Hello, I have a 3-4 year old river birch in my yard. The leaves started turning yellow about a week ago and continued to change the color. Within a week, about 60% of the leaves are yellow. I googled and learned that potentially there are two reasons. One is lack of water. I believe that is the case, because of the amount of rain we had in the area recently. Also it is a location where clay soil retains a lot of water anyway and we do not water the section of the yard. The second reason was lack of iron. The person I was watching a youtube said we could use Milorganite to supplement the iron deficiency. My question is are there any other reasons that need to be considered, do you recommend Milorganite, is there alternatives to Milorganite? Thank you so much.