OK, the average commodity gained 0.1% on Tuesday, according to the CRB raw materials index.

But that apparent lack of volatility concealed all manner of movement in agricultural commodities, one of which, cotton, chalked up a seven-month high in New York, where another, coffee, slumped 5%.

In Chicago, soybeans gained nearly 2%, while wheat lost early gains to end more than 1% down.

As Mike Mawdsley at broker Market 1 warned earlier, it looked indeed "time to get the Tums out again", referring to a US brand of heartburn tablets.

'Fund-led sell off'

Much of the movement reflected, at least in part, South American weather.

Rains in the centre of Brazil were viewed as helpful to 2013 sugar cane and coffee crops, fuelling a 1.4% drop to 18.12 cents a pound in prices of New York raw sugar for March, the weakest finish for a spot contract since August 2010.

Arabica coffee for March tumbled 4.9% to 148.60 cents a pound.

"In one hour, we fell 6 cents a pound from the high on pretty good volumes as a fund-led sell off stopped out the smaller speculators, almost into a void of buyers," Sucden Financial said.

Why the liquidation?

"Various thoughts began to circulate why we sold off so quickly. Index funds selling, but not sure why?

"Funds that had reduced their exposure putting it back on and even origin wanting to add again?" the broker said.

"Honestly it could be a mix of all three."

Macquarie flagged the role of producer selling, with the high prices tempting Brazil's growers to sell some of their hefty stocks left over from a strong 2012 harvest, and with potentially more on the way if prices rally again.

'Caught the market short'

And South American weather, as in the undue dryness south of central Brazil, was cited repeatedly as a reason for the strength in soybeans and, to a lesser extent, corn.

"The weather models for Argentina and southern Brazil have remained primarily warm and dry for the week, after two weeks of much warmer and drier than desired," Darrell Holaday at Country Futures said.

"This has caught the market a little short."

'Adding risk premium'

Benson Quinn Commodities said: "Argentina and southern Brazil will see only light showers over the next seven days with top soil moistures in decline.

"This drier pattern appears to be in place into early February raising some concerns for Argentine corn that is pollinating and Brazilian beans that are blooming and pod filling stages."

And US Commodities said that "the weather in southern Brazil and Argentina is expected to be warm and dry for the next 10 days. Thus the market is adding risk premium".

Richard Feltes at RJ O'Brien reminded investors to "be mindful that Argentine crop water use rates are increasing as crop development advances", exacerbating the impact of dryness.

Chinese buyers on the prowl

Indeed, there is plenty of talk that China, the top soybean importer, is already giving up on hopes of imminent South American exports reducing its dependence on high-priced US supplies.

"Rumours have China trying to source quick shipments of soybeans from the US gulf and the Pacific North West," US Commodities said.

Country Futures' Mr Holaday said: "The rumour is that Chinese inquiry for immediate shipment [soybeans] has picked up substantially."

That said, the only sales confirmed to China by the US Department of Agriculture were of 120,000 tonnes for delivery next season, and of "optional origin", meaning the order may not go to the US.

Technical pointers

Still, Chicago soybeans for March closed up 1.6% at $14.51 ¾ a bushel, their highest close for a month but just short of their first finish above their 75-day moving average for the first time since September.

Chart patterns in general "continue to turn positive and are nearing breakout levels", US Commodities said.

Indeed, corn, as another crop affected by South American weather, might have done better, and closed above its own 75-day moving average, were it not for the retreat in fellow grain wheat.

Chicago's March corn contract ended up 0.1% at $7.28 ½ a bushel, coming in short (as the contract has shown a recent habit of doing) of its 75-day line at a little over $7.32 a bushel.

'Increased chances of rainfall'

Chicago wheat for March dropped in a decline blamed on profit-taking, after rises in five of the previous six sessions, and a gain in early deals too to a one-month high of $7.99 ¾ a bushel.

That the contract failed to crack the $8-a-bushel mark was attributed as one reason behind the tumble which followed, with the prospect of moisture for the drought-hit US plains another.

"The GFS weather model is now pointing to increased chances of rainfall in the US hard red winter wheat area, primarily Oklahoma and Kansas, next week and that has lead to some selling in wheat and corn," Mr Holaday said.

There was some positive news from the former Soviet Union too, with Kazakhstan talking up prospects for its, spring sown, wheat after sizeable winter precipitation so far to water seedbeds.

Prices drop

Still, a refusal by Russia to lower its import tariff on milling wheat, despite bumper exports early in 2012-13 raising the prospect of buy-ins, was viewed as a bearish signal.

While US weekly wheat export data, as measured by cargo inspections, weren't bad for wheat at 21.9m bushels, (and improving for corn too, at 11.0m bushels, and strong for soybeans, at 48.1m bushels) Chicago's March wheat lot fell 1.6% to $7.79 ¼ a bushel.

The weakness hurt some European prices too, with London wheat for May closing down 0.8% at £216.00 a tonne.