You have three fields you can plant your crops and trees on. Each one has a different degree of fertility. The fields affect what types of crops can grow in each one. The trees can be planted in any one of the three fields. To help your crops grow a little faster, you could drop a bag of fertilizer on the tilled soil before you plant the plant seed.
The small field closest to your house is the Least Fertile of your fields. The crops you can plant here are the first generation group (tomatos, strawberries, etc.). If you plant a 2nd or a 3rd generation crop in your Least Fertile field, there's a very good chance that it will die before becoming mature.
The middle field next to your watering well is the Somewhat Fertile field. You can plant a 1st generation or 2nd generation crop here, but if you plant a 3rd generation crop it will end up dying. Since the watering well is right next to the planting area, you don't have to walk as far to refill your watering can.
The last field furthest away from your house is the Most Fertile field. You can plant any type of crop here. This field is the only one you can plant 3rd generation crops on, unless you use the Gemsoil enhancement with Tartan.