I suggest a slow release plant food such as Osmocote at planting time to provide continuous nutrition as the plant grows. Then when the dahlia plant is half the expected height you can use a stronger or quicker acting plant food to promote blooms such as Miracle Grow made for flowering plants. The long term answer is to improve your soil so you will not need to fertilize much. Adding organic matter such as compost or getting a soil sample to see if your soil is too acid or too basic can help you to unlock natural nutrients int the soil. Dahlias need warm soil to grow quickly so a raised bed or large container in a sunny site will speed up growth. Keep growing, Marianne Binetti