Given the nature of your data and how many attributes you have, you don't have any choice but to split it up into different tables. From the ArcGIS help:
Most size limits in a database depend on the DBMS edition and hardware
limitations. One exception is the number of fields (columns) supported
in a table or feature class; the maximum number is 500. Be aware,
though, that depending on the data types, the maximum number of
columns in a table could be less than 500. Most database vendors do
not recommend creating tables with over 200 columns of any data type.
You don't specify which DBMS you plan to use, so the capabilities offered might vary between choices. You mention making it Relational, which narrows down the choices a bit (ie an Esri file geodatabase isn't going to work).
Picking a DBMS is one thing, and actually organizing data in that DBMS is another. My comment mentioned Related tables from a file geodatabase perspective - you click on a county, it shows you all the records associated with that county from another table linked by a Relate. I don't know enough about database design to say what would be best (and such questions are generally closed as too broad or opinion-based here). Table per crop, table per year... there's a lot of options, and as my comment mentioned what you plan to access the database with may play a role (ie troubles GIS software has with one-to-many or many-to-many table relationships). My initial thought would be table by year with all crops, knowing that automatically puts you at 200+ fields but would be one-to-one. I can also see table by crop with all years in a single table for each crop, but again depends on how you're accessing it because that would be one-to-many.
Then you get into performance tuning...