Wash the horseradish root and peel off the outer skin using a vegetable peeler. Slice the root into thin pieces, about 1/4 inch thick.

Dehydrate the horseradish until completely dry and brittle. This takes 8-12 hours in a dehydrator, or 12-24 hours in the oven on the lowest setting with the door slightly ajar.

Grind the dried horseradish pieces into a fine powder using a spice grinder, blender, or mortar and pestle. Grind in small batches for best results.

Store the finished powder in an airtight container out of direct sunlight. It will keep for about 1 year.