Two percentage points of water can erase most of a physical trade's margin. Most trading desks still treat it as a footnote on an inspection certificate.

Picture a Panamax bulk carrier, 50,000 metric tons of soybeans in her holds, three weeks out from the discharge port. On the trader's screen, the deal is done. Purchase priced, sale priced, freight fixed, hedges placed. The book shows roughly $30 a ton of margin, or about $1.5 million on the cargo.