I use Sutton's Tomato feed - (50gm pack to make up to 900 litres of
food by mixing with water - good value).

I recently made too much up and used it to "water" some perpetual
spinach and also beet root.

However - I have noticed on some plants of both species that some
parts of some leaves are going brown.

Surely this cannot be because I actually fed them with the stuff-
perhaps I caught/wet the leaves when I watered the food on - would
this explain?