Hello. I have several trees 15 -20 years old that I have watered well over the years but have rarely fertilized. From my MG and some horticulture training, I'm thinking that my clay soil should be providing most or all of what they need, and have been searching for resources to clarify. Can you point me to something that will help me understand if I should be fertilizing my trees? They include pine, spruce, junipers, oak, serviceberry, ash, crabapple, ornamental pear, hawthorne, and a variegated maple, which is doing surprisingly well. Thanks very much for any assistance.