Question

if agriculture and commerce are state subjects why do you have a minister for agriculture and commerce in the union goverment

Answer 1

Although agriculture and commerce are the state subjects we have ministers of agriculture and commerce at the union because of the wider prospect of these subjects regarding inter-state trade. So a uniform policy has to be legislated.
Secondly, the agricultural and economic experts are part of the Planning Commission . So we require agricultural and commerce ministers in the Union for
“policy making” .