Investment legend Warren Buffett made the concept of “buy and hold” seem as reliable as gravity.

But now he’s having difficulty with the “buy” element of that equation, thanks to a drop in cash holdings at his Berkshire Hathaway.

The firm’s cash portfolio dipped below $20 billion in April from $47.1 billion in September 2007.

The drawdown came after Berkshire registered its biggest loss in at least two decades and Buffett spent billions on corporate debt and preferred stock in the likes of Goldman Sachs and General Electric.

Buffett says he wants to keep at least $10 billion of cash on hand to deal with any emergencies.

“He’s tapped out,” Jeff Matthews, author of “Pilgrimage to Warren Buffett’s Omaha” and founder of hedge fund Ram Partners, tells Bloomberg.

“He had to sell some of his stocks to buy stuff last fall. That’s why he’s not been making big stock purchases.”

Berkshire spent $624 million on stock in the first quarter, the smallest amount since at least 2005, according to Berkshire’s regulatory filings, Bloomberg reports.

To be sure, Buffett isn’t doing so badly with many of the investments he has made, locking in returns of 10 percent or more in preferred shares of Goldman, GE, and Swiss Reinsurance.

Still, some experts say the “buy and hold” concept is dead in this environment.

“There’s benefit now to being more active in your management style.” Matt Havens, a partner with Global Vision Advisors, tells CNBC.com.