Frosts that started over the weekend in parts of Argentina's Pampas farm belt may threaten late-planted soy if the unusually cold weather persists, climatologists said on Tuesday.

Farmers in grains powerhouse Argentina are halfway through harvesting 2011/12 soy, but some late-planted fields could still be hurt by early morning frosts over the days ahead. More beans were seeded late this year in order to avoid an earlier drought.

"A cold front moved in over the weekend, which caused strong frosts. The lowest temperatures came this morning, below zero in some parts of south central Buenos Aires province," said Ezequiel Marcuzzi, a climatologist for Buenos Aires-based consultancy Climacampo.

Buenos Aires is the biggest agricultural-producing province in Argentina, the world's top exporter of soymeal, used as animal feed, and soyoil, used for cooking and in the booming biofuels sector. It is also the No. 3 exporter of soybeans.

"Tomorrow should be more mild," Marcuzzi said. "But on Thursday and Friday we expect serious frosts in southern Buenos Aires. This is not good for the plants that are still developing and that are not ready to be harvested yet."

Argentina has been hit by uncommonly cold weather as the Southern Hemisphere autumn begins.

Buenos Aires-based agricultural economist Manuel Alvarado Ledesma said recent cold temperatures in northern Pampas provinces Cordoba, Santa Fe and Entre Rios did minimal damage thanks to the fact that most soy was already mature.

"The situation is different in the southern Pampas, where further frosts could limit crop development," he said, echoing Marcuzzi. "We'll have to wait and see what the weather does."

The government last week reduced its soy and corn crop estimates, citing the effects of a six-week drought that hit the Pampas in December and January. The Agriculture Ministry now expects 42.9 million tonnes of soy in the 2011/12 season and 20.3 million tonnes of corn.

Argentina needs all the farm revenue it can get as its economy slows due to global sluggishness, fallout from Europe's debt crisis and government-imposed currency and import controls that have hurt confidence.

Orthodox economists say foreign investment in Argentina has suffered due to state-centric policies, long criticized by the farm sector, such as the government's recent move to expropriate a controlling stake in the country's No. 1 oil company, YPF .

The move by President Cristina Fernandez to take control of YPF back from its current majority shareholder, Spain's Repsol , has met with a frosty response from Europe.

Some climatologists meanwhile say the weather is likely to moderate throughout the Pampas over the days ahead, sparring most late-planted soy.

"Parts of south central Buenos Aires have soy fields that are still in the pod-filling stage. If that area is hit by a serious cold front, it would be a problem for late-planted soy. But we're not forecasting that," said Liliana Nunez, chief forecaster at the National Meteorological Service.

"The worst of this cold snap is over," she added. "Early planted soy is fully developed by now and not at risk at all."

Grains exporters with operations in Argentina include Cargill Inc, Bunge Ltd, Molinos Rio de la Plata , Noble Group Ltd and Louis Dreyfus.