We have a lot of plants in pots in our garden and get through a lot of
fertiliser. The cheapest used to be the boxes of the powder Phostrogen in
our supermarket, but now we cannot see that anywhere locally.

What is the cheapest fertiliser to get hold of at the moment? We wouldn't
mind buying bigger boxes than the ones that go to local shops, although
obviously we dont want it on a commercial scale. Thanks.