This is my web page on fertilizing.
http://www.weareallabouttrees.com.au/fertilizing.html
Talks about trace elements. Mineral deficiencies or even too much of a mineral can affect the tree. Yellowing, perhaps not enough iron etc. Chances of deficiencies in a sandy soil is higher so regular supply of trace elements should help.
Compost is good, I suggested spent mushroom compost, any mushroom farms around?