One thing my Nurseryman husband used to do
for our own tomatoes, was to add plain old
Epsom Salt to the soil. He added it to everything
we grew, and they used it by the hundred pound
sacks at the nursery in potted plants.
It made our lemon tree produce larger, thinner
skinned fruit with sweeter juice. Works nicely on
all citrus, among other fruits and veggies.