A tad bit concerned about my Morus Issai...

[Letoatreides35081]

Hi. I'm writing from Lane County Oregon and I have a Morus Issai Dwarf Mulberry in a container in my backyard. I started the tree indoors when I lived in an apartment, and it grew well, and it's starting to sprout fresh leaves out in the yard of my new place... However, when I had it indoors in front of a window the leaves were a very deep green, and the new leaves, though growing rapidly are a greenish yellow. Is this normal for a second year of mulberry? And if not, is this a sign of malnutrition? If malnourished, are there any organic fertilizers which would be beneficial to mulberries?