I haven't played in awhile, but one of these updates I'll start a new game again. A few hints:
Rotate your fields, so you're harvesting and planting 1/3 of them each day, and not all of them in one day. This will save a lot of time and energy for other uses, and get you enough crops to cook for your monsters.
The cells are fertilizer, and you need them for higher quality crops. You only need to add it on the day before a crop grows to harvest, then it's good for 7 days, so it'll still be there next harvest. All fertilizer works the same, so you can buy the cheapest, usually Wild Cell. Wild Cell also shows up the best in any lighting conditions, any season. Milk your unassigned monsters daily for essence, and try to breed milk-specialized monsters with Beast of Burden (Holstaur Bloodline trait), Loads Cum (Common trait), and Quickshot (Common trait). Equip a Codpiece to a monster with those traits and it'll yield 12 cell per day.
For 5-star crops, you need water, fertilizer, and one or two field hands with a combined Physique of 100 and combined Swiftness of 100. You only need to assign them right before harvesting (assign them at the scarecrow at each field,) and if you have one monster with 100 in both stats, you could just place it before harvest, remove it after and place it at the next field you'll harvest. This may be an exploit to be closed later, though.
You can expand your farm through the slow opening door to the left of the bank, only after get the crystal from finishing the first quest. The door to the left of that is where you buy buildings, including special buildings.
Take every opportunity to dungeon, and get your stamina up to 200 minimum. 200 of every stat is all you need for the deepest dungeon levels atm, and all you need for farming. More energy is always better. Don't worry about becoming a badass adventurer because your companions will always be more effective fighters than you, and your monsters potentially far more effective.
Training monsters to max level is incredibly profitable, and all investment in it will persist and pay off more and more. Starting from Spring, plant a field of 8 Speedwheel/8 Shrubstick each day to cook Speedwheels for them, in Summer make Greasy Boys, Rouladen in Autumn. The recipes for Winter suck, but hopefully you can have 8 trainers at max Physique by then.