Since quality fertilizer requires fish to craft, and deluxe speed-gro requires coral, both of which are slow to acquire, it seems to me that an effective way to use them would be in the greenhouse with a recurring crop so that it provides a permanent boost. What is the most profitable recurring crop I could plant? Which fertilizer provides a bigger profit boost?
Ancient Fruit grows in Spring, Summer, and Fall and each fruit sells for 750g each. It regrows new fruit every 8 days. However, getting your hands on Ancient Fruit will be a bit difficult. You'll need to find it in the Mines or buy some seeds from the traveling gypsy. You can also randomly acquire Ancient Seeds by using the Seed Maker. Once you find an Ancient Seed in the Mines, you have to donate the artifact to the museum to get a plantable seed.
However, if you're looking for something a bit more feasible, the berry crops are usually the most profitable recurring crops in each season (Spring's strawberry, Summer's blueberry, and Fall's cranberry).
After some number-crunching, here some hard data. Hot Peppers and Eggplants are ready for harvest the earliest. This data also assumes that you are purchasing Ancient Seeds for 900g per seed.
- On Day 5, Eggplant is the only one showing a profit of 40g per crop. Hot Pepper breaks even while the other crops are still growing.
- By Day 7, Cranberries have ripened and shoot to the top of the leaderboard with 85g per crop. Even though they cost a whopping 240g per seed, each crop can produce 2 or 3 berries that sell at 130g each.
- Day 10, Eggplants have their second harvest and are leading at 100g profit per crop.
- As soon as Cranberries come into their second harvest, they are by far the most profitable crop available. Blueberries and Strawberries fight for a distant second place.
- By the time a full season (28 days) passes, Cranberries are still the most profitable, although Blueberries have narrowed the gap. Ancient Fruit has its first harvest, although it's still operating a loss considering its seed price (it is, however, third place if you didn't purchase the seeds).
- If you got your Ancient Seeds for free (i.e. from the Seed Maker or through crafting the artifact), they overtake Cranberries by Day 40 in terms of profitability (though they fall back to second when Cranberries ripen again before Day 56). If not, Cranberries are still king, though only ~300g more profitable than Blueberries at this point. Ancient Fruit is still third place.
Thus, I can conclude that Cranberries are, numbers-wise, the most profitable recurring crop until two seasons (56 days) have passed.