Brokerage execs can't say enough lately about commodities trading. On the various firms' third-quarter earnings conference calls over the past two months, executives heralded commodities trading as the one business they have that's totally untainted by the credit crunch -- and promising from a growth point of view. Outifts such as JPMorgan ( JPM), Lehman Brothers ( LEH), Merrill Lynch ( MER) and Morgan Stanley ( MS) -- which have otherwise been reeling from turmoil in the credit markets tied to the collapse of subprime mortgage securities -- have added substantially to their trading desks this year. But with signs of waning economic growth both in the U.S. and Europe, there's the danger that Wall Street is building up this business just as the commodities boom may be hitting a plateau. Commodities markets are small, and prices of energy, metals and agricultural products such as wheat have already come quite far over the past several years. The Goldman Sachs commodities research team made this very point Tuesday, when it warned in a note to clients that it is "time to take profits." The team said it is closing its long oil and related agriculture and gold recommendations. As if on cue, oil fell nearly 4% from its record high Tuesday o close at $89.90 per barrel. Stocks in that sector such as Exxon ( XOM), Chevron ( CVX) and Schlumberger ( SLB) likewise fell.