FARMING UP LINCOLN, Neb. (UPI) -Singer Willie Nelson finally . signed a contract with the University of Nebraska to hold the FarmAid III concert Sept. 19 at Memorial Stadium. Yesterday's contract signing ended weeks of troublesome negotiations during which FarmAid organizers, citing technical difficulties, announced they were dropping Elans to have the concert in incoln. Nelson assured Nebraska football coach Tom Osborne, that measures would be taken to protect the stadium's artificial turf. Nelson, with previous FarmAid concerts in Illinois and Texas under his belt, said he wanted FarmAid III in Lincoln because "it's right in the heart of the problem. "No matter how much we raise it's not going to be enough," he said. Nelson said the agricultural crisis is causing several hundred farmers daily to abandon their land. "FarmAid will help draw attention to the problem. The money is not the important thing. The important thing is the issue itself," Nelson said. Of the 70,000 tickets that will go on sale Saturday, the university will get 40,000, with the rest being sold through FarmAid outlets. The price is $20 per ticket.