What to do with plains squares

[joncnunn]

I normally farm plains and later post-Biology convert some of them into cottages.

And yes, I'll make an irrigation path to them.

As to the computer's recomendation; it basically recomends all unimproved tiles to all improvements it's currently eligible for.
And if you don't turn "leave old improvements", then it also tends to recomend changing a farm into a cottage and then changing the cottage into a farm, etc.