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CHICAGO -- No longer burdened by batting leadoff, Alfonso Soriano is doing what he was supposed to have been doing all along. Rookie right-hander Randy Wells, meanwhile, has exceeded the Chicago Cubs' wildest expectations.

"I wonder where we'd be without him," Cubs manager Lou Piniella said Wednesday after Wells pitched eight shutout innings in a 12-0 victory over the Houston Astros.

They probably wouldn't be seven games over .500 -- their high-water mark of the season -- and in a close race with St. Louis for the NL Central lead.

Despite the lopsided win, the Cubs did have one setback. Outfielder Reed Johnson sustained a broken left foot after fouling a ball off himself in the first inning, and could miss a month or more.

Wells (7-4) is the first Cubs rookie with seven victories since Kerry Wood won 13 in 1998 and his 2.84 ERA leads all first-year pitchers. But the 26-year-old, who didn't even make the team out of spring training, refuses to discuss his Rookie of the Year possibilities.

"I'm just trying to stay even-keel and continue this little ride I'm on," said Wells, who was promoted from the minors May 8. "I'm scared of getting comfortable, scared of getting overconfident and having the wheels fall off."

The Cubs, who are 10-3 since the All-Star break, made things quite comfortable for him Wednesday, scoring six first-inning runs off Mike Hampton. Soriano's three-run homer was the big blow.

Soriano, the $136 million ballplayer who had struggled in the leadoff spot, is hitting .383 with five homers and 15 RBIs while batting sixth the last dozen games. His 13th-inning grand slam won Monday's series opener.

Reluctant to leave the top of the order at first, Soriano has come to realize that his powerful bat can help the team more from the 6-hole.

"If you're hot and hit a leadoff homer, it's only one run," he said. "But being hot like now ... first at-bat, first pitch, I hit a three-run homer.

"I'm very comfortable. I don't have pressure to get on base. Just see the ball and hit it. I like to score a lot of runs, but now I can get a lot of RBIs."

Wells allowed six singles, walked two, struck out one and let only one runner get as far as second base. Using his sinker, he got Houston to ground into three double plays. He has won seven of his last eight starts.

He easily outpitched Hampton (6-8), who gave up nine runs in four innings. His Astros have lost four of five since using a 32-18 stretch to get back into contention.

"I just basically got my butt kicked and there's no other way to look at it," Hampton said. "It seems everything I threw they were putting the bat on ... and hitting them all over the park."

The Cubs sent 10 batters to the plate in the first. Johnson opened with a single, went to third on Ryan Theriot's double and scored on Derrek Lee's sacrifice fly. After an RBI single by Aramis Ramirez, Milton Bradley walked and Soriano followed with his 19th homer. Hampton then walked Jeff Baker, threw a wild pitch and gave up Koyie Hill's RBI single.

In the first innings of his 18 starts this season, Hampton has allowed 24 runs -- an ERA of 12.00, compared to 4.04 from the second inning on.

"Most of his starts he's been able to get right after he's given up one or two [runs in the first inning]," Houston manager Cecil Cooper said. "Today was a tough day for him."

Ramirez homered in the second inning, giving him at least one extra-base hit in seven straight games; he's batting .517 during that stretch. The Cubs scored two more off Hampton in the fourth when Lee hit an RBI double and scored on Bradley's single.

Andres Blanco led off the eighth with his first career homer. Jake Fox added two RBIs.

Mitch Atkins, making his major league debut after getting called up from Triple-A Iowa, completed the Cubs' third shutout of the season by working a hitless ninth.

Game notes

Johnson left after the third inning with a bruised left foot. ... Astros RHP Bud Norris, just called up from Triple-A Round Rock, made his major league debut. After taking over for Hampton, he allowed one run on three hits in three innings. ... It was Chicago's most decisive win this year but wasn't Houston's most lopsided loss. The Astros fell 13-0 to San Francisco on July 3.