How to cost effectively improve soil structure?

I have rocky, loamy soil with few nutrients and low organic matter.

I planted some fruit trees and attempting a fruit tree guild. I have a root mulch ring around all trees and I used black Kow compost when I put them in the ground. In the guild I planted comfrey (chop and drop), strawberry, marigolds, and clover in the grass surrounding the trees.

What else can I do to improve the soil structure?