Kevin Plank, the former U Maryland football player who founded Under Armour, has not been all that showy with his money.

In 2009, Plank gave himself an honorable pay cut, dropping his salary from $500,000 to $26,000. (Of course, with 22% ownership of the company’s stock, his corporate salary hardly matters; his equity is more than $900 million.) Yet in recent years, Plank has plunged his hands into a new and costly hobby; he’s trying to bring horseracing back to the state of Maryland.

It isn’t the kind of avocation one might expect from a football player. Horse-ownership is historically a wealthy, white-gloved pursuit. Then again, Plank is certainly one of the wealthy now, and everything about 530-acre Sagamore Farm, which he purchased in 2007, reflects that lifestyle. The Glyndon, Md. land (a 30-minute drive from Baltimore) was previously owned by the Vanderbilt family. Plank has 16 full-time employees tending to the farm and its 45 horses, almost half of which were born there. The property is absurdly bucolic; overlooking the stables is a hill that provided the setting for a scene in The Runaway Bride. Atop the hill sits an idyllic guesthouse, managed by friendly guest services coordinator Randy Lewis, who recalls that originally, Plank didn’t buy the guesthouse. “He’s very competitive,” says Lewis, “so [eventually] he said ‘I’m not going to let somebody live up on that hill and look down on my farm,’ and purchased the hill and guesthouse so he could own it all.” Inside the guesthouse, the only clue to Plank’s ownership is a framed photograph of him at the Secretariat film premiere with the horse’s original owner.

But Plank’s credentials are increasingly legit. One of his horses, Shared Account, won the 2010 Breeders’ Cup at Churchill Downs, a race that came with a $1.2 million winner’s purse. The filly was a 46-1 shot. Nearly all of the horses raised here have Under Armour slogans for names, like Humble and Hungry or Charged Cotton. Plank also named one of them Hamp after his friend Bill Hampton, who died this past July on the way home from the ESPY awards.

Quirkiest of all, though, and the best indicator that this farm’s proprietor is not your typical horse person, is the quarter-mile training track Plank had built in 2009. It’s made of soil, clay, and shredded bits of fabric from Under Armour t-shirts. Lewis and the other Sagamore staff have a slick answer about how the fabric soaks up moisture and makes for an efficient base, but at the end of the day, as Baltimore PR guy John Maroon believes, “He just did it because it’s so cool.” Lewis applies that notion a bit more seriously: “Kevin’s unspoken mission is to win the Triple Crown,” he says. If that does happen, Lewis reasons, Plank wouldn’t mind being able to say that his horses were trained on Under Armour shirts.

As he does with everything he touches, Plank has gone big with his rich-person hobby. Every year, Plank turns the days leading up to the Preakness into an “Under Armour hospitality weekend,” inviting friends of the company like athletes or coaches to the farm for an extravaganza the night before the race and a brunch the next morning. Recently, the New York Times called Plank, chef Bobby Flay, and Vitamin Water founder Mike Repole the “new faces of horse racing.” Plank’s lofty goal: to make the sport huge again in Maryland. Presumably any day now, Under Armour may end up getting into horse apparel.

Back to: Under Armour gets serious

This article is from the November 7, 2011 issue of Fortune.