I am using purchased topsoil, top grade. How deep under the maximum depth for the roots should I dig for drainage or fill in with soil? How much space on either side of blackberry bush do I need for soil? I will be planting erect branches to wire, unless you can suggest another way.
I would work in the topsoil at least 15 cm, 30 cm if you can, but 15 cm down will be enough. In terms of how wide to incorporate top soil, I would go .75 - 1 meter on all sides from where the plant will be planted. Your method of trellising sounds good, just remember that you will need to remove older cane as the plant grows, so just make sure that you will be able to do that.
I planted six plants last spring. All but one of the vines, that grew like crazy, are broken off about 6 inches above the crown. The leaves haven't opened yet, but the little stubs left on the crown are alive. Looks like new buds to me. These were grown on plastic-coated wire, spaced 1 1/2 ft, 4 feet, and 7 feet above the crown. Some leaves last year had some brown spots, but the whole vine seemed ok until fall frost. I am in Zone 5/6, 100 miles south of St. Louis.
Yes, the buds on the crown will grow into new canes. It may not fruit this year due to the canes being broken, but it should start fruiting again next year if the new canes are protected.
How do I propagate a blackberry plant?
This article should help you with that: https://www.gardeningknowhow.com/edible/fruits/blackberries/propagating-blackberries-cuttings.htm
I have two blueberries and two blackberry plants - planted last year (bare root). The blueberries are about 18" tall, whereas the blackberries are 4 -6 feet tall. Should they be pruned, and if so, how and when? I have raspberries also - planted three and have several suckers. I was going to prune these by cutting all of them to the ground in early spring - correct?
Thank you
Since these are new plantings, you may want to wait another year before pruning, but you can do a little. Here is more information:
This article will help you with pruning blackberries:
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberry-pruning.htm
This article will help you with pruning the raspberries:
https://www.gardeningknowhow.com/edible/fruits/raspberry/pruning-raspberry-bushes.htm
And this one will help you with blueberries:
https://www.gardeningknowhow.com/edible/fruits/blueberries/blueberry-plant-pruning.htm
Can I only plant one bush?
need to know what fertilizer for blackberry bushes
I would recommend using a fertilizer that has a slightly higher middle number (e.g. 5-7-5 or 20-25-20). This will give them a little extra phosphorous, which they need for fruit production.
Recently acquired thornless blackberry bushes. Our ground is acidic. Is this good for blackberry bushes or should I sweeten the ground with lime?
They like a pH between 5.5 and 7, so it really depends on how acidic your soil is whether it is too acidic. Have the soil tested and then adjust the pH of the soil into this pH zone according to your soil test.