Allotments 4 All

I grow my toms in the borders.  The last couple of years I have dug out the soil and replaced it with fresh but I read somewhere that you only have to do that every few years.  Would it be all right to leave the soil in and grow my toms in botomless pots on the top of the old stuff.  Altrnatively should I sterilise the soil with Jeyes fluid.

I don,t use the border but grow my Toms in containers which are one third of a 215ltr drum. All I do is replace about a 1/4 of the soil/compost mix, drench with Armillatox and leave for 2 weeks, then I add fish/blood/bone and pelleted chicken manure and keep the container covered until I plant out. The soil/compost mix removed is used as a mulch on the raised beds and then dug in, I use Armillatox on them also. So far (5 years ) it seems to work.