River birch is one of the fastest growing and forgiving trees! It can deal with a ton of different water/soil conditions and still do pretty well. But...it can't thrive if it is starved for water, drowned, or grown in alkaline soil.
We have hard clay soil with a ph above 8. And we have an 8 year old river birch that struggles each and every year. It has grown large and is extremely beautiful, but every year over 30% of the leaves turn yellow with green rib lines - iron chlorosis. See below:
The tree just can't get the iron because of the high ph. So this year, I've started trying to change things for good.
I peeled back the mulch layer under the tree and amended the hard clay soil with peat moss. I will also add elemental sulfur and start fertilizing the tree with coffee, acid loving fertilizer and rain water (the same as I do for blueberries.)
Changing the soil ph is a tough undertaking. It takes years and the soil will naturally work it's way back to it's "normal." So it's a life-long process. But it's worth it! Healthy trees are an investment worth making!
If you choose to use acid-loving fertilizers on your trees, be advised that you should not fertilize a river birch (or really ANY tree) in the summer. You should also be careful - or not fertilize in fall or winter. Trees put on almost all of their growth in early spring - so catch them before they wake up and give them a good feeding.
If you fertilize in the summer or early fall, the tree may respond with new growth that will a most certainly be killed in the winter. That will leave dead branches and possibly open wounds for insects and diseases to attack.