OK, let's examine the Plant Growth thing.
Usig the Jamis Buck town generator, based off the DMG info, I generated five large towns of populations 3,200-4,800. They averaged 1 cleric of 5th level or above, and 1 druid of 5th level or above each. Never more than one of each. That means that one town can manage one caster who can do Plant Growth (the Druid) and a small chance that the cleric can (I'd guess fewer than 20% of all celrics woudl have the Plant domain - probably less). The town only needs about 25 square miles of cropland to support itself, so that Druid can handle the job easily by himself. Assuming no evil clerics are countering it, and that druids are welcome in that area.
How about smaller towns, though? I generated 5 villages. 1 Druid (total) and 2 clerics (total). It starts getting dicey there, but the druids (if they are so inclined) can probably keep up the Plant Growth thing as long as they want to.
However - will they? Doing this will actually depress the value of the land - as there will be more crops sold to the same number of townspeople, and transportation is still an issue (outside of caravans equipped with Wall of Ice-powered refrigerator wagons, etc). Prices will drop. Standards of living will get worse. Famine may be avoided, but I can see the clerics and druids being very judicious on the use of this spell to regulate the agricultural output (divinations to determine the likelihood of drought, and spells only being cast to counter this).
Net effect: a 10-20% increase in yield seems plausible, but that's not going to make a massive impact on the visible game world from the PC/Player POV. Their meal costs them 5 sp instead of 6 sp per day, let's say.