1 farm engineer still allows you to build farms at half speed. 3 farms in a triangle will still produce 1.3 food/tick, which is more than the 1.2 used by a level 5 hq. You won't make much profit, but you can stop high food prices driving you into debt.

I doubt you're consistently in situations where food is the only viable moneymaker. You have full speed mines, quarries, pumps, reactors, windmills, and glass. At the very least, the large amount of farms your rivals use will push up the price of water, which you can easily acquire.