Temperatures two to four degrees below zero Celsius (4 to 7 degrees below freezing Fahrenheit) chilled the crop in the southwest of Cordoba and the west of Buenos Aires, National Meteorological Service forecaster Miguel Frutos told Reuters.

"In those areas, the freeze may have a light effect on very late-developing soybeans, which are the minority," said Hugo Visca of Enrique Zeni y Cia.

"But in the majority of spots, there is enough humidity and wind to mitigate the impact," he added. The south of Santa Fe was not touched by frost, but from the province's Rosario city on down through Buenos Aires the night air was extraordinarily brisk.

Late soybeans laid down in the crop's second planting make up roughly 25 percent of the 7.68 million hectares sown thisseason. The still-flowering plants are vulnerable to frost, particularly when they are dry as the cold rapidly permeates the cells holding their oil. "The upcoming forecast frosts could be more damaging than those from last night because they will hit a crop dried by wind that is not expected to get any rain," said agronomist Pablo Vernengo of the Argentine Agronomy Engineers Center.

Wet plants are more able to fend off temperatures less than six degrees Celsius (43 F) because they expell energy to freeze the water already on them, thus maintaining their temperature and "easing the frost impact," said Vernengo.

A dry crop appears to be on the horizon as Frutos said the weekend forecast does not include rain.