The little group I've joined up with needed a farmer so I've taken it on as a primary role. Observations:
A. Plowing (with a shovel, not a plow) seems to be the most efficient way to level up the skill. I suppose if you have lots of apples to harvest, that may be better, but our apple trees are taking forever to mature and yield apples.
B. Harvesting wild plant foods (with primitive sickle, we cannot make steel yet to make the sickle): just anecdotal but . . . potatoes seem to yield from sloped dirt piles more often. In the span of 40 tiles I harvested 6 potatos, 5 of which were from a sloped dirt pile at the base of our town palisade. Grapes MIGHT come from sandy soil more? Carrots no observed pattern. These are very small sample sizes so who knows; might be nonsense I'm spouting.
C. Not sure what the server's plant setting is on ours, but I doubt it is set to a long grow time. That said, planting on fertile soil seems to a long time for stuff to grow.
D. It seems to be possible to "flatten ground" both after it has been plowed, and after it has been planted. I don't know if this will tend to have a bad effect on it.
Few issues I'm not seeing discussed, and figure we can wonder outloud together:
1. Fertile soil vs. forest soil. Presumably fertile soil is better for crops?
2. Fertile soil quality: having prepared the area for our fields by getting it all to the same elevation and flattening it (maybe a mistake not sure) I've yielded a wide range of fertile soil quality. I've got about 2 cm worth of 100Q, 6cm of ~85, 10cm of ~70 and so on like that down to ~60 of ~15Q soil. One would think that given quality effects are important in most other aspects of this game, that using the highest quality fertile soil would be beneficial (as well as fertilizing it).
3. Flattening: depending on what this actually means, it might be very bad for crops, or it might not be so bad. Also might be okay for some, and not okay for others. Wheat for example, I believe does best on nice level soil. Potatos on the other hand, I have always seen planted in little humps.
4. Slope and water: One would think that proximity to water would impact the growth of crops. Not sure. I'm tempted to pour a strip of moderately high quality soil along a lake, and plow it and plant it and see how it does. As far as slope, IRL some crops like to be more moist than others, one would think that being upslope or down slope relative to the surrounding land might have some effects.
I'm impressed if the devs have actually considered half of this stuff, and if it is implemented or halfway implemented than I'm really impressed