When you already have better equipment, you can think of buying a new field. Think about areas no. 25 and 30. When you have enough money, go onto the field you choose and find the icon in the middle of it, which will allow you to buy it.
When having four pieces of land, you can encounter a problem - not being able to harvest all the fields in time (the sowing process will be faster than harvesting, despite having a better harvester). Remember about slowing the time in the game, to make it with harvesting on each field (you can also solve the problem by switching off plant withering). Continue to grow plants and earn money for buying another field. By doing so, you will have 5 pieces of land - 4 near the farm and one, no. 38, a bit farther.
At this stage of the game, your farm will be developed to some level. Your next step should be purchasing a new harvester (the costs will be high, but it is necessary). This transaction will allow you to work full time on all the five that you have and also to buy a sixth one. Two efficient sowing machines and harvesters will be enough to deal with such area, so make sure to work on every field (interchangeably), which will bring large crops and god profit. Consider buying another tipper.
Six pieces of land, two efficient harvesters and sowing machines and two large trailers - at this stage, you can truly say that you are an experienced farmer and you've obtained some nice equipment. The one think that will bother you, are the not so strong tractors of yours, so you should think about changing them next. Also, consider whether you want to continue growing plants or start a new business, like for example animal husbandry. There are quite a lot of possibilities, but don't change everything at once - purchase new machines slowly, so the work pace of for example your sowing machine wouldn't be much higher than of your harvesters. Watch out with buying new fields also - do not overinvest. Before finishing a transaction, make sure that you have enough machines to work on them. One set (a cultivator, a sowing machine, a tipper and a harvester) will be enough to work on three fields and more efficient equipment will manage on even larger area. Grow plants on another fields and sell the crop when the time is right, and you will quickly earn enough money to start new businesses.