And the first is weather. If you recently had a cold snap in your area, this can cause the leaves to turn yellow. So can too much water paired with poor drainage. If you suspect that the roots are staying wet, try adding some compost to the soil. Another cause is a high soil pH level. The pH should be 5.0-6.5, so be sure to check your soil's levels at first sign of changing leaf color.
A couple of other helpful hints - take a close look at the color of the leaves. If they are green with yellow tips, that can be a sign the plant needs iron. If it’s the opposite, yellow leaves with green tips, your plant likely needs magnesium.
Hope this helps!