Posted: 30/06/2015 at 09:02
I wouldn't use it on seedlings and I wouldn't let it get on the foliage, but it'll be fine watered onto the tomato compost (you don't want wet tomato foliage anyway). It won't affect the taste
When the butts are empty clean them out with a scrubbing brush and a very dilute solution of Jeyes Fluid or household bleach - dispose of it carefully down an outside drain (thus cleaning the drain as well).
Remember Jeyes Fluid in water is attractive to cats and dogs, and also deadly poisonous to them.
Keep a lid on the waterbutts to prevent the formation of algae and to stop stuff getting in and decomposing - that's what causes the smell.