It isn't difficult to grow pumpkins in suitable soil. However, there are a few tricks to being extremely successful and for having a nice crop of large, flavorful pumpkins.
Planting
Planting seedlings can give a jump-start on the amount of available growing season. However, the seeds germinate readily, so it isn't always necessary to purchase seedlings that have been already started. It isn't difficult to do this, and in many locations, it isn't necessary since the seeds will grow well in the ground, if the danger of frost is past.
Fertilizer
Broken down manure is a rich source of nitrogen, potassium, magnesium, and other elements. All of these encourage the pumpkins to grow healthy and strong, while promoting large fruits. A fourth cup of Epsom salts per plant can also be sprinkled around the young plants. Epsom salts are a magnesium compound, so this helps the young plants to become established and for the roots to grow large and deep.
Periodically mulching around the plants also helps, especially if you use seasoned compost for the mulch. This allows the plant to gain nutrients throughout the growing season. Used coffee grounds can be sprinkled around the plant without even composting. This will give the pumpkin plant extra nitrogen in a form it can use.
Location
Whether you plant seeds or seedlings, location is important. Pumpkins are sun-loving plants. They need to be planted in a sunny location where they can get many hours of sunlight a day, to do their best. In hotter areas, morning sun is better than evening sun; otherwise, the plant may be stressed during the heat of the day, every day. Droopy leaves or those that appear to wilt are a sign of stress, and while watering around the base of the plant at this time may help a lot, much of the plant's moisture is being evaporated instead of being used for setting and growing fruit.
The pumpkin plant also needs plenty of room to spread out. It isn't uncommon for one plant to cover an area of 12 square feet (4 square meters). If it is confined, the fruits will tend to be smaller.
Soil
Pumpkins will grow in nearly any kind of soil. However, if you want the largest fruit possible, the soil should be rich in organic nutrients such as compost and mulch, and it should drain well. On the other hand, sandy soils don't retain enough moisture, so they need the compost and mulch to retain enough water that the plant doesn't die of thirst.
The soil is a consideration often not taken seriously enough, despite its importance. Two years ago, a person noticed that in their two-year-old compost pile, a volunteer pumpkin plant had started to grow, long after pumpkins would normally be planted in the first place. The area was known for root crops, but pumpkins normally didn't do very well. Part of that was because the climate was too hot and dry.
Expecting no more than to at least give the plant the chance to grow, he started watering it. He was only mildly surprised when, by late July, blossoms started opening on the plant that was not quite a month old. By early August, there were several fruits on the vine. When hard frosts came at the end of September, killing the plant and uncovering the fruits, he was honestly astounded.
The total off the volunteer plant growing in good soil, given plenty of water, but not living in the best place for sunlight, was a 98 pound, 87 pound, two 62 pound, and one 47 pound pumpkins. All of this was in an area where it was difficult to even get a pumpkin to grow, and where the average was seldom more than 20 pounds, for those that did.
All the above is important for growing large healthy pumpkins, but soil and water are the most important. These two even trump the length of the growing season. Knowing what to expect and how to grow them, you too can have champion pumpkins.