Iam a third year bsc physics student.Can I do my PG in agricultural physics??

Yes you cam do a PG in agricultural physics.
It'll be called a MSC in agriculture
Looking a t your bachelor's degree in physics it'll be an easy passage for you.
I would recommend you to read
Agricultural Physics: The Commonwealth International Library: Physics Division
By c w rose
It is a good book, it'll help you to understand the base and then you can make an informed decision.
All the best
Yes, you cab defenetly do M.Sc. in agriculture physics
Its a 2 Years duration course.
Level:Post Graduation
Type:Degree
Eligibility:Graduation
I hope this helps.