

Marshall was in trouble in each of the first four innings against the Braves, but when he left after six, the Cubs trailed only 2-1. He ended up with the loss as that score held up, but it was an improvement from his previous start against the Florida Marlins in which he lasted only 3-1/3 innings, giving up seven runs on seven hits and four walks.

"It's about experience,'' manager Dusty Baker said. "He got out of several situations [vs. the Braves], but now it's about learning how to [avoid] those situations. It's learning what walking someone means as far as turning over the lineup. If you don't walk the eighth and ninth hitters, maybe you're not facing Andruw Jones in the ninth. It's part of the learning process.''

Learning about those aspects of a game are as important as learning command of pitches, and Marshall knows he's lucky to have one of the best teachers in future Hall of Famer Greg Maddux.

Camera shots of Maddux quietly counseling the rookie on the bench have become standard during Marshall's starts and when the two are not pitching.

"I got to meet him in spring training, and I was fortunate to have my locker near him,'' Marshall said of how their bond began. "He would give me tips on things, how to warm up, things about hitters. He's been great.

"I watched him pitch when I was growing up. We'd get WGN at home [in Virginia]. ... He was a guy I looked up to.''

Marshall also has tried to emulate some of his fellow lefties.

"When I pitched in college, I watched Barry Zito and Mark Mulder when they were with Oakland,'' he said. "I like watching Mark Buehrle pitch because he's similar to me with the changeup and curveball. You try to pick up things, but you also have your own style.''

Marshall knows Buehrle and Maddux have one common trait: pitching quickly.

"That's something I'm trying to do,'' he said. "I know your fielders love it because it keeps them in the game and keeps the game moving. The key to it is getting strike one. ... I'm learning to pound the strike zone early, and it makes the game quicker.''

And the learning curve all the better.

tginnetti@suntimes.com