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.
Sadly, mismanaged landfills are probably more common than you think. Even if having diminished yields might not be an issue, microplastics concentrating in edible parts of crops might be.
Edit: I'm not saying microplastics aren't a problem, they're just not likely much of a problem specifically for agricultural yields.