The dahlia's reputation as a finicky flower is well-earned. Even before you plant the bulbs into the ground, you will have to take special care to prepare the soil. Dahlias need fertile soil with a pH of around 6.5 to thrive. The fall before you plant, contact your local county extension office to have a soil test conducted. The results will inform you of any pH or nutritional amendments you must add to your soil to make it a suitable dahlia bed.
Till the top 6 inches of the soil in late winter or as soon as the ground thaws. Use a hand tiller to turn and mix the soil until it is an even consistency. Break up large clumps of soil and remove any rocks or plant and weed debris and roots that you find along the way.
Smooth the planting area with a rake and walk over it to compact the soil a bit.
Spread 3 pounds of a commercial 5-20-20 fertilizer per 100 square feet over the tilled area.
Till the soil again to evenly mix the fertilizer into the top 4 inches. Rake the soil smooth and walk over it a few times to compact it.
Plant your dahlias roughly one month after adding amendments to the soil or when the soil has warmed and all danger of frost has passed. Dig a hole that is just slightly wider than the dahlia tuber and deep enough to allow 6 to 7 inches of soil to cover the top.
Add a small handful (roughly one-fifth the amount of soil) of sphagnum peat moss to the soil excavated from each dahlia hole and mix it in well. Plant the dahlia bulbs with the amended soil.