U.S. farmers will plant more soybeans, sorghum and other grains than last spring, and less corn, a more expensive crop to grow, as they face their tightest finances in a decade, analysts said before the government's closely watched plantings report on Tuesday.

"When push comes to shove it's really going to be that corn acreage number that's going to be the main focus," said Jefferies Bache grains analyst Shawn McCambridge.

The U.S. Department of Agriculture's prospective plantings report, released at noon EDT (1600 GMT) on March 31, will be USDA's first official 2015 plantings estimate for the world's largest grain exporter.

The report is based on surveys of 84,000 farmers taken Feb. 27-March 18.

"If the corn acres are bigger than we think that would be a surprise to the trade," said Don Roose, president of U.S. Commodities.

Twenty-nine analysts polled by Reuters, on average, expect farmers to plant 88.7 million corn acres, or 1.9 million fewer than 2014, and 85.9 million soybean acres, or 2.2 million more.

That compares with USDA analysts' 2015 outlook last month that corn acres would fall by 1.6 million and soybeans would be down 200,000 acres.

USDA also projected total plantings for the nation's eight major field crops - corn, soybeans, wheat, rice, cotton, sorghum, barley, oats - to be down 3.3 million acres from 2014.

That plus 1.3 million acres coming out of the government's conservation reserve program would leave some 4.7 million acres of potential cropland unplanted this spring. USDA said farmers will plant fewer acres due to falling crop prices.

"We don't think in these modern times that producers are going to walk away from acres compared to last year," Terry Roggensack, an analyst at The Hightower Report, told a Chicago weather forum last week.

Roggensack acknowledged projected returns aren't good. The average cost to produce corn is near $4.15 a bushel, and Chicago Board of Trade December corn futures are trading at that level.

Also on Tuesday, USDA will issue its quarterly grains stocks data for the country's stored grain as of March 1, a key gauge of December-February consumption.

Analysts expect corn and soybean stocks to be the largest in at least three years given bumper harvests in 2013 and 2014. Wheat stocks should also be bigger amid soft export demand.

"These stocks reports in March can be very volatile on prices," Roggensack said.