Why are my Chinese elms yellowing and dropping leaves?

There are two main reasons the leaves on trees turn yellow and drop. First one is the tree could be lacking of nutrients needed. Have you fed the trees every year, every 6 months, or not at all? Second one is that the trees are getting too much water and not drying out in between watering. This could be because the soil is not draining properly or they are getting water too often. You can check this yourself by sticking a pencil down about six inches in the ground and seeing if the dirt is still wet.