I have a rougly 3.5 feet young Tangerine tree that i planted in early Spring here in Houston, TX. A few months ago, I noticed that the base of where the tree stem ended, some of the beginnings of the roots was exposed, probably due to soil washing away due to watering. I put some garden soil to cover it up, put some mulch on top of that as well to retain moisture. In the process, I also stuck a spike of drys slow releasing fertilizer (N-P-K ration is 16-4-8) about 2 feet away at a 45 degree angle into the ground. Since I did the above activities, a significant number of the leaves have started turning Yellow (see pic). It is really hot during the day and I end up watering the tree about every 3 days, but when I do water, i water heavily.
I am not sure if the Yellow'ing is due to me under watering, over watering, too strong fertilizer or a combination. Please suggest what to do.