Giant Pumpkins NZ - Soil
To get the most out of your garden, and giant pumpkin you need to have good soil.
A good measure of this is the PH level of the soil, a good PH level for giant pumpkin growing is 6.5 to 6.8. You should be able to get a PH test kit from any good gardening store.
To help adjust the soil to the correct PH level you can use sulphur or lime. Sulphur will lower the PH level, and Line will increase the PH level.
Soil Temp
The general consensus for soil temperature is that it should be above 22 degrees celsius. You can get a temperature probe from most garden stores, bunnings etc
I've found that as long as your not in freezing weather a lot, you should be ok. A couple of things can help though.
Planting from a seed might be better as it will already be used to the ground temp, growing from a seedling may not work as the shock from pot to ground may be too much, and the plant will go into shock. The safest bet is to have a couple of seeds and seedling and see which one grow best.
Mixing in manure and compost into the soil may also raise the temperature slightly, and for the ultra extreme in keeping the ground warm is to do what the americans do in some parts of the USA, and that is to put heating coils in the ground. I don't think many ppl in NZ would do that.
