It might be getting too much water? It could also just be slowing down for the winter, if you're still giving it a lot of water, I'd cut back a little.

Thank you iandyaz! I think that must be the reason. I'll cut back on the water.

It's good you're trying to keep the soil from drying out. But it is getting to be the winter season now and the plant does not need as much water.

And that potting soil you used does not look like it's going to be the best (it looks like mostly small wood chips), you will want more loamy soil that is going to be able to retain water better in the summer. If the soil does not retain water well, you will have to water it more frequently (in the summer I'm talking about).

The top layer is actually 3 inch mulch. I used garden soil mixed with native soil when I planted the tree. If it is magnesium deficiency, what should I do? I just cut back the water to once a month.

Could be an iron deficiency with the yellowing leaves. I would stay away from the Miracle Grow right now because the last thing you want is tender growth to be ravaged by an unexpected frost/freeze. Try to give it a foliar application of chelated iron and see if that does the trick. From what I can see in the photos, the tree overall looks fine, so don't overdue the "treatment."