Kimberlina,
We could use a bit more info. When were the trees planted? How often have you been watering and how much each time you water? I can't tell you how to read the meter as I've never used one. You can do the same thing with your finger. Just push a finger in a few inches a foot or so away from the trunk and if it feels dry you need to water. If it's moist hold off and check again in a few days. I usually soak a tree well when it's planted and then check the moisture with my finger every few days. You don't want to drown it, but don't let it dry out, either. Are the trees and shrubs mulched? That will help retain moisture. Any chance of you posting a picture? I hope this helps.

it is imperative that the ENTIRE root ball is damp.. but not sodden wet.. which i probably would not expect in NM ...

one gal of water on a large tree is NOT enough .. the roots need water. where the roots are.. and if this is a large tree that could be a foot or three down ....

dig out away from the original root ball a foot or two down.. and then near the root ball ...and see if it is damp or arid down there.. adjust watering appropriately ...

i doubt any snake oil [superthrive] is going to have any impact if watering is not proper...

good luck

ken