* Dockers union says strike continues, Rosario hit * But effects muted by slow shipping season * Union demands fuller staffing of work shifts * Argentina a top world supplier of soy, corn By Hugh Bronstein BUENOS AIRES, March 6 (Reuters) - Scores of grains ships sat idle outside Argentina's ports on Tuesday as the country's dockers union and port managers' chamber sparred over a strike that threatened to disrupt key soy exports. Agricultural powerhouse Argentina is prone to labor disputes such as the walkout called by dockworkers on Thursday. The CAPYM port chamber said the strike was suspended the next day, but union chief Omar Suarez said on Tuesday that the work stoppage continued uninterrupted. Roughly 150 ships were forced to drop anchor and wait for the protest to end before they could be moored and loaded, Suarez added, paralyzing operations in Argentina's main grains shipping hub of Rosario. "Not one ship more will enter or leave," Suarez said. "There could be about 150 ships halted." The union chief added, "This is the continuation of the strike that started last week. The strike did not end on Friday. It continues in all ports." The union, known by its Spanish acronym SOMU, demands that work shifts be better staffed. No negotiation sessions were set for Tuesday and without a deal between the dockers and their bosses, Suarez said the work stoppage will go on indefinitely. Argentina is a huge exporter of soy, a key source of protein for an increasingly hungry world, and farm sector revenue is important to the government's fiscal health. So port strikes in the South American country are followed by international grains traders and sovereign bondholders alike. The work stoppage fueled a rise in Chicago Board of Trade soy futures on Tuesday. At 12:30 p.m. CST (1830 GMT), CBOT May soybeans were up 7 cents at $13.32 per bushel. A CAPYM port chamber spokesman declined to say how many ships were being held up by the work stoppage. He insisted that the dockers, despite their own denials, ended the strike on Friday and then restarted it late on Monday. "The dockworkers, who are represented by SOMU, worked the weekend until 5 p.m. on Monday," the spokesman said. Argentina is the world's biggest exporter of soymeal, which is used as animal feed, and soyoil, used for cooking and in the booming biofuels sector. The Latin American country is also the No. 3 supplier of soybeans and a major corn exporter. As global population grows to an estimated 9 billion by 2050, demand for food will nearly double, according to the United Nations. Argentina, with its vast and fertile Pampas, will be important in meeting that demand. But the country is prone to jarring labor protests. Hefty wage demands, traditionally negotiated in March and April, are common as annual inflation is reported by private economists at more than 20 percent per year, one of the world's highest rates. Some unions have close ties to the government of President Cristina Fernandez, who was elected to a second term last year despite regular spats with farmers and orthodox economists who say her state-centric policies scare off investment. A grains export company executive said that this is not a busy time of year for Argentine ports, as corn and soy harvesting are just getting underway. Pressure on shipping has also been lowered due to a smaller-than-expected corn crop, after fields were hit by drought during the dog days of the Southern Hemisphere summer in December and early January. "This is usually a quiet period for the traffic of grains vessels, plus the corn crop was lousy," said the executive, who asked not to be named. Government officials initially estimated a 2011/12 corn crop of up to 30 million tonnes, but have since slashed that projection to 20.5 million to 22 million tonnes. The government forecast soy output at between 43.5 million and 45 million tonnes. "Normally dockworker strikes come later in the year, May or June, in order to give the unions more pull," the executive added. "So the outcome of this one should be gentler for the exporters than last years' wage increases of 35 to 40 percent."