By Michael Hirtzer CHICAGO, April 5 (Reuters) - A cool spring in the U.S. Midwest has farmers antsy for soils to warm up before rolling their big corn planters across fields to seed what is expected to be the largest area of the country's biggest crop since 1936. This week marks the first official days farmers can begin planting corn in many spots across the upper Midwest, according to crop insurance policies that cover costs if they have to replant in the event of a flood or killing frost. Farmers are hoping for a better season than 2012 when their yields were the smallest in 17 years due to the worst drought since the Dust Bowl years. "We're getting anxious. Things aren't quite where they need to be soil-temperature-wise, so we are trying to be as patient as we can," said Cory Ritter, who farms 2,000 acres with his father in central Illinois, not far from the Decatur headquarters of agribusiness giant Archer Daniels Midland Co . Soil temperatures were in the 40s degrees Fahrenheit (4 degrees Celsius) in Illinois this week, below the 50 degrees necessary for the seeds to germinate. March was one of the coldest on record for the No. 2 grain state of Illinois, a big contrast from a year ago when highs were in the 80s as far north as Chicago and Minneapolis, promoting a record-fast start to the U.S. planting season. "We've been ready to go since March 1. We've had the planter and field cultivator ready," said Maria Cox, who farms 3,000 acres with her father, Ethan, in White Hall, in the southwest part of the state near the Missouri border. The Cox farm last year planted corn the earliest in their farm's history, sowing acres in mid-March. The early-planted corn pollinated before the blistering drought conditions took effect in late-June and yielded 50 bushels per acre more at harvest than corn planted in April, she said. This season's cool spring means a more traditional start to the planting season and likely a later harvest, preventing farmers from cashing in on late-summer corn premiums. Those premiums are likely to be hefty amid the tightest corn supplies in nine years. "We are not going to be harvesting in the middle of August. We sold some grain for early-September delivery and if we can't deliver, we will have to roll the contract until the next month," Cox said. WET WEATHER NEXT WEEK TO STALL PROGRESS A few planters were starting to roll on Friday in southern Illinois and Missouri but rains are forecast across the Corn Belt from Sunday through Thursday, likely keeping farmers out of the fields. Soil temperatures remain below 50 degrees F across Iowa, keeping farmers side lined. Illinois and top grower Iowa are forecast to seed nearly 30 percent of the 97.3 million corn acres this year, according to U.S. Department of Agriculture. Texas and Louisiana farmers have already been planting corn over the past few weeks. USDA reported on Monday that Texas had seeded 54 percent of their corn crop and Louisiana farmers were nearly done, with 95 percent in the ground as of Sunday. Both are ahead of schedule due to dry weather, crop specialists said. The mid-South is struggling to get their crop in. "The weather is supposed to warm up but also get extremely wet. That may sideline us ahead of getting a lot of mid-April planting done," said Mike Zuzolo, president of Global Commodity Analytics, a farm advisor in Lafayette, Indiana. "Clients in Tennessee and in the Missouri 'boot heel' that I talk to are already starting to talk about pushing their corn acres into beans, especially if they get 3 inches of rain," Zuzolo said. "It will be hard for their low-level ground to dry out in time to get the corn in on time, and not miss the super-hot pollination window that they go through down there." The good news is all the rains are replenishing soil moisture after last year's historic drought. "I don't think very many people are nervous yet. The best days to plant are later than this anyway," said Emerson Nafziger, an extension agronomist at the University of Illinois.