I have found these two sites be be really helpful when describing Honey Bee Nectar sources.
I looked some text and I think that authors are " not very professional level".
For excample red clover is slended honey plant BUT it need wet soil before it starts blooming and it needs over 25C during one week warm in order to give so much nectar that bee reach nertar. Same with other clovers. I have much clover fields but just when clover is going to start blooming, farmer comes and harwest his yield :))
Heather gives nectar when it is moist and warm. Bogs are very good. Sand and rock areas gives nothing.
I have studied my self bee flowers 40 years. (by the way, I am master in science in botany) . It is very inportant because I carry my hives to better pastures. BUT every year I make mistakes when I valuate blooming pastures. It is difficult because bees fly so wide area.