Posted: 10/08/2013 at 22:32
I've never done double digging. Too much like hard work. Unless you have a solid clay layer, I wouldn't bother. It sounds like you have hungry sandy soil. Get loads of well rotted farm yard manure on bed 1 of 4, Leave it for the winter and then grow taters in it. add fertiliser/ garden compost to bed 2 , grow beans. bed 3 grow brassicas and salad veg. After you lift the potatoes, lay compost on the surface of bed 1. Let the worms do the work in the winter.
The following year grow beans in bed 1 , brassicas in bed 2, add FYM to bed 3 and grow potatoes.
Year 3 brassicas bed 1 FYM (pots) bed 2 and beans bed 3.
So every third year one of the beds gets a load of fibre to build up fertility and retain water. After about 6 years, the soil level will have gone up, and you will only need to fork it over. No double digging needed unless you get a "pan" of soil a foot down.