Dahlias benefit from a low-nitrogen liquid fertilizer (similar to what you would use for vegetables) such as a 5-10-10 or 10-20-20. You should Fertilize after sprouting and then every 3 to 4 weeks from mid-summer until early Autumn and then stop.
Do NOT over-fertilize, especially with nitrogen, or you risk small/no blooms, weak tubers, or rot. How to Dig Up Tubers
this is for those who either need to divide in spring or who have poor drainage or permafrost.
Digging up tubers is easy.
After the first frost has blackened the foliage, cut off all but 2 to 4 inches of top growth.
Carefully dig around tubers with a pitchfork without damaging them.
Lift and gently shake the excess soil off the tubers.
place on the grass to sun/air dry until the dirt is dried up.
treat any open wounds with dolomite lime or flowers of sulphur and remove any damaged or rotting tubers
using your chosen method* see storage post* - store in a cool, dry, dark area with airflow and check every few weeks for rot or mould etc