I’m trying to find a book where colonists land on a new planet but the food supply / plants turn out to be toxic to people. In the end the people realize the plants aren't actually toxic, but extremely concentrated with healthy minerals - and if they dilute them, the plants are very healthy and nutritious.
[edits added]: this book was read in the early 90s or late 80s so it is not Orson Scott Card's Pathfinder (from 2010) or The Green Book - which had glass like plants, not 'toxic' plants which were just super concentrated nutrients. Thanks for your help so far everybody, but we're still looking for the answer.