You will need two tables to do this.
First table:
FarmID, PrimaryCropGuid, other fields.
FarmID is the primary key.
Second table:
FarmID, SecondargCropGuid.
FarmID is the foreign key pointing to the FarmID in first table.
Now if you want all secondary crops of a given farm then you can do:
SELECT SecondargCropGuid from SecondTable WHERE FarmID='GivenID';
You can also find all farms which grow secondary crops with:
SELECT FarmID from SecondTable WHERE SecondaryCropGuid='GivenGuiD';
-Saurabh