When the umpire gives Little League pitcher Tucker Smith of Arlington, Mass., a choice of baseballs, Tucker knows exactly which ones he wants. And they're not the newest, shiniest balls. Tucker, who is 11, and other pitchers up through the major leagues know a secret about baseballs: They need a little dirt. That's because new balls just don't throw right. Dirt gives them some grip.

As baseball heads into the World Series, this humble substance is not just helping pitchers, but it's playing a key role underfoot as players bat, run, and slide. But not just any dirt will do. Baseball clubs use special mixes - from infield dirt that's mined in New Jersey to a secret rubbing mud found somewhere along the Delaware River.

"When you go into the umpire's room in Major League Baseball (MLB), there's a big tub of mud," says Nick Zibelli, director of umpiring for the Eastern College Athletic Conference. Before a game, an umpire or club attendant uses the mud to "rub up" new baseballs - about six dozen for each major league game.

There's a special technique to rubbing up a baseball. The mud - and a little water - goes on the leather, but not the seams. If it's not done right, the pitcher will throw the ball back to the umpire. "If you don't apply the mud uniformly," warns Mr. Zibelli, "the pitcher can't get the proper grip. At 90-plus miles per hour, it doesn't take much to throw the tracking off."

The quality of mud is important. It has to be just the right consistency, with enough texture to remove the shine.

All 30 major league teams use riverbank mud from a tidal tributary of the Delaware River in New Jersey. The exact location is an old secret guarded by Jim Bintliff. He owns Lena Blackburne Baseball Rubbing Mud, and its mud has the look and feel of chocolate pudding.

The secret mud was discovered in the 1930s. Back then, Lena Blackburne was a third-base coach for the old Philadelphia Athletics baseball team. Teams used infield dirt and water to rub on baseballs.

In 1938, an umpire complained to Blackburne about the condition of the balls. So when Blackburne went home to New Jersey, he discovered mud that seemed perfect for rubbing. He tested it, and players were impressed. By the late 1950s, the mud was being used by all major league teams. Blackburne passed the business - and the secret location - on to a friend. Now Mr. Bintliff's family is in charge. Each July, Bintliff goes to the secret spot, digs up 1,000 pounds of mud, and stores it over the winter to sell during the next season.

"I used to go only by boat for about 20 years, but I found a way to do it by going through the woods," he says. "I use my truck now; it's a 100-yard walk to get to the spot."

If someone is nearby, Bintliff will turn around and come back another day. His company sells the mud in tubs of various sizes. It costs $45 for the 32-ounce professional size, down to $2.99 for a trial size, enough to rub a dozen baseballs.

Other dirts used in baseball include mixes for the base paths, pitcher's mound, and home plate.

Baseball fields used to be packed with clay, which turned rock hard in the sun and melted into a slippery mess after a downpour. Today major league clubs often use a sports dirt mix called Beam Clay, made by Partac Peat Corp. Its dirts for the diamond, pitcher's mound, and home plate contain different ratios of red clay (for firmness) and orange sand (for drainage and softness). Both come from New Jersey. The pitcher's mound is the firmest, says Jim Kelsey, Partac's president. Its dirt must stay in place when a pitcher throws.

The mixture for home plate, however, is made with more sand so it drains well. The base paths and infield are coated in a diamond mix that provides traction for running but is soft enough for sliding onto base. Teams can even choose their color of dirt from four earth tones - gray, brown, red, or orange, Mr. Kelsey says. Red is the most popular, he adds.

Little Leaguer Tucker also knows the importance of infield dirt. "You can't really slide if it's bad dirt," he says. He admits to scooping up dirt from Fenway Park. "It's more like small pebbles. It grooms really nicely," he says. "You could definitely slide on that."

A trail of famous dirt

The National Baseball Hall of Fame and Museum in Cooperstown, N.Y., has some of the best dirt in baseball. Remember the final play of last year's World Series game, the Red Sox versus the Cardinals? A bounce to the mound and a soft underhand toss to first base made the final out that gave the Sox their first World Series win in 86 years. Members of the Hall of Fame's staff were at that game. They collected a pound of dirt from the pitcher's mound, and now it's on display under glass at the museum.

"We acquire dirt when there's a story to be told," says Jeff Idelson of the Hall of Fame.

Some of the other dirt on display includes soil from Ebbets field in Brooklyn where, in 1947, the Dodgers' Jackie Robinson became the first African-American to play in the major leagues.

"We collected dirt from the old Comiskey Park in Chicago when it closed [the park was demolished in 1991] and from the first Nationals game [in April 2005] in Washington, D.C," says Mr. Idelson.

There's even some dirt still stuck on the Hall of Fame's collection of Babe Ruth's spikes.