Last year I didn't really plan on growing a first prize pumpkin. My only goal was to grow a pumpkin that was the biggest on the street. By the end of August I knew that I had something special because I could literally see the pumpkin grow every day. Big pumpkins need great soil. When I first plant my seeds I like to use Happy Frog Light Warrior soil less medium with some mycorrhizal fungi added. I've tried other types of soils and soil less mediums and there is a 20% difference in the size of a seedling planted in Happy Frog verses regular top soils.
Once planted don't give your pumpkin any fertilizer. The plants are tender and don't need any special nutrients outside of what the compost, worm castings, and soil has to offer. There are lots of good sites on how to build a good compost but if you don't have a compost pile and a lot of time, try the following soil prep recipe. You can buy most of these items at Home Depot, Wal Mart or most good garden centers:
- Earth Gro Composted Steer Manure with humus
- Nature's Yeild or EKO brand compost
- Happy Frog Soil conditioner
- Earthworm Castings
- A mild organic fertilizer with mycorrhizal fungi such as Happy Frog 3-3-3
Dig the above amendments into your soil at least one foot deep and at least twelve feet wide from the center of where you intend to plant your pumpkin about a month before planting. After digging that mixture into your soil you should have a pretty decent planting area for your pumpkin plant. A soil test from the Colorado Extension service can help you know exactly what you need to put into your soil. Pumpkin roots can travel as far as 25 feet and tap roots can travel another 5 feet from the vine so the bigger an area you can prepare the better off you will be.
The difference between a giant pumpkin and a good sized pumpkin is the soil preparation that happens in the fall however. The above recipe is good when the planting season is upon you, but to really build your soil to world class levels you need to start in the fall and let it sit for 6 months. Good soil prep includes 2-3 yards of compost added to the soil in a 30x30 space along with humic acid and any other additional soil amendments that are recommended from your soil test. In Denver most soils tend to be alkali which means you need to drop the ph levels of the soil. The best way to do this is add sulfur and organic matter to the soil along with the rest of the amendments and rototill it all in. By spring you should see the ph levels of your soil drop a fair amount. In late winter or early spring you should do another soil test and then make the proper adjustments to your soil to make sure your organic matter, nitrogen, phosphorus and potash levels are just right.