We hear a lot about soil becoming contaminated with onion rot so that we are unable to grow onions or the like in that particular bed for many years to come!

My garden is in the alps and when I first moved here nearly 13 years ago, it soon became apparent that I was never going to grow good vegetables in the ground as it was, because the actual topsoil was barely 3/4 inches deep.

The garden is on a fairly steep slope so I set about terracing the garden and making flower and vegetable beds in terraces. Once the beds were made, I systematically over the years dug the sandy, rocky subsoil out, to a depth of about 18+ inches and replaced it with my own garden compost. The results were excellent for all my crops EXCEPT onions and shallots! I had seemingly infected my compost with onion rot and thus spread it throughout my garden!

Last year I had the idea of making long trenches in my proposed onion bed about 6 inches wide and 4 inches or so deep. I filled these with bagged compost and sewed my onion and shallot sets in this compost. 

The results were far better than I could have hoped! Not one single onion or shallot became infected. This year I did exactly the same and despite a really wet summer, both onions and shallots were a good size and again no sign of rot!

I would be interested in your comments and perhaps my experience will help many other gardeners with similar problems.