I have not used cornstalks but I have used lots of other dry stalks it's all carbon at the end of the day.

Been using some heat treated pallets for some burns too. I put some neodymium magnets in a plastic milk carton and fish out the nails with that once it's all been quenched and cooled.

Apparently you can make the biochar more paramagnetic with ferrous sulphate. Hope to find out one day wether just burning pallets with nails achieves something similar or not.