How much compost should I add to a clay soil for roses?

Question

A few months ago I removed a lot of over grown shrubs; the soil is clay based topsoil on top of solid clay. The top soil is about one spade deep. I don’t think the garden has had any compost for many years.

I am going to double dig before planning the roses, so how much compost should I put into the subsoil and how much into the top soil. I have a source of compost from wholesale market fruit & vegetable waste and cost is not a limiting factor. (Organic content = 100%, C:N ratio c. 15:1, pH c. 7.5)

I am also planning on adding more worms, as there seem to be very few in the ground. All of the beds will be covered with compost or wood chipping every year, but this is my only chance to properly dig-in compost.

(We are in the North West of the UK)

Answer 1

I would add as much as possible, however much that happens to be, as once the roses are established, they won't appreciate more being dug in around their roots, and using compost as mulch, while it works over time, is a slower way of incorporating it into the soil.

Answer 2

I would add 1" "a bunch"^* of compost and dig it into the topsoil, but I wouldn't mix it into the subsoil. The benefit of double-digging is that you can break up the subsoil. But you don't want to mix your soil layers, and adding compost to your subsoil isn't helpful.

I'm somewhat skeptical of importing worms: if you have poor soil, you won't have worms. But when you improve your soil, the worms will come. Adding compost and double-digging will help by breaking up the clay topsoil. Maintaining a deep mulch of wood chips on the surface will make a worm-friendly environment. (Though I do suppose that, assuming you have created the worm-friendly environment, importing the worms will hasten the process. Just make sure you've got a nice place for the worms to live or they'll leave and your efforts will have been pointless.)

*See discussion in comments, with good points raised by @jmusser. 1" is probably too little. 4" might be a better amount?

Answer 3

Maintaining a deep mulch of wood chips on the surface will make a worm-friendly environment. Though I do suppose that, assuming you have created the worm-friendly environment, importing the worms will hasten the process. Just make sure you've got a nice place for the worms to live or they'll leave and your efforts will have been pointless.