Berries are fun to eat and seem to go in groups. However, not all berries get along. Frequently, people plant blackberries right beside blueberries.
Trouble is, blackberries like alkaline soil and blueberries like acid soil. If one grows well, the other one won’t. How do you know which one will thrive?
Have a soil test done. They are usually around $10 and the bags and instructions are available from your county extension agent. When the test comes back, look at the pH.
If the number is below 5.5, you have a blueberry field. If the number is 5.5-7.0, you have a blackberry field. If the number is below 4.0 or above 7.0, you are in serious trouble. Not much grows in those ranges.
The good news is that if you want both blueberries and blackberries, the loser in your pH contest will grow just fine in a big pot. By big, you need to choose one that you can just barely encircle with your arms. That way, the plant has plenty of room to grow and the soil won’t dry out too fast.
For blueberries, you want a potting soil for acid plants. If you cannot find one that is labeled for blueberries, one for azaleas will usually do just fine. Peat moss is acidic, so mixing that in standard topsoil will work in a pinch. Just be careful not to inhale any of the dust particles from the peat moss, it carries a respitory fungus that can cause problems if you do.
For blackberries, normal potting soil will work fine. Most plants prefer the same range of pH as blackberries, so no special mix is required. You need the same big pot, though.
The advantage to having your plants in pots is that you can bring them in during the winter and set them out on nice days, so they produce earlier. You can also have both blueberries and blackberries without worrying about your soil pH.