I find the ports first and then farms helps more ... but trade is more my gameplay angle. Farms are good for your 'base' income and don't dissappear at times of war as ports being blockaded can cause seriuos cashflow problems. But I find I need a large trade income to upgrade my farming output.

I think all of my regions have upto crop rotation (normally 2400 florins) but the expense of building irrigation (normally 4800 florins) sometimes is not quite benificial for the outlay cost.

Obviously trade income is limited to landlocked countries.