Dahlias will grow in any soil type, but they might not thrive in every soil type.
Our soil is clay. Most people think that it is hard to grow dahlias in clay because of its water holding capacity and it is true that clay does pose some challenges but they can be overcome. Clay soil also has a good side to it. It requires less water and fertilizer, because of its holding capacity they do not leach through the root zone as fast as they would in a sandy soil.
If you are planting dahlias in clay soil wait until the soil is moist and not wet to plant. Do not allow the soil to dry out in the summer, Clay soil can become so dry that water will “run off” or flow to another area instead of penetrating into the root zone. If your clay soil stays wet there is an increased chance that the dahlia tubers will rot. If you live in a climate where you can leave tubers in the ground over winter but you often have standing water because the soil is saturated, the tubers should be dug up and stored over the winter. On the good side clay soils require less water than other soil types and that can be a big money saver. Also less fertilizer is required to maintain healthy plants. We fertilize two times a season, when we plant and when the dahlias start blooming. Water, fertilizer, and other chemicals bind with the small clay particles and move through the soil at a slower rate than in other soil types.
If you are planting in sandy soils you should have good drainage and if you live in an area with mild winters you can leave your dahlia tubers in the ground without too much to worry about. Dahlias do enjoy growing in sandy soil but there are also some challenges to growing in sand. Sand has almost to holding capacity therefore water, fertilizer, and chemicals move through it rapidly and need to be applied more often.
So what is the best soil to grow dahlias in? I feel that a sandy clay loam or a clay loam are the best soils for dahlias. These soils have a good balanced holding capacity (not too much not too little) and a good amount of organic material in them. Organic material is necessary for healthy soil. The microbial break down of organic materials makes nutrients available for plant to take up. Microbes are also needed to break down fertilizers for plant take up.
Can you change your soil? Yes. A good blend of soils would be 40% clay, 40% sand and 20% organic. If you have a loam soil add sand, clay and organic material to it. Remember that too much organic material is just as unhealthy as too much sand or clay. Once you have added the inorganic sand or clay to your soil it should not need to be added again for years. Because organic material is broken down into nutrients need to sustain the live in the soil and plants it will need to be replaced on a yearly bases. We add our lawn clippings, maple leaves, and the dahlia stocks to our gardens. Throughout the growing season grass clippings are added to the walking paths in the garden then in the fall leaves are added and the dahlia stocks are left.
Even with a good blend of soil PH is still something that needs to be at a good balance or the nutrients in the soil will not be available to the plant. The best PH balance for nutrient up take is 6.3
The best PH levels for dahlias would be 6.2 to 6.8 which is slightly acidic. If your soil is below 6.2 PH, lime can be added to raise the level. If it is above 6.8 sulfur can be added to lower the PH level. Call a local nursery and see if they can test your PH levels. They will tell you what you need to bring to them for the test, and after the test they can tell you what you need to do to get the PH level you need.
Remember dahlias will grow in almost any soil, but if you can give them a better soil (sandy clay loam) They will grow better and require less work. We grow in clay with around 5% organic material and they do fine for us.