fertilise your fields in spring time, just before or at the start of growth.
Recommend that you ask local farmers on neighbouring farms what they use as they will know what type of ground you have and therefore what ratios you need in fertiliser.
They should beable to tell you where to get it or if you are using a contractor then they should keep you right!
Plus if its had horses in it, always worth trying to get it rolled at the same time!