Posted: 23/04/2014 at 19:46
Any well rotted manure will inprove soil and as much as you can handle to be honest. I mean, four bags is alright, but eight might be better. It also depends on what plants you want in your bed. Med herbs will hate rich soil for instance.
Topsoil is okay if your soil is really rubbish but check its quality or you could end up with even more rubbish soil being dumped on your pavement. You'll know if its the good stuff, its colour will be appetising and it should have worms and feel nice in your hand.
Bark should only be added as a top layer, as thick as you can afford as it rots down slowly, improving the soil over time and also retains moisture and suppresses weeds to boot!