A sudden blast of Arctic air shocked late-planted soy fields in parts of Argentina's main grains province Buenos Aires over the weekend, putting pressure on harvest expectations in one of the world's top exporters of beans and soy byproducts.

The government expects a harvest of 51 million to 52 million tonnes of soybeans while consumer nations - in need of food supplies after disappointing grains crops in the United States, Russia and Australia - are counting on South American breadbaskets Brazil and Argentina to provide ample harvests.

Temperatures of 0 to 2 degrees Celsius (32-36 degrees Fahrenheit), unusually low for Argentina's late summer season, hit southern Buenos Aires on Saturday morning. The frosts returned on Sunday, concentrated in the southwestern part of the province.

"Surely it has done damage, especially to the very late planted soybeans that went into the ground in December. But we can't measure the damage for another seven to 10 days, when the damaged plants will turn from green to brown," said Anthony Deane, head of consultancy Weather-Wise Argentina.

Aside from being the No. 1 exporter of soyoil, used in the booming international biofuels sector, and soymeal animal feed, Argentina is the world's No. 3 supplier of soybeans and corn.

Its 2012/13 corn crop is not as likely to be damaged by the cold snap as corn fields are well past the flowering stages during which plants are most vulnerable to frost.

The government expects a 2012/13 corn crop of 27 million tonnes. More than 12 percent of the country's corn harvest has been collected, and soy harvesting is just getting started.

"This drop in temperatures could strengthen the probability that Argentina's soybean harvest will come in at the low end of expectations," said David Hughes, who manages soy, corn and wheat farms in Buenos Aires.

The Buenos Aires Grains Exchange expects Argentina to harvest 48.5 million tonnes of soybeans this season and 25 million tonnes of corn. Both estimates are down from earlier projections due to the difficult and widely oscillating weather.

The Pampas grains belt started the 2012/13 season with hard rains that flooded wide swathes of farmland. The same areas were parched by excessively dry, hot weather later in the season.

Argentina's record large soy harvest was 52.7 tonnes in the 2009/10 crop year, and a record high 23.8 million tonnes of corn were produced in the 2010/11 season.

(Reporting By Hugh Bronstein; editing by Jim Marshall)