It kind of is an unbelievable concentration except in certain exceptional circumstances. A 0.1% concentration by weight for just the first 30 cm of topsoil would require about 5 tons of microplastics per hectare (or about 2 tons per acre). That's a lot of microplastics. It would be the equivalent of about 52 plastic water bottles per square meter. Unless the farm is on an extremely mismanaged landfill, I don't think that kind of concentration is likely.

Edit: I'm not saying microplastics aren't a problem, they're just not likely much of a problem specifically for agricultural yields.