Answer: There are several possible reasons for leaves to yellow, unfortunately it is difficult to make a diagnosis long distance. First off, make sure the mulch is pulled away from the trunk by several inches and that it is no deeper than about three inches. Overly deep mulch or mulch against the trunk can invite fungal and disease problems, rodents (which may eat bark or roots), and may cause a lack of oxygen for the roots.
Next, make sure that the soil directly around the root ball and the surrounding soil is evenly moist but not sopping wet. You may need to dig down with your fingers to check. It is a good idea to water deeply in times of drought or those hot summer dry spells uring the first few years a tree is in the ground. Water as needed, wait a few hours, then dig down and see how effective your watering is, sometimes it is surprising. The soil should be kept moist up until it freezes in the late fall. Overwatering can actually cause yellowing, so take care that you are not keeping the soil totally saturated.
Next, any fertilizing should be done on the basis of soil tests rather than by guessing. Although a mulch could possibly tie up some of the nitrogen in the soil at first, eventually it begins releasing it, so after a year this is unlikely to be the cause of the yellowing. (A nitrogen deficiency would be more likely to show as an overall paler foliage, too.) In any case, late summer is not a good time to fertilize as it may encourage lush new growth that will not have time to harden off before winter.
Overfertilizing can also cause yellowing, so it is better to under rather than overfeed. Whatever you use, do not exceed the amounts indicated on the label unless a soil test tells you more is needed. Generally, an annual application of compost and/or a complete granular fertilizer is adequate.
Finally, there are a few problems that could cause yellowing of the oldest interior leaves first, but you would likely see additional symptoms such as spots on the leaves in addition to the yellowing. If careful inspection reveals additional symptoms, you might want to take a sample to your county extension or consult with professionally trained nursery personnel for a more specific diagnosis.
I hope this helps you trouble shoot.
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