This might help:

https://forums.totalwar.org/vb/showthread.php?t=117672

To grow your population and 'pop' the towns, it's as important to research agricultural techs and develop your farms as it is to run with low taxes.

Focusing on agricultural research in the early game is vital for states without much opportunity to trade (Prussia, Austria, Poland, Russia). It's more important for them than others simply because they don't really have an option of alternative income, but even GB, France, Spain & Marathas benefit from such a focus as farming is the only (non-trade) instantly available source of income to them too.

Personally, I only focus on Iron & textile research once I've done 75-80% of farming techs or when a significant number of towns have appeared that can be turned into industrial sites.