Corn question

I've got a question, I've been having a bit of a problem getting my corn to actually create full cobs, last year the yield was low, this year the yield was non-existant, and I hand fertilized. The thing is, as I pulled the corn out this week I found what looks like white sand in and around the roots, it has the rough texture of ground up cement. Anyone out there run into this before? I'm thinking that the corn issue has more to do with the depth of the raised bed, at 8 inches I'm pretty sure that the roots aren't getting down far enough, even though there is ground and not cement or plastic below it.
Thanks Philip
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